Ir/EPA
United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
EPA/600/2-87/018
March 1987
Research and Development
Leak Prevention in
Underground
Storage Tanks:
A State-of-the-Art
Survey
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EPA/600/2-87/018
March 1987
LEAK PREVENTION IN UNDERGROUND STORAGE TANKS:
A STATE-OF-THE-ART SURVEY
By
A. C. Gangadharan
Enviresponse, Incorporated
Livingston, NJ 07039
and
T. V. Narayanan, R. Raghavan, and G. Amoruso
Foster Wheeler Development Corporation
Livingston, NJ 07039
Contract Number 68-03-3255
Project Officer
Anthony N. Tafuri
Releases Control Branch
Hazardous Waste Engineering Research Laboratory
Edison, NJ 08837
HAZARDOUS WASTE ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OH 45268
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NOTICE
The information in this document has been funded wholly or in part by
the United States Environmental Protection Agency under Contract No.
68-03-3255 to Enviresponse, Incorporated. It has been subject to the
Agency's peer review and administrative review, and it has been approved
for publication as an EPA document. Mention of trade names or commercial
products does not constitute endorsement or recommendation for use.
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FOREWORD
Today's rapidly developing and changing technologies and industrial
products and practices frequently carry with them the increased
generation of solid and hazardous wastes. These materials, if improperly
dealt with, can threaten both public health and the environment.
Abandoned waste sites and accidental releases of toxic and hazardous
substances to the environment also have important environmental and
public health implications. The Hazardous Waste Engineering Research
Laboratory assists in providing an authoritative and defensible
engineering basis for assessing and solving these problems. Its products
support the policies, programs, and regulations of the Environmental
Protection Agency, the permitting and other responsibilities of State and
local governments, and the needs of both large and small businesses in
handling their wastes responsibly and economically.
This report reviews the state of the art of underground storage tank
(UST) leak prevention technology and identifies areas for further
research and development to aid in developing regulations for USTs as
mandated by the Resource Conservation and Recovery Act as amended in 1984.
For further information, please contact the Land Pollution Control
Division of the Hazardous Waste Engineering Research Laboratory.
Thomas R. Hauser, Director
Hazardous Waste Engineering Research Laboratory
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ABSTRACT
The overall objectives of this study were to examine the structural
design and operational practices associated with underground storage tank
(UST) systems in the context of preventing leaks from such systems and
identify areas for further research and .development to advance the
technology.
UST systems are conceptually simple. Many standards, guidelines, and
recommended practices for the design and operation of these systems are
currently promulgated by several professional and industrial
organizations. However, many of these procedures have overlapping
requirements and there is no way of confirming how widely they are
understood or followed in the field. Consequently, there is a need for a
cohesive and coordinated set of rules and standards that apply to various
types of UST systems, including those that store chemicals, and for
further work to assess and improve operating practices, including spill
prevention and leak detection methods and devices.
Other recommendations derived from this study include: (1)
establishing a national data base to provide information on failure rates
and mechanisms and their correlation to design, engineering,
installation, and operation practices and corrective actions; (2)
assessing the effectiveness of cathodic protection methods, their
interaction with the environment, and the performance of retrofitting
existing USTs; (3) developing compatibil ity protocols for the selection
of appropriate materials of construction and long-term protection; and
(4) developing methods to assess the life expectancy of both new and
existing systems and to extend their useful life.
This report was submitted in partial fulfillment of Contract No.
68-03-3255 by Enviresponse, Inc. under the sponsorship of the US
Environmental Protection Agency. This report covers a period from
October 1985 to September 1986 and work was completed as of December 3,
1986.
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TABLE OF CONTENTS
Foreword iif
Abstract iv
Figures vii
Tables viii
Abbreviations ix
Acknowledgements x
1. INTRODUCTION 1
Background 1
Factors affecting leak prevention ...... 2
2. CONCLUSIONS 8
3. RECOMMENDATIONS 10
4. DESCRIPTION OF UNDERGROUND STORAGE TANK SYSTEMS . 11
Tanks 11
Piping . 14
Accessories . . 15
Secondary containment 17
Discussion 20
5. DESIGN AND ENGINEERING PRACTICES 24
Properties of products 24
Mechanical forces .24
Corrosion 26
Materials of construction .......... 26
Codes and standards 27
State and local regulations . . . 35
Discussion 36
6. INSTALLATION TECHNIQUES ...... 38
Tank installation 38
Secondary containment system installation ...... 41
Piping and accessories installation 42
Discussion 44
7. OPERATING PRACTICES AND GUIDELINES 45
Overfill prevention . . 45
Trans'fer spill prevention 46
Vapor recovery systems ........... 47
Leak detection . 47
Discussion 53
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8. CORRECTIVE ACTIONS 54
Inspection 54
Maintenance and repair 63
Retrofitting 63
Tank system closure 67
Discussion 69
REFERENCES 71
Appendix - Corrosion Prevention 77
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FIGURES
Number
1 Dimensions of leak prevention 3
2 Basic UST system 12
3 Secondary containment using a flexible membrane 19
4 Secondary containment using concrete walls 19
5 Interaction of various groups in code-making and enforcement . . . . 37
6 Stage I vapor recovery.system 48
7 Tank evaluation graph .60
8 Construction of design fatigue curve 61
Al Sacrificial anode cathodic protection 86
A2 Impressed current cathodic protection . 88
VI 1
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TABLES
Number Page
1 Summary assessment of secondary containment components 21
2 Physical and chemical properties of some regulated substances . . 25
3 Applicable lining materials for six products stored in USTs ... 28
4 Major technical codes applicable to storage systems 29
5 Recommended UST installation practices 40
6 Acceptable leak detection requirements and alternatives
for existing tanks under California regulations 51
7 Required tank integrity testing schedule in Connecticut 52
8 Florida leak detection requirements for existing tanks 52
9 Structural integrity test methods 56
10 Basis for the evaluation of underground storage environment
(SAV systems) 59
11 Advantages and limitations of common lining materials 66
AT The galvanic series of metals and alloys 79
A2 Soil corrosivity vs. soil resistivity 81
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ABBREVIATIONS
ACI
ANSI
API
ASME
ASTM
AWWA
FRP
MEK
NACE
NFPA
RCRA
SSPC
STI
UL
UST
VOC
American Concrete Institute
American National Standard Institute
American Petroleum Institute
American Society of Mechanical Engineers
American Society of Testing Materials
American Water Works Association
Fiberglass-reinforced plastic
Methyl ethyl ketone
National Association of Corrosion Engineers
National Fire Protection Association
Resource Conservation and Recovery Act
Steel Structure Painting Council
Steel Tank Institute
Underwriters Laboratories
Underground storage tank
Volatile organic compound
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ACKNOWLEDGEMENTS
The authors gratefully acknowledge the intelligent and resourceful
guidance received over the life of this project from a series of monitors
from the'EPA Office of Underground Storage Tanks and its predecessors:
Stephen H. Nacht, David O'Brien, William Kline, and Stephen Glomb. The
continuing contribution of John S. Farlow, Releases Control Branch,
Hazardous Waste Engineering Research Laboratory, U.S. Environmental
Protection Agency, has been invaluable throughout this project.
In addition, the guidance from William Apblett, Jr., Gopal Das Gupta,
and Jeffrey Blough of Foster Wheeler Development Corporation and Seymour
Rosenthal of Enviresponse, Inc. is appreciated. Furthermore, the
information obtained both formally and informally from a number of oil
companies and state agencies was invaluable for the completion of this
report.
The editorial and word processing support and cooperation of Jane
Perrotta, Penny Thergesen, and Richelle Drummond of Enviresponse, Inc. are
deeply appreciated.
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SECTION 1
INTRODUCTION
BACKGROUND
Protecting the nation's groundwater resources from contamination by
regulated substances* that leak+ from underground storage tank (UST)
systems has emerged as a major issue on the nation's environmental agenda
and for valid reasons. More than 50 percent of the nation's population
draw drinking water from underground resources (1 ). There are between 2
and 3.5 million underground tanks buried across the nation, of which some
100,000 tanks are estimated to be presently leaking, and some 350,000 are
expected to leak within the next 5 years (2). The immensity of the
problem becomes all the more apparent with some additional statistics
(2-6):
o Almost 90 percent of existing underground tanks are made of steel
that are unprotected against environmental deterioration --
principally corrosion;
o More than one.mil lion existing USTs are 16 years or older;
o A recent analysis of over 12,000 leaks, approximately 65 percent of
which are from retail gasoline station incidents, indicated mean
and median tank system ages of 17 years at the time of the leak;
o An unknown, but presumably large, number of abandoned tanks ~ with
no information on their size, location, content, and ownership --
is strewn across the nation. (For example, some 28,000 abandoned
tanks are estimated to exist within New York State);
o The fate and transport of regulated substances in water-bearing
soil strata are complex phenomena. The potential for contamination
of groundwater a significant distance from the leak source and over
an extended period of time exists.
o Remediation and restoration of land and groundwater resources
contaminated by underground leakage of regulated substances are a
costly undertaking, with costs, in some instances, running into the
millions.
