EPA/530/UST-89/012
DETECTING LEAKS
Successful Methods Step-by-Step
November 1989
Office of Underground Storage Tanks
U.S. Environmental Protection Agency
Washington, D.C. 20460
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PREFACE
This handbook provides basic information on release detection that
you—as state and local regulators—will find useful in developing and
implementing the release detection portion of your program for regulat-
ing underground storage tank systems (USTs).
This information is meant to foster your understanding and use of the
release detection methods appropriate for your individual UST
programs. The handbook contains information on the methods of UST
release detection that were the most widely used at the time of publica-
tion; inventory control, manual tank gauging, tank tightness testing,
automatic tank gauging, vapor monitoring, ground-water monitoring,
secondary containment with interstitial monitoring, and piping release
detection methods.
111
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ACKNOWLEDGMENTS
This document was written by Cecily Beall, Linda McConnell, Albert
Nugent, and Julie Parsons for the U.S. Environmental Protection Agency's
Office of Underground Storage Tanks (EPA/OUST) under contract
No. 68-01-7383. Production assistance was provided by Maridene Amdt,
Keith Cox, Donna Kirk, and Doris Nagel. The Work Assignment Manager
for EPA/OUST was Thomas Young, and the EPA/OUST Project Officer
was Vinay Kumar. Technical assistance and review were provided by the
following people:
Tom Bergamini - Wisconsin Department of Natural Resources
Fermin de la Camara - Dade County, Florida, Environmental Resources
Management
Jon Gross - Nebraska State Fire Marshall
Sav Mancieri - Rhode Island Department of Environmental
Management
Enemute Oduaran - Delaware Department of Natural Resources and
Environmental Control
Michael Randolph - San Jose, California, Fire Department
Mike Scoggins - U.S. EPA Region VI
Helga Butler - U.S. EPA Office of Underground Storage Tanks
Steve McNeely - U.S. EPA Office of Underground Storage Tanks
Peg Rogers - U.S. EPA Office of Underground Storage Tanks
Phil Durgin - U.S. EPA Environmental Monitoring Systems Laboratory,
Las Vegas, Nevada
Tony Tafuri - U.S. EPA Risk Reduction Engineering Laboratory,
Edison, New Jersey
The Leak Detection Technology Association
American Petroleum Institute
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Disclaimer
This document has been reviewed in accordance with U.S. Environ-
mental Protection Agency policy and approved for publication.
Mention of trade names or commercial products does not constitute
endorsement or recommendation for use. Other alternatives may exist
or may be developed.
VI
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TABLE OF CONTENTS
I. INTRODUCTION
Purpose of the handbook
Contents of the handbook
Use of the handbook
1
2
3
II. INVENTORY
Summary
Brief description
Potential problems and solutions
Ensuring effective manual tank gauging
References
13
13
14
30
31
III. MANUAL TANK GAUGING
Summary
Potential problems and solutions
Ensuring effective manual tank gauging
References
33
34
43
45
IV. TANK TIGHTNESS TESTING
Summary
Brief description
Potential problems and solutions
Ensuring effective testing
References
47
48
52
73
76
vu
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V. AUTOMATIC TANK GAUGING
Summary
Brief description
Potential problems and solutions
Ensuring effective automatic tank gauging
References
77
78
83
92
94
VI. VAPOR MONITORING
Summary
Brief description
Potential problems and solutions
Approaches to ensuring effective vapor monitoring
References
95
95
96
123
124
VII. GROUND-WATER MONITORING
Summary
Brief description
Potential problems and solutions
References
127
128
130
159
Vin. SECONDARY CONTAINMENT WITH
INTERSTITIAL MONITORING
Summary
Brief description
Potential problems and solutions
Ensuring effective secondary containment monitoring
References
161
162
169
180
181
viu.
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IX. PIPING RELEASE DETECTION METHODS
Summary
Brief description
Potential problems and solutions
Ensuring effective release detection for piping
References
183
184
193
204
206
SUBJECT INDEX
APPENDIX A—LIST OF FIGURES
APPENDIX B—LIST OF TABLES
207
212
215
rx
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Chapter I
Introduction
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INTRODUCTION
PURPOSE OF THE HANDBOOK
This handbook provides basic information on release detection that
you— as state and local regulators—will find useful in developing and
implementing the release detection portion of your program that
regulates underground storage tank systems (USTs).
Even though tens of thousands of UST systems have had leaks or are
currently leaking, most existing UST systems are not currently being
monitored for releases. As a result, Federal regulations now require all
UST systems containing petroleum or certain hazardous chemicals to
have effective release detection.
Although some states and local governments have had active UST
programs for several years, most regulators are just beginning to acquire
the knowledge necessary to develop and implement UST management
programs. This handbook supplies information useful in acquiring that
knowledge.
Because the Federal release detection requirements will be implemented
through your state and local regulatory agencies, you need information
during the development of your UST program to answer the following
questions:
• What release detection methods are allowed?
• How do these methods basically work?
• What potential problems does each method have?
• What solutions to these problems are available?
• How can you make sure UST owners and operators are aware of
these potential problems and respond to them?
• How can you make sure that the providers of release detection
follow proper protocols and practices?
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You will find enough basic information in this handbook to answer
these kinds of questions as you develop and implement effective release
detection programs.
The handbook is not intended, however, to be a technical "how to"
manual for the release detection methods. It should not be used to
become familiar with the intricacies of different brands of test
equipment.
CONTENTS OF THE HANDBOOK
The handbook provides information about the following methods of
release detection for tanks allowed in the final rule
(53 FR 37196-37212):
• Inventory Control
• Manual Tank Gauging
Tank Tightness Testing
• Automatic Tank Gauging
• Vapor Monitoring
« Ground-water Monitoring
• Secondary Containment with Interstitial Monitoring
The handbook also contains information on the release detection
methods allowed in the final rule for underground piping.
The specific release detection methods allowed in the Federal rule have
been demonstrated to successfully detect petroleum releases from UST
systems and are currently allowed under several established state and
local programs.
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The chapters that follow describe the release detection methods allowed
under the Federal regulation. Each chapter focuses on a series of topics
presented in the same general order:
• Summary of the chapter
• Brief description of the release detection method
• Potential problems associated with the method
• Solutions to the problems
• Ways to oversee or enforce that proper release detection takes
place
• Technical references
The information in this handbook was initially gathered by EPA to
support development of the final federal regulations. It was gathered, in
part, from numerous visits to state and local programs and from
experienced vendors of the various methods. Additional important
technical information came from an extensive release detection research
and development program conducted by EPA laboratories in Edison,
New Jersey, and Las Vegas, Nevada, from 1985 through 1988.
USE OF THE HANDBOOK
On the next page, you will find a generalized flow chart (Figure 1) of
the process used by state implementing agencies to develop UST release
detection programs. This chart is based on information from several
states that have already developed UST programs. Agencies typically
begin by writing regulations on leak detection. The crucial decisions in
this area are what methods to allow and what design and performance
standards are necessary to ensure that the methods are effective. Once
the regulations are in place, the states face the issue of how to
implement their requirements. States have developed a variety of
oversight mechanisms to ensure that testers or installers follow the
design and performance standards.
The following sections provide expanded discussions of how you can
use the handbook in developing a leak detection program as outlined in
Figure 1.
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Regu ation
Development
Implementation
Activities
Write and Finalize Regulations
Which methods to allow
What restrictions on the methods
What
additional work
is needed
to implement effective
release detection
1
f
1
Develop
Procedures
for SITE
INSPECTIONS
1
REVIEW DATA
from Tests or
Monitoring Efforts
i
r
Develop
GUIDANCE
Materials
1
r
Develop
APPROVAL/
CERTIFICATION
Procedures
Figure 1. Development of a state or local leak detection program
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Which Release Detection Methods Should You Allow?
In writing the federal regulation, the U.S. Environmental Protection
Agency reviewed existing release detection methods and state UST
programs and selected the methods that were demonstrated to be
effective. States have the option of being more restrictive when
deciding which methods to allow within their jurisdiction. EPA
believes that, with appropriate restrictions and oversight, most, if not all,
release detection methods discussed in this handbook can be successful
for any given set of site conditions. Therefore, there should seldom be a
reason to exclude a type of release detection from a state program.
Many of the problems that are rumored about a method, such as
background contamination interfering with vapor monitoring, can often
be avoided or overcome. The chapters on the specific release detection
methods provide detailed discussions of solutions to potential problems
for both contractors and state officials.
None of the methods (including secondary containment with interstitial
monitoring) is fail-safe and assures detection of all leaks. EPA research
and state and local experiences in the field have shown, however, that
each of these methods has proven to be effective in detecting UST
releases when used properly and within the inherent design limitations
of the method. EPA has developed this handbook in the belief that a
better understanding of potential key limitations of each method will
convince you that each one can have an important role in your UST
program for detecting releases.
Although the handbook discusses the potential limitations or problems
with each method, it does not provide information on how frequently
these problems actually occur today in the field. In fact, many of the
reservations identified in the handbook concerning a method's proper
use are already well known and carefully controlled for by experienced
providers of release detection. Thus, many of the problems identified in
the handbook may not need special state or local consideration or
oversight to ensure that a particular method is being effectively applied.
For example, numerous tightness testers have recently adjusted their
protocols and training materials to incorporate the lessons learned from
EPA's tank testing research completed at the Edison, New Jersey,
laboratory in 1988. Accordingly, many of the concerns raised in the
tank tightness testing and automatic tank gauging chapters of the
handbook are now well known in the release detection service industry
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and have been used recently to increase the quality of the testing
services and equipment being provided nationwide.
In spite of the basic technical soundness of the different methods,
however, new entrants or otherwise inexperienced members of the
rapidly developing release detection service industry may need some
state and local oversight and guidance to assure their methods are
properly applied. Likewise, typical UST owners and operators may
need some help in selecting and using the right release detection method
for their sites. This handbook will help you develop regulatory
programs that assure release detection is used effectively at all UST
systems.
Table 1 on pages 8 and 9 presents a summary of the important factors
that affect the success of each release detection method. Included in the
table are summaries of the important design or operational elements that
each method should include to account for specific site conditions.
These elements are explained more completely in the chapters on each
release detection method. As can be seen in the table, there are very
few site conditions that completely eliminate from consideration any of
the approaches to release detection. Most methods will work at a site
given the selection of appropriate equipment and proper installation and
operation. Different site conditions favor different methods. For
example, ground-water monitoring is more effective in areas with
shallow ground water and with a product that floats. Methods that are
external to the tank itself, such as vapor monitoring or ground-water
monitoring, are more effective for large tanks. Methods that are internal
to the tank or its containment (interstitial monitoring, tank tightness
testing, automatic tank gauging, manual tank gauging, and inventory)
are more effective for highly contaminated sites. Tank owners and
operators will be best able to meet the regulations considering their
specific site conditions if all choices are left open to them. The
availability of a wide selection of release detection methods allows the
owner or operator to get an effective leak detection system for the
lowest cost.
The information listed in the table is applicable to both tank and piping
release detection. However, while manual tank gauging and inventory
control will effectively detect leaks from tanks, the sensitivity of these
methods is very low when applied to piping. Even the procedures
outlined in the table will not improve the performance of these methods
sufficiently to make them acceptable stand-alone release detection
methods for piping. The sensitivity of automatic tank gauging to piping
releases is still unproven.
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What Restrictions Should You Place on the Methods?
As illustrated in Table 1, most methods will work in most situations as
long as proper procedures are followed. In developing their UST
programs, implementing agencies may elect to place restrictions on
allowable release detection methods to ensure maximum effectiveness.
Each chapter contains a table titled "Indicators and Solutions for
Problems" that summarizes the information presented in the chapter,
and the column titled "Solutions" summarizes the actions that are
necessary to ensure effective release detection with that method. This is
the information that should be evaluated when an implementing agency
is considering regulatory limitations on the use of a method. For
example, the chapters on ground-water and vapor monitoring include
monitoring well network design requirements from several existing state
UST programs that have been confirmed as effective through a
combination of field experience and EPA research.
What Agency Oversight Mechanism Should You Use?
Once leak detection regulations have been adopted, there are four basic
approaches that an implementing agency can use to oversee the work of
release detection testers and installers to ensure that appropriate
methods have been chosen and that the correct procedures are followed
by testers and installers (see Figure 1). The "Indicators and Solutions
for Problems" table in each chapter contains a column titled "Agency
Oversight Options" that lists possible applications of these approaches
to the specific release detection method; the oversight options are
discussed in more detail in the text of the chapters. The following
discussions present general descriptions of the approaches and their
advantages and disadvantages.
Site inspections
Some local agencies have found that having their staff present
during release detection tests effectively ensures that tests are
conducted properly. Before such an approach can be implemented,
agency personnel have to be trained in proper procedures and
develop either a checklist of important features to be examined at
each site or an inspection procedures manual.
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Table 1. Effect of Site Conditions on Success of Release Detection Methods for Tanks and Piping
Manual Tank
Gauging and
Site Conditions Inventory
Automatic Tank
Gauging
Tank Tightness
Testing
Secondary Contain-
ment with Interstitial Ground-Water
Monitoring Monitoring
Vapor Monitoring
Ground water
Product type
Tank size
Shallow ground water
may interfere under
some conditions. Use
water-finding paste to
detect water in tank.
N/A
Less effective for
large tanks. Manual
tank gauging limited to
less than 550 gallons
when used alone or
less than 2,000 gal-
lons when combined
with tightness testing.
Shallow ground water
may interfere under
some conditions. Use
sensor to detect water
level in tank.
Effective for products
with viscosity and
thermal properties
similar to gasoline
and diesel fuel.
Shallow ground water
may mask a leak.
Can be used in high
ground water if
ground-water level is
measured and product
level in tank is raised
to overcome ground-
water pressure. Do
not test when ground
water is fluctuating.
Effective for products
with viscosity and
thermal properties
similar to gasoline and
diesel fuel.
Less effective for
large tanks. Generally
applicable to tanks
< 12,000 gallons.
Applicability to larger
tanks depends on
method and must be
demonstrated.
Less effective for
large tanks. Generally
applicable to tanks
< 12,000 gallons.
Applicability to larger
tanks depends on
method and must be
demonstrated.
Shallow ground water
may interfere with
sensors. Usefully
double-walled USTs,
excavation liners, or
jacketed tanks or
methods that are not
affected by water.
Depends on sensor
construction.
N/A
Deep ground-water Shallow ground water
delays detection. Do may interfere with
not use for sites where sensor. Do not use in
(a) ground-water depth saturated sites.
is > 20 ft or < 3 ft or (b)
ground-water fluctu-
ation exceeds well
screen interval for
more than 30 consec-
utive days. Place some
wells downgradient to
UST, if grade can be
determined.
Product must float on
ground water to be
detected (most petro-
leum products do).
N/A
Lower volatility products
delay and may prevent
detection. Effective for
gasoline. Response to
other products should be
verified. For less volatile
products, add tracer
compound, use aspir-
ated sensors and more
and larger diameter
monitoring wells, or set
lower alarm levels.
N/A
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Backfill
permeability
N/A
N/A
N/A
Existing site
contamination
N/A
N/A
N/A
With excavation liners,
backfill must be
permeable to allow
release to be detected
Use sand or pea
gravel as backfill.
Use monitoring
method that can
distinguish leak from
existing contamina-
tion or use full double-
walled tank or
completely sealed
excavation liner or
tank jacket.
Use sand or pea
gravel.
Floating product from
prior releases may
cause false alarm. Do
site assessment in
areas of suspected
background contam-
ination, and use other
methods if contami-
nation would cause
false alarm.
Use sand or pea
gravel.
Do not use at site with
high (background
concentration < 15,000
gallons ppm for gas-
oline) unless a manu-
facturer can show how
their device works at
high background levels.
Tracer compound may
also be used.
Temperature
Subsurface
conduits
Temperature differ-
ence between
newly delivered
product and product
in tank limits
accuracy. May use
simple temperature
measurement to
partially compensate.
N/A
Measure product
temperature for at
least three levels in
tank. Use longer
waiting times before
testing as tempera-
ture difference
between newly
delivered product
and product in tank
increases.
N/A
Requires frequent
measurement of
product temperature
for at least three
levels in tank; or that
the product mixed.
The greater the
temperature differ-
ence between added
product and tank,
the longer the wait
before testing. A few
methods are indepen-
dent of temperature.
N/A
Freezing of interstitial N/A
fluid in hydrostatic
systems will prevent
detection while high
temperatures may
cause false alarms
because of loss of
fluid through
evaporation. Add
antifreeze in cold
weather and add
more fluid in hot
weather.
Subsurface conduits Subsurface conduits
should not be in
backfill. May cause
leak to migrate so
that it will not be
detected.
should not be in
backfill. May cause
leak to migrate so
that it will not be
detected.
Low temperatures
reduce sensitivity.
Install sensors below
frost line. Use more
and larger wells or
aspirated systems to
compensate for reduced
volatility of product.
Subsurface conduits
should not be in
backfill. May cause
leak to migrate so
that it will not be
detected.
*N/A = No significant impact on test method.
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There are several advantages to having a knowledgeable regulator
observe the installation and/or operation of the test equipment.
Observation may cause the testers to be more conscientious about
following procedures. If deviations from proper procedure occur,
the inspector can catch them immediately and require a correction
or retest. An observer may be able to provide suggestions when
problems arise. A third-party observer may provide additional
credibility to the test results, in essence "certifying" that the test
results are valid. Finally, if a release is detected, the regulator is
on-site and can begin investigation immediately. Site inspections,
however, are expensive and labor intensive and may create
considerable scheduling difficulties when a limited number of
people are available to conduct the site visits. The following
discussion on data reviews indicates a possible approach that
requires less resources.
Data review
It is possible to requke that release detection installers, testers, and
tank owners keep detailed records of site conditions, events, time,
results, etc., during a test and submit these records to the
implementing agency. Regulators can then review all of the
reports, which would be somewhat time-consuming and expensive,
or a certain percentage of the reports, possibly on a random basis.
All aspects of the tests may be reviewed or, to save time, only
those aspects that have been determined to be most crucial for that
method in that locale. The most important things to check for in
the installation, operation, and interpretation of each method are
listed in the method-specific chapters. For example, jurisdictions
with a lot of clay deposits in the area may want to check the boring
logs for ground-water or vapor monitoring wells to verify that
product could get to the well if a leak occurred. For test methods
that requke calculation, a manual or computerized check of the
calculations may be performed.
A program to review test data costs less in time and money than a
site inspection program and has some of the same benefits,
although to a lesser degree. The obviously bad tests will be
identified with less effort than an inspection program would take.
Discovery of questionable results in a data review may be used to
target scarce inspection resources. There are two main drawbacks
to such an approach. Fkst, test reports may accumulate unread
while regulators work on problems of more urgency. Second,
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correcting a problem "after the fact" is more difficult than
correcting it on-site as soon as it happens, both in terms of the
regulator finding the time to do it and in enforcing additional action
by the owner/operator or tester after the tester has left the site. A
program to mail back deficient reports, inspect sites with deficient
reports, enforce retesting, etc., may be as expensive as doing the
site inspections initially.
Guidance or training
Release detection equipment installers and testers may conduct
tests improperly out of ignorance of the proper procedures.
Several implementing agencies have issued guidance to owners
and testers that identifies the worst procedural violations and helps
to ensure that release detection is effective. Guidance for testers
and installers may be in the form of booklets, videos, or training
seminars. Another approach to guidance would be to provide a
checklist of important procedures for each release detection
method to owner/operators for use during a test, so that they can
knowledgeably observe installation and testing at their sites.
This approach attempts to correct the problems before they occur
and can reach a fairly wide audience at relatively small expense.
By educating one tester or testing company, procedures at many
future tests have been improved. However, review of the guidance
material and adherence to its recommended procedures is
voluntary.
Approval/certification of release detection methods and personnel
Licensing or certification of release detection equipment and/or
operators is one potential means of ensuring valid test results. The
regulatory agency would have to set up some mechanism for
reviewing and approving equipment and/or personnel. There are
several ways in which equipment might be approved. A board of
knowledgeable regulators can review written evaluations of a
method's performance or hear presentations made by
manufacturers, during which any questions can be answered. A
regulatory agency may accept the findings of an independent
third-party evaluation. EPA is developing standard procedures for
testing leak detection equipment that can be used by independent
laboratories to evaluate the performance of commercial methods.
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Approving leak detection equipment has two main advantages.
First, it allows the agency to make clear and helpful
recommendations to owners about what methods are acceptable.
Second, it allows the agency to evaluate the claims made by
manufacturers and ensure that owners and operators use effective
equipment. The EPA test procedures should allow regulators to
compare the performance of various methods that have each been
tested the same way. Licensing of leak detection testers or
installers is more difficult than approving equipment. There are
many more contractors than manufacturers and the skills required
are diverse. Possible methods for screening personnel for licensing
include written examination, field observation, field test,
apprenticeship, and training requirements (manufacturer
certification). Any effective licensing/certification program also
requires a follow-up program to identify equipment or personnel
that is operating without a license. Also, some system of sanctions
against violators is needed. Such follow-up programs also may be
time-consuming and expensive. Studies of occupational licensing
programs in many areas have shown little improvement in service
quality after the program has been initiated, especially when an
examination is the only requirement. Controlling testers and
installers is important, however, because of the crucial role
procedure plays in determining leak detection effectiveness. Each
of the following chapters describes the available leak detection
methods and important aspects of procedure in greater detail.
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Chapter II
Inventory Control
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INVENTORY CONTROL
II
SUMMARY
To date, the practice of underground storage tank (UST) inventory
control primarily has been limited to use as a business practice to keep
track of product, and indirectly as a release detection method, for
motor fuels stored at retail distribution facilities. Inventory control,
also called inventory reconciliation is claimed to be the simplest and
most economical leak detection method. The technique is effective for
finding larger leaks (over 1 gal/h) if "recommended practices" are
followed. Because inventory control has low sensitivity, EPA requires
that it be combined with tank tightness testing. Recommended
practices for inventory control can be found in the American
Petroleum Institute's Publication API 1621, Recommended Practice
for Bulk Liquid Stock Control at Retail Outlets. Parts of the following
discussion are largely based on this publication.
The discussion presented in this chapter covers many of the possible
problems that may occur during inventory control. This does not mean
that all, or even most, of these problems will occur. Nor does it mean
that all of the problems are of equal importance, in terms of frequency
of occurrence or severity of impact on the effectiveness of inventory
control. Some problems occur infrequently, while others have limited
impact. This chapter presents a range of potential problems for
educational purposes, not to imply that they will always occur.
BRIEF DESCRIPTION
Inventory control is basically an ongoing accounting system, similar to
a check book. Its objective is to reconcile the inputs and outputs of a
stored substance in a given UST with the volume remaining in the
UST. Careful records of all product delivered, product dispensed, and
daily tank inventories are recorded on a ledger-like form and
reconciled on a monthly basis. The system "imbalance" at the end of a
month, the difference between book inventory and measured
inventory, is compared to a threshold value to help determine whether
the imbalance signifies a leak.
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Daily UST inventories are determined by using a gauge stick (similar
to a yard stick). The stick is inserted vertically through the fill tube
until the end of the stick touches the tank bottom. When the stick is
removed, the level of the product corresponds with a number on the
stick (similar to the way a car's oil level is indicated on its dipstick)
which, by using a calibration chart, can be translated to a volume of
product in the tank. A calibration chart often is furnished by the UST
supplier. The chart shows the number of gallons represented by each
inch on the gauge stick. Each chart is calculated for a specific brand of
tank with particular dimensions and capacity, and the chart used must
correspond to the tank being gauged.
Once the volume of product in the UST is determined, it is recorded on
a ledger form as the UST's daily inventory. The amounts of product
delivered to and withdrawn from the UST each day are also recorded.
At least once each month, these data are compared to determine if the
volume measured in the tank corresponds with sales and delivery
records.
The process of obtaining inventory information and its reconciliation,
can be divided into five steps: (1) tank gauging—the process of
measuring the stored substance or the water in an UST;
(2) calibration—the correlation of a gauge reading with the proper
calibration chart to determine the volume of the product in the UST;
(3) tank stock control—the determination of the amount of product that
was added to and withdrawn from the UST; (4) recording and
reconciliation—the use of an accounting form to record and reconcile
the information gathered; and (5) interpretation—the determination of
whether the result of a month of inventory records signifies an UST
release. The relationships among these five stages are shown in
Figure 2.
POTENTIAL PROBLEMS AND SOLUTIONS
A number of factors can affect the accuracy of inventory control as a
release detection method. Often an apparent loss of inventory may
occur even though the UST is sound; inventory records also may show
an overall increase in product. The following sections discuss
problems and solutions related to each of the five steps involved in
implementing inventory control. The order of discussion is not
intended to prioritize the importance of the problems, rather it is
intended to follow the order in which they would occur according to
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Testing
Analysis
Tank Gauging
Product gauge
Water gauge
Calibration
Volume of product determined
from calibration chart
Tank Stock Control
Withdrawals
Receipts
Recording & Reconciliation
\
Interpretation
Leak
No Leak
Figure 2. General procedure for inventory control
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the flow chart in Figure 2. The discussion of problems and solutions is
summarized in Table 2 on pages 18 and 19, in the order of the
discussion, and the most serious concerns have been marked by an
asterisk. Some agency oversight options are offered for problems,
when applicable, but not all of them need be undertaken.
Tank Gauging
"testing; f[ Assure the tank is gauged properly
Analysis
If a tank is not gauged properly, the gauge reading will not
correspond well to the actual volume of product in the UST.
Accuracy will decrease if a gauge reading is inadvertently taken
with the stick slanted or resting upon a protrusion (e.g., an internal
support ridge) in the tank, or if the stick is "bounced" off the
bottom of the tank. Accuracy is increased by careful, vertical
insertion of the stick, by taking more than one gauge stick reading
and wiping off the gauge stick between readings, then carefully
noting and averaging the results.
According to API, to properly gauge a tank, the stick is placed into
the tank through one of the openings in the tank until its tip touches
the tank bottom. Some tanks have a separate opening, called a
gauge hole, used for gauging the tank. The tank fill pipe opening
can be used to insert the stick if there is no gauge hole. The stick
should be inserted at the same point in the gauge hole each time a
gauge is taken and should be held in a vertical position. After the
stick is quickly withdrawn, the product "cut" (wet mark left by the
product on the gauge stick) is read on the graduated scale to the
nearest 1/8 inch. Once the stick is cleaned by wiping the "cut"
with a cloth, the procedure is repeated. Both readings should be
recorded. The average of the measurements should be used to
calculate the product volume in the tank.
Some UST owners prefer that two, consecutive gauge readings be
taken at both open and close each day or before each change in
working shifts. However, an accurate inventory reconciliation can
be obtained with only consecutive closing or opening gauges on a
daily basis. That is, one or the other should be chosen and the tank
gauged consistently at that time. Additionally, the tank meter
should be read at the same time of day that the tank gauges are
taken. Implementing agencies could provide some type of training
to owners and operators teaching a preferred method.
16
-------
Analysis
I
Analysis
Analysis
Avoid damaging the tank from careless gauging
Repeated gauging of a tank may wear a hole in the bottom of the
UST. When the gauge stick strikes the bottom of the tank, rust can
be chipped off of the surface and expose new metal, thus allowing
for quicker corrosion and the development of a hole in the tank
base. This can be protected against by careful gauging and by
outfitting the tank with a striker plate. A striker plate is simply a
layer of metal added to the tank to increase its strength in the area
that comes into contact with the gauge stick. Unless the striker
plate is of negligible thickness, whenever a striker plate is added to
an UST, the end of the gauge stick must be modified by cutting off
a length equal to the thickness of the striker plate so that true
volume conversions can be obtained.
Ensure accuracy of the product reading
If the gauge stick is used to gauge gasoline or other volatile
products, the side adjacent to the graduated side should be grooved
every 1/8 inch to keep the product from moving up the stick past
the measured level (creepage). Additionally, product-finding
pastes, applied over the stick in a light, even film, can significantly
improve the accuracy of gauging. Pastes improve adherence of
product to the gauge stick and prevent creepage. Product-finding
pastes change color in the presence of product.
Water in the tank must be identified
The presence of water in an UST results in an inaccurately high
gauge reading. Water intrusion may indicate a leak, especially in
high ground water situations. In addition, water may enhance
corrosion.
To identify the presence and measure the amount of water, a water
gauge must be taken at least once a month. A water-finding paste,
which is unaffected by the stored product but changes color in
water, is used to check for water at the bottom of USTs.
Information on satisfactory pastes may be obtained from an
equipment supplier.
A water gauge is taken in the same manner as a product gauge;
however, the length of time the stick remains in the tank should be
monitored carefully. The immersion time for a water "cut" is
17
-------
00
Table 2. Indicators and Solutions for Problems Encountered During Inventory Control
Problem
Indicators
Tester Solutions
Agency Oversight Options
* Assure tank is gauged
properly.
Slanted stick. Dipstick not
wiped. Large discrepancy
between consecutive gauges.
Take consecutive gauge
readings. Wipe dipstick
between gauges.
Provide training.
Avoid damaging tank from
careless gauging.
Loss of product.
Add a striker plate to tank base.
Gauge tank carefully.
Assure accuracy of product
reading.
No clear product line on gauge
stick.
Use notched gauge stick. Use
product-finding paste. Withdraw
pole quickly.
Water in the tank must be
identified.
Need to keep track of the
temperature when taking
gauges.
Water in gas. Unexplained gain
in product amount.
Large temperature changes.
Take a water gauge using water-
finding paste. Take water gauges
after delivery.
Record daily temperature next
to gauge readings. Do not take
meter readings shorty after a
delivery.
Require monthly water gauge.
Check UST for water during
site visits. Track customer
complaints of water in gas.
Account for product
evaporated during delivery.
Unexplained product losses.
Use vapor control. Use pressure
relief valves.
Check weights and measures
seal at retail stations. Check
calibration before further leak
investigation.
Assure the calibration
chart corresponds to UST.
Specifications on chart do not
match tanks.
Ask tank manufacturer to provide
a chart that corresponds with their
tank.
-------
Need to use the calibration
chart correctly.
Assure the accuracy for the
pump meter.
Need to unquantify
withdrawals or additions to
the LIST.
Imbalance in inventory.
Unexplained losses or gains.
Unexplained losses.
Use chart according to API
recommendations.
Calibrate pump meter. Read
meters when gauges are taken.
Quantify all losses as closely
as possible.
Review inventory forms.
Require pump meter
calibration.
Assure the data are recorded Imbalance in inventory.
completely and correctly.
*Assure proper reconciliation Imbalance in inventory.
of data.
Need to interpret data correctly. Imbalance in inventory.
Ask supply company for
recommended recording
practiice.
Follow proper reconciliation
process. Double-check
calculations.
Follow recommended
interpretation process.
Review inventory forms.
Review inventory forms.
Review inventory forms.
'Indicates the most significant problem.
-------
Analysis
Analysis
approximately 10 seconds for light products such as gasoline and
kerosene and 20 to 30 seconds for heavier products.
The quantity of water in the tank is calculated using the same
procedure described for product calculations using a tank
calibration chart. If the test shows more than 1/2 inch of water,
arrangements should be made for its immediate removal, the
product supplier should be notified that their product may contain
significant amounts of water, and further tests should be conducted
to ensure that the tank is not leaking.
Need to keep track of the temperature when taking gauges
Temperature increases or decreases can cause an expansion or
contraction, respectively, of the product within an UST.
Expansions and contractions cause level changes that may mask or
imitate a level change due to a leak. The difference between the
temperature of the stored product and that of the delivered product
will have the largest effect on the volume of product in the UST.
However, the outside daily ambient temperature will also affect the
system. On a daily basis it is important to be aware of the apparent
losses and gains of product that temperature changes may cause.
Ambient temperature should be noted when recording gauge
readings, for reference when interpreting results. Effects may also
be minimized by gauging the tank each day at the same time and
by not taking gauges immediately after product delivery.
Account for product evaporated during delivery
The more volatile (tendency of a liquid to change to vapor) the
stored product is, and the higher the temperature is, the greater
effect evaporation will have on the system. Evaporation losses
primarily occur during UST filling. For gasoline, these losses
average about 0.0012 gallon lost per gallon of throughput. This
effect can be minimized by using vapor control, such as vapor
recovery during filling, or by placing pressure relief valves on the
tanks to reduce the pressure within the UST during filling, which
reduces the amount of product that volatilizes.
20
-------
Calibration
J Ensure the calibration chart corresponds with the UST
Analysis
Underground tanks are fabricated as production items. The tank
manufacturer supplies charts intended to be used for all tanks of
the same nominal dimensions and capacities. Charts cannot
account for variations in the position of a tank in the ground. For
example, a tank may be tilted in the ground due to poor
installation, settling, frost heave, etc. A tilted tank will vary from
the chart in proportion to the degree it is tilted, and calibration
charts cannot account for such variations. If a tank is tilted and the
tank is being gauged from the middle, there is no effect. The
amount of product will be overestimated if the tank is gauged at the
low end and underestimated if the tank is gauged at the high end.
The amount of overestimated or underestimated product will,
however, be consistent as long as gauging is always done at the
same end. If the tank is tilted, it should be remembered as a source
of error. It is possible to create a tank chart specific to a tank by
adding small known volumes of product to an empty tank and
measuring the product level and repeating these steps until the tank
is full. This approach is very time consuming.
The calibration chart's specifications should correspond to the
UST's brand, size in gallons, and dimensions. Manufacturers
should provide a calibration chart for their particular tank. A
sample calibration chart is shown in Figure 3.
illMi&iil Need to use the calibration chart correctly
Analysis
If the calibration chart is read improperly or the calculations are
done incorrectly, the volume of product in the tank may be
estimated inaccurately resulting in false alarms or missed
detections.
21
-------
zz
essus
*
M
I:
**. ^*
SB
1
^8
S
0
S
s
>a
H
-------
33
34
35
36
37
38.
39
40
41
42
43
44
45
; 46
4?
48
43
SO
51
52
*».,, .
54
55
56
&
58
59
60
61
62
63
64
'. 36$
402,
415
428
440-
452
464
478
486
497
507
516
524
532
539
544
548
549
'm
.'., 73%
755
,776
, .80*
825
844
, , 865
i . ,. 885
90*
922
338
954
, 968
• 380
,990
997
999
"•
"
52*
341
56*
580
600 .
620
63S
659
678 r
69?
?1£
734
753
774
7S9:
806
823
849
856
872
887
302
&&
930
943
955 -
966
976
985
393
999
1002
* 784
at*
841
en '
901
930
953
988
•tet?
, -KJ46
1«74
tf02
1130
«S7
$183
tate
t23S
1260
1285
1308
1331
t353
1375
1895
1414
1432
1449
1464
1478
1490
1439
1504
1042 - - :
1982
1122
1161
12Ot
1240
1279
13*8
1 1356
, *39S
1432
1469
1506
1542
1578
1613
f647
1680
1713
1745
1775
18Q5
1833
i860
1886
1910
*932
1953
. 1S7t
1£8B
1998
2005
1303-
1353,
1482
1452
1501
I55G
1593
1648
f'O9&
1743
179O
1837
1883
1928
1973
2016
2059
2*01
2141
2181
22te
225$
2292
2325
2357
2388
2410
2441
2464
2483
2498
2506
1563 V
1623
1683:
1742
1802
1S6S
ISfSf
1977
2035
, 2092 , '•
2149
2204
2263
2314
. 2367
2420
'2471
2521
, 2570
26*7,
2663
2707
275ft
2794
2829
2865
2899:
2929
2957
2983
2998
3008
,2d8$
-2*6*
2244
23©3
2402
2481
1 2553
f 2637
[ 2713
, 2890
2865
2939
t 3013
1 3085
3456
3226
3295
3364 -
3426;
3490
355?
3610
3667
3721
3772
3823
3865
3906
., 3942
3973
• ^ 3937
4010
Figure 3. Sample Calibration Chart Source: Buffalo Tank
-------
The following procedure is excerpted from API publication 1621:
, " l^e obgrtsbdBjd, be «#d directly Sot & gauges which we ta the exact inch or
/Jeitifib^ove^icfa^oww^etlocbipafc, \ , /' ^ T
,- ' For gauges of 1/& inch (or wore) over or oflder iftte ex^ot Jacli the toll Wiag
' " * "
. , --" \- ^- ••'
a. ^jU , for a 1,000-gaiioft tanit (diameter ^4 inches; lengtb
'
s Chart reading at 47 inches » 7J
*' , CSiait reading at 46 iaches « 771 gaKlons
, ,%,^, ^ ,
,- IS gallons times 3/4* 13-5
' % ''"'•••••£
d. Tbis gattoaage Calculated jn step cVis added to the ga^oaage shown'ba
tt& chart jfor the Jower whole inch reading, i^
" ' "Total
7845 gallons
Therefore, a tank gauge of 46 3/4 inches, for the given UST, represents
784.5 gallons of product. Some companies offer computer programs
that perform these calculations automatically.
If a water gauge has been taken, the quantity of water contained in the
tank also is determined by using the above procedure. The total
amount of water should be subtracted from the total amount of liquid
in the tank (as determined in step d) to determine the net gallons of
product contained.
24
-------
Tank Stock Control
testing 'I Assure the accuracy of the pump meter
I
| Analysis
Analysis
It is impossible to calibrate dispensing meters to be 100 percent
accurate. Pump meter inaccuracy will cause a consistent, apparent
loss or gain in inventory depending upon whether the meter is slow
or fast. Once a meter has been calibrated, the system error can be
limited during reconciliation. The pump meter must be maintained
properly and calibrated frequently to keep it as accurate as possible
and to determine the degree and direction of error the meter may be
causing. All dispensing meters at retail outlets must be calibrated
to weights and measure standards of the locality. A procedure for
testing the accuracy of a dispensing meter is included in API
publication 1621.
Need to quantify all withdrawals or additions to the UST
Unaccounted for additions and withdrawals to the UST will cause
an imbalance in inventory reconciliation. Withdrawal sources
include all product withdrawn for personal use, any spills during
delivery of product or at other times, and any thefts. For the sake of
accuracy, withdrawals not shown by a meter should be quantified
as carefully as is possible and included in the inventory records and
reconciliation process.
All deliveries to the tank must be carefully recorded. A receipt
showing the delivery amount should be kept for the inventory
records. To check that the received product amount corresponds
with the receipt amount, the delivery receipt should be reconciled
with tank gauges taken immediately before and after delivery,
noting any withdrawals that occurred during the delivery.
Recording and Reconciliation
1
Analysis
Ensure the data are recorded completely and correctly
Recording inventory data is the first step in the reconciliation
process. The format for data entry varies greatly depending upon
owner preference, number of USTs at a facility, and the
inter-relation of these USTs. Because of the many different
accounting systems that can be used to record and analyze the
25
-------
inventory data, numerous different accounting forms are available.
Many oil supply companies advise operators as to the proper
accounting procedures to be used, including the use of suitable
accounting forms and where they may be obtained. All inventory
records should have a place to record daily receipts, daily
withdrawals, and the volume of product associated with the closing
inventory stick reading. A sample accounting form is shown in
Figure 4.
Ensure proper reconciliation of inventory data
During inventory reconciliation it is easy to make mistakes that
may improperly indicate that a tank is or is not sound. The basic
formula for daily reconciliation of an UST's inventory is as
follows:
Opening Inventory + Deliveries
- Sales - Unmetered Use - Closing Gauged Inventory
= Daily Overage or Shortage
Overage means that the gauged inventory is more than that which
is accounted for by deliveries, sales, and other use. An overage is
indicated by a final positive number. A shortage is indicated by a
negative result, and suggests that the inventory remaining in the
tank is less than that accounted for by deliveries, sales, and other
use. This overage or shortage is recorded on a monthly
reconciliation worksheet (see Figure 5 for an example). Inventory
monitoring records from an UST that does not have a leak should
have daily overages and shortages, that fluctuate randomly around
zero. Large overages or shortages for one day, when no history of
overages or shortages exists, should not be a cause for alarm unless
similar results are obtained in future testing.
Sophisticated statistical analyses can be performed to reconcile the
daily inventory records. These methods increase the sensitivity of
inventory control and may identify tank conditions interfering with
accuracy (e.g., tilted tanks). Owners/operators can purchase
statistical reconciliation services from outside companies; such
analyses typically cannot be performed by in-house personnel.
26
-------
Sample Inventory Control Program DAILY Reconciliation Form
Location Date:
•:, , Regular Regular Premium
Spofcnljnvantprsr,,,,,. Leaded Unleaded Unleaded Diesel
1,: Closing " 1 4-
2, 2 *
3. . 3 4-
4, ' 4 4-
*> , - 5 *
& B , 4-
7, "~ 7 " +
8. 8 +
9. 9 4-
10. ' fO 4-
1.1, Total Meters ±=
12. Meters Out -f
13, Meters In -
14. Dispenser Cal Test
^allonsi
I5ti Jotal Cfosfna Meters
16. Opening Meters -
17, Today's Sales -
••
•.
'
-
''
*
Regular Regular Premium
Leaded Unleaded Unleaded Diesel
Physfcaltnventory mk Gal, In. Gal. In. GaL In. Gal.
i.$.,.T.anki Product ,
i&tftiuri Water ,
20. Tank 1 Met
21. Tank a Product
22. Tank 2 Water
23. Tank 2 Net
24* Physical Inventory
Regular Regular Premium
Tank Reconciliation (gallons) Leaded Unleaded Unleaded Diesel
25,, Opening Physical Inventory
26, Today's Sales
27. Product Receipts 4-
28, Inventory Balance «
29> Pnvsical Inventory -
30. Tank Over (if 4-)
31, Tank Short (if -)
-
Figure 4. Sample of daily reconciliation form
27
-------
Sample inventory Control Program MONTHLY Reconciliation Form
-ocatlon , - "''"''', Dalev """ ,: --"-'.
-
s - „ . ~ "^ Dally Overage/Shortage
Regular .Regular
Jn'e Day Headed. . Unleaded
^
- • • 5
1
2
3
4
1
2
3
4
5
6
7
8
9 "'
10
11
12
13
14
1S ^
16
17
18
id
20
21
22
23
24
25
26
27
28
29
30
31
Cum. Over. Total
% thru.
Cum. Shrt.Total
% Thru.
••
« -.;-,/
-
••
- - ,
< -, f
-
- ,^.
,,v,,.
" ' '
- '
•* f
f
-
,v ,
. " '
:
>
' "' S
" '
' ,'/'" '
'" '
"
... svJ^
" '" , ' '
"
-
,,"•-,.,
- v ^'4- -;
f S /
,,,, + +
f fffff
•i '"•
% •> •> •>* I, ^
1J < s s'
•<• f
„«-,'"„,'
"
,
s, i
Gallons i
Premium
Unleaded Diesel
•.
_.
s s
,
--"
f *
•>
'
,
"•/,
V.
_,_,
-
'
,
-
<
'
Attention: the cumulative sum of monthly overages or shortages should I not exceed 1,0%
Df the monthly throughput plus 130 gallons! " »-•—.«.' ^^, <^.~
Figure 5. Sample of monthly reconciliation form. Source: API
28
-------
Interpretation
Analysis
Need to interpret the reconciled data properly
Improper interpretation may result in an inaccurate indication of
the UST's status. The federal regulation requires that an UST be
reported to the local implementing agency as leaking when
monthly reconciliation for two consecutive months indicates there
is a cumulative monthly overage or shortage equal to 1.0 percent of
total flow-through for the month plus 130 gallons. Using the form
shown in Figure 5, the following explains the steps for determining
this result:
Step 1. Enter the daily overage or shortage for each product, as
determined during manual reconciliation, for each operating day of
theanonth.
Step 2. When the daily overages and shortages from 31 operating
days (or one continuous month) have been entered, add all
overages and shortages and enter the value on line I or 3, as
appropriate.
Step 3. Calculate the total flow-through for the month for each
tank. (The total flow-through volume can be either the sum of the
monthly pump readings or the total amount of product delivered in
a month. Whichever method is chosen should be used
consistently.)
Step 4. Calculate one percent of the total flow through and add
130 gallons to it to determine a comparison number, using the
following formula:
(0.01 x flow - through) + 130 gallons = comparison number
Step 5. If the cumulative overage or shortage (determined in
step 2) exceeds the comparison number (calculated in step 4), a
leak in the UST system may be present. (It is likely that a
mathematical error, pump miscalibration, unaccounted for delivery,
or unaccounted for water is responsible for the discrepancy.)
These results should be rechecked carefully, remembering that
water may still be an indication of a leak. If the subsequent
month's results again exceed the comparison number, the results
29
-------
must be reported to the local implementing agency as a possible
leak.
This process should be performed individually for each tank.
ENSURING EFFECTIVE MANUAL TANK GAUGING
Chapter 1 provides a general description of the types of oversight that
can be used. The following sections discuss how these approaches
may be applied specifically to inventory control.
Site Inspections
Site inspections to review the inventory records is a possible oversight
mechanism. It would also be possible to observe the staff performing
the actual measurements, to see if the tank gauging is performed
correctly.
Data Review
Although all inventory control recording forms could be reviewed for
proper recording and interpretation of gauging data, this would entail
reviewing numerous forms. However, a smaller number of forms
would need review if submission of forms was only required when a
leak is suspected. Forms should be reviewed for accuracy in
recording, in conversion of gauge measurements to volumes and to
ascertain correct reconciliation.
Guidance and Training
Training could be provided to teach owner/operators proper tank
gauging techniques and how to conduct proper inventory control. An
alternative to training classes is guidance materials (e.g., a manual or a
video tape). The API 1621 is an excellent resource for tank owners
using inventory control.
Approval and Certification
Most errors in inventory control take place because the tank is gauged
improperly. An implementing agency could provide training and
30
-------
certification for all persons using inventory control. Training should
cover the proper tank gauging technique, recording, reconciliation and
interpretation of data.
REFERENCES
1. American Petroleum Institute. 1987. Recommended Practice 1621,
Bulk Liquid Stock Control at Retail Outlets.
2. American Petroleum Institute. June 5,1987. Review and Analysis
of Existing and Proposed Underground Storage Tank Inventory
Control Procedures, Vol. 1. Report by Radian Corporation, for the
American Petroleum Institute.
3. Mobil. 1984. Motor Fuel Inventory Verification Procedures.
4. Radian Corporation. August 1984. Analysis of Factors Affecting
Service Station Inventory Control.
5. Schwendeman, Todd G. and H. Kendall Wilcox. 1987.
Underground Storage Systems. Lewis Publishers, Inc., Chelsea,
Michigan.
6. U.S. EPA. January 1986. Underground Tank Leak Detection
Methods: A State-of-the-Art Review. Report by Shahzad Niaki and
John A. Broscious for Hazardous Waste Engineering Research
Laboratory, Office of Research and Development, U.S. EPA.
7. U.S. EPA. September 1988. Analysis of Manual Inventory
Reconciliation. Report by Richard F. Eilbert of Entropy Limited
for Midwest Research Institute for the Office of Underground
Storage Tanks, U.S. EPA.
31
-------
-------
Chapter III
Manual Tank Gauging
-------
-------
MANUAL TANK GAUGING
III
SUMMARY
Manual tank gauging, also called static tank testing, is an effective,
easy, and inexpensive release detection method for small volume USTs.
A study by EPA shows that manual tank gauging can detect leaks as
small as 0.2 gal/h for tanks less than or equal to 550 gallons in capacity.
The same study shows that for tanks of 551 to 2,000 gallons, manual
tank gauging has about the same sensitivity as inventory control. These
attributes make it a very appealing release detection method for smaller
UST operators.
The discussion presented in this chapter covers many of the possible
problems that may occur with manual tank gauging. This does not
mean that all, or even most, of these problems will occur. Nor does it
mean that all of the problems are of equal importance, in terms of
frequency of occurrence or severity of impact to the effectiveness of
manual tank gauging. Some problems occur infrequently, whereas
others have limited impact. This chapter presents a range of potential
problems for educational purposes, not to imply that they will always
occur.
Manual tank gauging is a weekly, short-term static test in which the
liquid level is measured in a quiescent tank at the beginning and end of
a 36-hour time period. Any change in liquid level is used to calculate
the change in volume, which is compared against established guidelines
to determine whether any disagreement in the measurements is
significant enough to indicate a leak in the UST system. Manual tank
gauging is sometimes confused with inventory control. Although both
methods involve "sticking the tank," manual tank gauging is a
short-term static test, while, in contrast, inventory control is an ongoing
record of all the activities at an operating UST for an entire month (for
more information on inventory control, see Chapter 2 of this manual).
Because the problems with inventory control and manual tank gauging
are similar, Chapter 2 can used as a cross reference for this discussion.
33
-------
The process of manual tank gauging involves the following steps:
(1) tank gauging—the process of measuring the product level in an
UST; (2) calibration—the correlation of a gauge reading with the
proper calibration chart to determine the volume of the product in the
UST; (3) recording—accurately recording gauging results; and
(4) interpretation—the determination of whether the result of tank
gauging signifies an UST release. The relationships among these four
steps are shown in Figure 6.
POTENTIAL PROBLEMS AND SOLUTIONS
A number of factors can affect the accuracy of manual tank gauging as a
release detection method. The following sections discuss problems and
solutions related to each of the four steps involved in implementing
manual tank gauging. The order of discussion is not intended to
prioritize the importance of the problems, rather it is intended to follow
the order in which they would occur according to the flow chart in
Figure 6. The discussion of problems and solutions is summarized in
Table 3, in the order of the discussion in the text, and the most serious
concerns are marked with an asterisk. Some agency oversight options
are offered for problems, when applicable, but not all of them need be
undertaken.
Tank Gauging
I
Analysis
Ensure the tank is gauged properly
If a tank is not gauged properly, the gauge readings will not
accurately reflect the amount of product in an UST. An
inaccurate measurement will occur if the gauge is read
incorrectly, or if the gauge is improperly taken with the stick
slanted or resting upon an extension in the tank. To take a gauge
properly, the stick is placed carefully into the tank through one
of the tank openings until its tip touches the tank bottom. Some
tanks have a separate opening, called a gauge hole, that should
be used for this procedure; otherwise the fill pipe can be used.
The stick should be inserted at the same point in the gauge hole
each time a gauge is taken and should be held in a vertical
position. The stick should not rest on a projection on the tank
bottom (e.g., a reinforcement rib in the base of a fiberglass tank).
After the gauge stick has been wiped off, the gauging procedure
should be repeated and both readings should be recorded. The
34
-------
Testing
Ana ysis
Tank Gauging
Measure product level
Calibration
Determine volume of product
from calibration chart
Recording
Gauging results
Temperature
Interpretation
Leak
No Leak
Figure 6. General procedure for manual tank gauging
35
-------
OJ
o\
Table 3. Indicators and Solutions for Problems Encountered During Manual Tank Gauging
Problem Indicators Tester Solutions Agency Oversight Options
'Assure tank Is gauged
properly.
Avoid damaging tank from
careless gauging.
Assure accuracy of product
reading.
Assure sufficient testing
period.
Identify water in the UST.
Need to keep track of
temperature fluctuating
during the test period.
Assure the calibration chart
corresponds to UST.
Need to use the calibration
chart correctly.
Record the data completely
and correctly.
Need to interpret the data
property.
Slanted stick. Gauge stick not
wiped. Large discrepancy
between consecutive gauges.
Loss of product.
No clear product line on gauge
stick.
Time elapsed < 36 hours.
Gain in product amount.
Large temperature changes.
Specifications on chart do not
match tanks.
Imbalance in results.
Imbalance in results.
Imbalance in results.
Take consecutive gauge
readings. Wipe dipstick
between gauges.
Add a striker plate to tank
base. Gauge tank carefully.
Use notched gauge stick. Use
product-finding paste.
Withdraw pole quickly.
Wait 36 hours or more between
beginning and ending gauge.
Take a water gauge using water-
finding paste.
Take gauges at same time: of
day. Record daily temperature
when gauging.
Ask tank manufacturers to
provide a chart that
corresponds with their tank.
Use chart according to API
recommendations.
Ask supply company for
recommended recording
practice.
Follow recommended
interpretation process.
Provide training.
Set a mandatory waiting period
Check beginning and ending
times of test.
Check during site visits for water
in tank.
Require recording of
temperature on gauging
records.
Review gauging forms.
Review gauging forms.
Review gauging forms.
"Indicates the most significant problem.
-------
Analysis
average of the measurements should be used in conjunction with
a calibration chart to calculate the product volume in the tank.
After at least 36 hours, two more measurements should be taken.
';•"•• - • \ * •
Because two gauges must be taken consecutively at both the
beginning and end of the time period, an error in stick placement
will be apparent if the gauges differ by more than a few gallons.
In this case, a third gauge should be taken to determine which of
the first gauges is correct.
Avoid damaging the tank from careless gauging
Repeated gauging of a tank may wear a hole through the bottom
of the UST. When the gauge stick strikes the bottom of the tank,
rust can be chipped off of the surface and expose new metal,
thus allowing for quicker corrosion which may result in a hole.
This can be protected against by careful gauging and by,
outfitting the tank with a striker plate. A striker plate is simply a
layer of metal, added to the tank to increase its strength in the
area that comes into contact with the gauge stick. Unless the
thickness of the striker plate is negligible, when a striker plate is
added the end of the gauge stick must be modified by cutting off
the exact length as the thickness of the striker plate so that true
volume conversions can be obtained. : . * ,
Ensure accuracy of the product reading
^
Analysis
If the stick is used for gauging gasoline or other volatile
products, the edge of the stick adjacent to the graduated side
should be grooved every 1/8 inch in order to keep the product
from moving up the stick past the measured level (referred to as
creepage). If desired, product-finding pastes can be used to
improve the accuracy of gauging. These pastes improve
adherence of the product to the gauge stick and prevent creepage
that would distort the reading. Product-finding pastes change
color in the presence of product, making it easy to identify the
line left on the stick by the product. Information on satisfactory
pastes may be obtained from an equipment supplier.
If product-finding paste is used, it should be applied in a light,
even coat to the stick before insertion into the UST. After the
stick is quickly withdrawn (to avoid creepage of the product),
the product "cut" (the mark left by the product on the stick) is
read on the graduated scale to the nearest 1/8 inch.
37
-------
Analysis |
f
| Analysis |
I
Analysis \
Assure sufficient testing period
Manual tank gauging should take place weekly over at least a
36-hour period during which no liquid is added or subtracted to
the tank. Because a shorter time period for testing reduces
manual tank gauging's ability to detect UST leaks, the 36-hour
period is required by the federal regulations. As the testing time
period is extended, manual tank gauging is able to identify
smaller leaks. Over a 36-hour testing period, manual tank
gauging should be able to accurately identify a leak of 0.2 gal/h.
Determine presence of water in the UST
A water gauge may be taken to determine whether water is
present in the UST (see Chapter 2). The presence of water in an
UST may indicate a leak. In addition, good management
practice dictates the detection and removal of any water in an
UST because water may enhance corrosion. A water-finding
paste, which is unaffected by the stored product, but which will
change color in water, is used to check for the presence of water
at the bottom of USTs.
Keep track of temperature fluctuations during the test period
Changes in temperature can affect the volume of the stored
product and the UST; temperature increases and decreases can
cause expansion or contraction, respectively, of the product
within an UST. An increase in volume due to a temperature
increase may mask a leak. Similarly, a decrease in volume due
to a temperature decrease may imitate a leak. Temperature will
have the least effect on dense liquids (e.g., used oil), which
expand and contract a very small amount per degree of
temperature change.
Because tank gauging is done over at least a 36-hour time
period, during which product is not delivered or removed,
temperature effects should be due only to ambient temperature
changes. To minimize the impact of temperature, if the change
in temperature is great, the testing period could be lengthened to
48 hours so that the beginning and ending measurements of the
gauge can be taken at the same time of day. Temperature effects
should be kept in mind as a potential source of error.
38
-------
Calibration
I
Analysis
Analysis
Ensure the calibration chart corresponds to the UST being tested
A calibration chart shows the number of gallons in the tank as
represented by inch marks on the gauge stick. Each chart is
calculated for a specific brand of tank of particular dimensions
and capacity, and the chart used must correlate to the tank being
gauged. If a chart is used that does not correspond properly, the
tank volumes determined will not be accurate. Manufacturers
will usually provide a calibration chart for their particular tank.
Figure 7 on pages 40 and 41 shows a sample calibration chart.
See Chapter 2 for additional discussion of calibration charts.
Use the calibration chart correctly
If the calibration chart is read improperly, the volume of product
in the tank may be estimated inaccurately.
API publication 1621 recommends the procedure shown below:
I -. the chart should be read directly for all gauges which aw to the exact inch or
1/1$ inch above or below an exact inch mark,
%. JFof gauges of 1/8 inch (at more) over o=f under the exact inch the following
procedure should be used;
«. the chart should be read fo* toe exact inch on the scale above and below
tfoe actual gauge stickjeading. Pot example^ if $» gauge sttck *e*te 46 3/4 inches,
the chart should be read at both 46 and 47 tacfaes.
b, The smaller gallonage shown oft the scale at these two readings should be
subtracted from the larger ie., fo* a 1,000-gallott tank (diameter 64 laches; length,
72 inches);
Chart reading at 47 inches s* 7$9 gallons
Chart reading at 46 inches «* 771 gallons
Subtracting s* 18 gallons
c. tliis gallouage is then multiplied by the fraction of an inch shown on the
original gaugevi.e.;
18 gallons times 3/4 * 13.5 gallons.
d, This gallonage (calculated in step c) is added to the gallonage shown on
the chart for the lower whole inch reading, ie<:
Gallons at 46 inches * 771 gallons
Gallons at Ifl inch **.1§«5 gallons
Total ' *> 784J gallons
39
-------
13:
r*
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-------
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40*6;
Figure?. Sample of calibration chart. Source: API
-------
Therefore, a tank gauge of 46 3/4 inches, for the given UST,
represents 784.5 gallons of product. Computer programs are
available to perform or check these calculations.
Recording
Analysis
Record the data completely and correctly
Tank gauging data should be carefully recorded. The format for
data entry will vary greatly depending upon preference, number
of USTs at a facility, and the interrelation of these USTs. All
tank gauging recordkeeping systems should have a place to
record all tank gauge readings, thek average reading and
associated volume, the time the gauge readings were taken, and
a final comparison of the beginning and ending volumes. It may
also be helpful to record the ambient temperature at the time of
the readings. This information might be useful when
investigating a possible release because temperature changes can
be a source of error.
Interpretation
.Testing :J Need to interpret the data properly
Analysis
If tank gauging data are not interpreted properly, the tank may
falsely be considered sound or leaking. All tanks using manual
tank gauging must test weekly and check the average of the
differences from the four previous weeks against the chart in
Table 4.
Table 4
Monthly and Weekly
Manual Tank Gauging Standards
Monthly Standard
Nominal Tank Weekly Standard (four-test average)
Capacity/gallons (one test) gallons gallons
<550
551 - 1,000
1,001 - 2,000
10
13
26
5
7
13
42
-------
After the tank is gauged for a given week, and the beginning and
ending volumes for the time period have been determined, these
two volumes must be compared to one another. The beginning
volume should be subtracted from the ending volume, and the
difference (including if it is a positive or negative number)
should be recorded. If the difference is positive, the UST has an
apparent gain. If the difference is negative, the UST has an
apparent loss. The difference should then be compared to the
information provided in Table 4 which correlates to weekly
results. If either a positive or negative difference is greater than
the numbers provided in Table 4, the tank has failed for that
week.
Manual tank gauging results must also be checked against a
monthly standard. To calculate an average monthly value, the
four previous weekly differences are added (the positive or
negative sign of the difference should be retained for this
addition) and the sum is divided by four. The result of this
calculation should then be compared to the monthly standard
provided in Table 4.
If the difference in the weekly beginning and ending volume
measurements, or the monthly average difference, is equal to or
greater than those shown for the appropriate size of tank in
Table 4, the UST may be leaking or may have holes that are
allowing water to enter.
ENSURING EFFECTIVE MANUAL TANK GAUGING
Chapter 1 provides a general description of the types of oversight that
can be used. The following sections discuss how these approaches may
be applied specifically to manual tank gauging.
Site Inspections
Implementing agencies could perform site inspections to review the
manual tank gauging records and to observe the staff performing the
tank gauging. The records could be reviewed for correct length of test
and proper recording and analysis of data.
43
-------
Data Review
Implementing agencies could require that all manual tank gauging forms
be submitted by the owner/operator to the agency to be reviewed for
accuracy in recording and conversion of gauge measurements to
volumes and to ascertain correct interpretation of results. Such an
approach, however, would be very labor intensive. Computer programs
could be developed to recalculate the submitted data. The records could
be reviewed for only the one or two most important factors, such as the
time between initial and ending measurements, to be sure the test was at
least 36 hours long. Another approach would be to require submission
of only some of the forms, such as twice each year.
Guidance and Training
Most errors in manual tank gauging take place because the tank is
gauged improperly. The implementing agencies could hold seminars or
training classes in proper gauging procedures for persons electing to use
this release detection method. An alternative to providing training
classes would be to provide guidance materials (e.g., a manual or a
video tape) explaining the proper process.
Approval and Certification
An implementing agency could provide testing and certification for all
persons using manual tank gauging. Such an approach would be time
consuming.
44
-------
REFERENCES
1. American Petroleum Institute. 1987. Recommended
Practice 1621, Bulk Liquid Stock Control At Retail Outlets.
2. American Petroleum Institute. February 2,1987. Analysis of
Static Tank Testing as a Leak Detection Technique for Used
Oil Tanks at Retail Outlets.
3. Mobil. 1984. Motor Fuel Inventory Verification Procedures.
4. U.S. EPA. January 1986. Underground Tank Leak Detection
Methods: A State-of-the-Art Review. Report by Shahzad Niaki
and John A. Broscious for Hazardous Waste Engineering
Research Laboratory, Office of Research and Development,
U.S. EPA.
5. U.S. EPA. April 1,1988. Review of Effectiveness of Static Tank
Testing. Report by Midwest Research Institute for Office of
Underground Storage Tanks, U.S. EPA.
45
-------
-------
Chapter IV
Tank Tightness Testing
-------
-------
TANK TIGHTNESS TESTING
IV
SUMMARY
EPA studies show that tank tightness testing can be done reliably and
affordably. For many existing tanks it is the best available release detection
option because permanent installation of equipment is not necessary, capital
requirements are limited, and many commercial methods are available.
Release detection methods are a combination of equipment and procedure.
EPA studies show that, for tank tightness testing, the equipment is generally
reliable and meets the manufacturers' specifications. However, tank tightness
testing procedures can be problematic. Getting effective tank tightness tests is
a matter of focusing on the manufacturer's protocol and the testers adherence
to the protocol. Consequently, this chapter explains key procedures, why they
are important, and how some existing state UST programs have been ensuring
that proper testing procedures are followed in their jurisdiction.
The discussion presented in this chapter covers a wide range of possible
problems that may occur with tank tightness testing. This does not mean that
all, or even most, of these problems will occur at the same time. Nor does it
mean that all of the problems are of equal importance, in tenns of frequency of
occurrence or severity of impact to the effectiveness of tank tightness testing.
Some problems, such as poor access, seldom occur, while other problems,
such as interference from evaporation/condensation have limited impact.
Experienced testers are well aware of these problems and how to deal with
them. For example, an experienced tester can recognize the presence of vapor
pockets. Release detection, however, is a growing industry, and new
companies are being formed with less experience. This chapter presents the
full range of potential problems for educational purposes, not to imply that
they will always occur.
Many of the descriptions and principles provided below for tank tightness
testing are also applicable to automatic tank gauging and to piping tightness
tests. Those release detection methods are discussed separately in Chapters 5
and 9, respectively.
47
-------
BRIEF DESCRIPTION
There are many commercial tank tightness test methods available. While there
is great diversity in the equipment and analysis schemes used, all tightness test
methods are based on the same general approach. This section focuses on the
general principles behind tightness testing and on procedures, not on specifics
or equipment. Details on many methods and their performance are available
in other publications (Ref. 1 and 6), and manufacturers and vendors of
equipment will provide literature on their equipment.
There are two main types of tank tightness tests: non-volumetric and
volumetric. There are only a few non-volumetric methods available, and they
are not covered in this document. Volumetric tightness test methods measure
the change in product volume or level over time to determine if there is a leak.
When a leak occurs in an UST, the loss of product causes a decrease in the
volume of product in the tank and, thus, a decrease in the level of product.
Other factors, such as changing product temperature, can also cause product
volume or level changes. Tightness test methods differ in how they measure
the volume or level change and how they account for the interferences.
The general procedure for conducting a volumetric tank test is quite similar
from one test method to another (see Figure 8). The three procedural aspects
common to all volumetric test methods are preparation, testing and analysis.
Fkst, the physical layout and condition of the tank and piping must be
evaluated to determine if tightness testing can be performed on that UST
system. There are situations in which the test equipment cannot physically be
placed into the tank. Other configurations may cause problems in obtaining
meaningful results. Details of physical concerns are discussed in the problems
section below.
To prepare for a test, the tank must first be filled to a gross approximation of
the level requked for testing. A waiting period must be observed to ensure
that thermal effects and structural deformation resulting from filling the tank
have stabilized. The instrumentation to measure product level and
temperature can be installed during or after the waiting period. Next, fine
adjustments may be made to the fluid level by adding or removing small
amounts of product. If fine adjustments of product level are made, a second
waiting period should follow.
48
-------
Preparation
Testing
Ana
ysis
Fill Tank
i
Wait for tank to stabilize
Install Test Equipment
Determine height-to-volume
conversion factor
Determine coefficient of
thermal expansion
Measure ground-water level
Top Tank
Walt
Conduct Test
1
Leak No Leak
• Temperature measurement
• Level or volume measurement
Make calculations
Plot graph
Apply detection criterion
Figure 8. General procedure for conducting a volumetric tank test
Source: U.S. EPA 1989
49
-------
Volumetric tank tests can be divided into two categories: overfilled and
partially filled. Figure 9 presents the two types of tests. In an overfilled-tank
test, the tank is filled until the level of the product reaches the fill tube or a
standpipe located above grade. Level changes occur in a small surface area,
so that small changes in volume cause relatively large changes in level. For
example, in a 4-inch fill pipe, a volume change of 0.05 gallons will cause a
level change of about 1 inch. In a partially filled tank, the test is conducted
with the product level somewhere below the top of the tank. Level changes in
these tests occur in large surface areas, where small changes in volume cause
very small changes in level. For example, in a half-filled 10,000-gallon tank
8 feet in diameter, a volume change of 0.05 gallons will cause a level change
of about 0.00006 inches. Level sensing devices must be considerably more
sensitive for partially-filled-tank tests than for overfilled-tank tests in order to
achieve the same accuracy.
During the waiting periods(s), the test operator must determine values for the
height (level)-to-volume conversion factor and the coefficient of thermal
expansion (see section below on problems during preparation for discussion of
these terms). Finally, the tester should determine the height of the water table.
The preparations are now complete, and testing can begin.
During the test, sensors take measurements of both the temperature and the
level of the fluid in the tank. The respective measurements are taken
repeatedly at specified intervals and are recorded for analysis at a later time.
Data collection can be manual or automated. The end of a test is based on a
criterion pre-determined by the manufacturer. Usually this criterion is
expressed in terms of time; for example, the test ends 60 minutes after the start
of the data collection.
By the time the test is complete, a considerable amount of data may have been
gathered. Generally, the more data that are gathered, the better the test.
Procedures for averaging the data, compensating for temperature, and
computing a volume leak rate are usually well defined by the manufacturer of
the test method. The end result of the analysis is a calculated volumetric
"flow rate" that indicates how fast fluid is escaping from the tank.
The detection criterion (usually a single threshold value) is applied after the
analysis has been completed and is used to determine whether the level
changes are due to a leak or to normally occurring volume fluctuations. If the
temperature-compensated volume change exceeds the detection criterion, a
leak is suspected; if not, it is assumed that the tank is not leaking. The most
common criterion is 0.05 gal/h.
50
-------
Partially Filled Tank
Overfilled Tank
A
(A) Adding a quart of liquid to this tank
would produce a barely noticeable rise In the
level of fluid. Level changes are distributed
over a large surface area, so that even large volume
changes produce only very small level changes.
Adding a quart of liquid to this tank would
cause the fluid to rise many Inches. Here the
surface area is very small. Thus, even a small
volume change can mean a drastic level change.
Figure 9. Comparison of partially filled and overfilled tanks.
Source: U.S. EPA (1989)
51
-------
POTENTIAL PROBLEMS AND SOLUTIONS FOR TIGHTNESS TESTS
From the practice of tightness testing and studies performed by EPA,
important procedural aspects have been identified. This section discusses the
most important aspects of tightness testing and the errors and problems
encountered. Table 5 presents a summary of the problems encountered during
tank tightness testing, the indicators and solutions for these problems, and how
implementing agencies might provide oversight to prevent or correct the
problems. A number of agency solutions are offered for each problem, but not
all of them need be undertaken. In the table, the problems that are most
serious, either because of frequency or severity of impact, have been marked
by an asterisk. The problems are discussed below in the order of the testing
procedure presented in Figure 8 (page 49). There is no ranking implied by the
order of discussion.
Site Considerations
Testing
Analysis
Many "real world" factors concerning the installation and condition of the
UST system can influence how well a tightness test performs and how much
effort is involved in obtaining meaningful test results. Most physical factors
can be accounted for with proper equipment and experienced testers.
Tightness testing will be most effective and efficient if as much as possible is
known about the setup and condition of the UST system prior to selecting the
method or beginning the test. The correct equipment can be assembled
beforehand, and any necessary modifications to the equipment, protocol, or
site can be made.
Joints, bungs, and manways must be tight for overfill methods
Common problems with UST systems are (1) fittings and joints that
were not tightened properly during installation; (2) gaskets at joints that
were not installed, were installed incorrectly, or have deteriorated over
time; and (3) joints that have become loose over time, such as piping
connections dislodged during frost heaves. Often gaskets are not
installed on manways. Also, temporary covers on openings used during
delivery are frequently not replaced with permanent bungs during
installation. During the overfill type of tightness test, the top of the tank
and the piping are filled with liquid, and product will leak from any
loose fittings or joints. Identification of these problems is desirable;
however, leaks due to loose fittings obscure identification of possible
leaks due to corrosion holes. Therefore, leaks due to loose fittings must
be stopped before testing of the tank for corrosion holes can begin.
52
-------
Table 5. Indicators and Solutions for Problems Encountered During Tank Tightness Testing
Ui
Problem
Indicators
Tester Solutions
Agency Oversight Options
Joints, bungs, and manways
must be tight for overfill
methods.
Assure that manifolded tanks
are tested appropriately.
Abandoned piping may require
special treatment (for overfill
methods).
Poor access may require use
of another method.
Assure that drop tubes do not
interfere with test method.
*Assure waiting time between
delivery and testing is
adequate.
Top of tank or fittings are wet with
product.
Tank drawings. Owner/operator
knowledge.
Tank drawings. Owner/operator
knowledge. Unexplained piping
from top of tank. Very large
losses when tank is overfilled.
Drawing of site showing tank near
or under large structure.
Object such as inventory stick
placed through fill pipe hits
sides of tube.
Erratic and large volume changes;
short-term volume decrease that
levels out.
Check fittings and gaskets before
testing. If results of test indicate
possible leak, uncover tank,
check fittings, and retest.
Disconnect tanks and test
separately.
Find out purpose of all piping.
Install shutoff valves on piping
with unknown purpose. Dig up
runs of piping.
Inspect site before testing to make
necessary equipment modifica-
tions. Use other opening in tank.
If unable to make necessary
physical modification, decline to
test.
Replace with temporary drop tube.
If permanent drop tube cannot be
removed easily or cost-effectively,
do not test.
Wait at least 6 hours after large
product additions to tank and at
least 3 hours after topping off.
Observe test. Inspect site after
test for new gaskets, etc., if
claimed by tester.
Review data sheets. Check
site after test for evidence of
disconnect.
Review data sheets for test
results. Check site after
test for evidence of digging
or shutoff valves.
Review site plans. Observe test.
Site inspection.
Review data sheets for volume
changes and testing times. Plot
test data to observe trends.
Continued
-------
(J\
Problem
Indicators
Tester Solutions
Agency Oversight Options
'Need to Identify and remove
vapor pockets.
Erratic or sudden large volume
changes; bubbles in fill pipe or
stand pipe.
Add known volume to tank, and
calculate expected and observed
level changes. If different,
bleed off vapor and start test over.
Review data sheets for volume
changes.
*Accounting for presence of
ground water.
Water in the tank.
Check existing observation well
or drill hole to level of bottom
of tank. If present, raise liquid
level in tank to counteract
pressure of ground water, or do
not test.
Review data sheets for ground-
water data. Inspect site to be sure
an observation well was available
or drilled.
Assure accuracy of height-to-
volume conversion factor for
each tank.