*Regulated substances are those defined in Section 101 (14) of the
Comprehensive Environmental Response, Compensation, and Liability Act of
1980, and petroleum, including crude oil or any fraction thereof which is
liquid at 60°F, and 14.7 pounds per square inch absolute pressure.
+In this report, the word "leak" is used to denote all types of
unauthorized releases.
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Accordjngly, the need to improve leak prevention methods is evident.
The first'step in any improvement strategy is to assess the problem and
evaluate solution options. This report is just such a step. Its purpose
is to examine the structural design and operational practices of UST
systems in the context of leak prevention and identify areas for further
research and development to advance leak prevention technology.
Specifically, the report:
(1) Reviews structural design, corrosion protection, installation,
testing, and operational practices;
(2) Examines the available statistical information on the
demographics of leaks;
03) Presents a summary of the most dominant failure mechanisms,
viz., corrosion, and other causes of leaks;
(4) Describes the applicable codes and standards for design,
installation, and operation of UST systems; and,
(5) Identifies gaps and deficiencies, and recommends topics for
future research.
FACTORS AFFECTING LEAK PREVENTION
System Characteristics
Preventing leaks in UST systems requires the consideration of several
factors (Figure 1). These factors include UST system characteristics,
elements of the solution scheme, and other factors such as time, cost,
regulations and standards.
The characteristics of the system that influence leak prevention
strategies and options include:
o Age of installations -- New installations, old but known
installations, abandoned installations;
o Ownership — Large industrial owners, small industrial/business
owners, local governments;
o Products stored -- gasoline and petroleum products, chemicals;
o Size of installation and quantity of fluid stored.
Age of installation--
The age of an UST system greatly influences the solution option. In
a new installation, leak prevention technology is designed and engineered
into the system. In an old, but known, installation, on the other hand,
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Figure 1. Dimensions of leak prevention.
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it is a re.active step that requires system monitoring, retrofitting,
remediation and restoration. In an abandoned installation, problem
identification is most significant in the solution scheme. Being
prepared to solve a problem even though it is not clearly defined thus
becomes a prudent strategy in this case. Material, personnel, and
organizational resources to detect leaks, to determine their sources, and
to contain and remediate the environmental damages should be the focus of
the prevention strategy. An appropriate analogy is the fire prevention
and firefighting strategies found in communities across the nation.
Ownership—
The owners of underground storage tank systems greatly influence the
effectiveness of leak prevention strategy. Large industrial owners
generally have the technical, managerial, and financial resources to
employ the most effective active and reactive leak prevention
strategies. Moreover, large owners have the economic incentive to
minimize product losses from underground systems and avoid risks of heavy
financial losses resulting from potential lawsuits. It can be reasonably
concluded, that cost reduction, generic research, improvements and
advancement of the technology, and wide dissemination of such
advancements are the most significant needs of. this ownership group.
Small owners* from industrial and business sectors, local
governments, school boards and others present a different set of needs.
This group usually lacks the financial resources and organizational
strength to develop their own methods, products, and procedures to
prevent and remediate leaks. The smaller the owner the less likely they
are to have in-house technical capabilities to discern the causes and
effects of leaks, and to apply effective prevention methods. This
group's needs are likely to include: (1) recognition and understanding
of the dimensions of the problem; (2) proven and reliable methods,
products, and procedures for solution; (3) a qualified, trained, and
price-competitive technical labor pool; and (4) incentives and rewards
that compel them to apply leak prevention programs continually.
Products stored—
By far the largest class of products stored in UST systems is
gasoline and other petroleum products. Understandably, the primary focus
of leak prevention investigations thus far has been UST systems that
store these products. The problem, however, extends far beyond this.
The list of regulated substances includes 698 chemicals which are stored
in USTs in varying quantities. For example, in California (9) close to
500 regulated chemical substances, most commonly sodium hydroxide,
sulfuric acid, toluene, acetone, and methyl ethyl ketone (MEK), are
stored underground. The differences in the physical and chemical
properti es,
*Small owners as used in this report are independent oil companies, gas
station owners, and oil jobbers, municipalities, small fleet owners,
etc.
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toxicity,,..transport, and fate characteristics of such chemicals and
petroleum.products require different approaches and strategies for leak
prevention.
Size of installation—
The size of UST installations has a significant influence on leak
prevention strategy. Leak rates from large installations are likely to
be higher than from smaller installations, and thus the potential for
environmental damages will most likely be higher. Size differences also
present unique problems and opportunities with regard to materials of
construction, design, inspection procedures, leak monitoring, repair,
maintenance, and replacement schedules.
Elements of the Solution Scheme
The objective of a leak prevention program is to insure the integrity
of the containment boundaries throughout the useful life of the system;
avoid or minimize accidental spills and overflows; provide early warning
of impending leaks; and prevent products from spreading should they
leak. These tasks require three necessary but sufficient steps:
1. Proper design, engineering, fabrication, and installation;
2. Correct operation;
3. Appropriate corrective actions through inspection, repair, and
maintenance.
Design and engineering—-
As the first step in leak prevention, proper design and engineering
of an UST system should follow a high level of standard engineering
practices, which include:
o Understanding the forces and environmental factors that impair the
containment boundaries;
o Applying valid principles of mechanics and other engineering sciences
to develop proper configurations and layouts;
o Selecting appropriate materials of construction to withstand the
forces of the system and environment;
o Providing appropriate means to control, monitor, maintain, repair,
and replace the systems once they are built;
o Insuring a desired level of quality in all aspects of the work by
implementing acceptable standards.
Fabrication and installation—
System fabrication and installation must adhere to high standards of
workmanship by:
o Insuring quality of materials of construction;
o Providing appropriate tools and instruments;
o Hiring trained, qualified workers and providing appropriate
supervision;
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o Adopting proven methods of construction and verifying quality of
workmanship.
Operations--
Correct operations of an UST system are necessary to prevent leaks
and spills. Several steps are required:
o
o
o
Establishing valid procedures for the range of operations: start-up,
normal, upset, emergency, and shut-down conditions;
Hiring qualified operators and providing them with proper training
and tools for operation;
Maintaining proper records of operation.
Corrective actions-
Appropriate corrective actions permit the identification and repair
of potential failure before they become major problems. These actions
require:
o Establishing appropriate schedules for inspection, repair, and
maintenance;
o Providing proper personnel, tools, access, and facilities for
corrective actions;
o Insuring quality of corrective actions to meet established standards.
Regulations and standards-
Regulations and standards enhance leak prevention by improving
quality, uniformity, and interchangeability of products. Too much or too
little regulation, and too early or too rigid standards, however, would
inhibit innovation and technological growth. A conscious appreciation of
these factors is essential in promoting leak prevention technology.
An equally compelling issue that applies to standards is user
participation in and acceptance of the standard-making process. Much can
be learned and applied to UST leak prevention from the successful history
of voluntary standards that are applied by many industries in the U.S.
Other factors--
Cost is an overriding consideration in the selection of a leak
prevention option. While prevent!"on-at-any-cost may be an ideal
solution, a rational decision process should include a consideration of
acceptable risk. Cost minimization would therefore be a norm rather than
an exception in the search for leak prevention strategies.
Time influences the choice of leak prevention strategies in many
ways. Problems that are current and more immediate, e.g., unprotected
tanks that are in place for 15 or 20 years, require more prompt attention
than a newly installed tank. Research priorities are thus influenced by
the time factor.
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In the following sections, the implementation of these steps in UST
systems is reviewed. Inadequacies of current practice are identified,
and appropriate remedial research and development work that should be'
initiated is suggested.
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SECTION 2
CONCLUSIONS
4.
5.
Leak prevention in UST systems can be achieved through improvements
in design, engineering, fabrication, installation, operational
practices, and corrective actions. Appropriate regulations and
standards would enhance leak prevention technology. Too much or too
little regulation and too early or too rigid standards, however, can
inhibit technological growth. A conscious appreciation of these
competing factors is essential to promote the technology.
The basic UST systems are conceptually simple, and include tanks,
pipes, and accessories. Some newer designs also have a secondary
containment system. The basic system parts are mostly made of carbon
steel. Fiberglass-reinforced plastic (FRP) is used in many newer
installations, particularly for tanks that store gasoline products.
Flexible liners and concrete vaults are the two most-developed
secondary containment concepts.
Hundreds of standards, guidelines, and recommended practices, many
with overlapping requirements, are presently followed for the design
and engineering of UST systems. Most of these documents apply to
systems that store gasoline products. There is need for a cohesive,
coordinated set of rules and standards that apply to various classes
of UST systems.
The available statistical information does not provide a correlation
of failure rates and failure mechanisms with different design
configurations, materials of construction, soil conditions, and
environmental and operational factors.
The effects of long-term exposure of materials of construction of UST
systems to different types of products and outside soil and backfill
materials are not known. There is a need to develop these data and
protocols for selection of materials applicable to specific
conditions.
Various agencies and institutions provide installation procedures and
guidelines. However, at present there is no way to determine how
well these procedures and guidelines are understood and followed by
installers.
Efforts to improve operating practices should focus upon three
areas: methods, equipment, and people. Adequacy of spill prevention
methods, leak detection methods and devices, and operator training
require special attention.
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Corrective actions include inspection, maintenance and repair,
retrofitting, and tank system closure. Methods and procedures to
perform these tasks have been^pjpepared by,j-severaT agencies and
professional groups. However, "information is required to determine
how well they are applied.