None.
Add or withdraw known volume
from tank, and observe level
changes.
Review data sheets for calculation
of conversion factor.
Use correct coefficient of
expansion of product.
Used average coefficient for product
type from published source.
Determine for each tank using
API hydrometer and tables.
Review data sheets for calculation
of coefficient. Compare coefficient
to published average values.
Assure collection of sufficient
test data.
Fewer than 25 data points collected.
Sample at least every 5 minutes
for at least 2 hours.
Review data sheets for duration
of test and sampling frequency.
*Provide adequate tempera-
ture compensation during
None.
Use at least three temperature
equally spaced vertically, or
mix the product and use one
sensor. Or use method that
measures a factor independent
of temperature.
Observe test equipment as it is
installed in the tank. Review
product literature.
-------
*Need to maintain constant
product level during test
(overfill methods).
Need to compensate for
condensation and evaporation
in very hot conditions.
*The number of tests per tank
should be fixed.
Decisions left to tester should
be minimized.
Need to follow protocol and
use correct threshold value.
Constantly changing product level
in fill pipe or stand pipe.
Vapors above stand pipe or fill pipe
or condensation on sides of pipe.
High percentage of tanks declared
"tight."
None.
High percentage of tanks declared
light" or leaking.
Assure calculations are
performed correctly.
A few data points are extremely
different from the rest of a test.
At frequent intervals, add or
remove product to keep level
constant.
Minor problem. No need to
compensate. Shade from direct
sunlight.
Protocol must include fixed
number of tests.
Protocol should be as explicit as
possible and cover as many
potential situations as possible.
Analysis scheme must be well
defined. Criteria for declaring
"tight" or leaking" must be clear.
Threshold value for declaring
leak should be smaller than
minimum detectable leak rate by
a factor of at least 2.
A second person double-checks
manual calculations. Computer
program is reviewed and tested
using known and verified data.
Approve only methods whose
protocol includes maintaining
constant level. Observe tests.
Provide guidance and education
on the importance of constant
product level. Review data
sheets for evidence of additions
and withdrawals.
None.
Review data sheets for results
of all tests. Compare tests
conducted to protocol. Keep
track of pass/fail ratios for each
testing company.
Compare activities on data sheets
with published protocol. Observe
during test. Approve for use only
those methods with adequately
defined protocols.
Review data sheets and
calculations to see if steps agree
with protocol. Keep track of
pass/fail ratio of each testing
company.
Manual or computer recalculation,
from raw data to tightness
declaration.
* Indicates the most significant problems.
-------
Testing
Analysis
Testing
Analysis
Testing the tank separately from the piping will help to distinguish some
of these causes of release. Any fittings on top of the tank that are
visible without removing backfill should be checked before beginning
the test for tightness and adequate seals/gaskets (e.g., on the manway)
should be installed; any visibly deteriorated gaskets should be replaced.
Checking all visible fittings on top of the tank during the test to see if
any product is present will help to discriminate sources of release. If the
visible fittings are not wet with released product but the test indicates a
leak, it may be necessary to expose the rest of the fittings on top of the
tank by removing the backfill. If an UST fails a tightness test and the
fittings are subsequently found to be loose and are tightened, the UST
may be tested again; if the UST is found to be tight, no report of a
suspected release is necessary.
Manifolded tanks must be tested appropriately
Sometimes tanks at a site that hold the same product are connected
together by piping. The siphon effect allows product to be drawn from
all of the tanks that are manifolded together. For overfill types of
tightness testing, it is possible to test manifolded tanks at the same time.
However, the uncertainty associated with the necessary temperature
measurements is high because it is unclear how to apply the temperature
measurements from two or more different tanks to one common volume
measurement. For test methods using partially filled tanks, there may
be some slight "wave action" in the tanks when they are connected due
to the siphoning effect of the connecting pipe. This variation in the level
of the liquid in the tank interferes with accurate level determinations.
For these reasons, manifolded tanks generally should be disconnected
from each other and tested separately. If the piping between the tanks
can be disconnected, either type of tightness testing can be used. This
approach can be very convenient for the overfill methods because the
extra product needed to overfill each tank is available from the tank
manifolded to it.
Abandoned piping may require special treatment
At older sites or sites where the use of the UST system has changed
frequently, there may be piping connected to the tank that is no longer
used. Rather than digging up the entire run of old piping, only the end
connected to the old pump or delivery source may have been removed.
This abandoned piping may be left open-ended. When the overfill type
of tightness test is used, this extra piping can cause several problems.
First, vapors can become trapped in the piping, and locating vapor
pockets in piping and removing them is very difficult. Second, if some
56
-------
Testing
Analysis
Testing
Analysis
of the piping is open to the ground, large volumes of product will leak
to the environment when the tester tries to fill the system. These
problems can be overcome by using a nonoverfill tightness test method
or by isolating the tank from the piping. Isolating the tank from old
piping may involve digging up part of the piping to install some type of
device that closes off the piping. Because an open-ended pipe is always
a potential source of large product losses, abandoned piping should be
removed or closed any time it is discovered, regardless of the type of
release detection being used.
Poor access may require use of another method
Some UST systems are located in sites where it is difficult to set up the
test equipment, such as near buildings or under other structures. The
space available to set up the test equipment and maneuver during the
test may be too limited for some tightness test methods. Another access
problem is the location of the fill pipe relative to the tank itself. Remote
fill pipes are sometimes used, where the actual opening is a long way
from the tank and connected to the tank by a run of horizontal piping.
Most tightness test methods rely on using the fill pipe to insert all of the
necessary equipment and sensors into the tank and, therefore, such
methods may be infeasible for a tank with a remote fill pipe. It may be
possible for such a test method to use other openings on the top of the
tank, such as the manway. Otherwise, another release detection method
should be used.
Drop tubes must not interfere with test method
To avoid agitation, wave action, and splashing during filling, many
tanks are equipped with a drop tube. This is a tube the diameter of the
fill pipe that extends from the opening to near the bottom of the tank.
The presence of a drop tube can interfere with proper temperature
measurement because the product in the tube is isolated from that in the
rest of the tank. The tube also can be an obstruction to placing
equipment and sensors in the tank. Drop tubes interfere with product
circulation for those tightness test methods that try to achieve even
temperature distribution using a circulating pump. Some drop tubes are
removable and, obviously, should be removed before a test is begun. A
permanent drop tube can be replaced with a temporary tube. Such a
replacement is difficult and expensive and may only be worth the effort
if it is expected that tightness testing will be performed routinely on the
tank for a long time. Otherwise, the tightness testing company should
be questioned about the ability of the method to perform well with a
permanent drop tube.
57
-------
Testing Considerations
| Preparation] Adequate waiting time between product delivery and testing
X
Testing
Analysis
The most common problem with tightness testing is not waiting long
enough between adding product to the tank and beginning collection of
temperature and level data. The fluctuating temperature and structural
changes of the tank following the addition of product cause volume and
level changes that are unrelated to changes caused by a leak. Not
waiting for these changes to stop results in erroneous test results.
There are two times when product may be added to the tank: (1) gross
product delivery, where thousands of gallons may be added to bring the
product to approximate testing level; and (2) "topping off', where
several gallons may be added to achieve the final level for testing. For
the reasons discussed below, each addition of product and increase in
product level causes changes within the tank that interfere with accurate
test results.
When product is added to a tank, its temperature is at or near ambient
air temperature. The temperature of the product already in the tank is at
ground temperature. The difference between ambient air temperature
and ground temperature varies with the season and location, but
differences of 10 to 20 degrees Fahrenheit are not uncommon. For
some time after delivery, the temperature of the product will fluctuate
rapidly and widely as the product mixes and eventually achieves an
equilibrium near ground temperature. As the temperature of the liquid
in the tank increases or decreases, the volume of the liquid will increase
or decrease, respectively (see Figure 10). For example, 1000 gallons of
gasoline will shrink by 0.7 gallons when the temperature drops by one
degree Fahrenheit. Increasing volume due to temperature increase may
mask a leak while decreasing volume due to temperature decrease may
falsely indicate a leak.
All tightness test methods must account for temperature changes.
However, for a period of time following delivery, the thermal chaos in
the stored product is too extreme to be adequately measured and
accounted for. In addition, the temperature changes immediately
following delivery are not the same at the ends of the tank as at the fill
pipe. Consequently, no matter how many temperature sensors are added
at the fill hole, the measured temperature at that point does not reflect
the average temperature in the tank.
58
-------
CAP
CONCRETE
CAP
f LEVEL
^^- FILL TUB
i—CONCRETE
PRODUCT LEVEL
•FILL TUBE
NEW, COOLER PRODUCT
(A) Product has Just btien added to an underground
tank that was already partly filled. The new
product Is cooler than the resident product, and
temperatures fluctuate greatly.
' '
As the old product cools and the new warms,
equilibrium Is reached. But tha temperature as a
whole Is cooler, causing the product to contract
and the level to go down. (The Inverse Is true
when warmer product Is added, causing the product
io expand and the level to rise.)
Figure 10. How temperature changes can be mistaken for a leak.
Source: U.S. EPA (1989)
59
-------
In addition to fluctuating temperature, the addition of product causes
structural deformation of the tank. Whether it is constructed of steel or
fiberglass and whether it is embedded in a dense backfill or in a loose
one that has more "give," the tank itself expands and contracts in
response to both temperature and level changes. When the tank
expands, the level of the fluid inside it goes down; conversely, when it
contracts, the level goes up (See Figure 11). The amount of volume
change due to tank deformation varies with the material of construction
and the type of backfill, but effects can easily be in the range of 1 to 10
gallons. Distinguishing between real volume changes and the apparent
changes brought on by structural deformation is generally not possible,
regardless of how accurate the equipment is.
The solution to the problems of temperature fluctuation and tank
deformation following addition of product is to wait until these changes
have stopped, i.e., until the product temperature has stabilized and the
tank has completely expanded. The exact waiting time that is necessary
will vary with the amount of product delivered and the temperature
difference between added and original product. As a guideline, a
minimum waiting time of about 6 hours should elapse after delivery of
product in the range of hundreds or thousands of gallons and about 3
hours after topping off of the tank. These minimum times should be
sufficient for both thermal fluctuations and tank deformation to
stabilize. A few tightness test methods avoid the problem of
temperature effects by measuring some aspect that is independent of
temperature, such as the mass of the product.
To determine that sufficient time has elapsed for the tank to stabilize,
the tester should watch the temperature and level changes. Preferably,
temperature and volume measurements versus time should be plotted on
a graph as the measurements occur. If the readings show large and
erratic changes, conditions in the tank are probably still fluctuating. If
the temperature-compensated level changes decrease and eventually
level off, it is an indication that the tank ends were continuing to relax
early in the test but finally stabilized. Some tightness test methods
include statistical analyses of the data as they are collected and, from the
randomness of the data, can determine that tank conditions are not
stable enough to begin the test. Regardless of which method is used to
determine tank conditions, any initial data indicating instability should
not be used in the final evaluation, and the length of the test should be
extended so that the minimum acceptable test duration occurs after the
tank has stabilized. That is, if the test protocol says that
60
-------
CAP W pCONC
RETE
PRODUCT.
LEVEL
FILL
TUBE
PRODUCT
(A) An empty underground tank has Just been filled
with product.
CAP
CONCRETE-
PRODUCT
LEVEL
FILL
TUBE
PRODUCT
In response to the pressure and/or temperature
of the product, the ends of the tank begin to
deflect ("structural deformation"), and the
level of the product goes down.
Figure 11. How structural deformation of the tank
can be mistaken for a leak. Source: U.S. EPA (1989)
61
-------
the test should last for 2 hours, the 2 hours of test data used in the
analysis should be obtained after the tank has stabilized.
I TeaHng |
~~
| Analysis |
Vapor pockets in overfilled-tank tests identified and removed
In overfilled-tank tests, vapor pockets may form. Although most test
operators claim to be able to identify vapor pockets easily, many tests
continue to be invalid or yield incorrect results because the problem was
not recognized or corrected.
Vapor pockets are almost always present after a tank has been filled to
or above the top of the tank because vapor becomes trapped in the
manways, deadend piping, etc. (see Figure 12). Temperature
fluctuations change the volume of the vapor pocket, and the expansion
and contraction of the vapor pockets changes the liquid level in the tank.
For example, a temperature change of about 0.25 degrees Fahrenheit
may change the volume of a 100-gallon vapor pocket by about 0.05
gallons. To a lesser extent, changes in barometric pressure also cause
vapor pockets to expand and contract, causing changes in liquid level.
An increase in liquid level due to expansion of a vapor pocket may
mask a decrease in liquid level due to a real leak, while a decrease in
level due to contraction of a vapor pocket would falsely indicate a leak
or exaggerate the rate of an actual leak. Vapor pockets in quantities as
small as 10 gallons can influence a test result.
The first step in solving this problem is to identify the presence of a
vapor pocket. While it is virtually impossible to determine the exact
size of the vapor pockets, there are several methods that can be used to
check for their presence. The most easily identifiable indication of a
vapor pocket is the presence of bubbling in the fill pipe or stand pipe
during the test. Vapor pockets may also be indicated by a sudden large
drop in product level, indicating a vapor pocket that just "released". If
the temperature-compensated volume changes fluctuate over time with
no obvious trend, then there may be a vapor pocket that is expanding
and contracting, thus confounding the results. This indicator, however,
is not conclusive unless sufficient waiting time has elapsed since
addition of product for temperature and structural deformation changes
to subside. Another method of identifying vapor pockets is to add a
known volume (of product or a solid object) to the tank and compare the
actual increase in product level to the increase that would be expected
from the geometry of the tank. If the actual level change is less than the
expected change, a vapor pocket may be present that compressed from
the pressure of the added product.
62
-------
Dispenser
Trapped
Vapor
Vent Pipe
o\
Figure 12. Location of vapor pockets in an overfilled tilted tank.
Source: Schwendeman and Wilcox (1987)
-------
If a vapor pocket is shown to be present, it must be removed. One
method of removal is to uncover the tank and install a bleed valve
on the high end of the tank. As the vapor is bled off, product will
fill the void. A relatively new method of removing vapor pockets
involves inserting a hose and bladder into the tank, inflating the
bladder so that it rises to the high end of the tank (where the vapor
pocket is), and suctioning out the vapors. After a vapor pocket is
removed, the test should be started over. Checking for signs of a
vapor pocket should continue because not all of the vapor may have
been removed or another pocket may have formed.
Accounting for the presence of ground water
T«atlng
Analyala |
The presence of ground water around the tank may completely mask an
actual leak or at least slow the rate at which product is leaking. Failure
to check for the presence of ground water and to take action when it is
present make the results of a tightness test questionable. In the National
Survey conducted by EPA at about 500 randomly selected sites around
the U.S., ground water was above the bottom of the tank at about 25
percent of the sites.
The water table of the soil in which a tank is buried can vary in height
depending on factors such as geographic location, season, and amount
of precipitation. As the illustrations in Figure 13 show, the height of the
water table in relation to the tank can have a direct effect on the leak
rate measured during a test. If the water table is above the location of a
hole or fissure in an underground tank, the ground water exerts a
pressure on that hole which counteracts the pressure exerted on the
same hole by the fluid in the tank. The best test results are obtained
when the water table is below the level of the tank. Flow of the leak
through the hole is then unrestricted, and measurement of the flow rate
will not be influenced by ground water.
Because it is virtually impossible to determine the location of a hole in
an underground tank, efforts must be concentrated instead on
monitoring the ground-water level. The tester should determine the
ground-water level or at least determine if it is below the bottom of the
tank. Hydrogeological information may be available from agencies
such as the U.S. Geological Survey or from boring logs from nearby
sites, but such data do not necessarily indicate conditions at the tank
being tested. There can be very localized hydrogeological formations
that result in a "pocket" of water in a small area of a region otherwise
characterized by deep ground water. The only sure way to determine if
64
-------
NO FLOW. Pressure exerted by
the product Is exactly balanced
by pressureof ground water
at hole. Product Is less
dense than water, so there
Is no flow in either direction
even though product level
Is higher than water table. The
dotted line shows the product height
required to produce an equal balance
of pressure given the height of the
water table.
FLOW INTO TANK. Pressure exerted
by ground water Is greater than
that of product, so water flows Into
tank. Product level would have to be at
the "equal pressure" line In order to
achieve an exact balance with the
ground water, given the height
of the water table.
FLOW OUT OF TANK. Water table
Is below the tank, so there is no
counter-pressure against the product
at the hole. Therefore, product flows out.
FLOW OUT OF TANK. Here, the pressure exerted
by the groundwater is less than that
of the product; therefore, the product
flows out.
Figure 13. Effect of ground water on the rate of flow through
a hole in an underground tank. Source: U.S. EPA (1989)
65
-------
the tank is surrounded by water is to check observation wells near
the excavation zone. If there are no existing wells, one can be made
fairly easily with a hand auger. A well drilled just for checking
water level during a tightness test does not have to be a formally
installed observation well. The important point is to determine
whether the tank is surrounded by water, so the well does not need
to be completed to the water table, only to the bottom of the tank.
A tightness test method should include a formal procedure for
dealing with high ground-water levels. For example, a test can be
postponed until the water table drops below the level of the tank.
This is often impractical, however. For overfilled-tank tests, another
approach is to raise the level of the product above grade until the
pressure on the bottom of the tank reaches a given level, such as 4 or
5 psi (the pressure must be within the safety margins of the design
of the tank). Under this approach, the pressure of the liquid within
the tank will be greater than that of the ground water surrounding
the tank.
Some tightness test methods conduct two consecutive tests, each at a
different fluid level within the tank, and compare the leak rates. If
there is a leak in the tank, the leak rates will be different because the
head pressures are different. Theoretically, no measurement of
ground water is necessary because the test is independent of ground
water, that is, the difference in leak rates will show up whether
ground water is present or not. However, the difference in head
pressure between two different test levels causes only a very small
change in leak rate, and the differences in leak rate from changes in
head pressure probably will be obscured by variations introduced by
other factors such as temperature.
Effective tightness test methods should include a procedure for
determining the presence of ground water and compensating for it.
To ensure that testers follow these procedures, the state of Rhode
Island checks all test reports for the information that the ground-
water level was checked and how it was compensated for. Some test
sites are visited after the test to check that a well is, in fact, present.
-------
Testing
Analysis
Accuracy of height-to-volume conversion factor for each tank
Most tightness test methods measure changes in product level over time.
To calculate a leak rate in gallons per hour, these level measurements
must be converted to changes in volume. The value used to make this
conversion is called the height-to-volume conversion factor and will be
different for each tank. If the wrong conversion factor is used, the
volume change that is calculated will be wrong, resulting in an
erroneous leak rate or even a false decision regarding the integrity of the
tank.
The height-to-volume conversion factor should be determined
specifically for each site because the geometry of each UST system is
slightly different. The conversion factor can be derived mathematically
based on the geometry of the tank. However, there can be differences
between the manufacturer's specifications for the general type of tank
and the actual tank that was installed, and there may be unknown factors
that change the internal volume of the tank, such as tank end deflection
or old piping that is still attached. For these reasons, theoretical
calculations of the conversion factor are considerably less accurate than
direct measurement. The conversion factor can be determined by
adding and withdrawing known volumes to the tank and measuring the
actual change in height of the product. The known volume added can
be either product or some solid object such as a metal bar. For example,
if 5 gallons is added to the tank and the height of the liquid in the fill
pipe increased by 15 inches, a height of 3 inches would equal a volume
of 1 gallon, making the height-to-volume conversion factor equal to 3
inches per gallon.
Correct coefficient of thermal expansion of product
Testing
Analysis
The coefficient of expansion of a liquid relates changes in its volume to
changes in its temperature. The coefficient of expansion is different for
each product. The units of the coefficient are change in gallons per
degree Fahrenheit. If the wrong coefficient of expansion is used, the
calculated leak rate will be incorrect.
Some tightness test methods use average coefficients of expansion for
general classes of product, such as gasoline or kerosene. However, the
coefficient of expansion varies within each type of product, and the
uncertainty inherent in assuming an average coefficient based on
product type is at least 10 percent. To more accurately determine the
67
-------
coefficient, the tester should take a product sample, measure its
density using an API hydrometer, convert the density to standard
temperature and pressure, and then read the coefficient from a set of
API tables. The uncertainty inherent in a coefficient of expansion
determined by measurement of the product density is about
5 percent.
Testing
| Preparation!
Analysis
Collection of sufficient test data
Another common problem with tightness testing is not collecting
enough data to make an accurate and statistically significant
determination of the status of the tank. Some tightness test
protocols requke sampling only every 15,30, or 60 minutes. As
discussed above, some important interferences to accurate tightness
testing such as tank end deflection and thermal fluctuation should be
monitored using the level and temperature data. If these data are
taken only infrequently, important trends in the data may be missed
and, thus, problems may not be identified. If the test does not last
long enough, very small leaks may not be identified.
More data may be collected by either sampling more frequently or
conducting longer tests. As a rule, obtaining more data increases the
probability of correctly identifying the presence of a leak. Ideally,
level data should be sampled at intervals of approximately one
second; however, such a sampling frequency is not practical with
most equipment. For manual tightness test methods, a sampling
frequency on the order of 5 minutes is generally adequate. Studies
have shown that tests at least 2 hours in duration (after appropriate
waiting periods have been observed) provide more accurate results.
Adequate temperature compensation during test
Measurement of the average temperature of the product throughout
the tank is important because the total volume of the product will
change in response to changes in temperature. If the temperature
changes in only one or two points in the tank are measured,
incorrect total volume changes for the tank will be calculated.
The stored product will expand and contract in response to
temperature changes. Once the appropriate waiting time has passed
following product delivery, the product in the tank will usually be
68
-------
stratified into layers of different temperatures. Very small
temperature changes continue to occur in each of these layers, even
after the tank is essentially stabilized. The extent and rate of
temperature changes will vary layer by layer, and thus, the change in
product volume will be different for each layer. The temperature in
one layer is not indicative of changes in the other layers. Therefore,
it is necessary to measure the temperature in as many layers as
possible to obtain an accurate total volume measurement.
Because of the problem of temperature gradients within the tank, the
tightness test method must have temperature sensors that provide
adequate spatial coverage of the tanks, so that the data they record
are representative of product conditions throughout the tank. A
vertical array of at least five thermistors provides the best spatial
coverage. An array of three sensors, arranged vertically, at the top,
middle, and bottom of the tank is considered adequate. Although
one sensor typically is not sufficient, methods that circulate the
product in the tank can obtain satisfactory results with a single
probe. By mixing the product, the problem of uneven temperature
distribution in the tank is eliminated. Methods using temperature
probes that average the temperature throughout the tank into a single
value also eliminate this problem.
Analysis
[Preparation] Constant product level maintained during overfill tests
* - -
Adding or removing product changes the hydrostatic pressure within
me tank' causin& me sides of me tank to expand or contract,
changing the apparent volume of product in the tank. Even relatively
small level changes that occur during data collection can cause some
tank deformation, leading to erroneous test results.
Tank tightness test methods can be categorized into those that
maintain a steady product level and those in which the product level
is allowed to fluctuate. In constant-level tests, small amounts of
product are added or removed periodically to maintain the product at
a specified level. The product can be added or removed either
manually or automatically. In variable-level tests, no such
adjustments are made. When the fluid level is kept constant during
the test, the tank will neither expand nor contract during the test in
response to the level changes, thus removing an interfering factor,
and measured volume changes will accurately represent actual
volume changes. The amount of product added during
constant-level tests is too small to introduce any error in the test
results due to temperature-related effects.
69
-------
Variable product level is a problem primarily with overfill tightness
test methods. For underfill test methods, the changes in product
level are too small to cause enough change in head pressure and,
thus, deformation of the tank ends.
If the product level fluctuates during a test, it is impossible to
compensate for the effects of tank deformation on volume. It is
advisable, therefore, to eliminate from consideration for use any test
conducted under variable hydrostatic pressure (i.e., variable product
level).
[Preparation!
..*....
\ •: Tasting ¥[
Analysis
Compensating for evaporation and condensation
Unless a tank and its fill tube are completely filled and no air or
vapor pockets are present, it is likely that, as temperature changes,
fluid will evaporate from the free product surface into the air space
and/or will condense along the inner surface of the tank walls and
drip back down. This activity produces volume fluctuations as
liquid product evaporates and condenses. Although a few test
methods attempt to account for the effect of evaporation and
condensation, these effects are not believed to be significant enough
to warrant special control or measurement during a tightness test. In
an overfilled-tank test conducted under extremely hot conditions or
with the standpipe in direct sunlight, it is possible that product may
evaporate in the standpipe; however, such conditions are very rare.
Shade could be provided for the standpipe, or testing delayed to a
cooler day. A record of ambient air temperature during the test may
help to pinpoint possible reasons for unusual test results.
I Preparation] Fixed number of tests in the testing protocol
Testing
Analysis
70
When the results of a tightness test indicate that a tank is leaking but
the leak rate is only slightly above the threshold value for declaring
a leak, some testers repeat tests on the tank until the results of one
test indicate that the tank is tight. This approach is a misuse of the
"more data are better" axiom and reduces the probability of
detecting a leak. A multiple-testing strategy is a valid approach to
tightness testing but it must be performed correctly and all of the
data must be considered, not just the data from the one test that
gives the desired answer. For example, the testing protocol could
require that five tests be performed and the tank declared tight if the
-------
results of three of the tests are below the threshold. Under this
approach, the minimum detectable leak rate is less than that for a
single tightness test. A multiple-testing strategy must be a
well-defined part of the testing protocol, and no deviation should be
allowed from the total number of tests conducted.
Minimizing decisions left up to the tester
In some test methods, the procedures to be followed by the tester are
ill-defined or are left to the tester to decide, such as the point at
which to end the test. Some circumstances may not be included in
the protocol at all, such as how to recognize vapor pockets. Much is
left to the discretion and experience of the tester. However, not all
testers have sufficient experience to make informed decisions. In
addition, even relying on experienced testers may result in different
decisions being made by different people in the same situation.
These factors decrease the likelihood of a test method achieving
accurate and consistent results.
The most reliable test methods are those least subject to operator
influence. Any test method which requires or allows the operator to
make subjective decisions during the test should be avoided. For
example, the test operator should not be allowed to decide when to
make a product-level adjustment or to decide how much product
should be added or removed. Adjustments of any kind should be
accomplished using a set, repeatable procedure, not an arbitrary one.
As discussed above, the number of tests and their length should be
specified by the test protocol and not left to the discretion of the
tester.
Analysis
| Preparation] Protocol followed and correct threshold value used
*
I—es*ng I Many test methods lack a defined data-analysis protocol and a clear
criterion for deciding if a tank is leaking. This deficiency allows
* testers to make subjective decisions and leads to unclear or even
false determinations of the status of the tank.
A reliable test method will have a well-defined procedure for
analyzing the data, either manually or by computer. The necessary
steps in a data analysis are presented here. The first step is to
calculate the volumetric flow rate, which can be accomplished in
71
-------
different ways. The goal of any calculation is to compute the
average rate of change, or flow rate, that indicates how fast liquid is
escaping from the tank. First, the level changes measured during the
test are converted to volume changes using the height-to-volume
conversion factor. Using the coefficient of thermal expansion,
compensation is made for any temperature changes that were
recorded. The temperature-induced volume changes are then
subtracted from the measured volume change, yielding the
temperature-compensated volume changes. These are used to
determine the volumetric flow rate. If performed manually, these
calculations must be explicitly defined for the tester.
The manufacturers of some test methods use all the data available
when they perform the analysis. Others employ averaging
techniques. One example of an averaging scheme is to subtract
end-of-test data from start-of-test data and divide by the time that
has elapsed between the two. Another example is to add all the
cumulative volume changes and then divide this sum by the duration
of the test. Finally, some manufacturers fit a line to the data; that is,
the data points are expressed as dots on a graph, and a line is drawn
as closely as possible through the points. Whatever analysis scheme
is used, it must be well defined, and the tester must adhere to that
protocol.
To determine if the tank is leaking or not, the volumetric flow rate
must be compared to a threshold value, which has been
predetermined as part of the test design. In order for a test method
to perform well against small leaks, the threshold value must be
smaller by a factor of 2 or more than the smallest leak to be
detected. Let us assume, for example, that a tank is leaking at the
rate of 2 gal/h. If the test method in question can discern leaks as
small as 1 gal/h, it will almost certainly detect the 2-gal/h leak.
However, if the tank is leaking at a rate of 0.5 gal/h, less than what
is discernible by this test method, the leak will go undetected.
The most commonly used threshold is 0.05 gal/h. This threshold is
often confused with the leak rate to be detected. If the threshold is
equal to the leak rate to be detected, the probability of detecting a
leak of that size is only 50 percent. The final federal regulation
requires a tightness test method to have a minimum detectable leak
rate of 0.1 gal/h. For a test method to meet this requirement, its
threshold must be less than 0.1 gal h. For most test methods, this
threshold will be around 0.05 gal/ h.
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[Preparation] Calculations performed correctly
Testing
For test methods in which the volumetric flow rate is determined
manually, one problem can be simple calculation errors, leading to
an incorrect conclusion. A tester should always have someone else
double check the calculations. In some local programs such as
Nassau County, New York, copies of all tightness test records must
be submitted to the implementing agency. Personnel at the agency
double check the calculations, either by hand or using computer
programs developed for the tightness test methods allowed in the
county.
ENSURING EFFECTIVE TESTING
To ensure that testers follow the procedures necessary to prevent the
problems described above from occurring, a number of state and local
implementing agencies have developed programs to oversee the practice in
their jurisdictions. Chapter 1 provided a general description of the four
types of oversight that can be used. The following section summarizes
briefly some of the specific actions taken by implementing agencies to
ensure effective tank tightness testing.
Site Inspections
In Rhode Island, the ground water is frequently very high and, therefore,
checking and compensating for its presence is very important. State agency
personnel routinely follow up a number of tests by visiting the site and
making sure that there is some way in which the ground-water level could
have been checked. Either an observation well was already on-site for some
other purpose or a hole was drilled by hand specifically for the tightness
test.
In Nassau County, New York, testers must call county officials when level
and temperature data collection is about to begin so that the official may
visit the site and observe the test. Which tests to observe is left to the
discretion of the regulator. In Austin, Texas, the owner/operator must
notify the implementing agency before a test is performed and agency
personnel observe all tests.
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Data Review
Several states and counties, such as Rhode Island, Nassau County, New
York, and Madera County, California, require tightness testers to submit
test reports to the implementing agencies. Regulators then review these
reports for adherence to key procedural elements such as: ground-water
level, actual calculation of coefficient of expansion, length of test, number
of data points, and appropriate testing levels. Recalculation of the test data
is also performed on all or part of the test reports. Personal computers have
been used to cut down on the time necessary for a review of test data.
Nassau County, New York, has written a computer spreadsheet to be used
with the data from two common test methods that double checks the tester's
calculations using the tester's raw data. Rhode Island enters all test results
into a computer program that performs statistical analysis on the pass/fail
ratios of the test companies. Whenever a company is passing or failing a
disproportionate number of tanks, the agency investigates.
Guidance and Training
Because proper procedure is so important to effective tank tightness testing
and because it is the major source of error as currently practiced, training
and guidance can be an important tool. Guidance can be aimed at
implementing agency personnel, so that they can provide effective
inspection and review, at owner/operators, so they can select and oversee
effective testers, and at testing personnel, to ensure that they perform the
tests correctly.
Approval and Certification
Some implementing agencies have tried to prevent tightness testing
mistakes by only allowing methods and personnel that they feel are
acceptable. Several different approaches are being used. Maryland requires
that manufacturers submit performance evaluation results to the state for
review, physically demonstrate the method to state personnel, provide
information on their personnel training/certification program, and then, if
accepted, train state agency personnel. In Nassau County, New York,
regulators review the test procedure manual for specific directions
addressing the key elements identified in NFPA 329. In Los Angeles
County, California, all tightness testers must be tested and approved by an
independent party.
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In other states, one step in the method approval process requires that the
manufacturer train agency personnel in the operation of the test method.
Massachusetts requkes that the manufacturer provide a video to be used for
training. Rhode Island and Maryland requke that testers train the
implementing agency personnel.
REFERENCES
1. U.S. Envkonmental Protection Agency. November 1988.
Evaluation of Volumetric Leak Detection Methods for Underground
Fuel Storage Tanks, Vol.1. EPA/600/2-88/068a. Prepared for U.S.
EPA by Vista Research, Inc.
2. Schwendeman, T.G. and H.K. Wilcox. 1987. Underground
Storage Systems - Leak Detection and Monitoring. Lewis
Publishers, Chelsea, Michigan.
3. National Fke Protection Association. 1987. NFPA329-
Underground Leakage of Flammable and Combustible Liquids.
Quincy, Massachusetts
4. U.S. Envkonmental Protection Agency. 1986. Development of a
Tank Test Method for a National Survey of Underground Storage
Tanks. EPA-560/5-86-014. Prepared for U.S. EPA by Midwest
Research Institute and Vista Research, Inc.
5. U.S. Envkonmental Protection Agency. May 1986. Underground
Motor Fuel Storage Tanks: A National Survey, Vol. 1. Technical
Report. EPA-560/5-86-013.
6. U.S. Envkonmental Protection Agency. 1986. Under ground Tank
Leak Detection Methods: A State-of-the Art Review.
EPA/600/2-86/001. Prepared for U.S. EPA by IT Corporation.
7. U.S. Envkonmental Protection Agency. July 1988. Common Human
Errors in Release Detection Usage. Prepared for U.S. EPA by Camp
Dresser & McKee, Inc.
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8. 40 CFR Part 280. Technical Standards and Corrective Action
Requirements for Owners and Operators of Underground Storage
Tanks (UST). 53 FR 37194-37212.
9. Maresca, J. W. and M. L. Seibel. September 23,1988. Volumetric
Leak Detection Systems. Vista Research, Inc.
10. U.S. Environmental Protection Agency. April 1989. Volumetric
TankTesting: An Overview. EPA/625/9-89/009.
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Chapter V
Automatic Tank Gauging
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AUTOMATIC TANK GAUGING SYSTEMS
SUMMARY
Automatic tank gauging (ATG) systems are permanently installed in
underground storage tanks and provide both leak testing and inventory
information. When the two modes are used together, ATG is an
effective release detection method. Two studies conducted on ATG
systems indicated that the leak test mode is capable of detecting leaks as
small as 0.2 gal/h with a probability of detection of 95 percent and a
probability of false alarm of 5 percent. The inventory mode of ATG
systems is more accurate than the comparable manual method.
ATG systems are selected by owner/operators because such systems
require minimal operator involvement, cause few service interruptions,
and can provide frequent automated release detection. In addition, the
inventory mode provides continuous product information helpful in
business management. For UST operations that close at least once a
month, which includes many sites, the leak test mode does not interrupt
operation. The inventory mode provides nearly continuous monitoring
for the large losses typically discovered by inventory methods and,
depending on how the owner/operator elects to operate the ATG system,
the tightness test mode can be used every time operations cease (e.g.,
nightly, at many service stations).
The discussion presented in this chapter covers a range of problems that
may occur with ATG systems. This does not mean that all, or even
most, of these problems will occur at the same time or at the same site.
Nor does it mean that all of the problems are of equal importance, in
terms of frequency of occurrence or severity of impact on the
effectiveness of ATG systems. Some problems, such as blended fuel
dispensers, seldom occur. Experienced ATG system vendors and
installers are well aware of these problems and how to deal with them.
For example, a reputable installer knows the proper wiring installation
materials and methods and will follow them. Release detection is a
growing industry, however, and new companies are being formed that
bring less experience to the field. Presented in this chapter is a range of
potential problems for educational purposes, not to imply that the
problems will always occur.
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BRIEF DESCRIPTION
General
Figure 14 presents a general schematic of an ATG system. All ATG
systems are based on the same general approach. This section focuses
on the general principles behind ATGs and not on specific equipment.
For details on the specific equipment, manufacturers and vendors will
provide literature.
ATG systems measure the change in product level within the tank over
time to determine if there is a leak. When a leak occurs in an UST, the
loss of product causes a decrease in the level of product. Other factors
also can cause product level changes. The most important of the
interferences are product temperature and tank deformation caused by
addition or withdrawal of product. ATG systems differ primarily in
how product level and temperature are measured.
ATG systems have two modes of operation: inventory control and leak
testing. When the system is on, it is in one of these modes and can be
switched to the other. The same equipment is used for both operations.
Installation of the equipment and the operation of the inventory and test
modes are described in the following sections. Figure 15 is a flow chart
of the general operation of an ATG system.
Installation
Installation of an ATG system involves equipping each tank with a
probe to measure product level and temperature. The probe is inserted
into the tank through a separate fitting (not the fill pipe) on top of the
tank. For most ATG systems, the fitting must be 3 or 4 inches in
diameter. Some older USTs do not have extra openings or the existing
bungs are too small; in this case, an opening of appropriate size for the
probe must be made by the installer.
A remote monitor and microprocessor are installed in a nearby building
to record the information read by the probe. The monitor usually has a
keyboard for programming and a display for presenting the required
data. Underground wking is installed between each probe and the
remote monitor. In some ATG systems, wking is also installed between
the dispensers and the monitor or the pump control console/point of
sales device and the monitor. National electrical codes requke that the
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Remote ATG Monitor
Fill Pipe
Pump or Pump Control
Console
Temperature, and Water
Sensor)
Figure 14. Schematic of automatic tank gauging system.
-------
Install Equipment
installation
J
Probes in tanks
Cables in conduits
Monitor nearby
Testing
or
Inventory
Program Monitor
Coefficient of thermal
expansion/product type
Tank depth and volume chart
Threshold criteria
Test times
Alarm levels
Shut Down Tank Operations
I
/ai
I
• Temperature to reach
equilibrium
• Tank deformation to subside
Take Temperature and Product Level Measurements
\
Reconcile Inventory Data
Ana ysis
Detection
Criteria
Analyze Test Data
Leak No Leak
Figure 15. General procedure for ATG Systems
80
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wiring be installed in some kind of conduit to protect it from dirt and
moisture and isolate it from the surroundings. For most ATG systems,
four to eight.probes can be connected to one monitor. The height-
to-volume conversion factor and the coefficient of thermal expansion
(see Chapter 4 for definitions of these terms) must then be entered into
the monitor by the installer.
Installation time varies with conditions at the site and the number of
tanksrbeing fitted with probes. For a four-tank site with no unusual
problems, it typically takes about a day to install the conduits and
wiring and a day to install the probes.
Leak Detect Mode
Tanks must be taken out of service when the ATG system is in leak
detect mode. Most tanks, therefore, are tested at night when operations
at the site typically shut down. Most ATG systems can be programmed
at the monitor to automatically switch to the test mode at a preset time.
Alternatively, on-site staff can manually switch the ATG system to test
mode at the appropriate time. The test mode can be run as frequently as
the owner/operator determines.
The test is conducted at whatever product level is in the tank at the time
of the test. The large surface area of the product in an underfilled tank
(product level below the top of the tank) means that any product loss
due to a leak would result in a very small change in level. Because
testing is performed on underfilled tanks, the product level is essentially
constant during the test, which is necessary for an effective tank test
(see Chapter 4 for a detailed discussion of constant-level vs.
variable-level tests).
When the ATG switches to leak detect mode, temperature and level
readings are taken automatically. The test can be programmed to last a
predetermined length of time. Also, some ATG systems can be
programmed for a desired minimum detectable leak rate; the
microprocessor then determines the necessary sampling frequency for
product level and temperature and for test duration. If a detectable leak
rate below 0.2 gal/h is selected, the probability of detecting such a leak
will probably decrease somewhat. Level and temperature readings are
usually taken every 1 to 2 seconds and averaged every 30 to 60 seconds.
The length of the test varies with the system and the level of sensitivity
desired. Tests generally range from 1 to 6 hours in length, with most test
lengths falling in the lower end of the range. As the number of probes
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connected to the monitor increases, the frequency of readings and the
test length decrease because the probes are "read" in series and it takes
longer to complete a circuit of eight probes than to read two probes.
The level and temperature readings are fed to the microprocessor, which
converts these readings to temperature-compensated volume
measurements, analyzes the data according to some statistical algorithm,
and determines a rate that indicates how fast product level is changing
in the tank. This rate must then be compared to a threshold value to
determine if the level changes are due to a leak or to normally occurring
volume fluctuations. If the temperature-compensated volume change
exceeds the threshold, a leak is suspected; if not, it is assumed that the
tank is not leaking.
Inventory Mode
The same level and temperature readings taken during the test mode are
taken in the inventory mode, which is in operation any time a test is not
being run. In addition to taking product level and temperature readings
in the tank and converting them to volume measurements, some ATG
systems measure and record the amount of product dispensed. For other
ATG systems, the dispenser information must be recorded manually by
on-site staff. Product deliveries also are recorded by the ATG system.
Increases in volume in the tank that are above a minimum rate and
volume are interpreted as a delivery.
If the dispensing information is collected by the ATG system, the
microprocessor automatically reconciles the inventory data at the
interval programmed into the monitor; one hour is often selected as the
interval. If staff record the dispensing information manually, the
volume and delivery data from the ATG must be combined with the
dispensing data and reconciled manually (see Chapter 2 on Inventory
Control).
As part of the inventory mode, the probe in most ATG systems also
measures the level of water in the bottom of the tank. This information
is converted to a volume and used in the inventory reconciliation. Other
features included in many ATG systems are alarms for high product
level, low product level, high water level, and theft (indicated by sudden
large loss of product). The levels at which these alarms are triggered
are programmed into the monitor.
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POTENTIAL PROBLEMS AND SOLUTIONS
This section presents a discussion of problems that have been
encountered with ATG systems. Many of the problems and solutions
are similar to those for tank tightness testing, which are discussed in
Chapter 4. To avoid repetition, this chapter includes only a brief
summary plus a reference to the relevant section of Chapter 4 for those
problems that are common to both methods. Table 6 presents a
summary of the indicators and solutions to problems with ATG systems
as well as possible approaches that implementing agencies can use to
prevent or overcome the problems. A number of agency solutions are
offered for each problem, but not all of them need be undertaken.
Table 6 and the discussion below are presented in the order of the
flow chart (Figure 15 on page 80). There is no ranking implied by the
order. In Table 6, the most serious concerns are indicated by an
asterisk.
Installation
Ensure that equipment is installed properly
Most problems with the installation of ATG systems are
associated with the installation of conduits for the wiring.
National electrical codes require that the cables be isolated from
other electrical wires. Most UST sites already have conduits
containing wiring for other equipment, such as the dispensers,
and this wiring is often contained within one main conduit.
Installers of ATG systems sometimes use existing conduits for
the ATG wiring because it is easier and less expensive than
installing new conduits. If existing conduits containing wiring
are used, the ATG wiring must be isolated in some manner, such
as with flexible plastic casing. Any time wiring is found that is
not isolated, power to the wiring should be immediately shut off
and the problem fixed before the system is allowed to operate
again.
Installers sometimes neglect to continue the conduit/isolating
material around the wiring where the wiring enters the building
and connects to the monitor. In addition, installers sometimes
use sealing compound for the last several feet of wiring instead
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00
Table 6. Indicators and Solutions for Problems Encountered With Automatic Tank Gauging Systems
Problem
Indicators
Tester Solutions
Agency Oversight Options
Assure correct installation
of wiring.
Use correct coefficient of
thermal expansion.
*Need for adequate waiting
time before testing.
No readings or erratic readings.
False alarm.
Erratic and large volume
changes. Short-term volume
changes that level out.
*Need for adequate temperature None.
compensation during the test.
'Preventing Interference due to None.
evaporation and condensation.
Isolate wire in separate conduits,
or encase in isolating material.
Whenever type of product
changes, reprogram monitor.
Wait at least 6 hours after fuel
delivery before testing. Wait
as long as possible after UST
operations stop to begin test.
At 24-hour sites, test following
period of lowest use.
Use at least three temperature
sensors equally spaced vertically,
or use temperature-averaging
probe.
Do not open any fittings on top
of the tank for at least 6 hours
before the test and during the test.
Observe installation. Review
installation plans.
Compare product type to
program value during site
inspecion or review of reports.
Review printouts for product
delivery and test times. Review
and approve testing schedule.
Observe installation. Review
product literature. Approve
only ATG systems with
appropriate designs.
Observe test Train/educate
staff at UST site.
*The number of tests to be
conducted at a tank must be
fixed.
High percentage of tanks declared
"tight."
Protocol must include fixed
number of tests. Use
progammable monitor that can
be locked.
Review printouts of tests;
compare to protocol. Approve
only programmable systems that
cannot be overridden. Train/
educate staff at UST site.
-------
Correct threshold must be
used to determine leak status.
(A threshold is a predetermined
value; measurements made
during a test are compared to
this value.)
Accounting for the presence
of ground water.
High percentage of tanks declared
"tight."
Assuring testing at a range
of product levels.
Use correct height-to-volume
conversion factor.
Assuring appropriate treatment
at UST sites with blended fuel
systems.
Assure calculations are
correct.
Water in the tank.
Monthly tests are always
conducted at low product level.
None.
None.
Improbable results.
Threshold value should be
smaller than the minimum
detectable leak rate by a factor
of at least 2. Program monitor
compares leak rate to threshold
and triggers alarm if leak is
suspected.
Water sensor in the tank as
part of the ATG system.
Program tests to take place
at varying product levels that
cover the range typically stored.
Add or withdraw known volume
from tank, and observe level
changes.
Treat multiple tanks as one
unit for purposes of
reconciliation.
A second person double-checks
manual calculations. Computer
program is reviewed and tested
using known and verified data.
Keep track of pass/fail ratio
of each type of ATG system.
Approve systems with adequate
thresholds or with alarm systems.
Approve only ATG systems
with water sensors. Observe
installation. Review release
detection plans and manufac-
turer's product descriptions.
Review printouts to compare
range of product levels during
the month to levels during tests.
Review testing schedule with
knowledge of delivery schedule
and UST operations.
Review data sheets for
calculation of conversion
factor.
Inspect site before system
installed. Review site plans
before installation.
Manual or computer
recalculation. Approve only
systems with automated
inventory.
indicates the most significant problems.
00
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of for the last few inches. Both of these practices violate
electrical wiring codes and are unsafe.
Sometime the conduits that are installed are too small for the
amount of wiring that must run through them. It takes extra time
and effort to install the wiring and creates the risk of damaging
the cables. Generally, conduits should have an internal diameter
of at least 3/4 inch.
1
r
Testing or
Inventory
Analysis
installation | Use correct coefficient of thermal expansion of the product
The coefficient of thermal expansion of a liquid relates changes
in its volume to changes in its temperature. The units of the
coefficient are change in gallons per degree Fahrenheit. The
coefficient is used to convert changes in temperature readings
taken by the probe to changes in volume as part of the
determination of leak rate. The coefficient of expansion is
different for each product. When an ATG system is installed,
the "average" coefficient for the product type in the tank is
programmed into the monitor. If the type of product stored in
the tank is later changed (e.g., from gasoline to diesel) and the
coefficient is not changed, the calculated leak rate will be
incorrect. When the ATG system is installed, it is important that
the installer inform the on-site personnel of the need to
reprogram the monitor and inform them how to accomplish this
should the product be changed.
Leak Test Mode
Allow adequate waiting time before beginning test
When product is added to a tank, the product in the delivery
truck is at or near the air temperature while the product in the
tank is at or near ground temperature. For some time after
delivery, the temperature within the tank fluctuates rapidly and
widely as the product mixes and eventually achieves
equilibrium. The addition or withdrawal of product causes the
ends of the tank to move outward or inward, respectively, in
response to the changing head pressure of the liquid on the tank
walls. The fluctuating temperature and structural changes of the
tank following the addition of product cause volume and level
changes that can be misinterpreted as changes caused by leaks.
See Chapter 4 for additional discussion of these phenomena.
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Unlike the tank tightness testing methods discussed in
Chapter 4, product is not deliberately added to a tank before a
test is conducted with an ATG system. Instead, the test is
performed at the product level in the tank when the monitor is
switched to the test mode. The level in the tank depends on
routine product deliveries and withdrawals. No standard
minimum waiting time between stopping tank operations and
beginning to test will be applicable to all situations because the
waiting will vary with the amount of use (i.e., number and
amount of withdrawals) at that tank before operations ceased.
The ATG system should be programmed to begin testing as long
as possible after UST operations have stopped. If the UST site
operates 24 hours per day, the test should be scheduled
following the period of least use, such as late at night (at service
stations). The tank to be tested must be shut down during the
test. Any time product is delivered to an UST, at least 6 hours
should elapse between delivery and testing.
Another temperature-related problem can occur when product is
added that is significantly higher in temperature than the product
in the tank. In this situation, ATG systems that determine
product level using capacitance probes may respond with a false
alarm during a leak test. Differences in temperature cause
differences in densities, which in turn cause differences in the
capacitance of the products. The waiting period discussed above
should reduce the chance of false alarms caused by this problem.
Most ATG systems using a capacitance probe also have a
program built into the analysis that can usually detect this
problem and declare the test invalid.
Adequate temperature compensation during the test
After the temperature in the tank stabilizes, the product is
usually stratified into layers of different temperature. Each layer
will continue to undergo small changes in temperature, the
extent and rate of the change being different for each layer. If
the temperature changes of only one or two layers is recorded
during a test, the temperature-compensated volume changes
calculated for the entire tank will be incorrect, resulting in an
erroneous leak rate determination. For additional discussion of
this issue, see Chapter 4.
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For the reasons given, the temperature sensors in the ATG
system probe must provide adequate spatial coverage of the
tank, i.e., the data recorded must be representative of product
conditions throughout the tank. At least five temperature
sensors arranged vertically provide the best coverage, although
three vertically arrayed sensors usually are adequate. One
temperature sensor generally does not provide sufficient
coverage of the tank, with one exception. At least one ATG
system uses a temperature-averaging sensor that provides a
single temperature value over the entire depth of the liquid.
Preventing interference due to evaporation and condensation
During a test with an ATG system, the tank is only partially
filled with product. If there are at least 15 cm between the level
of the liquid and the top of the tank, which is the case with most
ATG tests, evaporation and condensation are potential problems.
As temperatures change, fluid will evaporate from the product
surface into the air space at the top of the tank and/or will
condense along the inner surface of the tank walls and drip back
down. Evaporation and condensation of the liquid product cause
fluctuations in the product level that could mask or mimic a leak.
Eventually, the evaporation and condensation between the liquid
product surface and the air space above it will come into
equilibrium and no further change in the liquid level will occur.
If this equilibrium is upset, such as by opening the top of the
tank or adding product to the tank, the liquid level will fluctuate
until the tank regains equilibrium.
Before conducting an ATG test, the headspace of the tank must
be allowed to come to equilibrium. To achieve this, none of the
fittings on the top of the tank should be opened for at least 6
hours before the test begins. The equilibrium must be
maintained during the test, again by not opening any of the
fittings on the tank.
Fixed number of tests to be conducted
When the results of a test indicate a leak rate that is only slightly
above the threshold value for declaring a leak, some owners may
rerun the leak test mode on the tank until the results of one test
indicate that the tank is tight. Because conducting a test with an
ATG system is relatively simple and can be done during
nonbusiness hours when the tank is not in use, this approach to
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testing may be tempting. This approach, however, is a misuse of
the system and reduces the probability of detecting a leak.
The same ease of testing that leads to this type of misuse also
makes a valid multiple-testing strategy a likely option for ATG.
In a multiple-testing strategy, a fixed number of tests are run,
and the results of all the tests are used to determine the leak
status of the tank. This strategy increases the sensitivity of the
test and the likelihood of detecting a leak. An UST site that
closes operations for the night or weekend could easily run
several tests with the ATG system, and the microprocessor can
be programmed to perform the necessary statistical analysis.
Whether the ATG system uses a multiple-testing strategy or a
single test, the key to successful leak detection is to define
explicitly the number of tests and to carry them out. Many ATG
systems can be locked after they are initially programmed so that
the protocol cannot be changed except by the person with the
key. The manufacturer of the ATG system must determine the
number of tests to be run to meet the regulatory performance
standards and provide this information.
Correct threshold value used to determine leak status
After reading and analyzing the product temperature and level
data, the calculated volumetric leak rate in gal/h must be
compared to a threshold value to determine if the tank is leaking.
Some ATG systems just display the calculated leak rate and the
comparison and determination of leak status is made by on-site
personnel. Other ATG systems compare the calculated leak rate
to a programmed threshold and display PASS or FAIL along
with the leak rate. If the wrong threshold value is used, an
incorrect determination will be made. For a test method to
perform well in detecting small leaks, the threshold value must
be smaller by a factor of two or more than the smallest leak to be
detected. The federal regulation requires an ATG system to
have a minimum detectable leak rate of 0.2 gal/h. For an ATG
test to meet this requirement, its threshold must be less than 0.2
gal/h. For most systems, the threshold will be around 0.1 gal/h.
The exact value for each system should be determined by the
manufacturer and supplied to the owner/operator.
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Accounting for the presence of ground water
The presence of ground water around any portion of the tank
may lead to erroneous test results if there is a hole in the portion
of the tank under water. The pressure of the ground water
inward counteracts the outward head pressure of the leaking
product, either slowing or completely stopping the leak. The
ground-water level at a site generally fluctuates over time with
the seasons and the amount of precipitation. For further
discussion of the influence of ground water on leaking tanks, see
Chapter 4.
ATG systems conduct tests at the level of product in the tank at
the time of the test; therefore, the product level will be different
for each test. The ground-water level is also likely to be
different during each test. It is unlikely that for each test the
levels of product and ground water will be such that the
pressures are equal, preventing product from leaking out of the
tank and water from entering the tank, particularly if tests are
conducted daily or weekly. Therefore, the test mode will detect
either a decrease in product level due to product leakage or an
increase in water level inside the tank due to ground-water
incursion. If at all possible, it is still preferable, however, to
conduct the leak test at a time when the ground water is below
the bottom of the tank.
It is also important to have a water level sensor as part of the
ATG system and to program the monitor to trigger an alarm at
some preset water level. Accumulation of water in the tank
indicates a possible hole in the tank that must be investigated.
Ensuring testing at a range of product levels
ATG is conducted with the product level below the top of the
tank. Some portion of the tank walls will not be below the
product surface during a test and, therefore, will not be checked
for holes. Either deliberately or by chance, the monthly tests
may always occur at a very low product level, allowing a
significant portion of the tank surface to go unmonitored.
Given the variability in tank use, it is unlikely that, by chance,
the monthly ATG test would always be conducted at a low
90
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product level. Over the months, the tests probably will be
conducted over the range of product levels normally held in the
tank.
To counteract deliberate testing at low product levels,
implementing agencies could require that data on the product
levels in the tank throughout the month be submitted along with
the product level during the test. A review of this information
would show whether the tests are being conducted over the full
range of product levels.
Correct height-to-volume conversion factor used
ATG systems measure changes in product level over time. To
calculate a leak rate in gallons per hour, these level
measurements must be converted to changes in volume. During
installation of an ATG system, the tank manufacturer's tank
calibration chart giving depth measurements and corresponding
volumes is programmed into the monitor. Slight variations in the
manufacturing process means that the chart is not precisely
accurate for all tanks (see Chapter 2 for further discussion of
tank charts and how to use them). Usually, these minor
variations do not interfere with the accuracy of the ATG results.
Occasionally, however, the tank can differ enough from the
calibration chart to cause significant error. If repeated false
alarms occur, one possible factor to check is the height-to-
volume conversion. To do this, product should be added to the
tank in known increments, usually of 100 to 500 gallons, and the
level measured using the probe. These measured
height-to-volume values should then be compared to those
programmed into the monitor using the manufacturer's
calibration chart. If they are substantially different, the actual
measured values should be keyed into the monitor, replacing the
calibration chart values. "Strapping" the tank is not performed
at each ATG system installation because it is very time
consuming.
Ensure appropriate treatment at sites with blended fuels
At some service stations, the customer can select different
blends of fuel at the same dispenser. One dispenser is connected
91
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to more than one tank. The dispenser has several "blenders" that
mix different fuels in preset ratios, such as 60/40. To perform
accurate inventory control, the ATG system must have accurate
values of the amount of product withdrawn from each tank.
Some blenders do not have flow meters in the lines leading to
the blender to provide accurate product volumes supplied by
each tank. The blenders may have 1 to 2 percent error, so that
applying the stated ratios to the total volume passed through the
blender will not determine the actual volumes withdrawn from
each tank.
If blended fuel dispensers are encountered that are not
sufficiently accurate, one solution is to treat the product in all of
the tanks connected to one dispenser as a single volume. Each
tank must have a probe to measure product level and deliveries
as usual, converting the level readings to volumes. These
readings can then be combined and treated as one tank for
purposes of inventory control. The total amount withdrawn
from the dispenser is used to perform the reconciliation; no
determination of the amount attributable to each tank is
necessary because the tanks have been "combined." Combining
the tank readings is applicable only to the inventory mode; the
tanks must be tested separately in the leak test mode.
Ensure correct calculations
Some ATG systems are not connected to the dispensers. In this
situation, the dispenser readings must be recorded manually.
The delivery and tank volume data are taken from the ATG
monitor, combined with the manual dispenser data, and the
inventory reconciliation is performed by hand or the data may be
entered into a computer spreadsheet. Chapter 2 provides
information on how inventory data should be collected and
analyzed. Math errors may result from the hand calculations or
entry errors can occur when the data are input into the computer.
Any reconciliation other than that performed entirely by the
ATG system should be double-checked by another staff
member.
ENSURING EFFECTIVE AUTOMATIC TANK GAUGING
The four general approaches that implementing agencies can use to
ensure effective release detection are discussed in Chapter 1. The
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following sections discuss how these approaches may be applied
specifically to ATG systems.
Site Inspections
The site may be visited during installation of the ATG equipment. The
most important features to inspect would be that (1) proper conduits are
constructed for the cables; and (2) the proper values for the coefficient
and the threshold were programmed into the microprocessor. After
installation, random site visits might be made to check test records and
inventory results. The microprocessor and monitor also could be
inspected to ensure that no changes have been made in the
programming.
Data Review
The implementing agency could require that information on how the
ATG system is installed and programmed be submitted and approved
prior to actual startup of the system. In addition, copies of the test
and/or inventory results could be mailed to the agency for review. Most
ATG systems can be connected to a printer so that little extra effort is
needed on the part of the owner/operator to obtain copies. Agency
personnel then could compare the leak rate values to the threshold value
for the brand of ATG in use.
Similar to an approach used for tank tightness testing results
(Chapter 4), the implementing agency could keep a tally of the number
of passes and fails for each type of ATG system and investigate those
systems with abnormally high proportions of passes.
Guidance and Training
Guidance materials aimed at the owner/operator should emphasize the
proper timing of a test and the need to keep the tank closed both before
and during a test. Guidance material aimed at manufacturers should
emphasize the design needs, such as the number of temperature sensors
and the water level sensor.
Approval and Certification
An implementing agency can elect to review ATG systems and approve
for use in its jurisdiction only those systems that pass a review and
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approval process. Los Angeles, California, requkes data from an
independent third party demonstrating that the system meets the
performance standards. Massachusetts lists all of the elements that must
be contained in the written request for approval, such as the principles
of operation and whether the method tests the whole tank surface or just
the surface below the product surface.
Another approach an implementing agency can use is to require
certification or training of all ATG system installers. The agency could
run the certification program or could require a minimum amount of
training by the manufacturers of the equipment. Most manufacturers of
ATG systems already have some type of training program in place.
REFERENCES
1. Maresca, J. W., N. L Chang, Jr., and P. J. Gleckler. January
1988. A Leak Detection Performance Evaluation of Automatic
Tank Gauging Systems and Product Line Leak Detectors at
Retail Stations. Vista Research Inc.
2. Schwendeman, T. G. and H. K. Wilcox. 1987. Underground
Storage Systems - Leak Detection and Monitoring. Lewis
Publishers. Chelsea, Michigan.
3. U.S. Environmental Protection Agency. November 1988.
Evaluation ofVolumetric Leak Detection Methods for
Underground Fuel Storage Tanks. Vol.1. EPA/600/2-88/068a.
Prepared for U.S. EPA by Vista Research, Inc.
4. U.S. Environmental Protection Agency. 1986. Under ground Tank
Leak Detection Methods: A State-of-the-Art Review.
EPA/600/2-86/001. Prepared for U.S. EPA by IT Corporation.
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Chapter VI
Vapor Monitoring
-------
-------
VAPOR MONITORING
VI
SUMMARY
Vapor monitoring is a relatively new release detection method that has
been applied predominantly UST systems storing petroleum products.
It has been used extensively in California, and in studies by the EPA it
has been shown to detect small leaks quickly.
Before vapor monitoring is selected for release detection, a thorough
site assessment should be conducted to ensure that it is appropriate to
use this method at the site. Many of the problems that interfere with
vapor monitoring can be addressed during the site assessment stage. If
chosen as a release detection method, it should be noted that if a
monitor indicates the presence of a high concentration of vapors, it does
not necessarily mean there has been an UST release. Vapor monitoring
results must be carefully interpreted to differentiate between spills,
interferences, and releases.
The discussion presented in this chapter covers many of the possible
problems that may occur with vapor monitoring. This does not mean
that all, or even most, of these problems will occur. Nor does it mean
that all of the problems are of equal importance in tenns of frequency of
occurrence or severity of impact to the effectiveness of vapor
monitoring. Some problems, such as many of the environmental
interferences mentioned, occur infrequently, while others have limited
impact. Experienced vendors are well aware of these problems and how
to deal with them. For example, an experienced vapor monitoring
company knows better than to install a system in clay backfill or at a
site where the monitor does not respond to the UST product. Release
detection, however, is a growing industry, and new companies are being
fonned with less experience. This chapter presents the full range of
potential problems for educational purposes, not to imply that they will
always occur.
BRIEF DESCRIPTION
The two major components of a vapor monitoring system are the vapor
monitoring well and the vapor monitoring device (sensor). Vapor
95
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monitoring wells are small-diameter wells (typically 2 to 4 inches)
placed in the backfill material near the UST which are used for
collection of vapor samples. A monitoring device can be permanently
or temporarily placed in the vapor monitoring well to collect vapor
samples. A few monitoring devices are simply buried in the UST
backfill and do not requke a monitoring well.
Vapor monitoring works according to the principles of volatilization
(i.e., the change of a substance from a liquid to a gaseous state) and
diffusion (spreading of a gas). As a product leaks from an UST, some
of the liquid volatilizes, and the liquid and vapor phases of the product
spread throughout the surrounding soil. Vapor monitoring systems take
advantage of this phenomenon and are designed to detect the volatile
components of a stored substance. If the vapor sample that reaches a
sensor is above some predetermined concentration, the monitor
responds with an alarm.
A typical vapor monitoring system is depicted in Figure 16.
The successful implementation of a vapor monitoring system involves
six different stages: (1) site assessment—an evaluation of the site is
conducted to ensure vapor monitoring is an appropriate release
detection method and to determine the site characteristics; (2) sensor
selection—an appropriate type of vapor monitor is chosen; (3) network
design—the proper placement (lateral and vertical) of vapor well(s) is
planned; (4) construction and installation—vapor well(s) construction
and installation are conducted; (5) operation and maintenance—the
sensors are calibrated and monitoring begins; and (6) data
interpretation—the monitoring results are evaluated and leak status of
the UST is determined. The relationships among these six stages are
shown in Figure 17.
POTENTIAL PROBLEMS AND SOLUTIONS
If vapor monitoring is installed and operated correctly, it can be an
effective leak detection method. The following sections discuss
problems and solutions related to each of the six stages of the
implementation of a vapor monitoring system. The order of discussion
is not intended to prioritize the importance of the problems, rather it is
intended to follow the order in which they would occur according to the
flow chart in Figure 17. The discussion of problems and solutions is
summarized in Table 7, which starts on page 99; the more serious
concerns are marked by an asterisk. A number of agency solutions are
96
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Vapor
Monitprin
W
Vapor
Monitoring
Device
Backfill
Native Soil
Figure 16. Underground storage tank system
with vapor monitoring wells.
97
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Site Assessment
Product volatility
Backfill permeability
Background contamination
Other possible interferences
Sensor Selection Considerations
Installation
Device threshold and detection
range
Device specificity
Network Design
• Design of UST system
characterization
• Design of well placement
and depth
Construction & Installation
• Placement of well casing,
filterpack, bentonite seal,
surface seal, protective casing
• Well security
i ' • Documentation
Operation & Maintenance
Operation
I
Calibration of equipment
Setting alarm level
Maintenance
Interpretation
Ana ysis
Leak
No Leak
Differentiating between
interferences and leaks
Locating a leak
Figure 17. General procedure for vapor monitoring
98
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Table 7. Indicators and Solutions for Problems Encountered During Vapor Monitoring
Problem indicators Tester Solutions Agency Oversight Options,
Assure volatility of the
stored substance.
Assure LIST backfill is
permeable.
*Need to assess the level
of residual vapors at the site.
None.
Backfill is not sand or pea gravel.
False alarms.
Assure low soil temperature
won't interfere.
Assure backfill that is saturated
with water won't interfere.
Low temperatures (below 0°C) for
extended periods.
Standing water in vapor wells.
Assess possible interference
from methane contamination.
False alarm.
Choose proper monitoring
system; consult manufacturer to
verify that sensor responds to
product. Add tracer compound.
Test backfill permeability by
conducting tracer test. Increase
monitoring well diameter; use
more wells or aspirated sensors.
Add spill and overfill protection
to the tank. Determine back-
ground concentrations. Set
monitoring device threshold
above background. Add tracer
compound. Aerate the soil to
reduce concentration
Install monitoring wells below
frost line. Increase number of
monitoring wells.
For wet climates, use portable
monitor in dry areas.
Do not use for saturated sites;
consider ground-water
monitoring.
Check site for methane. Choose
monitoring system not sensitive
to methane.
Check product vapor pressure
in chemical handbook; review
manufacturer's literature.
Verify response to tracer.
Review results of backfill
permeability test Review
monitoring system plans.
Review background
determination. Check
that threshold is above
background level.
Review monitoring system plans;
compare to expected soil
temperatures.
Review local water table data
to ensure monitor is above high
water table.
Require testing of water content
of soils. Require monitor that
detects the presence of water.
Require testing for methane in
areas known to have problems.
Continued
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Problem
Indicators
Tester Solutions
Agency Oversight Options
o
o
Assess possible interference
from nearby UST sites.
Assure device responds to
the stored product.
Assure device will respond to
level of contamination.
Identify UST configuration.
Assure monitoring wells are
placed properly.
False alarm at monitored site.
Assure well screen is designed
properly.
Assure filter pack is properly
designed.
Assure well is sealed property.
Property secure and mark
monitoring well.
Device does not react to product.
Device shows erratic behavior
or no response.
None.
Delayed detection or no detection.
Assure proper well construction. Well collapses.
Well holds vacuum when purged
with hand pump.
Well holas vacuum when purged
with hand pump.
False alarms. Standing water or
product in vapor wells.
False alarms. Standing product
in vapor wells.
Install background monitoring
well near site boundaries.
Use tracer compound.
Use devices as recommended by
manufacturers.
Use devices as recommended by
manufacturers.
Examine construction records.
Use metal detector.
Install at least one sensor per
tank in highly permeable backfill.
Install at least two sensors per
tank in less permeable backfill.
Install sensors within 2 feet of
bottom of tank.
Construct well according to State
codes.
Use standard size No. 20 slots,
and maximize length of well
screen.
Use appropriately sized filter
pack material.
Seal well properly with cement/
bentonite.
Mark and lock well.
Check for nearby USTs that are
not monitored. Test vapor
concentrations near that site.
Test device during site
inspection.
Check device during site
inspection.
Request site plan with
monitoring plan.
Review monitoring system
plans—compare well
placement with local
regulations or manufacturer's
specifications.
Inspect well and documentation
of well development.
Review well designs (especially
slot size or filter pack).
Test with hand pump.
Review well design for proper
seal; check well box for damage.
Inspect site to see that well
is distinguished from fill pipe
and well cover is locked.
-------
Need to document well
construction.
Assure equipment is properly
calibrated.
Need to set the alarm level
correctly.
Assure proper maintenance
for equipment.
'Interpret monitoring results
considering possible
interferences.
Troubleshooting is time-consuming.
False alarm or lack of detection.
False alarm or no detection.
False alarm or no detection.
False alarm.
Locate wells so that the leak
source can be identified.
Provide well log or other
construction data.
Conduct at least annual
calibrations using standard based
on lightest compound of stored
product.
Set at least 50% higher than
background.
Maintain per recommendations
of manufacturer.
Verify proper operation of
equipment. Take second
reading.
Check for spills, other contamin-
ation. Check trends in monitoring
records.
Check for possible spills. Use
other methods to confirm.
Increase number of monitoring
wells. Use quantitative monitor-
ing device.
Require monitoring plans and
well drawings be kept on-site
or submitted to agency.
Require calibration by approved
contractor.
Inspect calibration records.
Review records of alarm level
settings—compare with initial
background levels.
Review maintenance records.
Inspect monitoring records.
Check to see if high readings
correspond to deliveries or
other possible spills.
If high readings do not appear
to be a spill, require additional
testing with other monitoring
methods.
* Indicates the most significant problems.
-------
offered in the table for each problem, however they are suggestions and
not all of them need be undertaken.
Site Assessment
I Installation I Assure volatility of the stored substance
Operation
Analysis
Volatility is the tendency of a product to change from a liquid to
a gas under standard temperature and pressure. As discussed
previously, vapor monitoring works according to the principles
of volatility and diffusion. A stored substance must be
sufficiently volatile or vapor monitoring will not work. Vapor
monitors are not appropriate for UST systems that contain
non-volatile products. The volatility of a substance is measured
by its vapor pressure. Table 8 lists the vapor pressures of some
common petroleum products.
Product
Table 8
Typical Vapor Pressures of Petroleum Products
Vapor
Molecular
Weight True Vapor Pressure in psia at:
@60°F 40°F 60°F 90°F
Gasoline
Gasoline
Gasoline
Jet naphtha
Jet kerosene
Distillate fuel No. 2
Residual oil No. 6
62
66
68
80
130
130
190
4.7
3.4
2.3
0.8
0.0041
0.0031
0.00002
6.9
5.2
3.5
1.3
0.0085
0.0074
0.00004
11.7
8.8
6.2
2.4
0.021
0.016
0.00013
The volatility of different products varies widely. For example,
gasoline, which contains lighter hydrocarbons, is more volatile
than diesel fuel, which is composed of heavier hydrocarbons.
Typically, vapor monitoring is an appropriate monitoring method
for most petroleum products.
102
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Operation
Analysis
Device manufacturers generally will provide a list of substances
to which their vapor monitoring device will respond. To verify
the manufacturer's claim, the monitoring device can be exposed
to a known concentration of the stored substance and checked to
see that the device gives an appropriate response. Several
California counties require inspectors to check a device's
response to the stored product before it is installed.
For less volatile products, a tracer compound may be combined
with the stored product to satisfy the volatility requirement. A
tracer compound is added solely for the purpose of providing a
volatile component to the product. When a stored product
containing a tracer is released, the tracer compound volatilizes
more easily than the pure stored substance, making release
detection by a vapor monitor easier. Freon or non-chlorinated
compounds (e.g., Stoddard solvent) are often used for this
purpose. If a tracer is used, it should be established that the
chosen monitoring device is sensitive to the compound being used
as a tracer, that the tracer and the stored substance can be mixed,
and that the tracer will not interfere with the normal use of the
stored substance. Under some circumstances when using a tracer,
a leak rate of as low as 0.0005 gal/h can be detected.
Assure UST backfill is permeable
Vapor monitors work best in permeable materials. If the backfill
surrounding a tank is not sufficiently permeable, vapors may have
difficulty moving throughout the monitored area. Loosely
packed, large-grained soils are more permeable than tightly
packed, fine-grained soils. For example, gravel is more
permeable than sand, which is more permeable than silt, which is
more permeable than clay. Sand, gravel, or other engineered
backfills typically are recommended when vapor monitoring is to
be used. However, many manufacturers of vapor monitoring
devices have successfully operated their systems in clay
materials. Figure 18 illustrates, for different types of soils, the
speed with which gasoline vapors would reach a sensor.
103
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6000
Dry gravel backfill
Dry sllty sand
Native soil
Dry gravel backfill
Dry silly sand
Native soil
Moist sand backfill
Wet silty sand
Natve soil
Moist sand backfill
Moist sand
Native soil
Wet sand backfill
Wet sand
Native soil
Wet sand backfill
Wet clay
Native soil
10
Number of Days
30
Figure 18. The effect of soil conditions on vapor
concentrations at a well 8 feet deep located 6.4 feet from
the source of the leak. Source: U.S. EPA (February 25,
1988).