Retrofitting of existing tank systems with cathodic protection is one
area that warrants special attention. Information on its
applicability and effectiveness is lacking.
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SECTION 3
RECOMMENDATIONS
As a result of this review, several topics have been identified and
are recommended for further research to improve leak prevention
technology. These topics are presented below in order of their
priorities.
1.
2.
3.
4.
5.
6.
Establish a national data base that will provide more refined and
detailed information on failure rates and mechanisms and their
correlation to different design, engineering, installation,
operation, and corrective action methods and procedures.
Establish organizational vehicles and mechanisms to integrate,
improve, and develop standards and procedures applicable to UST
systems.
Perform an in-depth assessment of cathodic protection methods,
including: (1) the effects of backfill, water tables, types of
anodes, and UST design configurations and details on the
performance of cathodic protection, and (2) retrofitting existing.
USTs with cathodic protection.
Study the effects of long-term exposure of materials of
construction with products stored, and with outside soil and
backfill materials. Develop compatibility protocols for the
selection of materials for construction and long-term protection of
USTs.
Develop methods to assess the life expectancy of new USTs and the
remaining life of existing USTs, and develop techniques to extend
the useful life of new and existing USTs.
Assess and establish effective means to train
adequate labor pool of installers, operators,
testers of UST systems.
and make available
inspectors, and
an
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SECTION 4
DESCRIPTION OF UNDERGROUND STORAGE TANK SYSTEMS
A basic UST system has three subsystems: tanks, piping, and
accessories. Tanks act as the primary containment for the product;
piping conveys and transfers the product from one point to another within
the system; and accessories -- pumps, valves, vapor vents, etc. --
control and regulate the flow of the product and operation of the
system. A basic UST system at a retail gasoline station is shown in
Figure 2.
Some modern USTs (those built mostly withfn the last five years) have
a secondary containment boundary that envelops the part of the primary
system that lies underground. The secondary containment acts as a
barrier to prevent any product leaking from the impaired basic system
from reaching the surrounding ground.
TANKS
Reports (2, 3) indicate that 89 percent of tanks in UST systems in
the U.S. are made of steel , mostly carbon steel. A large majority of
these steel tanks are unprotected against corrosion. For example, API
estimates that almost 66 percent of the tanks owned by its members as of
1984 were made of unprotected steel (4).
Tanks that have corrosion protection include: steel tanks with
internal and external coatings; cathodically protected steel tanks;
fiberglass-reinforced plastic (FRP) tanks; and steel/FRP-bonded composite
tanks. Tanks made of materials such as stainless steel, aluminum, and
various plastics are used in specialized applications.
Steel Tanks
The design, construction, and installation of underground steel tanks
usually follow one or more of the following standards:
o Underwriters Laboratories (UL) Inc. UL 58, Steel Underground Tanks
for Flammable and Combustible Liquids (7);
o National Fire Protection Association (NFPA). NFPA 30, Flammable and
Combustible Liquids Code (8);
o American Petroleum Institute (API) Publication 1611. Service Station
Tankage Guide (9);
o API Publication 1615. Installation of Underground Petroleum Storage
Systems (10);
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FRP tanks are normally designed to conform to one or more of the
following standards:
o UL Standard 1316. Standard for Glass-Fiber-Reinforced Plastic
Underground Storage Tanks for Petroleum Products (12);
o NFPA 30. Flammable and Combustible Liquids Code (8);
o NFPA 31. Standards for Installation of Oil Burning Equipment (13).
Steel/FRP Bonded Tanks
Steel/FRP bonded tanks are constructed of an inner shell of steel and
an outer layer of FRP fused together by a polyester resin bond. The
tanks have the advantage of the strength and stiffness of steel and the
corrosion resistance of FRP.
PIPING
Pipes used in USTs are made of a variety of materials:
o Carbon steel
o Cast iron
o Stainless steel
o Galvanized steel
o Rubber, plastic, or epoxy-lined
steel
o Plastic
o Fiberglass-reinforced plastic
Carbon steel pipes are compatible with petroleum; however, they are
susceptible to corrosion. On the other hand, cast iron pipes resist
corrosion well and can be used to carry concentrated acids. They are
brittle, however, and can break on impact or shock. Both carbon steel and
cast iron are relatively inexpensive.
Stainless steel pipes offer considerable corrosion resistance and
longer life, but they are expensive. Galvanized steel pipes provide some
protection against corrosion, although areas where galvanizing has been
impaired (e.g., through handling) are susceptible to corrosion.
Plastic-lined steel pipes combine the corrosion resistance of plastic
with the structural strength of steel. These pipes are expensive compared
with other types of steel pipes.
Plastic pipes, including FRP pipes, are used because of their superior
compatibility with a wide range of chemicals and petroleum products. They
are not susceptible to internal or external corrosion and do not induce
galvanic corrosion when joined with metal structures. However, they are
not structurally strong and are susceptible to failures when subjected to
frost heaves, excessive weights, and pressures. They also are not
suitable for complex piping layouts.
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ACCESSORIES
Accessories in an UST system include valves, pumps, joints and
fittings, and other components such as vapor recovery systems, overfill
prevention systems, and leak monitoring ports.
Valves
Valves are used to control the flow of fluid, isolate sections of the
system for maintenance, prevent backflow in pipelines, and relieve
pressure in pipelines and tanks. Most valve designs are modifications of
two basic types: gate valves for stopping and starting flow and globe
valves for regulating flow. Other types of valves include (14,15) angle,
ball, diaphragm, and control valves to regulate flow; plug valves to
flow; check valves to prevent backflow; and safety valves to relieve
pressures in the system.
stop
Valve bodies are usually made of metal or .FRP. Metals most commonly
used include cast iron, bronze, nickel alloys, steel, stainless steel,
aluminum, and titanium. Valve trim includes the internal part of the
valve body that comes into contact with the stored liquid and is made of
various alloys and plastics. It must retain its smooth finish for
successful operation.
Selection of valve material is based on the following criteria (19):
o Resistance to corrosion;
o Resistance to erosion by suspended solids;
o Prevention of galling by dissimilar or hard materials;
o Prevention of deformation or distortion.
The selection is also based on the viscosity, corrosivity,
temperature, and pressure of the liquid the valve is exposed to. For
example, cast iron and bronze are used for applications at temperatures up
to 260°C; nickel alloy steel is used in low-temperature applications for
temperatures down to -57°C. FRP or plastic is used where chemical
compatibility to the stored liquid is a primary design consideration.
Pumps move stored liquid by any of the following methods:
o Centrifugal force o Momentum transfer
o Volumetric displacement o Electromagnetic force
o Mechanical impulse o Gravity
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Pumps normally used in UST systems are either suction type or
submersible type. Suction pumps are either centrifugal, rotary, or
reciprocating and are located at grade or at the product dispenser.
Submersible pumps, usually centrifugal, are mounted inside the tanks.
Selection of a pump depends on the following factors (15,16):
o Characteristics of the fluid stored (temperature, viscosity, vapor
pressure, specific gravity);
o Desired capacity in gal/min;
o Static suction head;
o Static discharge head;
o Size, length, and type of pipe, hose, fitting, etc.
Most leakage in pumps occurs at the seals. Pumps seals are of two
types: packing or mechanical. Neither has a clear advantage over the
other, but the type of applications may dictate the selection of a certain
seal. Packing seals are comprised of fibers of cotton, asbestos, jute,
Teflon, silicon, or resins pressed between the two mating surfaces of the
pump where sealing should occur to provide a leak-tight fit. In
mechanical seals, the mating surfaces are kept in leak-tight contact by
springs. Packing seals can be tightened while the pump is in operation;
mechanical seals cannot.
Joints and Fittings
Joints and fittings connect various parts of a piping system. Joints
and fittings commonly used are:
o Couplings and unions to join two pieces of pipes;
o Elbows and tees to change pipe direction;
o Reducers and expanders to change pipe diameters;
o Plugs and caps to terminate a pipe;
o Tees, Ys, and crosses to join two or more streams together;
Expansion joints to prevent thermal stresses, eliminate noise and
vibration, and compensate for misalignment;
o Swing joints to give the pipeline rotational flexibility and
reduce torsional stresses that can result in pipe failure.
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Other Components
Overfill prevention systems are designed to prevent spills at the
product delivery transfer connection from the tank truck to the
underground storage tanks. These systems automatically shut off flow to
the underground tank at a level at which the delivery, including drainage
from the delivery transfer hose, is completed without overfilling the
tank. "Quick-disconnect" couplings are used on the end of discharge
hoses. The liquid released during disconnection is allowed to collect in
the spill container surrounding the fill pipe.
Vapor recovery systems prevent hydrocarbon vapor from escaping into
the atmosphere during delivery of product into service station underground
storage tanks or during product dispensation. Vapors generated during
delivery operations are forced to the top of the delivery tank. The vapor
release during dispensation is minimized by appropriate design of the
nozzles and hose connections at the product dispensation islands and by
adding vapor return lines to the underground tanks.