Backfill at new UST installations should be sand or pea gravel,
either of which ensures sufficient permeability. However, at
existing sites the soil used to backfill an UST may not meet this
criterion (e.g., the backfill may be either native soils, clay, or silt).
Some jurisdictions in California check the permeability of the
backfill at existing sites by using a tracer test. This is performed
after wells are installed by injecting a tracer in one well and
monitoring for its occurrence in another.
104
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[ Operation
I Analysis
If a site has soil of moderate or low permeability (e.g., silt or
clay), vapor monitoring can still be used if some modifications
are made to the monitoring wells or the monitoring well network,
or if the appropriate type of monitoring device is selected.
Depending on the soil, one or a combination of these solutions
may be necessary. The monitoring wells can be modified by
increasing the well diameter to approximately six inches. The
monitoring well network can be designed to include more wells
than typically would be required, which provides a greater
number of vapor sampling points. Typical monitoring well
networks are discussed later in this chapter under the discussion
titled "Assure monitoring wells are properly placed for effective
vapor detection."
A modification to enhance vapor movement in moderate to low
permeability soils is to choose an aspirated vapor monitoring
device. Vapor monitoring devices are available in both aspirated
and passive forms. Aspirated devices use suction to create a low
pressure area around the sensor, thus drawing the vapors through
the surrounding media to the probe. A passive device waits for
vapor to migrate to the sensor naturally.
Need to assess the level of residual background vapors at the site
Vapor monitors may respond to vapors remaining from previous
spills or leaks, falsely indicating a current leak. Use of a vapor
monitoring system is not recommended, without further
investigation, at sites with high background concentrations (e.g.,
above 1,500 ppm for gasoline). A new site with clean backfill
typically has levels of contamination that fluctuate between
0 and 500 ppm. The level of background contamination that
renders a monitor unusable varies for different vapor monitoring
devices. The manufacturer of a chosen sensor should be
contacted to determine the level of background contamination that
precludes the use of its sensor.
To determine background concentrations, a temporary vapor well
can be installed within the UST excavation area, and the chosen
monitoring device can be used to get an initial reading. If the
monitor indicates that background concentrations are high (e.g.,
above 1,500 ppm for gasoline), further investigation should be
undertaken to determine whether the concentrations are due to a
current leak (levels above 4,000 ppm for gasoline may indicate
this), a spill, or off-site interference.
105
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I Operation [
| Analysis |
When background contamination is due to a past release or
off-site interference, vapor monitoring is still appropriate if the
contamination levels are below the alarm threshold limit of the
chosen monitor (i.e., the level at which a monitor is set to react to
the presence of a substance as if there were a release, typically
about 500-2500 ppm). Some instruments have adjustable
threshold limits. If the background contamination levels exceed
the instrument's threshold limit, the site can be injected with air to
lower the level of contamination. This can be done by using an
air pump to inject low levels of air through temporary wells into
the soil. A vapor monitoring method should not be used when
background contamination levels cannot be reduced below the
instrument's threshold limit, unless a tracer compound is
introduced. The use of a tracer avoids the problem of background
contamination because the vapor monitor will react to the tracer
compound, not to the compounds that are contained in the
background contamination.
To prevent future background contamination, overfill protection
and spill containment should be installed when vapor monitoring
is used.
Assure environmental conditions will not interfere
Temperature can be an inhibiting factor for proper vapor monitor
operation at UST site; the colder the temperature, the less volatile
a substance will be. Figure 19 illustrates the difference in
volatilization rates for gasoline at different temperatures.
Generally, for approximately every 20-degree Fahrenheit increase
in temperature, the gasoline volatilization rate increases by about
one-third.
106
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.s
in
Number 01 Days
Figure 19. The effect of temperature on gasoline
volatilization rates. Source: U.S. EPA (February 25,
1988)
When monitoring wells extend below the frost line, temperature is
not a problem. Increasing the number of monitoring wells, s that
there are more sampling points to pick up the lower levels of
vapor, can compensate for continuous low temperatures.
If the backfill is saturated with water, because of a perched water
table, fluctuating water table, rainfall, etc., vapor monitoring
devices cannot be used. Saturated backfill conditions will inhibit
vapor movement. Figure 20 on the following page illustrates the
difference in the volatilization rate of gasoline for three different
soil moisture conditions.
107
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e
o
S
g
to
O
0,00012-
0.00010-
0.00008-
0.00006-
> 0.00004-
0,00002-
10
—I—
20
Dry gravel backfill
Moist sand backfill
Wei sand backfill
30
Number of Days
I Operation
3T~
Analysis
108
Figure 20. The effect of backfill moisture levels on
gasoline volatilization rates. Source: U.S. EPA
(February 25, 1988)
Additionally, if a vapor sensor is immersed in water, it will be
rendered ineffective in most cases. Often a portable vapor
monitor, rather than a permanent one, can be used in wet climates
because the sensor is not actually installed in the vapor well.
Assess possible interference from methane contamination
Some chemicals can interfere with the operation of a vapor
monitor, causing it to react to their presence as if there were a
release. Methane, which may migrate from near-by marshes,
landfills, sewage lines or sewage treatment plants, is the most
common chemical causing this reaction. Many vapor monitoring
-------
devices are able to distinguish among different volatile
compounds. If methane is present at a site, the chosen vapor
monitoring device should not be one that reacts to methane. This
information should be available from the manufacturer.
| Installation | Assess possible interference from nearby active or abandoned UST sites
Operation
+
Analysis
If there is an active or abandoned UST site with a history of
releases in proximity to the site being evaluated, it may cause
interference with vapor monitoring. Vapors from a nearby site
may travel to the monitored site causing the sensor to react as
though there were a release from the monitored site when there is
not. One way to differentiate between monitor alarms due to
on-site releases and off-site interferences is to install a
background well outside the excavation zone, on the side nearest
the source of the potential interference. If the vapor levels
increase in the background well while remaining relatively steady
in the on-site wells, the monitor alarm is most likely due to
outside interference.
Another method of differentiating between releases and off-site
interferences is to introduce a tracer to the monitored UST
system. The vapor monitor will then react specifically to the
tracer, thus ensuring that when the monitor alarms, it is reacting
to the monitored UST.
Sensor Selection
Operation
Analysis
Assure vapor monitoring device responds to the stored product
Specificity is the ability of a device to detect vapors from a
particular substance being stored on site (e.g., hydrocarbons or a
tracer). If the monitoring device does not react to the stored
substance, it is totally ineffective. Equally important, of course, is
a device's ability to avoid reacting to substances for which the
site is not being monitored (e.g., methane).
To determine which vapor monitoring device is suitable for
different stored products, literature provided by the device
manufacturer should be consulted. Most vapor sensor
manufacturers list the types of stored products that their device
will effectively monitor.
109
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Operation"]
*
Analysis
Assure selected device responds to the level of contamination
For a device to be effective at a site that has high background
concentrations, it must be able to record and monitor a high level
of vapors. Such a monitor is necessary because the background
level at a contaminated site is significantly higher than zero, and
the monitor must be able to evaluate and record levels higher than
the threshold. If a device does not have a suitable threshold level
or a broad enough detection range (i.e., the lowest and highest
levels of vapor a monitor will detect), it will not accurately reflect
changes in concentrations at a site, thus making the identification
of a release difficult if not impossible.
The appropriate monitoring device to use when there are high
background levels is one that is responsive to a high level of
vapors. In any case, the level of background contamination and
the desired range of detection should be considered before
choosing a monitoring device. Specific information about a
device's threshold and detection ranges should be available
through the manufacturer.
Network Design
[Installation'] Identify system configuration to prevent damage
Jr'"
Operation
*
Analysis
The number and location of USTs and associated piping must be
identified to help determine where the monitoring devices should
be installed to ensure efficient leak detection and to avoid
damaging the UST system during installation of the monitoring
wells. Information about the UST system should be available
from past installation records and construction plans.
Information obtained from available records should be reviewed
with the site owner and operator to determine if any alterations
not indicated in the records were made to the UST system.
Additionally, a metal detector or ground penetration radar can be
used to determine the general location of tanks and piping.
110
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T
Operation I
Analysis
Assure monitoring wells are placed for effective vapor detection
Improper location of vapor sensors will increase the amount of
time before a release is detected and may, in extreme cases, allow
a leak to go undetected. Detection time is a function of the
distance between monitoring wells and a leak. The placement
and depth of monitoring wells are affected by the configuration of
the UST system, the mobility and volatility of the stored product,
the permeability of the backfill and surrounding soil, and the type
of vapor monitoring device.
The federal UST regulation requires that vapor sensors be placed
in wells that are installed in the UST excavation area (backfill).
In addition, vapor wells are normally placed as close to the tank
as is technically feasible; sometimes this is accomplished by
installing the wells at a slant instead of vertically.
Although travel times vary, a rule of thumb is that hydrocarbon
vapors will migrate 15 feet in about 15 to 20 days in unsaturated
sand or gravel backfill. This seems to be a reasonable assumption
based upon sandbox experiments and computer models (U.S.
EPA, February 1989). State and local agencies have adopted a
variety of network design requirements for vapor monitoring (see
Table 9).
Although the requirements are diverse, they tend to require wells
to be separated by no more than 20 to 35 feet. These
requirements seem reasonable based on EPA research and field
experience indicating that a design that includes at least one well
every 40 feet should be sufficient for gasoline tanks in a clean dry
backfill. In general, this translates to one well for a single tank.
If the backfill is not highly permeable (e.g., it is native fill
material) or the migration of vapors is impeded by other factors, it
is recommended that the number of sensors be increased by a
factor of two.
The ideal depth of a vapor well, as indicated by industry
recommendations, would be a depth at least equal to that of the
base of the tank, and preferably one to two feet deeper. Even if
vapor monitoring wells are installed at this ideal depth, few wells
will be deeper than 10-15 feet. Research indicates that diffusion
(spreading of vapor in many directions) is the dominant type of
111
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Table 9
Typical State Network Design Requirements for Vapor Monitoring
State
Vapor Well Location Requirements
Vapor Well Depth Requirements
California
Santa Clara County
City of Torrance
City of Vemon
Delaware
Maine
Iowa
General: every 35 feet of long dimension (if
passive monitoring device, need more
wells).
At station: 1 per tank
1 piping
1 pump island
For 1 tank: one at each end
More than 1 tank: every 20 feet
Design for 15 feet diameter of influence
1 per tank within 5 feet of tank
According to manufacturer's specifications.
At a minimum:
- 1 within 5 feet of each dispenser
- 1 at each piping joint
- no piping run > 15 feet from well
- 1 at each end of tank
- 1 at longitudinal ends of a single tank
- if cluster of tanks where tanks ^ 10 feet apart,
at least 4 wells, 1 on each side of cluster
- all wells > 1 feet from nearest tanks
- all wells within excavation zone
At least to bottom of tank
N/A
N/A
2 feet below tank bottom or to ground water,
whichever is less
Manufacturer's specifications
2 feet below bottom of tank
Source: Reference 15.
112
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vapor movement over time, but because most leaks occur in the
bottom of the UST, a deeper monitoring well may reduce the leak
detection time. Deeper wells, however, are not mandatory for
effective leak detection. A rule of thumb is that the less
permeable the backfill is, the closer the vapor well should be to
the recommended ideal depth. Figure 21 gives a comparison of
the time it takes for gasoline vapor to reach wells at various
depths and distances from a release.
6000-
7000-
SENSOR PROXIMITY
DEPTH TO LEAK
Shallow Near
(2 ft) (2.2 ft)
Deep
Intermediate
(6.4 tt)
Shallow Intermediate
«) (6.4)
Deep Distant
(Btt) (13.1ft)
Shallow Distant
(2 ft) (13.1ft)
Number of Days
Figure 21. The effect of vapor sensor placement on leak
detection time. Source: U.S. EPA (February 25, 1988)
Whether an aspirated or passive sensor is chosen also affects well
depth and placement. For most soils, an aspirated system will
detect vapors more quickly than a passive system at a given
depth, since the aspirated system draws the vapor to the sensor.
However, if the soil is very permeable (e.g., gravel), an aspirated
sensor system and a passive sensor system will perform similarly
113
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at the same depth. Some counties in California require slant wells
that extend beneath the tank if a passive vapor monitoring device
is to be used.
Figure 22 shows a typical configuration for well placement at a
gas station. This configuration includes wells within the tank
backfill area, and wells located so that sensors will detect releases
from piping at the end of pump islands in the pipeline backfill
area. A background well, although not necessary for effective
leak detection, can be installed at a location upgradient from the
pipelines and the tanks to evaluate surrounding soil vapor levels.
Construction and Installation
| Installation^ Assure proper monitoring well construction
Operation
Analysis
Improper construction of monitoring wells can render vapor
monitoring ineffective (e.g., surface runoff could enter the well,
the well casing could collapse, etc.). Construction of vapor
monitoring wells and the installation of vapor monitoring devices
should be done by a qualified contractor who is aware of any
specific state requirements or any industry codes that may affect
construction and installation. Figure 23 shows a cross-section of
a typical monitoring well.
There has been discussion about monitoring well construction for
vapor monitoring wells, especially when ground water in the area
is deep and the wells are installed in the UST backfill. Santa
Clara County in California has thousands of vapor monitoring
wells that are modified in construction. In many cases a modified
well structure, as shown in Figure 24, may be perfectly
acceptable.
A typical vapor well is less than 6 inches in diameter. The casing
may be polyvinyl chloride (PVC), cast iron, galvanized steel,
polyethylene, polypropylene, fluorocarbon resins, Teflon®, or
stainless steel. In choosing the type of casing, the local site
conditions should be considered. For instance, galvanized steel
casings deteriorate in corrosive environments. The most
commonly selected materials for a monitoring well are PVC or
stainless steel. Both of these materials meet the structural needs
of the vast majority of vapor monitoring wells.
114
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Pump Island
Vapor Well In
Product Line Backfill
Tank Backfill Area
Product Lines
Background Vapor Well
Vapor Well
In Tank Backfill
TANK
TANK
TANK
Official Storage
Figure 22. Sketch of typical underground storage tank site.
115
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Existing Grade
Watertight Manhole
Threaded Cap
Concrete
Annular Seal
Casing
Gravel Pack
Porous Backfill
Well Screen
Well Plug
Figure 23. Typical vapor monitoring well cross section.
116
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Existing
Grade
Water-tight manhole,
security box,
or concrete cap
Native Soil
Tank Backfill
Well Cap
Seal of Less
Permeable ~
Material than
Backfill
Well Casing
Perforated
Casing
Well Plug
Figure 24. Modified vapor monitoring well cross section.
117
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jlnstaljiiiitip'f^l Assure the well screen is designed for proper influx of vapors
Operation
I
Analysis
I Installation I
I Operation I
Analysis
[installation]
' * .
Operation
Analysis
The casing has a slotted or perforated section (called the well
screen) that allows for influx of vapor to the monitoring device. If
the well screen has perforations that are too large it may become
clogged with surrounding soil particles, thus blocking the influx
of vapors. Gathering vapor samples is similarly inhibited if the
perforations are too small or only cover a short length. A typical
well screen would have the standard size #20 slots. The well
screen section usually begins from 2 to 5 feet below the ground
surface and extends to the base of the casing. In general, the well
screen extends over as much length as is possible.
Assure filter pack is designed to prevent clogging of the well screen
The well screen area should be surrounded by a filter pack that
allows for passage of vapors while preventing passage of
fine-grained soil particles that could clog the well screen. If the
filter pack material is of too small a size, it may block the passage
of vapors and clog the well screen; if it is too large, soil particles
may migrate through the filter pack and clog the screen.
Typically, several inches of filter pack are placed in the bottom of
the borehole before the well casing is installed. The filter pack
should extend 1-2 feet above the well screen. Materials other
than carefully graded gravels that are acceptable for filter pack
include clean quartz sand, silica, and glass beads.
See that well is sealed to eliminate introduction of contaminants
The area outside the casing, above the well screen, should be
sealed (annular seal) to prevent contaminants such as infiltrating
surface water or other liquids from entering the well that may
interfere with monitoring or reach the ground water. A
cement-bentonite mixture, bentonite chips, or antishrink cement
mixtures are normally used for this purpose. The annular seal
usually extends for 1 or 2 feet above the filter pack. A concrete
seal is placed above the annular seal up to the ground surface to
provide additional protection to the well casing from
contamination and physical damage. Ideally, the interface of the
bentonite seal and the cement seal should be located below the
frost line to protect the well from damage due to frost heaving. A
118
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Operation
Analysis
JL
Operation
JL
Analysis
protective steel casing of a diameter larger than the monitoring
well can be placed in the cement seal to provide additional
protection to the well.
Secure and mark vapor monitoring well properly
If a well is not secured and marked properly, it may be damaged
accidentally causing interference with its integrity. Monitoring
wells may be mistaken for other pipes, such as a tank fill pipe, in
which case the well may be inadvertently filled with product. To
prevent tampering, the well must be secured with a threaded cap,
covered and locked. The well should be visibly marked to
prevent accidental damage, and service stations should protect
monitoring wells with a traffic box to prevent vehicle damage.
Document well construction properly
To aid in future identification of well problems and to prove that
the monitoring wells comply with state codes, the design and
construction of each monitoring well should be documented on a
well completion diagram. This diagram indicates well design
specifications, including the type and depth of filter pack, annular
seal, concrete seal, well diameter, and well screen design. In
addition, drilling and boring logs should be completed indicating
the depth of the well and the type of backfill in which it was
placed. This documentation should be useful to state or local
agencies to determine whether correct procedures relating to
design and installation were followed.
Operation and Maintenance
| Installation] Calibrate equipment properly to detect vapors from stored product
Analysis
The most important step for successful operation of a vapor
monitor is the initial calibration of the device, which should be
performed by a professional. Calibration consists of exposing the
monitor to a pure gas standard to ensure that the monitor correctly
responds to vapors. If a device is not calibrated correctly, it is
likely to give either false positive or false negative results. False
negatives occur when the device is calibrated to indicate low
concentrations of vapor when a high concentration is present.
False positives occur in the opposite situation.
119
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Monitoring devices should be calibrated at least annually,
specifically to the substance stored in the monitored UST, and
preferably to the lightest component of that substance. The
lightest component is that which volatilizes the easiest. Santa
Clara County in California requires that devices used at sites
containing petroleum products be calibrated annually using
certified 1000 ppm isobutane (the lightest component of
gasoline). Some areas require that tank owners use an approved
contractor or agency to perform an annual calibration.
Installation Need to set the alarm level to avoid false alarms
Operation:
"
Analysis
An alarm level is the level of concentration of vapors in the soil
that triggers an audible or visual alarm. The alarm level is
determined by the professional after die device is installed. If an
alarm level is set too low, the alarm will be triggered by small
spills, interferences, or normal fluctuations and become a
nuisance; if set too high, it will not be triggered by a real release.
Some monitors do not have alarms, in which case the operator
should be aware of the vapor reading level that indicates a
possible leak.
To select an appropriate alarm level, a background reading is
needed to determine the site's current condition. The alarm level
should be set to a value at least 50 percent higher than the
background concentration (EMSL-Las Vegas). In general, for
gasoline, vapor levels of 3,000-4,000 ppm with an increasing
trend will be indicative of a leak. However, this level will vary
from site to site and for different brands of monitoring devices.
Assure adequate maintenance for vapor monitoring equipment
Typically, maintenance of vapor monitoring systems includes
cleaning, calibration, and operational checks. For manual
systems, maintenance may consist of recharging the electrical
component by plugging it into an electrical source or changing the
batteries, and keeping the device clean. In addition, some
systems may require periodic replacement of a filament or a lamp.
Automatic systems are often self-checking. Self-checking
systems verify the integrity of the sensor outputs, inputs, power
supply, alarms, and displays, thus ensuring every aspect of the
system is operational at all times. If an automatic system is not
self-checking, periodic calibration and checks of the electrical
120
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system should be performed. The power source used for
automatic systems should be periodically checked to ensure that
the supply has not been cut off. The final maintenance item is
simply the replacement of paper for the device's recorder.
For any type of vapor monitoring system, major maintenance
items should be performed by a qualified professional following
the manufacturer's instructions.
Interpretation
I installation [ Consider normal fluctuations and any other circumstances
Operation
x
Interpretation of monitoring results is a critical step in vapor
monitoring. Interpretation is hindered by normal vapor level
fluctuations of 100-400 ppm over the course of a week. An alarm
or a high reading does not necessarily indicate a leak; it may be
the result of a spill, or simply be the result of background or
other interference.
When an alarm level is recorded, the operator should first verify
the vapor monitoring system's integrity. This entails determining
that the system is working properly and that it is calibrated
correctly. If after this initial check the monitoring device is found
to be operating properly, an additional reading should be taken to
verify the results. Should further monitoring confirm the
preliminary results, any potential interfering factors (excessive
rains, a spill from a product delivery, a leak from a nearby tank,
etc.) should be evaluated next.
If no interfering factors are found and monitoring results have
been confirmed, the monitoring alarm can be attributed to a tank
release of some sort. Several owners have reported that an alarm
always followed product deliveries before spill and overfill
equipment was added. When there is not an obvious spill, one
way of differentiating between leaks and spills is by looking at
past monitoring records. Records typically apply only to the
numerical information given by quantitative monitoring devices;
differentiating between a spill and leak is more difficult when
using a qualitative device which simply indicates the presence of
vapor. Figure 25 show graphically the difference between a leak
and a spill.
121
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.0
I
CD
O
o
O
*_
o
O.
CO
Time
A Leak
c
o
CD
O
C
o
O
l_
8.
Time
A Spill
Figure 25. Interpretation of vapor monitoring results.
A leak is normally indicated by a gradual increase in vapor
concentration that eventually reaches a high level plateau. Spills,
on the other hand, are depicted by a sharp increase in vapor
concentration, followed by a gradual decrease. If a manual vapor
monitor is being used, readings taken on a daily basis following
the recorded high vapor level (also the high point on the graph),
should follow one of the trends indicated in the figure.
122
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[ Installation] Place monitoring wells so that leak location is identifiable
±
[ Operation
IT"
I
Generally, vapor monitoring is non-specific; a vapor monitor
cannot locate the source of the vapor to which it is reacting.
Because of this, confirmation of a release location (i.e., the
release is attributed to the tank, piping, or interference) should
always be done before corrective action begins to avoid
unnecessary work.
Specificity of vapor monitoring can be greatly improved by
increasing the number of monitoring wells and using a
quantitative monitoring device. With these improvements, it is
reasonable to assume that the well at which the highest vapor
levels are recorded is also nearest the source of the release. To
increase specificity, monitoring wells should be adjacent to
individual tanks, not between two or more tanks.
APPROACHES TO ENSURING EFFECTIVE VAPOR MONITORING
Chapter 1 provides a general description of the four types of oversight
that can be used. The following sections discuss how these approaches
may be applied specifically to vapor monitoring systems.
Site Inspections
The site could be visited prior to installation of vapor monitoring to
ensure that the method is appropriate for the specific site. Particular
attention should be paid to the background concentrations found in the
UST backfill, the type of UST backfill, and the volatility of the product
stored in the UST. Site inspections could also take place during
installation to confirm the proper location and installation of monitoring
wells. Finally, periodic visits to ensure that vapor monitoring
instruments are properly calibrated would be beneficial.
Data Review
Before vapor monitoring is implemented, the implementing agency
could require that a pre-installation site assessment report and that the
proposed monitoring well network design be submitted for review.
Whenever a vapor monitor indicates a suspected release monitoring
records and their determined interpretation could be submitted for
123
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review. Because this may result in numerous submissions, records
should only be required for releases that require reporting, not simply
for any vapor monitoring alarm.
Guidance and Training
Guidance developed for owners/operators should emphasize proper
calibration of equipment and the correct manner of interpreting results.
Network design guidance would be useful for both owners/operators
and installers.
Approval and Certification
One approach an implementing agency can use is to require certification
of all vapor monitoring system installers, either through agency
programs or those performed by manufacturers. Some vapor monitoring
equipment manufacturers already train installers, and the implementing
agency could review these programs. Another option for implementing
agencies is to allow installation of only the types of vapor monitoring
devices that meet specified requirements. This would involve setting up
an approval program to which vendors could apply.
REFERENCES
1. Eklund, B. and W. Crow. March 1987. Survey of Vendors of
External Petroleum Leak Monitoring Devices for Use With
Underground Storage Tanks. Report for J. Jeffrey van Ee,
Environmental Monitoring Systems Laboratory, U.S. EPA.
2. Geonomics, Inc. [not dated]. Soil Vapor Monitoring for Fuel Leak
Detection.
3. Hanselka, Reinhard and Paul M. Allen. September 1985.
"Aspirated Vapor Sensing for Leak Detection." In: Sensors
Magazine.
4. Kaman Tempo, March 1988. Evaluation of U-Tube Underground
Tank Systems for Soil Vapor Testing.
5. Kaman Tempo. March 1988. Fuel Vapor Background
Concentration Measurement and Tracer Testing in Underground
Storage TankBackfill,
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6. Levine Fricke, Inc. February 11,1985. Capabilities of Soil Sentry
Underground Tank Leak Detection System Under Field Test
Conditions. Report for Genelco, Inc.
7. Weston, R. F. August 1986. Underground Storage Tank Leakage
Prevention, Detection and Correction.
8. Weston, R. F. May 14,1987. Summary of Available Monitoring
Technologies for Underground Storage Tanks.
9. U.S. EPA. [not dated]. Development of Procedures to Assess the
Performance of External Leak Detection Devices. Report by
Radian Corporation for Environmental Monitoring Systems
Laboratory, U.S. EPA.
10. U.S. EPA. [not dated]. Proposed - Guidance Document for
External Monitoring of Underground Storage Tanks. Report by
Dennis Weber and Klaus Stetzenbach for Environmental
Monitoring Systems Laboratory, U.S. EPA.
11. U.S. EPA. [not dated]. Soil Gas Sensing for Detection and
Mapping of Volatile Organics. Report by Radian Corporation, and
Environmental Monitoring Systems Laboratory for Environmental
Research Center, U.S. EPA.
12. U.S. EPA. March 1987. Soil-Gas Measurement for Detection of
Subsurface Organic Contamination. Prepared for Environmental
Monitoring Systems Laboratory, U.S. EPA.
13. U.S. EPA. June 1987. Processes Affecting Subsurface Transport
of Leaking Underground Tank Fluids. Report for Environmental
Monitoring Systems Laboratory, U.S. EPA.
14. U.S. EPA. October 20,1987. Underground Fuel Storage in
Barnstable County, Massachusetts. Report for Region I Office,
U.S. EPA.
15. U.S. EPA. February 1988. Interim Report: Summary of Several
State Underground Storage Tank Regulations. Report for
Environmental Research Center, U.S. EPA.
16. U.S. EPA. February 25,1988. Research for Abatement of Leaks
From Underground Storage Tanks Containing Hazardous
125
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Substances. Report by Camp Dresser & McKee, Inc., for Phil
Durgin, Environmental Monitoring Systems Laboratory, U.S. EPA.
17. U.S. EPA. February 29,1988. Background Hydrocarbon Vapor
Concentration Study for Underground Fuel Storage Tanks. Report
for Phil Durgin, Environmental Monitoring Systems Laboratory,
U.S. EPA.
18. U.S. EPA. March 24,1988. Standard Practice for Evaluating
Performance of Under ground Star age Tank External Leak!Release
Detection Components and Systems. Report by Radian
Corporation for Environmental Monitoring Systems Laboratory,
U.S. EPA.
19. U.S. EPA. August 1988. Leak Lookout - Using External Leak
Detectors to Prevent Petroleum Contamination from Underground
Storage Tanks. Prepared by John D. Kotler for the Office of
Underground Storage Tanks, U.S. EPA.
20. U.S. EPA. February 1989. Soil Vapor Monitoring for Fuel Tank
Leak Detection - Data Compiled for Thirteen Case Studies.
Prepared by On-Site Technologies for Environmental Monitoring
Systems Laboratory, U.S. EPA.
126
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Chapter VII
Ground-water Monitoring
-------
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GROUND-WATER MONITORING
VII
SUMMARY
The application of ground-water monitoring as an UST release detection
method involves the use of one or more permanent observation wells
that are placed close to the tank and are checked periodically for the
presence of free product on the water table surface. When properly
designed and installed, ground-water monitoring systems can result in
effective detection of releases from UST systems.
Factors that have the greatest impact on the proper operation of a
ground-water monitoring system are those associated with
environmental conditions of the site (e.g., depth to ground water, range
of ground-water table fluctuation), characteristics of the UST system
(number and size of tanks, type of stored product), and the presence of
other subsurface structures. These site-specific characteristics will
determine the design and complexity of the monitoring well system.
In general, ground-water monitoring is most effective at sites where the
water table is within or very near to the excavation zone of the tank. The
method is also more effective at UST sites where no residual product is
present in the subsurface materials due to prior releases.
The discussion presented in this chapter covers a range of problems that
may occur with ground-water monitoring. This does not mean that all,
or even most, of these problems will occur at the same time.
Furthermore, the problems addressed below are not necessarily of equal
importance, in terms of the frequency of their occurrence or in the
severity of their impact on the effectiveness of ground-water
monitoring.
Professionals experienced in ground-water monitoring will know how
to identify and correct these problems. For example, a qualified firm
would not install a ground-water monitoring system at a site where the
water table is 50 feet below the ground surface. Release detection,
however, is a growing industry and new companies with little
experience in ground-water monitoring are opening up.
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BRIEF DESCRIPTION
A ground-water monitoring system consists of two main components:
the monitoring well and the free product detection device. Monitoring
wells are constructed of small-diameter (2-4 inches) well casing
extending from the ground surface to several feet below the lowest
water table level. The portion of the well casing that is perforated (or
slotted) is referred to as the well screen and extends from the bottom of
the well to several feet above the water table surface, allowing liquids to
enter the well. Figure 26 shows the components of a typical
ground-water monitoring system.
When a leak occurs from an UST, the released product will migrate
downward through the backfill material and underlying soil. When
liquid-phase (free) products less dense than water encounter the
ground-water table, they will float and spread out horizontally on the
surface of the water table. Monitoring wells properly installed next to a
leaking tank will intercept the free product layer that accumulates on the
water table surface.
The presence of free product can be measured by sensing devices which
can be either permanently installed in the monitoring well or manually
inserted in the well to take a discrete measurement. Devices which are
permanently installed can be operated automatically on a continuous
basis.
Although EPA is requiring only that free product be detected when
ground-water monitoring is used as a release detection method, free
product monitoring wells also can be used for sampling and analysis of
dissolved product. Several states have been conducting analysis of
dissolved product in ground water as a requirement for release detection
monitoring.
The process of designing, constructing, and installing a ground-water
monitoring system can be separated into six phases:
1. Site assessment
2. Selection of a monitoring device
3. Design of the monitoring well network
4. Construction and installation of monitoring wells
5. Operation and maintenance of the monitoring system
6. Interpretation of the monitoring results
128
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MONITORING
WELL
PAVEMENT
BACKFILL
WATER TABLE
SURFACE
t
STORAGE
TANK
FREE PRODUCT LAYER
PRODUCT/WATER CONTACT
WELL SCREEN
PERIMETER OF
TANK
EXCAVATION
Figure 26. Monitoring wells installed in the excavation zone
will quickly detect a release when the ground-water table is
within the tank excavation. Source: U.S. EPA 1987
129
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Figure 27 shows the relationship of each of these phases and identifies
the important components of each phase.
POTENTIAL PROBLEMS AND SOLUTIONS
Successful operation of a ground-water monitoring system depends
upon a number of factors. In general, operational problems will be due
either to the monitoring well or to the product monitoring device.
Operation of a monitoring well can be affected by a variety of site
factors. Once identified, such site problems can sometimes be
overcome by careful system design. Problems related to the monitoring
device typically are easier to overcome than problems related to the
monitoring well; another device better suited to the site-specific
conditions usually can be selected.
Table 10 is a summary of the problems that may be encountered during
the installation and operation of a ground-water monitoring system and
ways to identify these problems. More than one solution may be offered
to agency personnel for a particular problem and not all of the solutions
need to be undertaken. The problems identified in Table 10 are
presented in the order that the method process is implemented; the order
does not indicate any prioritization or measure of importance of the
problems. The most serious concerns are identified in the table by an
asterisk.
Site Assessment
The characteristics of the site environment and of the stored product that
may affect proper operation of a ground-water monitoring system are
discussed below.
Assure that depth to ground water is less than 20 feet
Ground-water monitoring is a very effective release detection
method for UST sites where the surface of the ground-water
table is within the tank excavation zone (see Figure 26). This
occurs at approximately 25 percent of UST sites based on results
of EPA's national survey data (U.S. EPA 1986a). Product
released from a tank will be quickly intercepted by monitoring
wells placed in the backfill of the excavation.
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Site Assessment
LIST system layout
Hydrogeologica! characterization
Other onsite/offsite factors
Exploratory borings
Sensor Selection
Installation
I
• Continuous or intermittent
• Permanent or manual
Network Design
• Number and location of wells
• Design specifications for wells
\ ' • Drilling method
Construction & Installation
Borehole drilling
Installation of well, filterpack
and seals
Development of well
Securing the well
Documentation of well installation
Operation
Operation & Maintenance
I
Calibration of equipment
Maintenance of well and sensor
Interpretation
Analysis
1
Leak
No Leak
Differentiating between
interferences and leaks
Locating a leak
Figure 27. General procedure for ground-water monitoring
131
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to
Table 10. Indicators and Solutions for Problems Encountered With Ground-Water Monitoring
Problem
Indicators
Tester Solutions
Agency Oversight Options
Assuring that ground water is
less than 20 feet or greater
than 3 feet deep.
Assuring that hydraulic
conductivity is greater than
0.01 cm/sec.
Assuring that water table
fluctuation does not exceed
well screen.
Responding to steep ground
water gradient.
Assuring that stored product
is soluble in water.
Assuring that specific gravity
of stored product is less than
water.
Determining presence of
background contamination.
Assuring proper selection of a
sensor.
None.
None.
Well is dry. Water table is at top
of well screen.
Ground surface slopes steeply.
Product mixes with water and is
not observable as a separate
liquid phase.
Product is observable as a
separate liquid phase and will
not float on water surface.
Stains are observed on ground
surface. Contamination is found
during performance of site
assessment
Device is not sensitive to the
stored product. Environment
conditions adversely affect
sensor operation.
Take water level measurements.
Perform exploratory borings.
Use another release detection
methods.
Inspect boring log. Perform slug
test. Replace material with
appropriate backfill. Select
another release detection method.
Replace well with one that is
properly screened. Select
another release detection method.
Place wells downgradient of
tanks.
Obtain data on product solubility
from chemical handbooks or from
manufacturer. Select another
release detection method.
Obtain data on product solubility
from chemical handbooks or from
manufacturer. Select another
release detection method.
Cleanup residual product
Select another release detection
method.
Select another sensing device.
Select another release detection
method.
Review water level data. Oversee
performance of exploratory
borings.
Review boring logs. Review slug
test data.
Inspect well boring and
construction logs. Evaluate
long-term water level data
taken in same aquifer.
Review water level data from
a minimum of three wells.
Review topographic map of site.
Conduct site visit to observe
topography.
Review solubility data. Review
data from chemical/petroleum
supplier.
Review chemical data on
specific gravity of product.
Review site assessment data
collected by owner/operator.
Conduct a site visit.
Review site assessment data
collected by owner/operator.
Review information on sensor
from manufacturer.
-------
Assuring adequate number of
monitoring wells present.
Assuring proper well
placement.
Determining if conduits are
present near the tank field.
* Assuring adequate
maintenance of sensors.
Site-specific (see Table 7).
Site-specific.
Product or vapors are observed in
utility trench or basement.
Sensor float is hung up on the
well casing. Occurrence of false
alarms.
*Assure diameter of well casing Sensing device will not fit into
Is not too small. the well.
* Assure well screen slot size is Flow rate of water into well is
not too small. restricted. Well is dry.
* Assure well screen slot size is
not too large.
* Assure well was properly
developed.
Filter pack is collecting in the well.
Flow rate of water into well is
restricted. Well is dry.
'Ensuring proper interpretation Recording of false alarms.
of environmental Influences.
Review site assessment data.
Check maintenance schedule for sensors.
Install additional wells.
Install additional wells in
proper location (s).
Identify and mark location of
utility lines. Install wells
between tank field and subsurface
conduits.
Visually check sensor on a frequent
basis.
Select another sensing device.
Install a new well of larger
diameter.
Redevelop well to increase flow
rate of water into well. Install
a new well that is properly
designed.
Bail out well to remove
particulate material. Install a
new well that is properly designed.
Redevelop well to increase flow
rate of water into well. Install
a new well if flow rate cannot be
increased.
Check sensor to make sure that
it is operating property. Conduct
a site assessment to determine if
any background contamination is
present.
Review site assessment data.
Conduct a site visit.
Review site assessment data.
Conduct a site visit.
Review site plans and utility
maps. Conduct a site visit.
Have owner/operator keep a
record of sensor checks. Conduct
a site visit to confirm that sensor
is operating properly.
Review well construction
log. Conduct a site visit to
observe well design.
Review well construction
log. Conduct a site visit to
observe well design.
Review well construction
log. Conduct a site visit to
observe well design.
Review well construction log
to determine the method used
to develop well and the length
of time it was conducted.
U)
* Indicates the most significant problems.
-------
Ground-water monitoring is not an allowable release detection
method when the water table surface is greater than 20 feet
below the ground surface (BGS). Although monitoring wells
installed under this situation do not necessarily present an
operational problem, a leak at a site with a deep water table
could go undetected for months until the product migrates down
to the water table. Restricting use of ground-water monitoring
to sites where the water table is less than 20 feet deep will
minimize the potential for widespread environmental
contamination and ensure relatively rapid detection of a release.
Ground-water monitoring is also not recommended for use at
UST sites where the water table is less than 2 to 3 feet BGS.
Monitoring wells used for free product detection cannot be
properly constructed (with a surface grout seal) when the water
table is very shallow. Free product will be excluded from the
well screen if the surface seal extends below the water table
surface (see Figure 28). Monitoring wells that are not properly
sealed may be susceptible to contamination from surface spills
and runoff, which may result in the false reporting of a tank
release (see Figure 29).
Depth to the water table can be determined at existing
monitoring wells by taking water level measurements. Well
boring and completion logs can be inspected to determine where
the top and bottom of the well screen are located. An
improperly constructed well should not be used for free product
detection or for sampling of dissolved product.
At new UST installations, the depth to the ground-water table
can be determined by taking water level measurements at wells
located close to the site. Another alternative is to conduct
exploratory borings on-site. If water is encountered at a depth
less than 3 feet BGS or is not observed down to a depth of
20 feet BGS, ground-water monitoring would not be an
appropriate release detection method.
Determine hydraulic conductivity of backfill or native soil
Monitoring wells may be placed in either the excavation backfill
or in the soil surrounding the tank excavation. If the backfill
material or the soils situated between the UST system and the
134
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GROUND SURFACE
IXXXXXXXXXXXXXXXXX>
FREE PRODUCT LAYE
WATER TABLE
SURFACE
Figure 28. The well seal will prevent interception of free
product when the water table surface is very shallow.
Source: U.S. EPA 1987
135
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Spilled Petroleum Product or
Contaminated Water
Free Product Layer
Product/Water Contact
Figure 29. Monitoring well that does not have a proper
surface seal placed above the filter pack will be
susceptible to contamination from surface runoff.
Source: U.S. EPA 1987
136
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monitoring well have a low hydraulic conductivity (less than
0.01 cm/sec), movement of product released from an UST will
be restricted. Product released from an UST either will not
reach the monitoring well or will reach the well only after a long
period of time. Even though backfill or native soils with a low
hydraulic conductivity may limit the amount of product that can
be released from a tank (because the migration rate of the
product away from the tank is very slow), a slow leak over a
long time period may result in relatively high concentrations of
dissolved product in the ground water.
Hydraulic conductivity is a measure of the rate of flow of a fluid
in a porous medium and is a property of both the soil and of the
stored product. In general, the hydraulic conductivity of a soil
increases with an increase in soil porosity and grain size, and a
decrease in fluid viscosity. The relative hydraulic conductivities
of the major soil classes are: gravel > sand > silt > clay (see
Figure 30). Materials that are considered to be suitable for
backfill include clean, graded sands and gravels. Silts and clays
are much less permeable than sand and gravel and are generally
not appropriate for use as backfill.
Ground-water monitoring is most effective when the hydraulic
conductivity of the backfill or soils situated between the tank
and the well is greater than 0.01 cm/sec. The hydraulic
conductivity of materials can be estimated from information
provided on boring logs for wells installed onsite or near to the
site. If materials consist of well-sorted sand or coarser, as is
required by national installation codes for new tanks, the
hydraulic conductivity is most likely greater than 0.01 cm/sec. If
there is some uncertainty about the hydraulic conductivity of the
medium, it can be estimated by conducting an in situ test, called
a slug test, in the well.
The hydraulic conductivity of soils and backfill at existing sites
can be improved only by replacement with materials having a
greater permeability. A more cost effective solution may be the
selection of another release detection method.
Need to evaluate the range of ground-water table fluctuation
Floating product will not be detected in a monitoring well if the
surface of the ground-water table falls below the bottom, or
extends above the top, of the well screen. When the ground-
137
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CLAY
GLACIAL TILL
SILT
SAND
GRAVEL
-10 -8 -6 -4 -2 2
10 10 10 10 10 1 10
K (CM/SEC)
Figure 30. Range of hydraulic conductivities (K) for
the major soil classes. Source: Dragun 1987
138
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water level is below the bottom of the well screen, water will not
be encountered in the well (the well is dry). When the water
table surface is higher than the top of the well screen, product
floating on the ground-water table will not be able to enter the
monitoring well. Free product would enter the well once the
water surface falls below the top of the well screen.
The surface of the ground-water table fluctuates in response to
seasonal variations in ground-water recharge and other
influences such as ground water extraction. The location and
design of a monitoring system must consider the range of water
table fluctuation at a site (see Figure 31). Wells should be
screened over the entire interval of ground-water levels. The
range of fluctuation can be determined based on long-term water
table level measurements taken in the same aquifer. The high
and low water table levels can sometimes be determined from
soil characteristics such as color observed during drilling of the
well borehole. To help the regulator identify problems with
water table fluctuations, the owner/operator should obtain and
record water level measurements on a monthly basis.
Need to assess the ground-water flow gradient
Monitoring wells placed upgradient of the tank field may not
intercept a free product plume at UST sites where the
ground-water table surface has a relatively steep slope. Product
released from an UST under these conditions will migrate down
through the soil to the ground-water table. When product
reaches the water table it will generally migrate laterally in the
direction of ground-water flow. Monitoring wells placed either
upgradient of the UST system or perpendicular to the direction
of ground-water flow may not intercept a product release under
these conditions.
A rough estimate of the direction and gradient of ground-water
flow can be made from observation of the ground surface
gradient. For example, when there is a steep decline of the
ground surface, ground water will most likely flow in the
direction of the decline (downgradient). Ground water will also
tend to flow towards surface waters, which are discharge points
for ground water. The ground surface gradient can be
determined from onsite observations and from information
presented on topographic maps, which are available from the
state or federal geological survey.
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FREE PRODUCT LAYER
GROUND SURFACE
RANGE OF WATER TABLE
FLUCTUATION
Figure 31. The well screen is placed to extend over the
entire range of water table fluctuation.
Source: U.S. EPA 1987
140
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The installation of monitoring wells on all four sides of the tank
field is usually adequate to ensure detection of a release at sites
located on a relatively steep gradient or if there is some
uncertainty about the ground-water flow gradient.
Stored product must not be soluble in water
Products that are highly soluble in water will not be detectable
as a separate liquid phase. When a soluble product is released
from a tank and reaches the ground-water table, it will mix with
the water and disperse throughout the aquifer. Therefore,
ground-water monitoring for free product detection cannot be
used when the stored product is highly soluble in water. Most
petroleum products are not highly soluble in water. Though
gasoline and diesel fuel are composed of many individual
chemicals that are soluble to some extent in water, these
products will be observable as a separate liquid layer.
The type of product stored in each tank should be recorded on
the UST notification form. This form does not require reporting
information on the solubility of each stored product, but this
information can be found in standard chemical handbooks. The
manufacturer of the product also should be able to provide this
type of data.
Specific gravity of the stored product must be less than that for water
Products that are denser (heavier) than water (i.e., specific
gravity is greater than 1.0) will not float on the water surface.
Instead, these products will migrate down through the
unsaturated and saturated (aquifer) zones until an impermeable
zone is encountered. The free product will accumulate at the
interface of an impermeable zone, such as a clay layer below the
aquifer.
Products that are denser than water include the halogenated
hydrocarbons (e.g., trichloroethene, dichloroethane) and coal tar.
Ground-water monitoring is not an effective release detection
method under these conditions since no product will enter the
well. Furthermore, the presence of these types of products would
be difficult to detect with most of the currently available free
product sensors. As with product solubility, information on
141
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density can be obtained from either chemical handbooks or from
the manufacturer of the product.
Need to determine the presence of background contamination onsite
Ground-water monitoring is most effective for use as a release
detection method when there is not any background
contamination present due to prior spills or releases. Residual
product present in soils above the water table (unsaturated) can
reach ground water by vertical migration due to gravity forces
and from infiltration of rainfall and surface runoff. Also, if the
water table rises up to an area where residual product is located,
free product may accumulate on the water table surface. The
observation of free product in a monitoring well due to residual
product will result in a false indication of a leak.
The site owner/operator should be interviewed to determine if
any prior releases or large spills have occurred at the site or near
to the site. Confirmation of background contamination can be
made by conducting a preliminary site assessment involving
investigation of the subsurface using field analysis techniques
(e.g., soil vapor surveys). If residual free product is discovered,
it should be removed prior to initiating ground-water
monitoring.
Sensor Selection
Assure selection of appropriate sensor
Selection of an improper monitoring device may result in either
a release not being detected or going undetected for a long time.
For example, a monitoring device that is not sensitive to the
stored product will not detect a release, regardless of the
thickness of the product layer. A particular device may be
adversely affected by some environmental conditions such as
very low temperatures. The problem can be resolved by
selecting a device that is appropriate for the site-specific
conditions. Currently, however, free product sensing devices are
not available for all types of stored products. Furthermore, as
discussed in the previous section of this chapter, "Site
Assessment," ground-water monitoring is not an appropriate
release detection method for all stored products.
142
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A variety of devices are available for measurement of free
product. Devices may be permanently installed in the well for
automatic, continuous measurements of product thickness.
Manual devices range from grab samplers used for collection of
a liquid sample for visual inspection of free product, to devices
that can be inserted into the well to electronically indicate the
presence of free product. None of these devices measure the
leak rate from an UST system, however; they only indicate the
presence of free product.
Any of the automatic sensors can be permanently installed in a
monitoring well to take continuous measurements.
• Differential float devices operate using a system of two floats;
one float reacts only to liquids with a density similar to water,
and the other float responds to liquids lighter than water (most
hydrocarbons).
• Product soluble devices are coated with or constructed from a
material that degrades when exposed to hydrocarbons
resulting in a change of pressure (i.e., an air leak) or a change
in resistance (for electrical resistivity devices).
• Thermal conductivity devices measure heat loss when the
floating probe comes into contact with a non-polar liquid.
Thermal conductivity is the most commonly used continuous
device.
Some of the manual devices that can be used include the
following:
• Grab sampling devices, such as a bailer or bucket, obtain a
ground-water sample from the well which is visually
inspected for the presence of free product (i.e., a sheen on the
surface of the water) or electronically analyzed on-site.
• Chemical sensitive pastes are attached to a weighted tape
measure that is lowered into the well and which react (by a
change in color) when hydrocarbons are present.
• Interface probes detect polar versus nonpolar substances using
properties of thermal conductance and reflection/refraction of
infrared light.
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Network Design
The monitoring well network is designed after an initial assessment of
the tank field layout and local hydrogeological conditions. This phase of
the process involves determining the number and location of monitoring
wells and their approximate depth. The well network design must be
based on site-specific conditions and should be developed by a qualified
professional. For common system configurations, however, a generic
design recommended by the manufacturer is probably sufficient.
Problems which may be encountered due to improper well network
design are addressed below.
Ensure an adequate number of monitoring wells
If the number of monitoring wells at an UST site is inadequate, a
release from a tank may not be intercepted and, thus, may not be
detected. For example, if a monitoring well cannot be placed in
the backfill and is situated in soils which are highly fractured, a
release from the tank may not be intercepted by the well (see
Figure 32).
In general, the use of a single monitoring well may be adequate
for UST sites with only one tank. However, one well may not
always be reliable in detecting free product. If the site consists
of multiple tanks, more than one monitoring well should be
provided. The exact number of wells should be based on the site
hydrogeology and the UST system configuration. A general rule
of thumb recommended by professionals is the placement of at
least one monitoring well on each side or comer of the tank
field.
A number of states and localities currently have specific
requirements for design of a ground-water monitoring well
network. Table 11 summarizes the network design criteria
included in several regulations. Although the regulations each
have a different approach, most require that single tanks have
one to two wells and that typical multiple tank systems have
three to four wells (or more for a larger tank field).
Identification of this problem is difficult and requires knowledge
of the site hydrogeological characteristics. To assist the
regulator in evaluating this type of problem, the owner/ operator
144
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Free Product Layer
Free Product
Layer
Product-Filled
Fractures
V
Water-Filled
Fractures
Water-Filled
Cavities
Note: No Free Product in Well
Fractured Rock
Karstic Limestone
Figure 32. Free product will preferentially flow
through fractures and cavitites; wells that do not
intercept these structures will not detect product.
Source: U.S. EPA 1987
145
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Table 11
Typical Network Design Requirements for Ground-Water Monitoring
Fremont, California
Torrance, California
Iowa
Maine
Nebraska
Delaware
Vernon, California
Florida
Maryland
South Carolina
Wisconsin
Single tank: 1 downgradient well within 10 feet of the excavation
perimeter
Multiple tanks: 1 well placed every 35 feet on the longest dimen-
sion of the excavation with a minimum of two wells
Single tank: 1 downgradient well. If the ground-water gradient is
not known, 2 wells on opposite sides of the tank
Multiple tanks: To be evaluated
Single tank: 1 well at each longitudinal end of the tank
Multiple tanks: 4 wells placed on each side of the tank field
Wells must be placed within 1 to 20 feet of the nearest tank
Ground water < 15 feet: No fewer than 2 wells at either end
of the tank
Ground water > 15 feet: No fewer than 4 wells for each tank or for
multiple tanks located in the same excavation, one well at both
ends of each tank and at each end of the excavation
Ground water > 15 feet: 1 well per tank
Ground water < 15 feet: 2 wells per tank
New installations: 4 wells placed around tank excavation fields
Existing installations: 3 wells placed in the excavation around
the tank(s)
Wells downgradient of tank(s) being monitored
4 wells placed in the excavation around the tank(s)
2 wells placed at opposite corners of the tank field
Minimum of 2 wells placed every 30 feet
3 wells required only for new UST installation
Source: Reference 11.
146
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could be required to provide a site plan of the UST facility
showing the layout of the UST system (number and size of
tanks) and the locations of existing monitoring wells. Other
useful information includes the ground-water flow gradient and
geological characteristics of the site.
Assure proper placement of wells relative to the tank field
Improper placement of monitoring wells may result in delayed
or missed detection of a product release. For example,
monitoring wells placed too far away from the tank field may
not detect a release until a large volume of product has leaked
from the tank. Monitoring wells should be placed as close to the
tank excavation as possible. Care needs to be taken when
selecting a monitoring well site to ensure that the installation of
the well will not interfere with any subsurface structures such as
utilities or UST system piping. In some cases, the installation of
a well upgradient of the UST system might protect owners from
false alarms at sites where there are other (offsite) petroleum
UST systems located near the site being monitored (see
Figure 33).
Need to check for subsurface conduits near the tank field
Buried fill material or subsurface utility conduits (e.g., trenches
constructed for telephone, power, gas, sewer, and water lines)
may act as preferred pathways for free product migration. Free
product generally will flow more easily through these open
subsurface structures than through surrounding soils because the
material is more permeable and offers less resistance to flow.
Therefore, free product may not reach the ground-water table
and will be channeled away from monitoring wells (see
Figure 34).
This problem may be indicated when product or odors are
observed in utility conduits but floating product is not observed
in the monitoring well. At UST sites where there are subsurface
conduits present, monitoring wells should be placed between the
tank field and the conduit to ensure that product released from
the tank is intercepted by the well before it reaches the conduit.
The locations of buried conduits can be found by examining site
construction maps and maps of all utilities present in the area.
The utility companies should be notified, prior to conducting
147
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CO
SERVICE
STATION
NO. 2
MONrrORING
WELL
DISSOLVED AND
FREE PRODUCT
PLUME
DIRECTION OF
GROUND-WATER
FLOW
Lack of upgradient well implicates Station No. 1
TANK
BUILDING
Figure 33. Off-site sources of contamination should be
considered when designing the monitoring well
network. Source: U.S. EPA 1987
-------
DIRECTION OF
GROUND-WATER FLOW
PRODUCT IN
UTILITY TRENCH
MONITORING
WELL
X
TANK
SERVICE
STATION
BUILDING
Figure 34. Subsurface utility conduits will act as a
preferential pathways for free product migration.
Source: U.S. EPA 1987
-------
well installation, that drilling will be performed at the site. To
help troubleshoot any operational problems that may occur over
the life of the monitoring system, the location of all subsurface
conduits should be indicated accurately on a current site plan.
Construction and Installation
This section addresses problems that may occur as a result of poor or
improper design, construction, or installation of a monitoring well. It is
important to keep in mind that the design and construction requirements
for a monitoring well used for free product detection are not the same as
for a drinking water well or for a monitoring well constructed solely for
sampling and analysis of dissolved product. The proper design and
construction of a monitoring well is crucial to effective detection of free
product and these tasks should be performed by an experienced
hydrogeologist.
Assure proper design of the monitoring well
Improper design of a monitoring well may result in a release
either not being detected at all or going undetected for months,
resulting in widespread environmental contamination. Figure 35
shows the construction of a typical monitoring well used for free
product detection. Design factors that are considered to be the
most important to proper operation of a monitoring well are
addressed below.
Before beginning design of a monitoring well, any specific state
or local construction requirements should be identified. Some
states, such as Florida and California, have developed specific
criteria that must be met for length of well screen, screen slot
size, filter pack specifications, etc.
Well casing and screen material: Most materials used for
ground-water monitoring wells are compatible with petroleum
hydrocarbon products. However, some materials, such as steel,
may deteriorate in highly corrosive environments. This could
result in the collapse of a well casing or infiltration of paniculate
material through holes created by corrosion.
A variety of materials are available for construction of the well
casing and screen, including fluorocarbon resins, cast iron,
150
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GROUND SURFACE
WELL CAP
VENT HOLE
BENTONITE OR
CEMENT GROUT
WATER TABLE
WELL SCREEN
BOTTOM
CASING PLUG
\
PROTECTIVE STEEL
CASING
ANNULAR SEAL
FILTER PACK
Figure 35. Components of a typical ground-water
monitoring well installed in a borehole.
Source: U.S. EPA 1987
151
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galvanized steel, polyethylene, polypropylene, Teflon , stainless
steel, and polyvinyl chloride (PVC). The selection of an
appropriate material should be based on environmental
conditions, structural requirements, and compatibility with the
contaminants of interest.
Typically, the material of choice for monitoring well
construction is PVC or stainless steel. Both of these materials
meet the structural needs of a monitoring well. Some types of
PVC will deteriorate when in continual contact with petroleum
hydrocarbons. However, the use of an impervious PVC material
is acceptable and is common for construction of monitoring
wells. The type of material used for construction of the well
screen and casing should be indicated on a well completion log
that is kept onsite for inspection by the regulator.
Well diameter: The inner diameter of the well casing and
screen typically ranges from 2 to 4 inches. Though smaller
diameter casing is available, diameters less than 2 inches are not
recommended. A monitoring well that is constructed of very
small diameter casing could limit the type of hydrocarbon
monitor selected for use. Monitoring devices are also more
likely to get hung up on small diameter casing. Monitoring
wells larger than 4 inches in diameter can be installed; larger
diameter wells could later be used as extraction wells if
remediation of ground water is required in the future due to a
release incident.
The diameter of well casing and screen used should be
documented on a well completion log. Regulators can refer to
this log to determine the design of the monitoring well.
Screen slot size: The size of the well screen slots is very
important to the proper operation of a monitoring well. If the
slot size is too large, soil particles will be allowed to pass into
the monitoring well, which may eventually become filled with
soil. If the slot size is too small, it may prevent the flow of
product into the monitoring well.
The slot size should be wide enough to permit the flow of
ground water and free product into the well, but not so wide as
to allow the passage of filter pack or fine-grained soils into the
well. Therefore, the slot size should be determined based on the
type and texture of surrounding soils and should be selected by
152
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an experienced hydrogeologist. Premanufactured well screens
are available in slot sizes typically ranging from 0.008 to 0.120
inches.
This problem can be evaluated by requiring the slot size of the
well screen to be indicated on the well completion log. If the
slot size of an existing well is inappropriate and causing an
operational problem, the well should be properly closed and
another well installed with the correct slot size.
Length of well screen: The total length of the well screen will
depend on the depth to the water table and the range of water
table fluctuation levels. The screen must extend over the entire
interval of the high and low water table levels as discussed in the
section of this chapter titled "Site Assessment."
Ensure proper installation of the monitoring well
The process of well installation involves borehole drilling and
placement of well in borehole; installation of filter pack,
bentonite seal, surface seal, and protective casing; well
development; well security; and documentation of well
construction and installation.
Borehole drilling generally does not need to be conducted for
monitoring wells installed in the excavation field of new UST
facilities. For these sites the well may be located during tank
installation and the backfill placed around it. Operational
problems associated with borehole drilling for ground-water
monitoring wells installed outside of the excavation zone
(typically for well installation at existing UST facilities) will not
be discussed in this document. Though selection of an
inappropriate drilling method could affect the hydraulic
conductivity of the subsurface materials, this situation is not a
common problem. A discussion on the types of available
drilling methods is provided in U.S. EPA 1986.
The use of an inappropriate filter pack may allow the
introduction of sediment into the well, eventually causing it to
become filled or restricting the flow of fluid into the monitoring
well. Typically, several inches of filter pack are placed in the
bottom of the borehole before the well is installed. The filter
153
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pack, which may consist of clean sand, silica, or glass beads, is
placed in the borehole to extend 1 to 2 feet above the top of the
well, screen.
The annular space (the space between the wall of the borehole
and the outer well casing) above the filter pack is then sealed to
prevent migration of contaminants to the well screen. Materials
such as a cement-bentonite mixture, bentonite chips, or
antishrink cement mixtures can be used as the sealant in
unsaturated zones. This seal should extend at least 2 feet above
the filter pack.
The area above the annular seal should be filled to the surface of
the ground with a cement-bentonite seal to prevent migration of
liquids from the ground surface and to protect the well casing
from structural damage. Ideally, the interface of the annular seal
and the seal above it should be located below the frost line to
protect the well from damage due to frost heaving. A well
casing (referred to as a protective casing) with a diameter larger
than that of the monitoring well (4 to 8 inches) can be placed in
the cement seal around the monitoring well to provide additional
protection from physical damage. This protective outer casing is
typically made of black steel or PVC.
Monitoring wells installed in the backfill of new UST sites can
be constructed using the backfill as the filter pack if the material
meets the requirements of an appropriate filter pack (see
Figure 36). These wells still should be constructed with a
surface grout to prevent infiltration of contaminated runoff from
migrating vertically down to the filter pack and to help stabilize
the well casing and screen.
Flow of liquids into the well may be prevented or severely
reduced if a monitoring well is not properly developed. The
process of well development involves the surging (mixing) and
removal of ground water from the well casing. The purpose of
this procedure is to dislodge material trapped in the well screen,
to remove sediment in the well introduced during the installation
process, and to restore the natural flow rate of ground water
(hydraulic conductivity).
154
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CEMENT PAD
BACKFILL
MATERIAL
Figure 36. Components of a monitoring well
constructed with the backfill material used as the filter
pack. Source: U.S. EPA 1987
-------
A variety of techniques can be used for well development. The
well can be surged using surge blocks, bailers, or pumps. Well
development is continued until ground water in the well is
relatively free of turbidity (cloudiness).
If well development is not continued for an adequate length of
time, the result will be a reduction in the flow of fluid into the
well. This problem can be evaluated by inspecting the well
completion log to determine how long the well was developed
during installation. The problem can sometimes be corrected by
redeveloping the well until the recharge rate of the well is
improved.
Need to secure the monitoring well
A monitoring well that has not been properly secured is
susceptible to tampering or accidental contamination. For
example, product could accidentally be delivered to the
monitoring well instead of the tank fill pipe. To secure a
monitoring well, a threaded or flanged cap is placed at the top of
the well to prevent the introduction of any foreign matter into
the well. The well cap should be locked in place to prevent
tampering. The well should be unlocked only when entrance to
the well is requked. The equipment used to secure a well should
be documented on the well completion log. Monitoring wells
should also be clearly marked (e.g., by color coding or labelling)
to distinguish them from tank fill pipes. The security of a well
can be easily checked by inspection of the well head.
Need to document well construction and installation
The first step that should be taken when a problem with
operation of a monitoring well occurs is to obtain a copy of the
boring log and well construction diagram for each well. It is
much more difficult to troubleshoot operational problems if
these documents are not provided. The boring log describes the
soil types and texture of different geologic strata encountered
and thek interval depth, drilling method(s) used, depth to ground
water, etc. The well construction diagram depicts the well
design specifications, including the type and depth of filter pack,
annular seal, cement seal, well diameter, and well screen design.
This information should be documented on the well boring and
completion logs for each monitoring well.
156
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Operation and Maintenance
This section describes the most common operator-induced problems that
will affect proper performance of ground-water monitoring. The most
common types of human errors are described below.
Need for proper maintenance of sensors
Ground-water sensor devices that are improperly maintained can
falsely indicate that a release has occurred or fail to detect a
release. Inadequate maintenance of sensor devices can result in
a build-up of ice or algae, which is a common problem with
continuous monitoring devices. Thermal conductivity devices
can get caught on the float. Ice also is a maintenance problem
with product-soluble devices. Maintenance generally is not
considered a problem with intermittent devices. To prevent
maintenance problems with monitoring devices, inspections of
• the sensor should be conducted on a regular and frequent basis
to ensure that it is operating properly.
Calibration of manual ground-water monitoring devices is
typically not necessary. Automatic devices should be calibrated
if this is recommended by the manufacturer, according to their
specifications.
Ensure the integrity of sensor coatings
Electrical resistivity sensors and hydrocarbon-soluble devices
use hydrocarbon sensitive coatings that degrade when exposed
to hydrocarbon products. These coatings may also biodegrade
over long periods of time. After the sensor has been exposed to
hydrocarbon product, it must be replaced. If the sensor is not
replaced, it will fail to detect a future release.
157
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Interpretation
Ensure proper interpretation of environmental influences
False alarms in continuous monitoring systems are often caused
by environmental influences. The most common error is failing
to identify changes in electrical resistance that are due to
equipment shorts or power surges. This is primarily a problem
that affects thennal conductivity devices and can result in a false
negative measurement.
Ensure proper interpretation of monitoring results
Another potential source of errors is the failure of the operator to
determine that the source of an alarm or positive result is from
an offsite source of contamination or from the accidental
introduction of product into the monitoring well (e.g., surface
spills). False alarms can also occur when the local water table
rises and contacts residual product from previous spills (see the
discussion in the section of this chapter titled "Site Assessment,"
addressing background contamination). This is more of a
problem with intermittent devices because these devices rely on
the results obtained from a discrete sample and not on data
trends.
Malfunction of monitoring devices, in particular thermal
conductivity devices, may result in false positives. Another
potential source of errors is the long (up to 10 hours) response
times exhibited by electrical resistivity sensors. Although this is
not documented as a common error, an operator unaware of this
time constraint may prematurely decide that a resistivity test
indicates no leak. The response and lag times for all other
continuously operating devices mentioned in this manual are less
than 30 minutes.
The interpretation of monitoring results from ground-water
monitoring wells does not require a high degree of technical
expertise. Manual systems require inspection of a liquid sample
obtained from the well, or the use of a sensor which will either
electronically indicate the presence of free product (i.e., a
change in thermal conductance) or will change color.
Automated detection systems require the greatest degree of
158
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interpretation. For systems with an alarm, the operator must
determine if a triggered alarm is legitimate or a false alarm.
Systems using a continuously operating strip chart recorder may
require interpretation of the data trends indicated on the chart.
REFERENCES
1. American Petroleum Institute. 1986. Observation Wells as
Release Monitoring Techniques. Prepared by Weston for API
Marketing Operations and Engineering Water Subcommittee.
2. Dragun, J. 1988. The Soil Chemistry of Hazardous Materials.
Hazardous Materials Control Research Institute, Silver Spring,
Maryland.
3. Electric Power Research Institute. 1987. Summary of Available
Monitoring Technologies for Underground Storage Tanks -
Draft. Prepared by Roy F. Weston, Inc.
4. U.S. Department of Energy. 1985. Procedures for the
Collection and Preservation of Ground Water and Surface
Water Samples and for the Installation of Monitoring Wells,
2nd Edition. Prepared by Bendix Field Engineering Corporation
for the U.S. Department of Energy, Nuclear Energy Programs,
Division of Remedial Action Projects.
5. U.S. EPA. 1985. September 1985. Practical Guide for
Ground-Water Sampling. Prepared by the Illinois State Water
Survey for the Robert S. Kerr Environmental Laboratory.
EPA/600/2-85/104.
6. U.S. EPA. 1986. Septmeber 1986. RCRA Ground-Water
Monitoring TEGD. Office of Solid Waste and Emergency
Response, OSWER-9950.1.
7. U.S. EPA. 1986a. May 1,1986. Underground Motor Fuel
Storage Tanks: A National Survey. Vol. 1, Technical Report.
Prepared for the Office of Pesticides and Toxic Substances. May
1, 1986. EPA-560/5-86-013.
159
-------
8. U.S. EPA. 1988. February 1988. Free-Product Release
Detection for Underground Storage Tank Systems, Vol. 1.
Capabilities and Limitations of Wells for Detecting and
Monitoring Product Releases. Prepared by Geraghty and Miller,
Inc. for the U.S. EPA Office of Underground Storage Tanks.
9. U.S. EPA. 1988a. February 1988. Free-Product Release
Detection for Underground Storage Tank Systems, Vol. 2. The
Effectiveness of Petroleum Tank Release Detection Monitoring
with Wells in Florida. Prepared by Geraghty and Miller, Inc. for
the U.S. EPA Office of Underground Storage Tanks.
10. U.S. EPA. 1988b. July 1988. Common Human Errors in
Release Detection Usage. Prepared by Camp Dresser & McKee
Inc. for the Office of Underground Storage Tanks.
11. U.S. EPA. 1988c. February 1980. Interim Report: Summary of
Several State Underground Storage Regulations. Prepared by
the Environmental Research Center, University of Nevada-Las
Vegas for the Environmental Monitoring Systems Laboratory.
160
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Chapter VIII
Secondary Containment
with Interstitial Monitoring
-------
-------
SECONDARY CONTAINMENT
WITH INTERSTITIAL MONITORING
VIII
SUMMARY
The use of secondary containment with interstitial monitoring as an
UST release detection method for petroleum storage tanks involves a
barrier outside the primary tank with a release detection device between
the inner and outer barriers. The space between the barriers is called the
interstitial space. The outer wall or liner contains the leak long enough
for it to be detected by the monitoring system. This method is required
in several states and is considered to be the most protective of the
environment because leaks are generally detected before they can
contaminate the environment.
The factor that has the greatest impact on the proper functioning of
secondary containment is installation. The factors that have the greatest
impact on the interstitial monitoring systems are system installation and
operation and maintenance factors.
The discussion presented in this chapter covers a range of possible
problems that may occur with secondary containment with interstitial
monitoring. This does not mean that all, or even most, of these
problems will occur at the same time. Nor does it mean that all of the
problems are of equal importance, in terms of frequency of occurrence
or severity of impact. Some problems, such as false alarms caused by
curing of fiberglass tanks, seldom occur, while other problems, such as
residual contamination, may have limited impact. Experienced
installers are well aware of these problems and how to deal with them.
For example, an experienced installer would use waterproof electrical
junction boxes to prevent detection system failure during precipitation.
Release detection, however, is a growing industry, and new companies
are being formed with less experience. This chapter presents a range of
potential problems for educational purposes, not to imply that they will
always occur.
161
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BRIEF DESCRIPTION
An interstitial monitoring system is intended to detect any leak from the
tank under normal operating conditions and not to measure the leak rate.
Secondary containment with interstitial monitoring for tanks consists of
two components. The first is an outer barrier, which directs the leak
toward the interstitial monitoring system. The purpose of the outer
barrier is to retain the leak for a sufficient time so that it can be detected.
The barrier is not requked to contain the leak from a petroleum tank so
that it does not contaminate the environment (as is requked for
hazardous substance tanks). The second component is the interstitial
monitoring system, which detects the leak and alerts the operator.
The outer barrier can be either an outer shell (on a double-walled tank),
a synthetic liner around the tank (a tank jacket), or a liner in the
excavation of the tank system that is between the soil and the backfill
material (Figures 37 through 39). The outer shell of a double-walled
tank is generally made of the same materials as the tank, e.g., steel or
fiberglass reinforced plastic (FRP), while a liner can be made of various
synthetic materials, such as high-density polyethylene, polyester
elastomers, epichlorohydrin, and polyurethane.
The outer barrier either completely surrounds the tank (the fully or
completely enclosed design) or covers only the bottom half of the tank
(partially enclosed design). In the completely enclosed (double-walled
or jacketed) design, a leak from any part of the tank is trapped in the
interstitial space between the inner and outer barriers and is detected. In
the partially enclosed design, leaks from the bottom half of the tank
would be contained for detection by the outer shell, but leaks originating
above the outer shell could potentially enter the backfill and avoid
detection by an interstitial monitoring system.
Excavation liner systems also may be either fully or partially enclosed.
A fully enclosed liner system would have a liner section across the top
of the tank that would be sealed to the sides of the excavation liner. A
partially enclosed design might not have the top liner, and its excavation
liner might not reach the top of the tank.
Concrete vaults are also used for secondary containment but are not
commonly used for petroleum products because vaults are more costly
to construct. Other references are available on concrete vaults, which
are not discussed further in this chapter.
162
-------
/o\
Inner Tank Wall
• Outer Wall
Inner Wall
Sampling
Standpipe
or
Electronic
Detection
"Double-Walled Steel Tank
Interstitial Space
•Double-Walled FRP Tank
A cross section of a double-walled tank is shown in Figure 40
Figure 37. Two double-walled tank configurations.
Source: U.S. EPA (January 1989)
163
-------
Extrusion Welded Seams
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Jacket
Figure 38. Jacketed tank. Source: U.S. EPA (August 23,1986)
-------
Monitoring Point Suitable
for Leak Detection and
Withdrawal of Accumulated
Water
Collar to Connect
Pipe Trench to
Tank Liner
Pavement Liner Turnback
Trench Top Liner
Backfill
Tank
Excavation
Liner
Some Partially
Enclosed Liners
Would End Here
Pipe Leak
Detection
Monitoring
Point
Slotted Pipe for
Leak Detection and
Withdrawal of Water
Native Soil
Interstitial. Space
Figure 39. Tank with excavation liner. Source: U.S. EPA (August 22,1986)
-------
Interstitial monitoring systems operate to detect leaks based on the
following mechanisms:
• Electrical conductivity
• Pressure sensing
• Fluid sensing
• Hydrostatic monitoring
• Manual detection
• Vapor monitoring
The applicability of the monitors to the types of secondary containment
systems is summarized in Table 12.
Vapor monitoring is discussed in Chapter 6 of this handbook.
Therefore, it will not be discussed in this chapter except where the
problems are specific to interstitial monitoring systems.
The electrical conductivity monitor depends upon the leaked product
changing the resistance of sensing wires that are placed in the interstitial
space of tank systems. The leaked product completes an electrical
circuit and allows current to flow, thus activating an alarm. Designs are
available for both electrically conductive (e.g., water or a water-based
detergent solution) and for non-conductive products (e.g., like
petroleum). This type of system is used in secondary containment
systems that utilize an excavation liner, a jacket, or a double-walled
tank.
Pressure sensing systems are used only in the interstitial space of
double-walled tanks. The space is either put under a vacuum or
pressurized, and leaks are identified by the detection of pressure
changes that occur when either the inner or outer tank shell develops a
hole or crack.
Fluid sensing systems are used in the interstitial space of double-walled
tanks to detect a leak into the normally dry space of either the product in
the tank or of ground water. One system uses an optical principle in
which the change in the reflectance of a mirror is detected when the
product or ground water leaks into the space and covers the mirror.