SECONDARY CONTAINMENT
Secondary containment retains leaks from a basic UST system, aids
their detection, and facilitates their cleanup (17-19). Secondary
containment can be accomplished in two ways: (!•) by building a barrier'
between the basic system and the surrounding ground with flexible membrane
liners, a concrete vault, clay liners, or soil sealants; or (2) by using a
double-wall structural configuration for tanks and pipes. Systems
constructed of these materials can be "fully lined," "partially lined," or
"unlined" (11). In a "fully lined" system, the entire tank excavation
pipe trench are lined with either flexible membrane liners, concrete
vaults, or clay/soil liners before backfilling. The backfill
maintained in a dry
double-walled tanks
clay/soil) enclosing only the pipe trench. "Unlined systems" are
double-walled tanks, pipes, and fittings, thereby obviating the need for
additional liners. However constructed, all secondary containment systems
require one leak monitor; double-walled tanks require one monitor for each
tank.
Flexible Membrane Liners
Flexible membrane liners are made of polymeric materials manufactured
in sheet form. Polyester elastomer, high-density polyethylene,
epichlorohydrin, and polyurethane products have been used for a variety of
LIST system applications, including the storage of petroleum products.
Flexible membrane liner materials with bases of polyvinylchlorides,
chlorinated polyethylenes, neoprene, ethylene propylene diene monomer,
butyl rubber, and chlorosulfonated polyethylene are used for the storage
of compatible chemicals, but are inappropriate for petroleum product
storage because of their poor resistance to hydrocarbons (17). Flexible
can then
or wet condition. In a "partially lined" system,
are used with liners (flexible membrane, concrete,
and
be
or
-17-
-------
membrane liners must be compatible not only with the stored product but
also with'the surrounding soil and groundwater. They should also be
resistant to bacterial deterioration.
A fully lined system in which the entire tank excavation pit and pipe
trenches are lined with a continuous flexible membrane liner is shown in
Figure 3. Two variations of this concept are: (1) a dry system in which
both the tank excavation pit and pipe trenches are kept in a dewatered,
dry, state; and (2) a wet system in which the tank excavation pit is kept
wet with aqueous saturated backfill, and the pipe trenches are kept dry.
Concrete Vaults
Vaults made of reinforced concrete can be constructed on site to house
one or more storage tanks and associated piping. As concrete is a porous
material, all concrete vaults must have an internal lining or coating to
make them leak proof. Concrete vaults are structurally stable and durable;
however, they are brittle and subject to cracking. A typical concrete
vault secondary containment system is shown in Figure 4.
Clay Liners and Soil Sealants
Clay liners and soil sealants are relatively inexpensive secondary
containment methods. They ,are both generally used in UST systems that
store chemicals. Although some clays may effectively contain petroleum
products, they are generally considered unsuitable because of their
susceptibility to dessication by hydrocarbons. Natural soils can be made
impermeable by treating them with either sealants (e.g., sodium bentonite)
or cements (e.g., hydrated Portland cement). However, sealants are
susceptible to reactions with groundwater, and may rapidly degrade when
exposed to hydrocarbons. The long-term performance of clay liners and
soil sealants is not well established.
Double-Walled Tanks and Pipes
Double-walled tanks and pipes are essentially a tank within a tank,
and a pipe within a pipe, respectively. The outer walls act as an
additional containment boundary in the event of leaks in the primary inner
walls. The annular space between the two walls is monitored for leaks.
Double-walled tanks are available in steel, fiberglass, and composites of
steel and fiberglass. Double-walled pipes are not available commercially
on a large scale.
Double-walled UST systems allow near-conventional installation
practices and easy replacement of individual tanks and pipe sections.
However, there are disadvantages, including: fabrication difficulties at
tank and pipe interfaces and other connections; a requirement to have one
leak monitor for each tank or pipe section; an inability to protect
against product spills and tank overfills; and difficulty in preventing
corrosion in the annular space between the tank and pipe walls.
-18-
-------
VENT
LINES
DISCHARGE
OVERFLOW
OBSERVATION
WELL
LINED TRENCH
PRODUCT
LINES
Figure 3. Secondary containment using a flexible membrane. (Adapted
from (2).)
VACUUM SUCTION LINE
FOR SOLVENT REMOVAL
FILL LINE (GRAVITY)
MANWAY FOR
TANK CLEANING
4 INCH SUMP
PIPE MANWAY — ^=n
LEAK DETECTOR/ "
VACUUM SENSOR
LEVEL SENSOR
i,'4'
4i?
PEA GRAVEL -
FILL
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• *
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^
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a..fr:*-t
STORAGE TANK
i i'
i,1
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GRADE '
CONCRETE
TANK VAULT
Ul
%,
PVC WATERSTOP (TYP)
A LIQUID DETECTOR
°'. A- ^~^- ikicinc
x^s^sST •"<*""-
CATHODIC CORROSION
PROTECTION
BOTTOM, SCREEN TO
BOTTOM OF SUMP
Figure ^. Secondary containment using concrete walls. (Adapted from
W.D. Bellamy and R.B, Brummett. Designing for Underground
Tanks. Pollution Eng., Nov. 1986, p. 24.)
-19-
-------
A comparative evaluation of different types of secondary containment
system components is presented in Table 1.
DISCUSSION
The descriptions of the system configurations and design details given
in the preceding paragraphs, although not exhaustive, provide an
indication of the state of the art of UST structural design technology.
In order to assess the adequacy of these design concepts, an understanding
of their performance histories, and failure locations and causes are
required. Information on type of product released, type and age
distribution of a leaking system and the influence of secondary
containment, materials of construction and corrosion-prevention systems is
also required. Unfortunately, the quality and completeness of failure
data in the USTs are limited. The most comprehensive and current
statistical information on leaks -- subsurface and underground releases --
is provided in a recent EPA report prepared by Versar, Inc. (3). The
report contains an analysis of 12,444 documented leak incidents in all 50
states. Key findings of the Versar study are summarized below.
o 10,300 (83 percent) of the reported 12,444. leaks occurred at UST
facilities covered under Resource Conservation and Recovery Act (RCRA)
Subtitle I.
o About 65 percent of the reported releases involved retail gasoline
stations; commercial establishments accounted for 11 percent of the
releases; manufacturing facilities, 5 percent; municipal facilities, 4
percent; and the remaining facility types accounted for 15 percent of
the total.
The reasons for the high proportion (65%) of the reported releases
from gasoline stations as compared to the proportion (48%) of retail
gasoline station tanks in the total tank population have not been
determined.
o 95 percent of all reported leaks occurred at operating facilities; the
remaining 5 percent involved abandoned facilities.
o Gasoline accounted for more than 70 percent of the reported volume of
leaked products.
o Of the 25 reported leaks that released 50,000 gallons or more of
product, 14 involved gasoline, 5 heating oil, 4 diesel fuel, 1
aviation fuel, and 1 unspecified substance. Eleven incidents occurred
at gasoline s'tations and the rest at commercial transportation and
manufacturing facilities, military facilities, and other locations.
o Only 10 percent of the leak incident reports specified the age of the
tank system. The available data, however, indicated the mean and
median tank age for the nation as a whole to be 17 years. Except in
the Southwest region (New Mexico and Arizona), the mean age ranged
-20-
-------
TABLE 1 . SUMMARY ASSESSMENT OF SECONDARY CONTAINER COMPONENTS
Secondary
Containment for
Major Advantages
Major Disadvantages
TANK PROTECTION
Double Wall
Dry Hole Lined
with Flexible
Membrane
Wet Hole Lined
with Flexible
Membrane-
-Near conventional in-
stallation
-Backfill contamination
unlikely
-Easy monitoring of
annular space for pri-
mary containment
failure and outer shell
fa i1ure
-Allows individual tank
replacement
-Large capacity
-One monitor for all
tanks
-Provides overfill and
spill protection
-Large capacity
-One monitor for all
tanks
-Leak prompts system
shutdown
-Secondary containment
system continuously
monitored
-Provides overfill and
spill protection
-Isolates failure
-Piping interface may be
difficult
-One monitor per tank
-Depending on material,
may require corrosion
protection in annular
space
-Depending on material,
weight and increased
dimensions may require
special transport and
heavy duty cranes
-Does not in itself pro-
vide for tank overfill
or detection
-Significant installation
training/inspection
-Testing difficult
-Future 1iner repair
-Backfill contamination
cleanup
-Difficult to maintain
dryness
-Difficult to monitor
product in soil in
small quantities
-Seams if field applied
are difficult to seal
-Significant installation
training/inspection
-Future 1iner repair
-Maintenance of water
level
-Backfill contamination
cleanup
-Seams if field applied
are difficult to seal
-Disposal of excess water
-21-
-------
Table 1. SUMMARY ASSESSMENT OF SECONDARY CONTAINMENT COMPONENTS (Cont'd)
Secondary
Containment for
Major Advantages
Major Disadvantages
Concrete Vault/
Encasement (Dry)
PIPE PROTECTION
Double Wall
Flexible Mem-
brane Lined
Trench
Concrete Lined
Trench
•Generally recognized
-Structural stability
and durability
-Large capacity
-Easy leak monitoring
-Backfill contamination
unlikely
-Isolates failure
-Applicable to double
wall and tank liner
systems
-Easy to monitor
-Applicable to double
wall and tank liner
systems
-Easy to monitor
-High cost
-Not impermeable
-Lacks plasticity (may
crack)
-Specially trained
installers
-Piping interface experi-
ence in industry limited
-Testing difficult
-"Grade" problems due 'to
pipe size, requires deeper
tank hole
-High cost
-Difficult tank interface
-Significant installation
training
-Future liner repair
-Backfill contamination
-Significant installation
training
-Future liner repair
-Backfill contamination
Reprinted from (17). Used by permission of American Petroleum Institute,
Washington, D.C.
-22-
-------
from 14 to 18 years. The mean age in the Southwest region was 28
years. The results seem to suggest that soil conditions in different
regions of the country do not significantly affect corrosion rates and
tank lifetimes. This is contrary to the generally held belief that a
stronger correlation exists between soil condition and age-to-leak.
o The mean age of steel tanks that leaked was 17 years, FRP tank age was
5 years.
o The mean and median ages of leaks in pipes were 11 and 9 years,
respectively.
o Steel tanks represent 81 percent of leak incidents, and fiberglass, 19
percent. It should also be noted that 89 percent of tanks in the UST
population are made of steel and the rest FRP.
o More than half of the tanks that leaked ranged between 4,000 and
11,990 gallons in size. Analysis of the size distribution further
showed that large tanks are as likely to leak as medium or small tanks.
o 43 percent of leaks were reported to occur in tanks. 18 percent
occurred in pipes and 17 percent as a result of overfill.
o Structural failures caused by vehicle impact, ruptures caused by
excessive pressure during tank tightness tests and ruptures due to
improper excavation were the most commonly specified causes of leaks,
followed by corrosion, loose fittings, improper installation, and
natural phenomena. Relatively few of the structural failure incidents
involved corrosion-related problems. The results strongly suggest
that a program to minimize leaks in UST systems must address a variety
of causes of structural failures in addition to corrosion-related
releases.
The statistical information provided by the Versar study, although very
valuable, does not permit a detailed analysis of failure mechanisms and
failure rates that correlate to different design configurations, materials
of construction, protective design concepts, soil conditions and
environmental and operational factors.
-23-
-------
SECTION 5
DESIGN AND ENGINEERING PRACTICES
Key factors that influence design and engineering of UST systems and
their components are: properties of the products stored; mechanical
forces imposed on the structural components; corrosion factors; properties
of materials of construction; and applicable codes and standards.
PROPERTIES OF PRODUCTS
Physical, chemical, and hazard characteristics of the stored product
are important considerations in the design of an UST system and its
components. Critical characteristics include the product's physical state
at storage temperature, melting point, boiling point, specific gravity,
vapor pressure, explosivity, flammabil ity, combustibility and
corrosivity. Table 2 summarizes these properties for ten chemicals that
are in liquid state at 20°C. A more complete set of data for other
chemicals is found in (20).
When products stored in an UST system comprise a mixture, the likely
consequences of combining the constituent chemicals must also be
evaluated. One tool that is widely used for determining these
consequences is a chemical class compatibility matrix (21). The matrix is
prepared by grouping chemicals into 38 classes based on similar molecular
structure (classes 1 thru 31), and based on similar reactivity
characteristics (classes 32-38). For example, mixing of caustics (class
10) with aldehydes (class 5) will generate heat. On the other hand, the
consequences of mixing caustics with organic nitro compounds (class 24)
include generation of heat and explosion. A clear understanding of such
consequences of mixing chemicals is an essential first step in the proper
design of UST systems that store chemicals.
MECHANICAL FORCES
Mechanical forces imposed on an UST system and its components include:
o
o
o
Dead loads due to product weight, self weight, weight of soil
overlay, reaction forces, etc;
Live loads due to internal pressure, thermal expansion forces,
vehicular traffic;
Environmental loads due to traverse and buoyancy pressures due to
groundwater table, seismic load in earthquake-prone zones.
finite
the
Very sophisticated design methods and analytical tools, e.g.
element programs, are presently available to determine optimal
configurations, dimensions and layouts of UST systems. However,
current design approach used in UST systems is largely based on
manufacturers' specifications and requirements, and industry and
professional standards. This approach is not unlike those employed in
many other industrial designs.
-24-
-------
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-25-
-------
CORROSION
Corrosion is one of the major causes of deterioration and failure of
metallic underground tanks and pipes. Corrosion can occur both internally
and externally. Internal corrosion is largely due to incompatibility of
the stored product with materials of construction. External corrosion is
primarily an electrical process in which a flow of electricity from the
metal surface to the surrounding ground (which acts as the electrolyte) is
induced under favorable conditions. The flow carries material, at the
atomic level, that results in the thinning of the underground structure
and causes its eventual failure. Factors that influence external
corrosion are: soil resistivity; type and concentration of salts in the
soil; presence of certain types of bacteria; temperature; permeability of
surface film; presence of adjacent underground metallic structures, and
stray underground electrical current. Anong them, soil resistivity is the
most important factor and is a function of moisture content, soil salt
concentration, and temperature; the higher the values, the lower the
resistivity of soil and the higher the possibility of corrosion.
External corrosion can be prevented by providing cathodic protection
that forces the electric current to flow toward, rather than away from,
UST components, or by selecting materials of construction
electric flow altogether. A detailed description of
and preventive methods is given in Appendix A.
that inhibit
the corrosion process
MATERIALS OF CONSTRUCTION
Materials used in UST systems include various types of metals and
polymeric materials. Structural strength and compatibility with products
and soil environment are two key factors that determine the choice of
materials.
Most universally accepted standards for materials of construction are
those developed by the American Society for Testing Materials (ASTM).
Many industries accept the ASTM requirements by adoption or adaptation
into their own standards. However, the scope of these standards and
specifications does not necessarily include exposure to all hazardous
chemicals listed as regulated substances. Specifications and guidelines
proposed by manufacturers of UST system components are the standard tool
that is presently used for the selection of appropriate materials of
construction.
Structural strength and deformation characteristics of most materials
of construction are well known; but data related to their compatibility
with various chemicals and environmental conditions are less known. Two
excellent references that provide guidelines for the selection of
compatible construction materials for the containment of various chemicals
are: Corrosion Data Survey (22) prepared by the National Association of
nnrrosion Engineers (NACE). and Perry's Chemical Engineering Handbook (20),
-26-
-------
NACE has also published standards and manuals for the proper
coating/lining protection of tank interiors (23-25). Table 3 summarizes
the applicability of various lining materials for six products that are
among those commonly stored in LIST systems.
CODES AND STANDARDS
Design and engineering of LIST system components are presently based on
standards and recommended practices established predominantly by
professional, trade, and industrial organizations. These standards and
practices represent a consensus of design and engineering approaches used
by various manufacturers and technologists participating in the
standard-making process. The basic approach used in these documents is
what is generally termed as "design-by-rule" in which the "rules" specify
the minimum acceptable design parameters, e.g., wall thickness, materials
of construction, and coating and lining requirements.
into:
For the purpose of design classificaton, UST systems may be grouped
>;
o.
0
Atmospheric systems that operate essentially at atmospheric
pressure;
Low pressure systems that operate at pressures up to 15 psig;
High pressure systems that operate at pressures higher than 15
psig.
Many documents provide technical standards, guidelines, and recommended
practices that apply to these systems. As a broad generalization, most of
the documents that apply to atmospheric systems are developed by the API,
AWWA, and UL. Most standards and design guidelines that apply to
lower-pressure systems are prepared by the API and the ASME. The ASME
Boiler and Pressure Vessel Code, by adoption, is a legally binding
standard in most states and local jurisdictions in the U.S. for the
design, construction and operation of high-pressure systems. The API and
the NFPA also have developed some codes pertinent to higher-pressure
systems. A complete listing of codes and standards from these various
organizations are given in Table 4. Some of the more pertinent industrial
standards, and recommended practices used for the design and engineering
of UST systems are:
Standard UL-58, Steel Underground Tanks for Flammable and Combustible
Liquids (7)--Out1 ines requirements for cylindrical and horizontal
atmospheric-type steel tanks for the storage of underground flammable and
combustible liquids. It provides for tank design configurations, metal
thicknesses, and construction materials. The standard also addresses
details of design and fabrication, including shell seams, heads, head
joints, bracings for unflanged and flanged bulkheads, and compartment
tanks containing single and double bulkheads; pipe connections; size of
pipe vent fittings; and internal pressure leak testing requirements. The
standard recommends that the length of the tank be no more than six times
its diameter.
-27-
-------
TABLE 3. APPLICABLE LINING MATERIALS FOR SIX PRODUCTS STORED IN USTs
Products
Lining Material
Chlorostrtfonated Polyethyl
Chlorinated Polyester
Epoxy
Epoxy Resin Cement
Fur fury! Alcohol
Furnace Res-in Cement
Neoprene
Phenol ics
Phenolic Resin Cement
Polyethylene
Polyester Reinforced
Polyester Resin Cement
Polyvinyl Chloride
Rubber
Teflon
Urethane
Vinyl
Vinyl idene Chloride
Adapted from (25)
1.
2.
3.
4.
5.
6.