The hydrostatic method (Figure 40) is used in double-walled tanks and
is based on detection of the change in level of a fluid that completely
fills the interstitial space. When a breech in the inner wall occurs, the
166
-------
Table 12
Applicability of Leak Detection Methods to Secondary Containment Systems
Containment
Full double-
walled tank
Tank jacket
Partial double-
walled tank
Fully enclosed
excavation liner
Partially enclosed
excavation liner
Electrical Pressure
conductivity sensing
X X
X
X X
X
X
Fluid Hydrostatic
sensing monitoring
X X
X
X
X
X
Manual Vapor
methods monitoring
X X
X X
X X
X X
X X
Source: U.S. EPA.
167
-------
o\
CO
Concrete
Traffic Pad
.Optional Reservoir
Liquid Level Sensor
•Stable Reservoir
Liquid Level
Stored
Product
Double-Wall
Underground Tank
Normal Conditions
The reservoir liquid level will
be stable if both the inner and
outer tank are tight.
The optional reservoir sensor
will activate an alarm if the
reservoir drains or overfills.
Outer Wall Breach
If the groundwater is below
the tank top, the monitor fluid
drains into the ground causing
the reservoir to drain.
Reservoir
Drains
Inner
Wall
Breach
Inner Wall Breach
Monitor fluid drains into the
primary tank causing the
reservoir to drain. No petroleum
product escapes from the
primary tank to pollute the site.
'S^"',""""™' '"•r
' jfitoundwater ^
- Res&ilwflf ;' "^
If the groundwater is over the
tank top, the reservoir will
overfill with groundwater and
activate the high level alarm
on the reservoir sensor.
Figure 40. Hydrostatic monitoring system. Source: Owens Corning Fiberglass Corporation
-------
fluid leaks into the tank, and the fluid level in the space is lowered. If
the breech occurs in the outer wall, the fluid will leak into the
surrounding soil, and the fluid level in the space will decrease, or
ground water will leak into the interstitial space and cause it to overfill.
Alarms are set so that either a decrease or an increase in fluid level will
be detected.
The manual detection method is simply the use of a pole with a cloth or
a petroleum-detecting paste on one end. When the pole is inserted in a
pipe that extends into the interstitial space, any leak into the space can
be detected from visual observation of the cloth or from a change in
color of the paste. This method is used in both double-walled tanks and
in liner systems of secondary containment.
To be successful, most methods rely on the leaked product being
dkected by the containment to the position of the sensing device so that
detection can be accomplished. That is, the containment is usually
sloped toward the sensor in such a way that leaks entering the interstitial
space will be detected. Some electrical conductivity methods, however,
rely on a continuous sensing wke that is capable of detecting leaks
along the sensor's length and, thus, do not require a sloped containment.
The successful implementation of secondary containment with
interstitial monitoring involves the following two phases: (1) instal-
lation, including site assessment, an evaluation of the site environmental
conditions to assist in the determination of what type of secondary
containment and interstitial monitoring system is appropriate; and
(2) operation and maintenance. The relationship between these phases
is shown in Figure 41 along with some of the important factors that
should be considered for successful implementation. The problems
associated with the use of a secondary containment system with
interstitial monitoring are discussed below in reference to these two
phases. The order of discussion does not necessarily reflect the priority
of the problems discussed.
POTENTIAL PROBLEMS AND SOLUTIONS
The main problems in the implementation of an interstitial monitoring
system result from inadequate attention to installation, such as an
inappropriate choice of containment for the site conditions or faulty
installation of the monitoring system or containment, and lack of
attention to the system during operation. The major problems and some
169
-------
Insta
Installation
ation
Assess hydrogeological conditions
Evaluate residual contamination
Select partial or complete
containment system
Choose interstitial monitor
appropriate for site
Prepare site
Use qualified professionals
Select proper backfill
Follow installation codes
and manufacturer's
recommendations
Operation
Operation
Ana ysis
• Set threshold level
• Calibrate periodically
• Maintain and conduct routine
checks per manufacturer's
recommendations
Analysis
Leak
No Leak
Confirm leak detection events
against spills, overfills, and
environmental conditions
that may cause false alarms
Figure 41. General procedure for secondary containment with
interstitial monitoring
170
-------
solutions are discussed below, including regulatory approaches to
oversee the implementation effectively. A number of Agency solutions
are offered for each problem, but not all of them need be undertaken. A
summary of the problems and solutions is given in Table 13. An
asterisk identifies the most serious concerns with containment and
monitoring systems.
Installation
Operation
Analysis
Assuring proper installation of secondary containment
The proper installation of secondary containment is important to the
success of interstitial monitoring. If the containment fails to collect
the leaked product, the monitoring system may be unable to detect
the leak before it reaches soil and ground water.
For excavation liners, rocks, tree roots, or debris in the excavation
may cause damage to the liners, which could cause leaks to go
undetected. All rocks, tree roots, debris and other protruding objects
should be removed prior to the installation of these secondary
containment systems.
If improper backfill is used, two problems might arise. FRP
double-walled tanks might not be supported sufficiently so that,
when the tank is filled, the settling of the tank under the load of the
fuel might cause cracks in the outer wall and produce leaks that
escape detection. With excavation liner systems, if the backfill is
not of sufficient permeability, leak detection may be delayed due to
slow migration of the leaked product to the monitoring system.
Both problems can be avoided by using sand or pea gravel as the
backfill material. These materials both provide adequate support to
double-walled tanks and are of sufficient permeability so that leak
detection will not be delayed. To prevent excessive settling of
double-walled FRP tanks, additional precautions should be taken,
such as compaction of the backfill under the tank and the use of a
filter fabric in the excavation to prevent the backfill from migrating
in shallow ground-water or tidal areas.
Surface water run-on and precipitation can cause the backfill to
become saturated. If this occurs, a vapor monitoring system will not
function because the vapor cannot migrate to the monitoring system.
Manual leak detection methods may also be adversely affected if the
water is sufficient to float leaked product away from the monitoring
171
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Table 13. Indicators and Solutions for Problems with Secondary Containment with Interstitial Monitoring
Problem
Indicators
Tester Solutions
Agency Oversight Options
Account for shallow ground
water.
Ground water leaks into partial
containment causing failure of
vapor monitoring and manual
methods.
Use double-walled tank, fully
enclosed liner or jacket, or
monitoring method not sensitive
to water.
Require site assessment to
establish ground water depth
and fluctuation.
*Prevent faulty electrical
Installation.
False alarm or inoperative system.
Use waterproof, corrosion-proof
junction boxes; follow
manufacturer's installation
procedures and local codes; use
manufacturer's representative to
supervise.
Certify installers; review
installation plans; inspect
installation.
*Prevent faulty containment
Installation.
Leaks escape detection. Sudden
catastrophic product loss.
Install per local and national
codes; use representative of
manufacturer to supervise.
Review plans; inspect; certify
installers; require supervision
of installation by manu-
facturer's representative.
Prevent interference from
surface water run-on or
precipitation.
Water leaks into containment and
causes failure of vapor monitoring
or manual methods.
Cover containment with sloped,
impervious synthetic membrane,
or use method unaffected by
water in containment.
Review manufacturer's
literature to ensure sensor
will respond or require test
with the product.
Assure proper operation and
maintenance.
System failure or false alarm.
Electrical conductivity. Replace
sensor wires after detection.
Pressure systems. Check
tightness of plumbing connections
on routine basis.
Review plans; train staff on
monitoring systems; inspect;
require demonstration of
system and system tests
before operation.
-------
Fluid sensing systems. Set
threshold to distinguish product
from water.
Hydrostatics monitoring systems.
Add fluid in hot weather; add anti-
freeze in cold weather.
Manual detection. Use consistent
procedure.
Vapor monitoring. Waft for tank to
cure so that emission of gases in
interstitial space ceases. Set
threshold above gas concentration.
Prevent false alarm due to
residual (background)
contamination.
False alarms.
Use system that can distinguish
leak from background contamina-
tion. For site conditions where
background would interfere with
detection, use double-walled tank
or other completely sealed
secondary containment.
Require site assessment; review
plans; require system tests to
show that leaks can be
distinguished from background
contamination.
* Most frequent causes of failure.
-------
Operation
Analysis
location. To avoid this problem, a synthetic liner should be placed
over the tank and its backfill to prevent water from entering the
backfill. Another solution would be to use release detection
methods that function adequately in water, such as electrical
conductivity or ground-water monitoring methods.
Improper sealing of the seams of liners can result in leaks escaping
the containment systems without being detected. The installation of
liner systems should be conducted only by professional installers
who are experienced in liner installation.
The installation of secondary containment should be supervised by a
representative of the manufacturer or by an experienced professional
who has been trained for the task in order to minimize the potential
for damage to the containment and the consequent inability of the
monitoring system to detect the leak. Some states, for example,
California and Rhode Island, requke that the installation be
supervised by the manufacturer or by a representative of the
manufacturer. Other regulatory agencies, such as Dade County,
Florida, require inspection of secondary containment systems during
installation. Dade County also requkes prior agency review of the
secondary containment design and installation plan. The City of
San Jose requkes integrity testing of all secondary containment
systems before they are accepted for use.
In general, state and local installation codes are in effect that, when
followed, will promote proper installation and that will help
minimize the possibility of damage to the secondary containment
system. State and local installation codes must be followed, and the
reference section at the end of this chapter lists other national codes
which may be used for guidance.
Accounting for shallow ground water
Shallow ground water may adversely affect vapor monitoring and
manual methods of interstitial monitoring. The problems related to
vapor monitoring in shallow ground-water areas were discussed in
Chapter 6. Vapor monitoring will not function properly in saturated
soils because the movement of vapor is slowed or prevented.
Manual methods might not detect a leak under high ground-water
conditions if enough water is present to float the product away from
the test location.
174
-------
Operation
*
Analysis
A site assessment to determine the depth to ground water and the
ground-water fluctuation should be made to assist in the choice of
an appropriate secondary containment system. If the ground water
at any time is expected to reach the level of the containment, a fully
enclosed containment system could be used to prevent the ground
water from transporting the leaked product away from the detection
system. The use of the fully sealed containment would also allow
vapor monitoring to be used at sites where vapor monitoring
normally would not be effective. Pressure sensors, fluid sensors,
and hydrostatic methods are used in double-walled tank systems
and, therefore, should not be adversely affected by ground water as
long as the containment does not leak.
At sites where shallow ground water exists, the forces on both
double-walled tanks and excavation liner systems caused by the
ground water may cause shifting of the containment and consequent
damage that would allow leaks to escape and go undetected. These
sites should be dewatered before installation of the containment, and
the containment should be properly anchored to prevent shifting.
Preventing false alarms from background contamination
Background contamination of soils and ground water can cause false
alarms in interstitial monitoring systems that are not completely
sealed. When the site has been previously contaminated or when a
threat of contamination exists from neighboring properties, it may
be necessary to use a monitoring method that can distinguish a leak
from background contamination or to use a double-walled tank, a
tank jacket, or an excavation liner that is completely sealed. This
precaution will ensure that detected leaks are only from the tank
system of concern and did not originate elsewhere. The background
contamination problem and its solutions are discussed in more detail
in the ground-water monitoring and vapor monitoring chapters.
Assuring selection of proper interstitial monitoring system
—
Operation
Analysis
Interstitial monitoring systems that are inappropriate for the stored
product will not allow detection of leaks. An understanding of the
monitoring method and its ability to detect the specific product is
necessary. For example, electrical conductivity systems function
differently for conductive products than for non-conductive products
such as petroleum. For petroleum products, one common design
175
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Operation
Analysis
(Figure 42) is based upon the ability of petroleum products to
complete an electrical ckcuit by degrading a coating that separates
two metal conductors. Another design (Figure 43) allows the
petroleum product to penetrate a braided cover on an electrical
cable, which causes a conductive polymer jacket to swell and
contact conductive metal wires, thus completing an electrical circuit.
A non-petroleum electrical conductivity monitoring system used in
an UST containing petroleum might not detect a leak. The coatings
and conductive polymers of these systems are formulated to be
specific to non-petroleum products and will not allow detection of
petroleum.
In addition, if the stored product is changed over the life of the tank
an evaluation of whether to change the monitoring system also
should be made to ensure that the system is appropriate for the new
product. As discussed in the chapter on vapor monitoring, the vapor
monitor's ability to detect a leak depends upon the volatility of the
product. If the product were changed from gasoline to used oil
during the tank's lifetime, the vapor monitor might not be able to
detect leakage of the new product due to its lower volatility. All
monitoring systems should be evaluated for use for the specific
stored substance by checking the information available from the
manufacturer and by asking the manufacturer to verify adequate
response to the stored product, if necessary.
Assuring proper installation of monitoring system
One major problem of all non-manual interstitial monitoring
systems is improper electrical installation. If junction boxes are not
waterproof and corrosion proof, short circuits may occur, which will
cause system failure and inability to detect leaks. Waterproof
junction boxes are more expensive than non-waterproof ones and,
therefore, are sometimes left out of the as-built system by
installation contractors to save money. Another common problem
caused by "short cuts" taken by installation contractors is the use of
two-conductor electrical cable in place of three-conductor cable.
This substitution can cause a false alarm in some systems or may
cause a diagnostic trouble alarm in other systems to alert the
operator that a fault is present. Some systems require that bridge
resistors be used to span.the unused sensor channels on the system's
electrical control board. When these bridge resistors are omitted, a
false alarm also may be given. It is important, therefore, that the
manufacturer's recommended procedures be followed during
installation, that local electrical codes be adhered to, and that short
176
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Metal Conductors —.
Coating Degradabte
by Hydrocarbons
Figure 42. Cross section of electrical conductivity sensor
using degradable coating. Source: DETEX Systems, Inc.
Sensor Wires
Continuity Wire
Conductive
Polymer
Layer
Overbraid
Figure 43. Cross section of electrical conductivity sensor
using polymer jacket that swells. Source: Raychem
Corporation
177
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cuts that attempt to ckcumvent these recommendations and codes
not be used.
When vapor monitoring systems are used in double-walled FRP
tanks, the tanks must be allowed to cure a sufficient length of time
so that outgassing of organic vapors from the tank interior walls will
not cause false alarms. This outgassing should occur at the factory,
but it occasionally does occur in the field. In the field, the primary
solution is to set the threshold setting of the vapor monitor above the
level of the gas being produced and then to check the threshold
setting periodically to see if it can be lowered as the tank continues
to cure. These periodic checks must be made or the potential will
exist for small leaks to go undetected due to the higher threshold
settings.
Operation
[installation) Preventing false alarms and undetected releases
Analysis
False alarms and undetected releases may be caused by operational
factors. These factors and some solutions are discussed below for
the specific type of system in which they may occur.
Electrical conductivity systems. False alarms will result if
electrical conductivity systems are not replaced after exposure to a
leak. The degradable coating system sensing wires must be replaced
after exposure to leaks; otherwise, the system will give a false alarm
because the coating has been degraded. The sensor cable in a
conductive polymer system also must be replaced after exposure
because the conductive polymer has closed the electrical circuit by
swelling and the cable can not be cleaned sufficiently to avoid a
false alarm if reused.
Pressurized systems. Losses of pressure or vacuum in these
systems will cause an alarm even if the leak that results in the
pressure loss is not caused by leaking product. The pipe fittings,
vacuum tubing, and vacuum or pressure gauges used in these
systems can become loose due to vibration from traffic or due to
physical contact with vehicles or with personnel. When a leak is
detected, the first action should be to check all plumbing
connections to the interstitial space, to retighten any loose fittings,
and to re-establish the pressure or vacuum level. If the plumbing is
tight, then it can be assumed that a leak in the inner or outer tank
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wall has occurred because environmental conditions, such as
temperature differences between the tank and the delivered product
or the effect of barometric pressure, have not been found to cause
false alarms.
Fluid sensing systems. Optical devices, which are a class of fluid
sensing systems, allow the leaked product to deposit on a mirror.
The reflectivity of the mirror is decreased by the deposits, and the
amount of light reflected from a light source, thereby, can be
detected and can cause an alarm to be actuated. Any deposit on the
mirror, including condensation water, will cause the mirror's
reflectivity to decrease. The system's threshold should be set so that
condensation water can be distinguished from leaked product.
Hydrostatic monitoring systems. Temperature extremes may
cause false alarms or system failure in hydrostatic monitoring
systems. Because hydrostatic systems use fluids to detect leaks,
precautions must be taken in hot and cold weather to ensure that
evaporation does not cause false alarms and that freezing
temperatures do not disable the system and prevent it from detecting
leaks. During cold weather, an antifreeze solution that is compatible
with the tank, the secondary containment, and the interstitial
monitoring system should be used to prevent freezing of the fluid.
Additional fluid must be added periodically to the interstitial space
to compensate for evaporation during hot weather.
Manual detection. Manual methods may not detect a leak if
improper operating procedures are used. Manual methods consist of
using a pole with either a cloth or a petroleum-detecting paste to
indicate that a leak has occurred. If the pole is not always inserted
in the same manner into the access hole to the interstitial space, the
pole may not sample leakage from the lowest point in the
containment and, therefore, may miss leaks. Although this problem
would probably not cause large leaks to go undetected for long, a
small leak might not be detected within the required monthly
monitoring period. Specific spots should be designated as test
positions, these positions should be the lowest points in the
containment to which all leaks will drain, and the process of
inserting the pole should be done as consistently as possible from
one test to the next. That is, the pole should not be inclined and
should be inserted until it touches the bottom of the containment.
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Vapor Monitoring. The primary solution is to set the threshold
setting of the vapor monitor above the level of the gas being
produced and then to check the threshold setting periodically to see
if it can be lowered as the tank continues to cure.
APPROACHES TO ENSURING EFFECTIVENESS
Installation
Several options are available to regulators to allow them to check for
installation errors in secondary containment and interstitial monitoring
systems. For example, regulators could require site assessments and
then review plans for secondary containment systems before installation
to ensure that proper systems are chosen for shallow ground water and
contaminated sites. A manufacturer's representative could supervise the
installation or could certify installers. Inspection of the installations
during construction and integrity testing are other alternatives.
The regulatory agency could review monitoring system plans and
specifications before installation to check the selection of the
monitoring system for specific site conditions, such as existing
contamination.
Operation
Manufacturers of monitoring equipment could be requested to provide
training on the equipment for regulatory personnel, as is done for tank
tightness testing in Rhode Island. Regulators could also request that
manufacturers submit videos on the use of thek equipment, as is done in
Massachusetts. In this way, agency personnel would become familiar
with the equipment and would be better able to inspect monitoring
systems and recognize both installation and operational problems that
would affect monitoring.
Regulatory agency personnel could conduct periodic inspections of
monitoring systems during operation, as is requked by the city of
Austin, Texas. This approach allows calibrations and threshold values
180
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to be checked and maintenance and leak detection records to be
examined to ensure that proper attention is being given to the system.
Regulatory agencies could also require demonstration of the monitoring
system and a demonstration of the system tests on-site before operation
and periodically thereafter to promote regular system checks by owners
and operators and to ensure that the monitoring system is both
responsive to any leaks and can distinguish existing contamination from
leaks.
REFERENCES
1. U.S. EPA. January 1986. Under ground Tank Leak Detection
Methods: A State-of-the-Art Review. EPA 600/2-36-001.
Hazardous Waste Engineering Research Laboratory, U.S. EPA.
2. Baker/TSA, Inc. June 1987. Final Report: Study to Evaluate Cost
and Effectiveness of Secondary Containment Methods for
Underground Storage Tanks. Prepared for the Utility Solid Waste
Activities Group.
3. Radian Corp. September 7, 1984. Secondary Containment for
Underground Petroleum Products Storage Systems at Retail Outlets
Final Report. Prepared for American Petroleum Institute, Ground
Water Task Force.
4. Government Institutes, Inc. 1987. Underground Storage Tank
Management, 2nd Edition.
5. Petroleum Marketers Association of America. November 1986.
Underground Storage Tank Leakage Prevention, Detection, and
Correction.
6. U.S. EPA. August 22,1986. Guidance Manual for the Storage and
Treatment of Hazardous Waste in Tank Systems.
EPA/530-SW-84-004. Policy Dkective No. 9483.00-1A. Report by
Fred C. Hart Associates, Inc., for Waste Treatment Branch, Waste
Management Division, Office of Solid Waste, U.S. EPA.
7. National Sanitation Foundation. 1983. " Flexible Membrane
Liners," NSF Standard 54.
181
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8. Steel Tank Institute. 1988. "Standard for Dual Wall Underground
Steel Storage Tanks," Standard F 841-88.
9. Underwriters Laboratories, Inc. 1987. "Standard for
Glass-Fiber-Reinforced Plastic Underground Storage Tanks for
Petroleum Products," UL 1316.
10. Petroleum Equipment Institute. 1987. "Recommended Practices
for Installation of Underground Liquid Storage Systems," PEI/RP
100.
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Chapter IX
Piping Release
Detection Methods
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PIPING RELEASE DETECTION METHODS IX
SUMMARY
Information collected by the U.S. Environmental Protection Agency
demonstrates that about 25% of the underground storage tank systems
in the United States are leaking. Piping and loose fittings are
responsible for the majority of these leaks, and piping is responsible for
most of the large, catastrophic releases. Thus, an important part of any
release detection program is the use of equipment to prevent or
minimize the releases from piping.
There are a number of piping release detection methods available,
representing a wide variety of approaches. Each method has advantages
that make it appealing under certain conditions.
• Flow restrictors provide nearly continuous release detection for a
small capital investment. They are readily available and require little
owner/operator involvement.
Flow shutoff devices also provide nearly continuous leak detection
and, because they are automated, require little effort from the on-site
staff. As their name implies, these devices completely stop a leak when
it is detected.
• Line tightness tests require no permanent equipment and, therefore,
no capital investment. Performed infrequently, line tightness tests
interfere little with the daily operation of an UST. Line tests can
often be perforated conveniently as part of a tank tightness test.
• Interstitial monitoring within secondary containment minimizes the
environmental damage while providing sensitive release detection.
Interstitial monitors can also be coupled with automatic sensors,
shutoffs, and alarms.
• External monitoring of underground piping using ground-water or
vapor monitoring can easily be integrated into external monitoring
systems for USTs.
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Available studies demonstrate that flow restrictors, flow shutoffs, and
line tightness tests are capable of meeting the required performance
standards when designed and operated properly.
The discussion in this chapter focuses primarily on the first three types
of piping release detection: flow restrictors, flow shutoffs, and line
tightness tests. Interstitial, vapor, and ground-water monitoring for lines
are essentially the same as for tanks, and the discussions in the chapters
covering these methods are applicable to piping. The aspects of those
release detection methods that apply only to underground piping are
included in the sections below.
The discussion presented in this chapter covers a range of possible
problems that may occur with each piping release detection method.
This does not mean that all, or even most, of these problems will occur
at the same time or at the same site. Nor does it mean that all of the
problems are of equal importance, in terms of frequency of occurrence
or severity of impact to the effectiveness of the release detection
method. Some problems, such as use of incorrect threshold value,
happen less often, and other problems, such as tampering, are relatively
easy to fix. Experienced testers, vendors, and installers are well aware
of the problems and how to deal with them. For example, an
experienced tester recognizes a vapor pocket in the line and knows the
methods to use to try to remove the vapor pocket. Release detection,
however, is a growing industry, and new companies are being formed
with less experience. This chapter presents a range of potential
problems for educational purposes, not to imply that they will always
occur.
BRIEF DESCRIPTION
To understand the workings of the piping release detection technologies,
it is important to understand how the different types of UST piping
systems work. This section presents descriptions of common piping
systems followed by descriptions of piping release detection
technologies.
184
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Piping Systems
Figure 44, which follows on the next page, has been prepared to
illustrate the following discussion of UST piping systems.
Pressurized lines
In a pressurized piping system, a submerged centrifugal pump
located near the bottom of the tank moves the stored product from
the tank to the point of end use (e.g., a dispenser at a service
station). The delivery piping lines extend from the pump discharge
point to the dispenser. The product is essentially "pushed" from
the tank, typically at positive pressures of 28 to 32 pounds per
square inch (psi), although some piping systems are pressurized up
to 60 psi. Very large releases can occur very quickly if a hole or
break occurs in a pressurized UST pipeline because the pump
continues to push product through the line and through the hole or
break. The higher the operating pressure of a line the higher the
leak rate when a hole is formed. Pressurized systems generally are
chosen for high-volume sites because the product can be delivered
very quickly.
Suction lines
Typical suction systems use a positive displacement pump at or
near the point of end use to draw the product from the tank to the
pump. The pump creates a lower pressure at the pump end of the
pipe, thereby allowing atmospheric pressure to push the product
along the pipe to the delivery point. Typical suction lines in the
U.S. operate at a vacuum of 3 to 5 psi. When the pump is shut off
or a hole or break develops, suction is interrupted, and the product
flows backwards through the pipe, away from the dispenser and
towards the tank. One or more check valves in the pipe close when
product begins to flow backwards through the pipe. Product is
held in the pipe between the check valve and the point of end use
or between check valves if more than one is present. Product in the
pipe between the tank and a check valve drains back into the tank.
Suction systems are characterized as "European" or "American"
systems. In the European system, the check valve is located
immediately below the pump. When the pump is turned off or
185
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Vent Pipes
Product Dispensers
Submerged Pump
Assembly (Inside Tank)
Line Leak
Detectors
Figure 44. Typical retail gasoline station.
186
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lot®
Operation
Analysis
there is a line failure, suction is broken and most of the product
drains directly back into the tank. In the American system, the
check valve is located near the top of the tank, where it is often
called an angle check, or at the bottom of the suction line within
the tank, where it is called a foot valve. When there is a line
failure, product cannot drain into the tank and is released to the
environment. Although the total release is small, it can occur each
time product is dispensed over a long period, resulting in a
significant cumulative effect.
Although suction piping is environmentally safer than pressurized
systems, it has some limitations, including:
• Potential to vapor lock at high altitudes and high ambient
temperatures;
• Tank location restricted to within 50 feet of the pumps for
proper operation;
• Slower delivery of product from the UST to the point of end use
than with pressurized systems; and
• Larger diameter (higher cost) pipe than required for pressurized
systems.
Piping Release Detection Technologies
Figure 45 is a flow chart of the process used for establishing release
detection systems for underground piping.
Automatic flow restrictors
Currently, most UST systems with pressurized piping delivery
systems use a device that restricts the flow of product from the
pump to the point of end use in the event of a leak. These devices
are installed only on pressurized lines and do not entirely shut off
the flow of product. Flow restrictors are self-contained mechanical
devices installed directly in the pipe using special fittings or in the
pump housing. The device has a diaphragm or piston that is
activated by the pressure in the pump delivery system. Each time
the pressure in the piping system drops below a preset threshold,
typically 1 to 2 psig, a test of the system is performed. No leak test
is conducted if the system pressure remains above the threshold.
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Assess Site and Needs
Whether to combine with tank
release detection system
Automated vs. manual
Suction vs. pressurized piping
Installation
Select Methods
Flow restrictor or flow shutoff
Tightness test, vapor monitoring,
ground-water monitoring, or
interstitial monitoring
Installation
Proper procedures
Trained and experienced
installers
Operation & Maintenance
Operation
I
Proper procedures
Routine inspection
and maintenance
Ana ysis
Interpretation
Leak
No Leak
Figure 45. General procedure for piping release detection
188
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Operation
*
Analysis
After the dispenser is turned on, product flows through the line at
1.5 to 3 gal/h. If there is no leak in the line, the line pressure
reaches about 10 psig in about 2 seconds, and the diaphragm or
piston opens completely, allowing full product flow and
pressurization of the line. If there is a leak of at least 3 gal/h, the
line pressure will not reach 10 psi, and flow is restricted to 3 gal/h.
Leaks smaller than 3 gal/h are indicated if more than 2 seconds are
required to fully pressurize the line. Reference No. 5 cited at the
end of this chapter contains a more complete description of the
workings of a flow restrictor.
Automatic flow shutoff devices
Automatic shutoff devices are relatively new piping release
detection devices and have had limited actual use at operating UST
sites. Shutoff devices can be used only on pressurized lines.
Several types of shutoffs are available, but all rely on detecting
changes in line pressure. There are essentially two groups of
shutoffs: those that monitor pressure increases and those that
monitor pressure decreases. These devices respond to a suspected
leak by completely shutting off the flow of product. All shutoff
devices are permanent installations. The degree of automation can
vary. Some systems are run entirely by personal computers and,
once a leak is suspected and the piping has been shut down, cannot
be overridden by the on-site staff.
One group of shutoff devices checks for leaks by monitoring line
pressure decrease over time. A pressurized line will not be able to
maintain a constant pressure in a static situation if a leak is present.
Some shutoff devices monitor the decay of line pressure over
5-minute intervals and shut off the line if the rate of decay exceeds
predetermined values (e.g., from 16 psi to 6 psi in 5 minutes). At
least one shutoff device measures the time it takes for pressure to
decay from one predetermined value to another (e.g., time to go
from 10 psi to 5 psi). Most of these shutoff devices require more
than one test indicating a leak before shutting off the line. Tests of
the line are not run while the product dispenser is on, and most of
the shutoff devices require a minimum amount of time between
dispensings to run a test. Another type of shutoff system combines
a pressure decay test with the flow restrictor described above.
Another shutoff device monitors the rate of pressure increase in a
piping system once the pumps are activated. A leak in a
189
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pressurized line will cause the line to pressurize at a slower rate
than usual. For any given length of piping between the pump and
the dispenser, the amount of time it should take for the length of
piping to become fully pressurized can be calculated and
programmed into the detection device. Should the pressure not rise
quickly enough, a leak will be indicated, and the system will be
shut down.
Line tightness testing
Operation
Analysis
A variety of line tightness tests are available. The following
descriptions are specific to pressurized lines. The tests may be
performed on suction lines using variations on these procedures.
Tightness tests on suction lines are typically performed at about 7
psi positive pressure (not vacuum).
No single pressure value is recommended for a line tightness test.
The method must be capable of detecting a leak of 0.1 gal/h at 1.5
times the line operating pressure. However, the actual line test
may be done at any pressure as long as the detectable leak rate is
mathematically equivalent to the federal performance standard.
This means that the evaluation to demonstrate that a line tightness
test meets the performance standards can be conducted at any line
pressure and then converted to a value equivalent to 1.5 times the
typical line operating pressure.
In a direct volumetric line tightness test, a hand pump or the
dispenser and the submerged pump is used to pressurize the piping
leading back to the pump in the tank. Under one approach to line
tightness testing, if pressure decreases in the piping system,
product is added to the piping system to bring the pressure back to
the level at the beginning of the test. The amount of product added
over time is recorded to estimate the leak rate. Another approach
to volumetric line testing observes the volume of product lost over
time in a tube above ground that is connected to the pressurized
piping, and no attempt is made to maintain constant line pressure.
An alternative approach is to pressurize the line pressure using a
pressure gauge on the hand pump or temporarily installed on the
dispenser. Some method of converting pressure change over time
to a leak rate is necessary. The conversion method will be specific
to the type of test and will be supplied by the manufacturer.
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Operation
Analysis
In an indirect line tightness test, the piping is tested as a component
of a full system test. First, a tightness test is performed of the
entire UST system, as described in Chapter 4. An overfill test
method must be used in order to include the piping. If no leak is
indicated by the test results, then both the tank and the lines are
assumed to be nonleaking. If the total system test indicates a leak,
the tank is isolated from the piping and tested by itself using the
same test procedure. If the results of the tank tightness test
indicate that the tank is tight, then the leak is presumably from the
piping. If the tank is found to be leaking, then the condition of the
piping is unknown, and the lines must be tested directly. The
indirect approach is not a practical approach to conducting a line
tightness test if the line is the only part of the UST system of
concern at the time of the test.
A relatively new type of line tightness test is the helium gas test, in
which helium gas is injected into an empty product line. While the
line is pressurized, a tester holding a portable helium detector
walks over the piping route to detect the presence of helium rising
from the ground. This method not only indicates a possible leak, it
helps to locate where along a run of piping the leak is occurring.
Interstitial monitoring within secondary containment
Another method of detecting leaks from underground piping is to
place a monitor in the interstitial space between the piping and an
outer barrier. The containment is a barrier between the piping
containing the product and the environment. If a hole forms in the
piping and the product leaks into the interstitial space, the barrier
will direct the release towards the monitor, which detects the
release. The types of barriers and interstitial monitors for piping
are essentially the same as those for tanks, and the details are given
in Chapter 8. A summary of the information applicable to
underground piping is provided below.
When secondary containment is used for piping, care must be
taken that the containment extends the full length of piping, from
the connection to the tank directly to the dispenser. Containing
either end of a piping run is the most difficult portion and is often
neglected as a result.
One common method of secondarily containing underground
piping is the use of trench liners (see Figure 39). The trench that is
dug to install the piping can be lined with a flexible membrane
191
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liner that is impervious to the stored product. The backfill and
piping then are placed within the lined trench. The liners often are
thermoplastic or polymeric sheets, typically 50 mm thick. Instead
of flexible liners, rigid U-shaped pieces of plastic may be used to
line the bottom and sides of the piping trench. When liners are
used, the trench may be sloped away from the tank excavation to
help differentiate between tank leaks and pipe leaks.
The interstitial space to be monitored is the backfill between the
trench liner and the piping. The simplest monitor consists of a
sump at the lowest point of the piping system to "collect" the liquid
from any leaks. This sump can be monitored by visual inspection,
a dipstick, hydrocarbon sensors such as those used in ground-water
monitoring, or vapor monitors in the airspace of the sump. A
single monitor at the sump may indicate that a leak has occurred
but does not help to locate the leak along a run of piping.
Interstitial monitors placed at intervals along the run of piping can
help to identify the location of the leak so that less piping must be
dug up.
When vapor monitoring is used, a typical well may be used that
extends to the bottom of the trench; such a well will be shorter than
that used for tank monitoring. Another approach is to use a
horizontal slotted tube at or below the level of the piping rather
than the conventional vertical well; these horizontal wells may be
up to 10 feet long.
Another form of secondary containment for piping is double-
walled piping. The primary, or inner, piping that carries the
product is contained within an outer pipe of larger diameter. The
inner and outer piping both may be made of fiberglass-reinforced
plastic (FRP) or the inner pipe may be made of galvanized steel
and the outer pipe of FRP. In a few specialized cases, both pipes
may be made of steel. Care must be taken that the product in the
lines is compatible with the FRP. For service stations, the
diameters of the inner and outer piping often are 2 and 3 inches,
respectively.
A monitor is placed in the space between the inner and outer pipe.
Double-walled piping is often sloped to a containment structure or
observation well that can be monitored for the presence of
hydrocarbon liquids or vapors. Small sumps may be placed
periodically along the run of piping and monitored for liquids or
vapors.
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Operation"]
External monitoring
Both ground-water and vapor monitoring can be used to monitor
Analysis I f°r releases from underground piping. The descriptions provided
1 1 in Chapters 6 and 7 for tanks are applicable to piping. For both
methods, wells a couple of inches in diameter are installed at
intervals along the run of piping. For ground-water monitoring,
the wells extend below the ground water, and a sensor detects the
presence of free product floating on the water. For vapor
monitoring, any leaked product will evaporate and diffuse through
the soil, and the vapor monitor will detect its presence. It is
theoretically possible to connect piping monitoring wells to shutoff
devices, so that whenever a predetermined hydrocarbon liquid or
vapor level is detected, the delivery of product is halted. At least
one automated piping shutoff system has been designed to
incorporate a vapor monitoring system.
POTENTIAL PROBLEMS AND SOLUTIONS
This section presents a discussion of problems that have been
encountered with piping release detection methods. Some of the
problems and solutions are similar to those for tank release detection
methods. To avoid repetition, this chapter includes only problems
unique to the piping release detection methods; for additional potential
problems, see the discussions in Chapters 4 and 6 through 8. The
problems for each release detection method are presented generally in
the order of importance. Table 14 presents a summary of the indicators
and solutions to common problems as well as possible approaches that
implementing agencies can use to prevent or overcome the problems
with piping release detection methods. A number of agency solutions
are offered for each problem, but not all of them need be undertaken.
The most serious concerns are indicated by an asterisk. Table 14 and
the discussion below are presented in the order of the flow chart
(Figure 45 on page 188), not in order of importance. The most serious
concerns have been indicated in Table 14 by an asterisk.