1
ene
X
X
X
X
X
X
X
X
X
Product
Gasol ine
MEK
Potassium
2 3
X X
X
X
X
X
X
X
X X
X
List
4
X
X
X
X
X
X
X
5
X
X
X
X
X
X
X
X
X
X
X
X
X
X
6
X
X
Hydroxide
Sodium Hydroxide
Sulfuric
To! uene
Acid (50%)
-28-
-------
TABLE 4. MAJOR TECHNICAL CODES APPLICABLE TO STORAGE SYSTEMS
Code No.
Title
Applicable
Tank Types
API
Spec.
Spec.
Spec.
RP
Std.
RP
RP
Std.
Std.
Std.
Publ.
Publ.
Bull.
Publ.
Bull.
Bull.
Std.
RP
RP
Publ.
12B
12D
12F
12RI
510
520
521
526
620
650
1587
1604
1615
1621
1623
1628
2000
2001
2003
2009
Bolted tanks for storage of production liquids
Field welded tanks for storage of product liquids
Shop welded tanks for storage of production liquids
Setting, connecting, maintenance, and operation of
lease tanks
Pressure vessel inspection code
Design and installation of pressure-relieving systems
in refineries
Pressure relief and depression systems
Flanged steel safety relief valves
Design and construction of large welded, low-pressure
storage tanks
Welded steel tanks for oil storage
Waste oil round-up
Abandonment or removal of used underground service
station tanks
Installation of underground petroleum storage systems
Bulk liquid stock control at retail outlets
Bulk liquid loss control in terminals and depots
Underground spill cleanup manual
Venting atmospheric and low-pressure storage tanks
Fire protection in refineries
Protection against ignitions arising out of static,
lightning, and stray currents
Safe practices in gas and electric cutting and
welding in refineries, gasol ine plants, cycling
plants, and petrochemical plants
A
A
A
A
L,H
H
H
L,H
L
A
A
A
A
A
A
A,L,
A,L
A,L,
A,L,
A,L,
H
H
H
H
Publ. 2013
Publ.
Publ.
2015
2015A
Publ. 2023
Cleaning mobile tanks in flammable or combustible
1iquid service
Cleaning petroleum storage tanks
A Guide for controlling the lead hazard associated
with tank cleaning and entry
Safe storage and handling of petroleum-derived
asphalt products and crude oil residues
A,L,H
A,L,H
-29-
-------
TABLE 4. MAJOR TECHNICAL CODES APPLICABLE TO STORAGE SYSTEMS (Cont'd)
Code No.
Title
Applicable]
Tank Types
API
Bull. 2202
Publ.
Std.
PSD-2207
2510
Bull. 2519
NFPA
11
11A
11B
12
Dismantling and disposing of steel from tanks which
have contained leaded gasoline
Preparing tank bottoms for hot work
Design and construction of LP6 installations at
marine terminals, natural gas plants, refineries, and
tank farms
Use of internal floating covers and covered floating
roofs to reduce evaporation loss
Guide for Inspection of Refinery Equipment:
- Ch. II - Conditions causing deterioration or
failures
- Ch. Ill - General preliminary and preparatory work
- Ch. IV - Inspection tools
- Ch. V - Preparation of equipment for safe entry
and work
- Ch. VI - Pressure vessels
- Ch. XI - Pipes, valves, and fittings
- Ch. XII - Foundations, structures, and buildings
- Ch. XIII - Atmospheric and low-pressure storage
tanks
- Ch. XIV - Electrical systems
- Ch. XV - Instruments and control equipment
- Ch. XVI - Pressure relieving device
- Ch. XVII - Auxilliary and miscellaneous equipment
- Appendix - Inspection of welding guide for
follow-up inspection of interior tank coatings
Foam extinguishing systems
High expansion foam systems
Synthetic foam and combined agent systems
Carbon dioxide extinguishing systems
A,L,H
L,H
A,L,H
A,L,H
A,L,H
A,L,H
L,H
A,L,H
A,L,H
A,L
A,L,H
A,L,H
A,L,H
A,L,H
A,L,H
A.L.H
A,L,H
A,L,H
A,L,H
-30-
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TABLE 4. MAJOR TECHNICAL CODES APPLICABLE TO STORAGE SYSTEMS (Cont'd)
Code No.
NFPA
12A
16
17
30
43A
49
58
59
68
69
70
72A
72B
72C
720
72E
77
78
231
231A
321
325H
327
329
419H
495
1221
Title
Halogenated fire extinguishing agent systems
Installation of foam-water sprinkler systems and
foam-water spray systems
Dry chemical extinguishing systems
Code for flammable and combustible liquids
Liquid and solid oxidizing materials
Hazardous chemical data
Storage and handling of LPG
Storage and handling of LPG at utility gas plants
Explosion venting
Explosion preventing systems
National electrical code
Installation, maintenance, and use of local
protective signaling systems
Installation, maintenance, and use of auxiliary
protective signaling systems
Installation, maintenance, and use of remote
protective signaling systems
Installation, maintenance, and use of proprietary
protective signaling systems
Automatic fire detectors
Recommended practice on static electricity
Lightning protection code
General indoor storage
General outdoor storage
Classification of flammable and combustible liquids
Fire hazard properties of flammable liquids
Cleaning small tanks and containers
Underground leakage of flammable and combustible
liquids
Code for explosive materials
Identification of fire hazards of materials
Installation, maintenance, and use of public fire
Applicable
Tank Types
A,L,H
A,L,H
A,L,H
A,L,H
A,L,H
A,L,H
L,H
L,H
A,L,H
A,L,H
A,L,H
9 9 • '
A,L,H
A,L,H
A,L,H
A,L,H
A,L,H
A,L,H
A,L,H
A
A
A,L,H
A,L,H
A,L,H
A,L,H
A,L,H
A,L,H
A,L,H
service communications
-31-
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TABLE 4. MAJOR TECHNICAL CODES APPLICABLE TO STORAGE SYSTEMS (Cont'd)
Code No.
ASME
Section II
Section V
Section VIII
Section X
AMA
Dl 08-67
D101-53
0102-64
ACI
344
NACE
RP-01-69
No. 1
No. 2
No. 3
No. 4
RP-03-72
Title
Applicablel
Tank Types!
Boiler and Pressure Vessel Code: L,H
- Materials specifications
- Nondestructive examination
- Pressure vessels
- FRP pressure vessels
A
A
Standard for steel tanks, standpipes, reservoirs, and
elevated tanks for water storage
Standard for inspecting and repairing steel tanks,
standpipes, reservoirs, and elevated tanks for water
storage
Standard for painting and repainting steel tanks,
standpipes, reservoirs, and elevated tanks for water
storage
Guide for Protection of Concrete Against Chemical A,L,H
Attack by Means of Coatings and Other Corrosion-
Resistant Materials
Manual of Concrete Practices A»L,H
Design and construction of circular prestressed A,L,H
concrete structures
Control of external corrosion on underground or A,L,H
submerged metallic piping systems
Surface preparation for tank linings A,L,H
Surface preparation for some tank linings and heavy A,L,H
maintenance
Surface preparation for maintenance A,L,H
Surface preparation for very light maintenance A,L,H
Method for lining lease production tanks with coal A,L,H
tar epoxy
-32-
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TABLE 4. MAJOR TECHNICAL CODES APPLICABLE TO STORAGE SYSTEMS (Cont'd)
Code No.
Title
Applicable
Tank Types
SSPC
5063
10-63
6-63
7-63
White metal blast
Near-white metal blast
Commercial blast
Brush off blast
A,L,H
A,L,H
A,L,H
A.L.H
LEGEND:
Organization:
API = American Petroleum Institute
NFPA = National Fire Protection Association
American Society of Mechanical Engineers
Water Works Associaton
ASME =
AWWA = American
ACI = American Concrete Institute
NACE = National Association of Corrosion Engineers
SSPC = Steel Structure Painting Council
Code Number:
A numerical designation assigned to a code, etc., by the promulgating
organization.
Spec. = specification
RP = recommended practice
Std. = standard
Publ. = publication
Bull. = bulletin
Applicable Tank Types:
A = Atmospheric
L = Low Pressure
H - High Pressure
Adapted from (21).
-33-
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Standard UL-1316, Glass Fiber Reinforced Plastic Underground Tanks for
petroleum Products (Ik!)—Covers requirements for spherical or horizontal
cylindrical, atmospheric-type FRP tanks for the underground storage of
petroleum-based flammable and combustible liquids; the requirements do not
cover tanks storing alcohol or alcohol-blended fuels. The standard
describes UST system design and construction, as well as performance
testing for leaks, bending moment, and water load; strengths of lifting
fittings; external and internal pressures; earth load; and physical
properties of tank materials.
UL of Canada CAN4 S615M83, Standards for Reinforced Plastic Underground
Tanks for Petroleum Products (21)—Covers the fabrication andjnstal 1 ation
of horizontal FRP tanks used for the underground storage of flammable and
combustible liquids, such as highly aromatic premium grade gasoline and
middle distillate fuels. The standard includes design details of
connections, tank capacity, supplemental equipment, metal coatings,
lifting lugs, manholes, and internal protection (impact pads under
ooeninq). It also covers requirements and tests for internal pressure,
concentrated loads, flood loads, subsidence, drop strength, torque and
bending strength of tank connections, leakage, immersion, and aging to
determine deterioration due to the action of stored materials or
surrounding soil conditions.
NACE Standard RP-01-69, Recommended Practice for Control^ of^External
Corrosion on Underground or Submerged Metal 1
RP-Q22-&5. Recommended Practice tor Control ot
c Piping Systems \e.i
External Corrosion
and
,..,. Liquid Storage Sys
installation, operation, and
Metallic Buried, Partially
(43)—Encompass criteria tor
Buried, or Submerge
the design,
terns
i tj.,3 j — —triL.UIUjJCl:>;> ^l i I«GI • <* lul •"<•- uw^i3.., • -• — , •
iaTntenance of cathodic protection systems; control of interference
currents; and corrosion control records. They also include a
comprehensive index of coating references and testing standards.
API 1630 Cathodic Protection of Underground Petroleum Storage Tanks
and Piping Systems (28)-Addresses the theory and practice of cathodic
protection, advantages and disadvantages of the two cathode protection
systems, and criteria for determining whether impressed-current cathodic
protection has achieved corrosion protection of underground petroleum
storage tanks and distribution piping.
API 620. Recommended Rules for Design and Construction of Larger
tJoiriPrt. low-Pressure Storage Tanks, Seventh Edition (29)--Covers the
various aspects of structural design and construction or large, welded,
low-pressure storage tanks used for the containment of petroleum products
and other materials. Although this standard contains little information
on corrosion, it establishes thickness allowances for corrosion.
Steel Tank Institute (STI) Specification for st1-P3 System for
rnal Corrosion Protection of Underground_ steel storage^ Tanks
tXueniai ourruaiuu r i ^ ^^.^^•^•- ». ~ j• -.j.^,,.1—-t-anl/c
"(30)—Addresses external corrosion control of underground stee tanns.
ITiFTudinq sacrificial cathodic protection, protective coatings, and
electrical isolation of tanks from other underground metallic structures.
-34-
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MSI Pressure Piping Code B31.1, Power Piping (31)—Considers
allowable stresses in UST piping systems due to internal and external
pressures, fluid expansion effects, dynamic effects (impact, wind,
earthquake, vibration), weight effects (live, dead, test), thermal
expansion and contraction loads, as well as the cyclic behavior of
expansion stresses. Provisions are also made for protection against
corrosion and erosion.
ANSI Pressure Piping Code B31.3, Chemical and Petroleum Refinery
Piping (32)— Considers the loads on LIST piping as well as dynamir Affartg
from fluid discharge reaction; thermal loads caused by restraints; loading
caused by temperature gradients and by differences in expansion
characteristics; effects of support, anchor, and terminal movements; and
effects of reduced ductility. It also specifies design criteria for
nonmetallic piping.
ANSI Code B31.4 Code for Liquid Petroleum Transportation Piping
(33)—Prescribes minimum requirements for design, materials, construction,
assembly, inspection, and testing of piping that transports petroleum
products. Piping components covered in this code include pipes, flanges
butting, gaskets, valves, relief devices, fittings, hangers, and
supports. As this Code does not cover pipes designed for internal
pressure less than 15 psig, use of this code for UST systems is limited.
ASME Boiler and Pressure Vessels Code, Section VIII, Divisions 1
and 2 (11 )—Covers rules that are applicable for design and
of pressure vessels and piping systems used for storage of chemicals. The
code is usually invoked as a result of requirements specified by owners of
the storage facility.
Other applicable standards include the following:
o NFPA-30, Flammable and Combustible Liquids Code (8)
o NFPA-329, Underground Leakage of Flammable and Combustible
Liquids (34)
o National Association of Corrosion Engineers (NACE) Technical
Practices Committee, TPC Publication 2, Coatings and Linings
for Immersion Service (35)
o NACE Coatings and Linings Handbook, 1985 edition (36)
o American Society for Testing on Materials (ASTM) Specification
D4021-81, Standard Specification for Glass-Fiber-Reinforced
Polyester Underground Petroleum Storage Tanks (37)
STATE AND LOCAL REGULATIONS
States that have already established UST regulations are California
(38), Connecticut (39), Florida (40), Maryland (41), and New York (42).
Several other states and local governments, e.g., Maine, New Jersey, New
-35-
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Hampshire,-Rhode Island, Dade County, Florida, and Suffolk County, New
York are in the process of finalizing similar standards. These
regulations incorporate, and may adopt as a requirement, industrial
standards and codes developed by trade and professional associations.
DISCUSSION
In the case of high-pressure systems, the ASME Boiler and Pressure
Vessel Code is a legally applicable standard for the design of UST
systems. However, for low-pressure and atmospheric systems, no such
single document or professional group that prepares such documents
exists Instead, hundreds of standards, guidelines, and recommended
practices, many having overlapping requirements prepared by several
organizations, are presently followed for the design of UST systems.
There is a need for a cohesive and definitive set of rules and standards
that can be applied universally.
U S industries have a successful tradition of preparing and
implementing voluntary standards that have improved in public safety and
technology. The ASME Boiler and Pressure Vessels Code is a prime example
of such a tradition. The success in preventing explosions in high
pressure boiler systems as a result of the use of this Code is well
documented (43).
Preparation of the ASME Code and other similar industrial voluntary
standards in the U.S. are coordinated by one general umbrella
organization, viz., the American National Standards Institute (ANSI).
This national body provides an organizational framework that brings
together various groups that may have an interest in the process, content,
and implementation of a particular set of standards. For example, Figure
5 illustrates the various groups involved in the development and
implementation of the Boiler and Pressure Vessel Code. The voluntary
interaction and participation of groups such as these generally result in
standards that are more comprehensive and complete, more acceptable to the
user community, and readily enforceable by regulatory agencies. This
successful organizational interaction is worthy of consideration to
improve standards applicable to UST systems.
-36-
-------
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-37-
-------
SECTION 6
INSTALLATION TECHNIQUES
Improper installation of UST systems is a common cause of leakage.
Adherence to proper installation and testing procedures that are based on
sound engineering principles can reduce such leakages. These procedures
should include:
o Investigation of soil conditions and characteristics;
o Selection of materials of construction appropriate for design
conditions;
o Selection of proper bedding and backfill material;
o Handling and care of equipment during construction;
o Tightness testing requirements;
o Supervision requirements.
Although state regulations vary, most of them require installation of
secondary containers, overfill protection, leak monitoring wells, and
alarm systems to signal the presence of leaks.
TANK INSTALLATION
Improper handling of storage tanks before installation and poor
installation practices can damage the protective coatings of steel tanks,
puncture fiberglass tanks, and result in poor foundations, inadequate
anchoring, and improper tank levelling. Damage to coatings can accelerate
corrosion. Failure to level a tank properly can create air pockets in the
tanks, which leads to inaccurate inventory measurement and masking of
leakage.
Chemical properties and electrical characteristics of backfill around
an UST can affect the corrosion rates of steel tanks. The coarseness of
pea gravel, one of the more commonly used backfills, can damage a coated
tank during direct contact. Pea gravel can also permit water to collect,
and hasten corrosion.
Most manufacturers (44-48) provide step-by-step procedures for tank
installations which must be followed to validate the warranties.
Professional organizations such as the API, and the Petroleum
Engineering Institute (PEI) recommend that an owner's best protection
against UST system leakage is provided by (10,49):
o Strict compliance with applicable Federal and state regulations;
-38-
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0
0
0
0
0
Proper planning and design of the UST system;
Appropriate choice of materials based on site-specific conditions;
Capable and adequate supervision and inspection during
installation;
Strict adherence to design and installation requirements by
installers and contractors;
Appropriate tests at different stages of installation;
Proper registration of the UST with the appropriate agency in the
state.
Some typical provisions for clearance, depth of excavation, anchoring
backfill requirements, etc. by PEI, API, and New York State are summarized
in Table 5. PEI further requires that tank excavations have adequate
space for tanks, liners, monitoring wells, cathodic protection, anchoring
and other equipment, and for placement and compaction of backfill
materials. Definitions of adequate space and acceptable backfill
materials differ for steel, FRP, and other tanks.
Most installation practices require that filter fabric be placed
between the backfill and adjacent unstable soil, bogs, swamp areas, and
landfills to prevent the backfill from migrating (49). FRP tank
manufacturers also recommend the use of filter fabric in wet
installations. If sand and pea gravel are used to backfill the tank
excavation, they should also be separated with filter fabric.
Tanks should be ballasted with the intended product as soon as
possible after backfilling (48) to prevent the tank from floating in a
high groundwater table. Water ballast may be used as an alternative, but
it must be removed before installation of submerged pumping units in the
tank. Tanks should not be set directly on a concrete slab or placed on
hard or sharp objects that could cause damage to e-ither the tanks or tank
coatings.
FRP tank manufacturers and major oil companies recommend two methods
of anchoring: concrete anchor pads or prefabricated deadman anchors
(46,47,50). Anchor straps should be installed so that the tank and
coatings are not damaged. The tanks must be electrically isolated from
the anchor straps by placing a section of rubber tire between the tank and
anchor strap.
Pressure testing underground storage tanks is recommended at several
times during the course of installation: when the tanks are delivered to
the site; when they are placed in the excavation pit but before
backfilling; and after the installation is completed (48,49).