Automatic Flow Restrlctors and Shutoff Devices
In addition to meeting the regulatory requirement for continuous
monitoring of large line leaks, automatic shutoff devices may also be
used to meet the regulatory requirement for less frequent monitoring for
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Table 14. Indicators and Solutions for Problems Encountered During Piping Release Detection
Problem
Indicators
Tester Solutions
Agency Oversight Options
• Flow Restrictors and Shutoffs
Need to prevent tampering. None.
*Need to identify and remove
vapor in the line.
Assure that necessary line
conditions for a valid test are
reached.
*Need for properly functioning
check valves.
• Line Tightness Testing
Assure sufficient waiting
time between filling line
and beginning test.
Assure that sufficient test data
are collected.
*Need to recognize and remove
vapor pockets.
Slow product delivery.
None.
False alarms.
Erratic readings. Readings that
increase or decrease, then level off.
None.
Excess product from line after
test.
Lock restrictors or shut off
control panel. Educate staff.
Increase pressure to absorb
vapor. Flush line with product
at high rate.
Select device based on
knowledge of device design
and UST operations.
Replace, clean, or repair
valves and retest.
Wait at least 3 hours between
filling lines and starting data
collection.
Collect data for at least 1 hour.
Empty line, valve off blind ends,
and retest. Increase pressure to
absorb vapor. Flush line with
product at high rate.
Inspect and perform test
to see if device functions.
None.
Review plans prior to
installation.
Observe test. Check that
valves are actually replaced
or repaired.
Review test reports for
reasonable waiting times
and data trends. Observe
test.
Observe test. Review test
results.
Observe test. Review test
results.
-------
Number of tests to be
conducted must be fixed.
High percentage of lines declared
light."
Assure use of proper protocol
and correct threshold.
High percentage of tanks declared
light.'
*Need for properly functioning
check valve.
False alarms.
Interstitial Monitoring Within Secondary Containment
*Assure correct installation of
containment.
Product observed outside
containment.
• External Monitoring (Ground-Water and Vapor)
*Assure that monitoring wells None.
are properly placed.
Develop clear protocol with
specific number of tests and
follow it.
Criteria for determining "tight"
or "leaking" must be clear.
Threshold value for declaring
leak should be smaller than
minimum detectable leak rate
by a factor of at least 2.
Replace, clean, or repair valves
and retest.
Follow manufacturer's
specifications, with special
attention to seams and joints.
Pressure-test double-walled pipes.
No more than 40 feet between
wells. Conduct site assessment
before installation.
Approve multiple-testing
strategies. Observe tests.
Receive test results, and
track pass/fail ratios for
companies and methods.
Review test results and
calculations to see if
they agree with protocol.
Keep track of pass/fail
ratios for companies and
methods.
Observe test. Check that
valves are actually replaced
or repaired.
Observe installation.
Certify/license
installers. Review pressure
test results.
Review site plans and
monitoring plan before
installation.
* Indicates the most significant problems.
VO
U)
-------
smaller leaks. Shutoffs may be used in place of line tightness testing,
ground-water monitoring, or vapor monitoring if they are sensitive
enough to meet the performance standard for line tightness testing.
I Installation | Need to prevent tampering
Analysis
Even when there is no leak in the line, flow restrictors can cause
slight delays in the delivery of product after the dispenser is turned
on. When a leak occurs in a line with a flow restrictor, the delay of
delivery is even longer, causing the rate of delivery to be much
lower than usual. When a shutoff device detects a leak, product
flow stops altogether. Such conditions can cause customers to
complain about poor service. At production facilities, the delayed
delivery of product from an UST may slow down production. In
reaction to the slow-down, production facility sometimes
deactivate the line leak detectors by removing them completely,
thus defeating the purpose of their operation.
A seal that can be installed on flow restrictors to indicate that
tampering has occurred has recently become available. Shutoff
devices usually are automated, and the control box can be
programmed and locked so that only management personnel with
keys can override the shutoff function. Training on-site staff in the
importance of piping leak detection and the proper response to
warning signals also helps to overcome the problem of tampering.
One approach that owner/operator representatives or implementing
agency personnel can use to combat the problem of tampering is to
perform periodic checks on the line to see how the restrictor or
shutoff performs. Proper operation can be checked by simulating a
leak in the system and monitoring the line pressure. A defective
device will not restrict or shut off the flow of product. Some
shutoff devices have an automatic test mode that will perform this
type of test.
| Installation] Need to identify and remove vapor in the line
Analysis
At high altitude or high temperature, liquids volatilize more
quickly. As a result, product vapors may form in the piping. These
vapors can significantly increase the amount of time required for
the product to reach operating pressure because additional product
and time are needed to compress the vapor pocket. Such delays
may be interpreted falsely by the device as leaks, and product flow
will be restricted or shut off. If additional time is spent
196
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pressurizing the line, the vapors may eventually be absorbed back
into the liquid.
| installation | Line conditions need to be right for a valid test
Analysis
As described above, most flow restrictors and shutoff devices have
minimum requirements for line conditions that must be met to
conduct a valid test, such as a period of at least 5 minutes in which
the line is not being used or a decrease in line pressure below a
threshold level (e.g., 2 psi). At a busy service station, there may
not be enough time between dispensings to conduct a test with
some types of restrictors or shutoff devices. A fully pressurized
line without a leak or without large thermal changes may not drop
as low as 2 psi for days, even with no withdrawals occurring in the
line.
The level of use of an UST system and the design of the restrictors
or shutoffs should be considered together when selecting the line
leak detection device. If a device requires a minimum amount of
time to conduct a test, the typical time between dispensings at the
UST should be determined before selecting that device. For
systems that require the line pressure to fall below 2 psi to conduct
a test, if there is a leak in the line, the pressure will decrease after a
dispensing so that a test can be performed. High pressure can be
maintained in a line for long periods of time only if there is no
leak.
I installation] Need for properly functioning check valves
As described above, there are usually check valves in a line that
prevent product from draining backwards towards the tank any
further than the valve. During a line tightness test, the pressure is
being maintained between the check valve and the hand pump or
dispenser. If the check valve does not close tightly, it may allow
product to seep through the valve and drain to the tank. For line
leak detection methods such as flow restrictors and shutoffs that
rely on pressurizing the line, this loss of product (and
corresponding loss of line pressure) due to a bad check valve
would falsely indicate a leak.
If the flow restrictor or shutoff indicates a leak but there are no
other indications that the line is leaking, it is possible to service the
check valve(s) in the line and retest the line. If a check valve is
faulty, it must be replaced. Sometimes, dirt or foreign particles
197
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become trapped in the check valve, preventing it from sealing
completely. In this case, cleaning the valve should be sufficient. If
the line tests tight after the replacement or repair, then no
additional work is needed.
Line Tightness Testing
I Analysis
This section discusses the problems of tightness tests that are performed
only on the line. When lines are checked for leaks as part of tank
tightness testing, the problems are the same as those discussed in
Chapter 4 on tank tightness testing.
| Installation] Need to allow sufficient time between filling and testing
The pressure in a pipeline is a function of the temperature,
coefficient of thermal expansion of the product, and the
compressibility of the product. For line tightness testing,
temperature is the single most important variable. As the
temperature of the product increases or decreases, the volume of
the product also increases or decreases, thus changing the line
pressure. Shrinking of product as it cools may imitate a leak
because the line pressure is decreasing, and swelling of product as
it warms may mask aleak because of the increased line pressure.
When product moves from the tank into a line, there may be a
temperature gradient between the product and the surrounding
backfill. If the product remains in the line, as it does during most
line tests, its temperature changes towards that of the backfill. The
extent and rate of this change vary with the material of piping
construction, the backfill material, and the product in the line. For
a 2-inch steel pipe in gravel backfill, if the temperature differential
between the product and the backfill is 5 to 15 degrees Centigrade,
the temperature in the product may take 3 hours to stabilize.
Temperature changes of 0.1 to 0.5 degrees Centigrade can cause a
5 to 10 psi change in pressure. The maximum changes in product
temperature occur immediately after product has been delivered to
the tank, when the differential between product and backfill
temperatures is the greatest.
The solution to this problem with line tightness testing is to wait at
least 3 hours after filling the line with product before beginning
data collection for the test. This time period allows the
temperature of the product to stabilize. An increasing or
decreasing trend in the data that eventually levels off indicates that
198
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the temperature continued to change during part of the test but it
eventually stabilized and that there is no leak. Determination of
leak status should use only those data obtained after the readings
have leveled off.
Installation Ensure that sufficient test data are collected
As discussed in Chapter 4 on tank tightness testing, insufficient test
data may not allow important trends in the data to be identified
and, thus, problems or leaks may be missed. In addition, if the test
does not last long enough, small leaks may be missed.
As a rule, obtaining more data increases the probability of correctly
identifying the presence of a leak. For line tightness tests, data
should be collected for at least one hour.
Analysis
Installation] Need to recognize and remove vapor pockets
Another factor that may affect the results of a line tightness test is
the presence of vapor pockets. When a line is completely filled
with product, vapor may become trapped in some areas, such as
deadend piping, bends in piping, or vertical stubs. This vapor
expands and contracts in response to temperature and pressure
changes more quickly and to a greater degree than the product in
the lines. Changes in vapor pocket size affect the line pressure,
thus masking or imitating a leak. Any test conducted with a vapor
pocket in the line is invalid. For further discussion of vapor
pockets, see Chapter 4.
One approach to determining if a vapor pocket is present is to
measure the amount of product that drains from the line when the
pressure is released after the test. If no vapor pocket is present,
only a small amount of product will drain out as the walls of the
piping relax under the reduced pressure. If a vapor pocket was
present, a larger amount of product (at least 0.05 gal) will drain out
because the vapor pocket was compressed significantly under
pressure and expands when the pressure is released, pushing more
product from the line.
There are several approaches to removing vapor in a line that can
be tried. Sometimes increasing the line pressure will recondense
the evaporated product. Sometimes flushing the lines with high
velocity product using the dispenser will remove vapors.
Sometimes pulling a vacuum on the line will remove the vapors,
199
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although if there are any slight cracks in the line or if the pump is
exposed, air will be drawn back into the line. These approaches
will not work if the air is trapped in piping stubs. In that case, it
may be possible to uncover those portions of the piping and install
valves that shut off the blind end from the main run of piping.
Whatever method is used to try and remove the vapor, the line
should be retested after the 3-hour wait between filling and data
collection. It is not always possible to remove trapped vapor from
piping and conduct a valid tightness test.
I Installation [ Need for properly functioning check valve
!... i_,
The problems with bad check valves for line tightness testing are
the same as for flow restrictors and shutoff devices. The preceding
section contains a discussion of these problems (page 197).
Analysis [
| Installation") Number of tests must be fixed in the protocol
JL
Operation |
When the results of a line tightness test indicate that the line is
leaking but the leak rate is only slightly above the threshold value
for declaring a leak, some testers repeat tests on the line until the
results of one test indicate that the line is tight. As discussed in
Chapter 4 on tank tightness testing, this approach is invalid and
reduces the probability of detecting a leak. A multiple-testing
strategy is a valid approach to line tightness testing, but all of the
data from all of the tests must be used in the analysis unless the
protocol specifically excludes them (e.g., vapor pockets, bad check
valves). The number of tests to be performed and how the data are
analyzed must be explicitly defined in the line testing protocol, and
no deviations should be allowed from the protocol.
| Installation] Proper protocol and correct threshold must be used
I Operation |
As discussed in Chapter 4 on tank tightness testing, lack of a
well-defined data analysis protocol and clear criterion for declaring
a leak allows testers to make subjective decisions, leading to
unclear or false determinations of the status of the line. A reliable
data-analysis protocol will have clear and detailed instructions on
how to convert raw data on pressure or volume changes to an
estimated leak rate. The protocol should specify how to determine
which data to use; when, If ever, it is permissible to discard data;
200
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what conversion factors to use; how to determine the conversion
factors; and what mathematical computations are needed.
To determine if the piping is leaking, the estimated volumetric leak
rate must be compared to a threshold value. This threshold value
must be predetermined as part of the test design and its use must be
well defined in the test protocol. Discretion on the part of the
tester in determining the leak status should not be allowed.
As discussed further in Chapter 4, in order for a test method to
perform well in detecting small leaks, the threshold value must be
smaller by a factor of 2 or more than the smallest leak to be
detected. The federal regulation requires a line tightness test
method to have a minimum detectable leak rate of 0.1 gal/h. For a
test method to meet this requirement, its threshold must be less
than 0.1 gal/h. The most commonly used threshold for line
tightness testing is 0.025 gal/h.
Interstitial Monitoring Within Secondary Containment
Operation
Analysis
The problems and solutions specific to interstitial monitoring for piping
are discussed below. Additional information on interstitial monitoring
for tanks that may be applicable to piping is included in Chapter 8.
Ensure correct installation of secondary containment
As discussed on pages 191 and 192, there are two types of
secondary containment: trench liners and double-walled piping.
Incorrect installation is a problem for both types.
Incorrect installation of the liner is the most important potential
problem with trench liners. Piping trenches are very narrow and
long, and piping usually joins to a building or dispenser. To cover
a very narrow trench and difficult areas such as near buildings or
dispensers usually requires piecing together smaller pieces of liner.
Seams are the most vulnerable to leakage, and trench liners can
have many seams. A trained and experienced professional is
necessary to ensure that the liner is designed for as few seams as
possible for the site and that the liner is installed correctly.
201
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Incorrect installation also is a problem with double-walled piping.
Joining segments of piping so that the joint is tight is more difficult
for double-walled than for single-walled piping. As part of the
installation procedure, the inner pipe should be tested before the
outer pipe is installed and tested. The inner pipe should be tested
at 50 psi or 1.5 times the working pressure of the system. The
outer piping should be tested at 5 psi. In addition, double-walled
piping sometimes left "pen" (single-walled) where it joins the tank
or dispenser. Sumps may need to be placed at these points if
complete containment cannot be installed. Trained and
experienced personnel should be used to install double-walled
piping.
External Monitoring
Operation
Analysis
The problems and solutions specific to using ground-water or vapor
monitoring as release detection for piping are discussed below. The
problems with external monitoring of piping that are the same for
external monitoring of tanks are discussed in Chapters 6 and 7.
Assure that monitoring wells are properly placed
The area that a monitoring well network must cover for piping is
very large because piping runs can be very long, and leaks can
occur in any portion of the line. Detection is, in part, a function of
the distance between the monitoring wells and a leak. Because of
the large area covered by piping systems, monitoring well
networks are sometimes designed with too few wells, to reduce the
cost. If monitoring wells are placed too far from each other or
from the pipe, the amount of time before a leak is detected may
increase or, in extreme cases, a leak may go undetected.
Table 15 presents a summary of the requirements for various states
on the placement of vapor and ground-water monitoring wells with
regard to piping. Although the requirements are diverse, in
general, wells must be separated by no more than 20 to 35 feet.
These requirements are reasonable based on-EPArresearch
indicating that a design that includes at least one well every 40 feet
should be sufficient for gasoline tanks in a clean, dry backfill. If
the backfill is not highly permeable (e.g., it is native fill material)
or the migration of liquid product or vapors is impeded by other
factors, the number of sensors should be increased by a factor of
two.
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Table 15
Typical State Network Design Requirements For
Vapor and Ground-Water Monitoring of Piping
Maine
Vapor
According to manufacturer's
specifications
At a minimum: 1 well
at each piping joint
No piping run > 15 feet from well
Santa Clara County, California
Vapor, aspirated systems
South Carolina
Ground water
Vernon, California
Vapor
General: 1 well every 35 feet
At station: 1 well for each set of piping
1 well at each pump island
Minimum of 2 wells every 30 feet
Design network for 15-foot diameter of
influence
Source: U.S. EPA
The Federal regulation requkes that ground-water monitoring wells
for tanks be placed as close as possible to the tank and that vapor
monitoring wells be installed in the backfill. These placement
criteria should be followed for piping leak detection as well.
For additional information on monitoring well design and
installation, see Chapters 6 and 7.
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ENSURING EFFECTIVE RELEASE DETECTION FOR PIPING
Site Inspections
Site inspections could provide useful oversight for several of the piping
release detection methods. Proper installation of secondary containment
is crucial to successful interstitial monitoring, so observation during
installation could be a powerful tool. Development and use of a
checklist of important elements of proper installation during the
inspection would increase the usefulness of the site visit. The City of
San Jose, California, has developed several secondary containment
inspection techniques. In one technique, a lined piping trench is filled
with water, the water level immediately after filling is marked, and the
water level 24 hours later is measured; 1/4 inch of water loss due to
evaporation is assumed. If returning the next day is infeasible, then
paper can be placed under the seams of the trench liner before it is filled
with water. A leak will mark the paper within minutes; and the effects
of evaporation are avoided. For double-walled pipes, the City of San
Jose performs either a hydrostatic or pneumatic pressure test. If a
pneumatic test is performed, soapy water is applied to all pipe
connection during the test. A leak will be indicated by bubbles.
If selected as the release detection method, line tightness tests are
required annually or every 3 years, so the number of tests that would be
conducted within a jurisdiction each year is relatively small.
Implementing agency personnel could be onsite for some of these tests
to ensure that the proper waiting time and test duration are observed and
that no vapor pockets are present.
For sites where external monitoring is planned, a visit to the site before
installation to ensure that conditions are, in fact, appropriate is an
option. A checklist of pertinent features, collection of soil samples, and
measurement of the depth to ground water might be considered while on
site.
Because tampering is the primary problem with automatic flow
restrictors, random inspections by agency personnel to check for
tampering would be effective. At a minimum, the seal on the restrictor
should be checked. It is also possible to simulate a leak in the part of
the line under the dispenser and observe the response of the restrictor.
The records of monitoring results and repair and maintenance could be
checked whenever a site visit is made.
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Data Review
The implementing agency could requke that all line tightness test results
be submitted to the agency for review and recalculation. See Chapter 4
for discussion of the types of data review and compilation that could be
useful. In particular, the length of the waiting and test times should be
checked.
The agency could also require submittal of the results of external and
interstitial monitoring. Because these methods are performed each
month, the volume of data may be overwhehning. In this case, only the
results of tests from every other month or every six months could be
requked.
Guidance and Training
Education of owner/operator staff on the importance of piping leak
detection might help prevent some problems such as tampering.
Education in how the methods work might help owner/operators
provide meaningful oversight during installation of equipment or
operation of a method. Review of manufacturers' in-house training
programs is another possible oversight mechanism.
Approval and Certification
For external and interstitial monitoring the design of the release
detection system is particularly important to its success. For these
methods, it may be appropriate to requke submittal of the plans for
review and approval prior to any installation. The plans should include
a site map and pertinent hydrogeological data, manufacturer's
information, and the installer's recommendations.
Because proper installation is so important to interstitial monitoring
with secondary containment, certification or licensing of installers could
be requked. Either the implementing agency could run the program or
they could requke that installers have a minimum amount of training by
third parties, such as manufacturers. Similarly, line tightness testers
could be licensed or certified to conduct tests to ensure that they
understand the important aspects of the testing and analysis protocol.
Line tightness testing is often performed in conjunction with tank
205
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tightness testing. If there is an existing licensing program for tank
testers, it would be relatively easy to include line testing.
REFERENCES
1. U.S. Environmental Protection Agency. July 1988. Common
Human Errors in Release Detection Usage. Prepared for U.S.
EPA by Camp Dresser & McKee, Inc.
2. Maresca, J.W., J. L. Chang, Jr., and P. J. Gleckler. January 1988.
A Leak Detection Performance Evaluation of Automatic Tank
Gauging Systems and Product Line Leak Detectors at Retail
Stations. Vista Research, Inc.
3. Maresca, J. W., and J. S. Farlow. December 17,1987. Pipeline
Leak Detection Modeling and Analysis Results. Presentation to the
U.S. EPA's Office of Underground Storage Tanks.
4. National Fke Protection Association. 1987. NFPA329:
Under ground Leakage of Flammable and Combustible Liquids.
Batterymarch Park, Quincy, Massachusetts.
5. Schwendeman, T.G. and H. K. Wilcox. 1987. Underground
Storage Systems - Leak Detection and Monitoring. Lewis
Publishers, Chelsea, Michigan.
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SUBJECT INDEX & APPENDICES
Subject Index
Appendix A—List of Figures
Appendix B—List of Tables
207
212
215
-------
-------
SUBJECT INDEX
Air pockets (see vapor pockets)
Alarm levels
automatic tank gauging 82
vapor monitoring 106, 110,120
Allowable methods 5,7
"American" piping systems 185-186
Annular seal (see well seals)
Approval of methods 11-12, 30-31,44,
74-75, 93-94, 124, 205-206
Aspirated vapor monitoring systems 105,
113
B
Backfill
ground-water monitoring 134-137
interstitial monitoring 171-172
tank tightness testing 60
vapor monitoring 103-105, 113
Background contamination
ground-water monitoring 142
interstitial monitoring 175
vapor monitoring 105-106,110
Bailer 143
Barrier (see liners and double walls)
Bentonite seal (see well seals)
Blended fuels (ATG) 91-92
Borehole (see installation, ground-water
monitoring)
Bucket (see bailer)
Bungs (see fittings, loose)
Calculations, performed correctly
automatic tank gauging 92
tank tightness test 73
Calibration chart (see tank chart)
Calibration
ground-water sensor 157
vapor sensor 119-120
Cement seal (see well seals)
Certification of methods or personnel (see
approval)
Check valves 185, 197-198
Chemical-sensitive pastes 143
Coefficient of thermal expansion
automatic tank gauging 86
line tightness test 198
tightness test 50,67-68,72
Condensation (see evaporation)
Conduits (ATG) 78, 83, 86
Constant product level 69-70
Creepage 17, 37
D
Data review 10-11,30,44,74,93,123-124,
205
Detection criteria (see threshold)
Delivery of product
automatic tank gauging 86-87
tank tightness testing 58, 60
Diffusion 96, 111
Dip stick
piping 192
tanks 169,179
Dissolved product 128
Double walls
piping 192,202
tanks 162-163
Drop tubes 57
207
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E
Electrical conductivity interstitial monitors
166,175-176,178
"European" piping systems 185-186
Evaporation
automatic tank gauging 88
inventory 20
tank tightness testing 70
Excavation liner (see liners)
F
Filter pack
ground-water monitoring 153-154
vapor monitoring 118
Fittings, loose 52,56
Fluid-sensing interstitial monitors 166
Free (floating) product 128,179
Flow rate (see leak rate)
Flow resttictor 183,187-189
Flow shutoff 183,189-190
Fractured rock 144-145
G
Gaskets (see fittings, loose)
Gauge stick (pole) 14,17,
Gauging procedure
inventory control 16-17
manual tank gauging 34,37
Gravel pack (see filter pack)
Ground-water depth
ground-water monitoring 130-137,
139-141
tightness testing 64-66
vapor monitoring 107-108
interstitial monitoring 174-175
Ground-water flow gradient 139-141,
147-150
Ground water influences
automatic tank gauging 90
tank tightness testing 64-66
Ground-water monitoring sensors 142-143
Guidance 11,30,44,74,93,124,205
H
Height-to-volume conversion factor
tank tightness testing 50,67,72
automatic tank gauging 91
Helium gas 191
Hydraulic conductivity 134-137
Hydrostatic interstitial monitors 166-167,
179
Installation
automatic tank gauging 78-81, 83, 86
ground-water monitoring 150-156
interstitial monitoring 161,176-178,180
secondary containment 161,171-174,
201-202
vapor monitoring 111-114
Interpretation of results
automatic tank gauging 89
ground-water monitoring 158-159
inventory control 29-30
line tightness tests 200-201
manual tank gauging 42-43
tank tightness testing 50, 71-73
vapor monitoring 110, 121-123
Interstitial monitors, types
tanks 166-169
piping 191-192
Interstitial space 161-162
Inventory mode (ATG) 78, 82
Inventory reconciliation (see reconciliation)
Joints (see fittings, loose)
Junction boxes 176
K
K (see hydraulic conductivity)
Leak detect mode (ATG) 78, 81
Leak rate 50, 71-73, 89, 200-201
208
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Line tightness testing 190-191,198-201
Liners
piping (see trench liners)
tanks 161,165
M
Manifolded tanks tightness testing 56
Manual tank gauging
monthly standard 42
weekly standard 42
Manways (see fittings, loose)
Moisture
interstitial monitoring 171,174
vapor monitoring 107-108
Methane 108-109
Multiple-testing strategy 70-71, 88-89
N
Network design for monitoring wells
ground water, tanks 144-150
piping 202-203
vapor, tanks 105,109-114,123
Nonvolumetric tank tightness tests 48
o
Operation and maintenance
ground-water monitoring 157
interstitial monitoring 161,180-181
vapor monitoring 119-121
Optical interstitial monitors 166,179
Outgassing 178
Overfill tightness tests 48-50
Passive vapor monitoring systems 105,
113-114
Permeability of soil (see backfill)
Piping, abandoned 56-57
Porosity (see backfill)
Poor access 57
Pressure-sensing interstitial monitors 166,
178-179
Pressurized piping 185
Product-finding paste
interstitial monitoring 169
inventory control 17
manual tank gauging 37
Product-soluble devices
ground-water monitoring 143
interstitial monitoring 176-177
Protocol
automatic tank gauging 88-89
line tightness test 200
tank tightness testing 50, 70-73
Pump meter 25
Purging (see well development)
Purpose of handbook 1-2
R
Reconciliation
inventory control 190-193
manual tank gauging 42-43
Residual vapors (see background
contamination)
Restrictions on methods 7
Sampling frequency
automatic tank gauging 81-82
tank tightness testing 68
Seals (see well seals)
Seams of liners
piping 201
tanks 174
Secondary containment (see also liners and
double walls)
piping 191-192
tanks 162-165
Site assessment
ground-water monitoring 127-128, 130,
interstitial monitoring 174-175
vapor monitoring 95
Site inspection 7,10, 30,43, 73, 93,123,
174-175, 204
Slot size (see well screen)
Soil moisture (see moisture)
209
-------
Soil porosity (see backfill)
Solubility of product 141
Specific gravity of product 141-142
Specificity of sensor
ground-water monitoring 142-143
interstitial monitoring 175-176
vapor monitoring 109
Spill and overfill protection for vapor
monitoring 106
Sticking (see gauging)
Stock control (see reconciliation)
Striker plate 17,37
Subsurface conduits 147,149-150
Suction piping 185,187
Surface grout (see well seals)
Surface seal (see well seals)
T
Tampering
ground-water wells 156
piping 196
vapor wells 119
Tank chart
inventory control 13-14, 21-24
manual tank gauging 39-42
Tank deformation
automatic tank gauging 86-87
tank tightness testing 60-61
Tank end deflection (see tank deformation)
Tank jacket 162,164
Temperature
automatic flow restrictors 196
automatic tank gauging 86-88
ground-water monitoring 157
interstitial monitoring 179
inventory control 20
line tightness testing 198-199
manual tank gauging 38
tank tightness testing 58-62, 68-69
vapor monitoring 106-107
Test length and frequency
automatic tank gauging 81,82
line leak detectors 187-188,197
line tightness testing 199
manual tank gauging 33,38
tank tightness testing 68
Threshold value
automatic tank gauging 89
tank tightness testing 50,72
line tightness testing 200-201
Tilted tanks
inventory control 21
tank tightness testing 63
Tracer compounds 103, 106, 109
Training (see guidance)
Trench liners 191-192, 201
U
Underfill tightness testing 48-51, 81
Use of handbook 3-12
V
Vapor pockets
flow restrictors/shutoffs 196-197
line tightness testing 199-200
tank tightness testing 62-64
Vaults 162
Volatility of product
interstitial monitoring 176
vapor monitoring 96,102-103,106-108
Volumetric tank tightness testing 49-51
w
Waiting times
automatic tank gauging 86-87
line tightness testing 198-201
tank tightness testing 58-62
Water in tank
automatic tank gauging 82,90
inventory control 17, 20
manual tank gauging 38
210
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Water-finding paste 17, 20,
Water sensor (ATG) 82,90
Water table (see ground-water depth)
Wells, ground-water
casing 150-152
depth 153
development 154, 156
diameter 152
documentation 156
number (see network design)
placement (see network design)
screen 139-140,150-152
seals 134-136
security 156
Wells, vapor
casing 118-119
depth 111-114
diameter 96,105,114
documentation 119
number (see network design)
placement (see network design)
screen 118
seals 118-119
security 119
Wiring
automatic tank gauging 78, 83, 86
interstitial monitoring 176
211
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LIST OF FIGURES
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16
Figure 17
Figure 18
Figure 19
Development of a state or local leak detection program 4
General procedure for inventory control 15
Sample calibration chart 22-23
Sample inventory control daily reconciliation form 27
Sample inventory control monthly reconciliation form 28
General procedure for manual tank gauging 35
Sample calibration chart 40-41
General procedure for conducting a volumetric tank test 49
Comparison of partially filled and overfilled tanks 51
How temperature changes can be mistaken for a leak 59
How structural deformation of the tank
can be mistaken for a leak 61
Location of vapor pockets in an overfilled tank 63
Effect of ground water on the rate of flow
through a hole in an underground tank 65
Schematic of automatic tank gauging system 79
General procedures for ATG systems 80
Underground storage tank system with vapor monitoring wells 97
General procedures for vapor monitoring 98
The effect of soil conditions on vapor concentrations
at a deep vapor well 104
The effect of temperature on gasoline volatilization rates 107
212
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Figure 20 The effect of backfill moisture levels
on gasoline volatilization rates 108
Figure 21 The effect of vapor sensor placement on leak detection time 113
Figure 22 Typical underground storage tank site 115
Figure 23 Typical vapor monitoring well cross section 116
Figure 24 Modified vapor monitoring well cross section 117
Figure 25 Interpretation of vapor monitoring results 122
Figure 26 Typical ground-water monitoring system 129
Figure 27 General procedure for ground-water monitoring 131
Figure 28 Well seal will prevent interception of free product
when water table is low 135
Figure 29 Well without proper surface seal may be contaminated by
surface runoff 136
Figure 30 Range of hydraulic conductivities (K)
for the major soil classes 138
Figure 31 The well screen is placed to extend over the entire range
of water table fluctuation 140
Figure 32 Free product will preferentially flow through
fractures and cavities 145
Figure 33 Off-site sources of contamination should be considered
when designing the monitoring well network 148
Figure 34 Subsurface utility conduits will act as preferential
pathways for free product migration 149
Figure 35 Components of a typical ground-water monitoring
well installed in a borehole 151
Figure 36 Components of a monitoring well using backfill material
as the filter pack 155
213
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Figure 37 Two double-walled tank configurations
Figure 38 Jacketed tank
Figure 39 Tank with excavation liner
Figure 40 Hydrostatic monitoring system
Figure 41 General procedure for secondary containment
with interstitial monitoring
Figure 42 Cross section of electrical conductivity sensor
using degradable coating
Figure 43 Cross section of electrical conductivity sensor
using polymer jacket that swells
Figure 44 Typical retail gasoline station
Figure 45 General procedure for piping release detection
163
164
165
168
170
177
177
186
188
214
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LIST OF TABLES
Table 1
Table 2
Table 3
Table 4
Table 5
Table 6
Table 7
Table 8
Table 9
Table 10
Table 11
Table 12
Table 13
Table 14
Table 15
Effect of Site Conditions on Success of Release
Detection Methods for Tanks and Piping 8-9
Indicators and Solutions for Problems Encountered
During Inventory Control 18-19
Indicators and Solutions for Problems Encountered
During Manual Tank Gauging 36
Monthly and Weekly Manual Tank Gauging Standards 42
Indicators and Solutions for Problems Encountered
During Tank Tightness Testing 53-55
Indicators and Solutions for Problems Encountered
with Automatic Tank Gauging Systems 84-85
Indicators and Solutions for Problems Encountered
During Vapor Monitoring 99-101
Typical Vapor Pressures of Petroleum Products 102
Typical State Network Design Requirements
for Vapor Monitoring 112
Indicators and Solutions for Problems Encountered
with Ground-Water Monitoring 132-133
Typical Network Design Requirements
for Ground-Water Monitoring 146
Applicability of Leak Detection Methods
to Secondary Containment Systems 167
Indicators and Solutions for Problems with
Secondary Containment with Interstitial Monitoring 173
Indicators and Solutions for Problems Encountered with
Piping Release Detection 194-195
Typical State Network Design Requirements
for Vapor and Ground-Water Monitoring of Piping 203
215
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July 1992
, * ffcwtor: 'A Swamry o/Leak&mGftm Mett&ds/dr
Mf^^^tme^gnmd Storage Totd® $y*tem$ describes & leak detection method not Included.
In tli§ o^ginal publication,
Statistical Inventory Reconciliation
Will I be in compliance?
Statistical inventory reconciliation (SIR), when performed according to the vendor's specifica-
tions, meets Federal leak detection requirements for new and existing underground storage tanks
(USTs) and piping as follows. SIR with a 0.2 gallon per hour leak detection capability meets the
Federal requirements for monthly monitoring for the life of the tank and piping. SIR with a 0.1
gallon per hour leak detection capability meets the Federal requirements as an equivalent to tank
r-tightnessJesting..-SIR could, in some-cases, meet-the-Eederal requirements-for line tightness.
testing as well. (For additional leak detection requirements for piping, see the sections on leak
detection for piping.) You should find out if there are State or local limitations on the use of SIR
or requirements that are different from those presented below.
How does it work?
Statistical inventory reconciliation analyzes inventory, delivery, and dispensing data collected
over a period of time to determine whether or not a tank system is leaking.
• Each operating day, you measure the product level using a gauge stick or other tank level
monitor. You also keep complete records of all withdrawals from the UST and all
deliveries to the UST. After data have been collected for the period of time required by
the SIR vendor, you provide the data to the SIR vendor.
• The SIR vendor uses sophisticated computer software to conduct a statistical analysis of
the data to determine whether or not your UST may be leaking. The SIR vendor provides
you with a test report of the analysis results.
What are the regulatory requirements?
--rrs^To be;allowable as-monthly-monitoring,a-SIR-method must-be-able-to detect-a-leak at-—=--
least as small as 0.2 gallons per hour and meet the Federal regulatory requirements
regarding probabilities of detection and false alarm. Data must be submitted monthly.
• To be allowable as an equivalent to tank tightness testing, a SIR method must be able to
detect a leak at least as small as 0.1 gallons per hour and meet the Federal regulatory
requirements regarding probabilities of detection and false alarm.
• The individual SIR method must have been evaluated with a test procedure to certify that
it can detect leaks at the required level and with the appropriate probabilities of detection
and false alarm.
• If the monthly test report is inconclusive, you must take the steps necessary to find out
conclusively whether your tank is leaking.
• You must keep on file both the test reports and the documentation that the SIR method
used is certified as valid for your UST system.
d8) Printed on Recycled Paper
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Win it work at my site?
• Generally, few product or site restrictions apply to the use of SIR.
• A SIR method may be used on tanks with up to 1.5 times the volume at which that
method was evaluated. If you are considering using a SIR method for tanks greater than
18,000 gallons, discuss its applicability with the vendor.
• Water around a tank may hide a hole in the tank or distort the data to be analyzed by
temporarily preventing a leak. To detect a leak in this situation, you must check for water
at least once a month.
What other information do I need?
• Data, including product level measurements, dispensing data, and delivery data, should
all be carefully collected according to the SIR vendor's specifications. Poor data
collection may produce inconclusive results and non-compliance.
• The SIR vendor will generally provide forms for recording data, a calibrated chart
converting liquid level to volume, and detailed instructions on conducting measurements.
• Statistical inventory reconciliation should not be confused with other release detection
methods that also rely on periodic reconciliation of inventory, withdrawal, or delivery
data. Unlike manual tank gauging, automatic tank gauging systems, or inventory control,
SIR uses a sophisticated statistical analysis of data to detect releases. This analysis can
only be done by competent vendors of certified SIR methods*
• You should "shop around," ask questions, get recommendations, and select a method and
company that meet the needs of your site.
How much does it cost?
• There are no installation costs. Equipment costs are minimal, although you should ensure
that dispensing meters are in calibration and that your gauge stick or other tank level
monitor is in good condition. Annual costs for the service may depend on your data
quality, how you provide data to the vendor (paper, diskette, or modem) and the number
of tanks and sites you have.
• Here are possible costs for a typical station with three tanks:
- Used as a monthly monitoring method (with 0.2 gallon per hour leak detection
capability), SIR could cost $840 to $ 1200 yearly; or
- Used as the equivalent to tank tightness testing (with 0.1 gallon per hour leak detection
capability), SIR could cost $225 to $540, to test three tanks one time. If piping is also
tested using SIR, additional costs would be added.
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