Multiple tanks storing the same product should be connected through a
siphon to permit product equalization in the tanks (48). Such
-39-
-------
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-40-
-------
Interconnected tanks should be of the same diameter and placed at the same
depth in the ground. Siphon action at deliveries greater than 300 gal/min
is too slow to maintain the same level in two or more tanks. To fill such
interconnected tanks to near capacity, either the delivery rate must be
slowed to provide time for liquid leveling, the delivery hose must be moved
from one tank to the other, or, more conveniently, manifold fittings and
withdrawal piping should be installed.
SECONDARY CONTAINMENT SYSTEM INSTALLATION
The installation of secondary containment systems is often a complex
task that can only be performed by qualified contractors.
Flexible Membrane Liners
General guidelines for installation of flexible 1iners include
(18,49,51):
o
o
Installation should be done during dry, moderately warm weather;
The excavation base and wall should be firm, smooth, and free of
sharp rock or debris;
o Factory-trained personnel should perform the thermowelding or
adhesive bonding that may be required. If bonding is to be done
at the site, prior training of these personnel should include
working under typical site conditions. Proof of personnel
qualifications should be available at the site;
o A protective layer of puncture-resistant fabric may be required
under the liner to prevent damage from paving, rocks, etc.;
o Liners should be pitched toward sumps; the pitch should be tested
by pouring harmless test liquids at the high point of the system
and measuring when these liquids reach the sumps.
Concrete Vaults
Concrete vaults are mandatory for the underground storage of gasoline
and other fuels in some local jurisdictions, e.g., New York City. Concrete
vaults must be designed and constructed to insure that joints do not leak
or walls do not crack when exposed to a freeze-thaw weather cycle.
Standards and codes established by professional associations such as the
American Concrete Institute (ACI) should be followed in their design and
construction.
Clay Liners and Soil Sealants
The installation of a clay liner involves a series of steps (2,51,52).
The excavation should first be drained and stabilized. A bottom layer of
clay must be laid and compacted using steel wheel rollers. The thickness
-41-
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of the layer depends upon the soil, its clay content, its density, and
local regulations; the minimum acceptable thickness is 6 in (51).
Soil sealant can be made of soil cement or bentonites. Soil cement is
a compound mixture of Portland cement, water and in-place soil. Some of
the procedures to be followed are:
o The base and wall should be properly finished and well moistened
before placing the mix;
o The mix should be plastic enough to consolidate well, but not
loose enough to slip on side slopes;
o" Liners must be properly cured;
o Bentonite sealants should be wetted to saturation and then
compacted.
Double-Walled Tanks
Bed and backfill requirements and installation procedures for
single-walled tanks apply to the installation of double-walled tanks as .
well. Special care should be taken in handling double-walled tanks.
Depending on the construction material, weight, and size, double-walled
tanks may require special transport and heavy-duty cranes for placement.
Once installed, both their inner and outer walls should be tested for
tightness.
PIPING AND ACCESSORIES INSTALLATION
Selection, installation, and testing of piping for underground tank
systems must be based on appropriate standards and guidelines. Care must
be taken to ensure that field attachments are made properly and are
protected against corrosion. Such care includes cleaning and preparation
of the surfaces to be connected, proper use of thermowelding or mechanical
clamps, and application of effective corrosion protection to the bare
metal surfaces before backfilling. The following are some recommended
practices for the installation of piping systems for USTs storing
petroleum products (48):
o Product lines should be run in a single trench between the tank
area and the pump island area. Similarly, vent lines between the
tank area and the building (or other structure to which the
above-ground vent 1ines are attached) should be placed in a
single trench;
o All underground pipe lines (both metallic and nonmetallic) should
be laid on a bed of at least 6 in of well-compacted noncorrosive
material, such as clean sand or gravel. Bedding and backfill
should be of the same material;
-42-
-------
o Ripe lines should not cross over underground tanks;
o Vent lines should have a uniform slope of not less than 1/8 in/ft
toward the tank;
o
o
Product lines should be at least 12 in below the finished surface;
Pipe failures can be minimized by installation of swing joints
where pipes connect with tanks, or at multiple pipe junctions.
Since fiberglass piping is flexible, it does not require swing
joints if there is at least 4 ft of straight run at pipe junctions
where directional change exceeds 30 degrees;
o FRP pipes are normally joined with adhesive; all joint surfaces
must be cleaned before adhesive is applied.
Galvanized steel piping can be used where size, complexity, and design
characteristics preclude the use of fiberglass material (50). Fill pipes
and other vertical risers under dispensers or vapor equipment are usually
made of galvanized steel pipes with standard galvanized malleable iron
fittings. Joints with steel piping should be made with an approved
gasoline pipe compound or Teflon tape.
Steel tanks with cathodic protection require nonmetallic tank bushings
in tank openings at all points of connection between product and vent
piping to the tank, with separate protection provided for steel piping
(48). When remote pumps are used, an insulated fitting should be installed
in the electrical conduit at the pump. After piping has been tested, all
exposed threads of galvanized pipes should be coated with a coal-tar
product or tape film. This prevents the formation of an electrolytic cell
between the galvanized fitting and the threaded area.
General guidelines for testing of piping systems before backfilling are
as follows (48):
o The piping must first be isolated from the tanks, pumps, and
dispensers. The piping should then be subjected to an air test of
1.5 times the working pressure, but not less than 50 psig. The
pressure should be maintained for a minimum of 60 minutes;
o Leaks can be detected by applying soap suds to all joints while
the piping is under pressure.
Shell Oil specification (50) recommends a low-pressure, 5 psig, leak
test before the 50 psig test. It also specifies a 30 min hold time for the
higher pressure.
Another major oil company specification calls for pipes to be tested
before the trenches are backfilled. Piping lines are isolated from tanks,
and then pressure-tested (50 psi) from the tank connection to the base of
the pump/dispenser. Pressure should be maintained for at least 15 min.
-43-
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All cathodic protection components should be inspected and tested. A
negative voltage of at least 0.85 volt (as measured by a copper-copper
sulfate reference half cell) is deemed to be adequate (49). During this
test, anodes and reference cells must be backfilled with the same material
used for tanks and piping and soaked in water to accelerate conductivity.
Electrical continuity between the tank and associated piping should also be
tested. No continuity should be detected, where dielectric bushings and
unions are installed, indicating that they have been effectively isolated.
DISCUSSION
Since most state regulatory codes and new Federal acts regard the owner
as having primary responsibility for LIST system compliance, owners can take
the following steps after installation to protect themselves from potential
allegations of poor design, workmanship, and operational practices (80):
o Maintain a file of pre-operational test results at least for one
year;
o Prepare and file "as-built" drawings or photographs of underground
piping, monitoring, and other system components. The documents
may be in the form of a "marked-up" set of installation drawings;
photographs showing the location of piping, conduit, and other
significant system components; or both;
o Maintain a record of installation instructions, test procedures,
and preventive maintenance schedules, including tank charts
indicating gallonage at various depths;
o Train personnel in the operation of the system, inventory control
procedures, and operation of leak-detection and monitoring systems;
o Establish a program of preventive maintenance and periodic testing
procedures.
Installation procedures and guidelines provided by various agencies and
institutions address most steps that are necessary to prevent impairment of
UST systems containing gasoline. If the procedures and guidelines are
followed, leaks caused by improper installation would be reduced to a great
extent. However, at present, there is no way to determine whether these
procedures are understood or in fact followed in the field. Procedures
that apply to storage systems for other products are also not readily
available. Training and, if necessary, certification of installers, and
development of installation procedure for a wider range of applications are
needed.
-44-
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SECTION 7
OPERATING PRACTICES AND GUIDELINES
h™0hH9 ?roc*d"res and guidelines for UST systems that store gasoline
have been developed by manufacturers and professional and trade
organizations. These procedures are designed to prevent release of
products during filling and transfer operations, and to enable prompt
recognition of underground leaks that result from impairment of tanks
pipes, or accessories. The procedures include: overfill prevention '
transfer spill prevention, vapor recovery, and leak detection and '
monitoring.
OVERFILL PREVENTION
An ideal overfill prevention system should include (2 53 54)- a
level-sensing device with an alarm to alert the operator of impending
overfill, and an automatic product shutoff when the tank is full.
Level-Sensing Devices
Available level-sensing devices operate on the basis of one or more of
the following principles:
o Buoyancy
o Hydrostatics
o Capacitance
o Thermal conductivity
o Optics
o Ultrasound
Detailed descriptions of these devices are available in product
literature (55-65). Devices that operate on the principles of buoyancy
hydrostatics, capacitance, and thermal conductivity depend on fluid flow
rates, pressure, and temperature. Those that operate by reflected light
(optics) or sound waves (ultrasound) are generally not affected by fluid
temperature and pressure variables. All these sensing devices can be
equipped with an audible or visual high-level alarm. These devices are
found to measure liquid levels with an accuracy wthin 1/16 in to 1/2 in.
Retrofitting existing USTs with level-sensing devices with high-level
alarms would require considerable site excavation and retrofitting.
Automatic Shutoff
Automatic shutoff systems stop product delivery at a level that
permits drainage of the transfer hose without overfilling the tank. Flow
can be partially shut off by an inexpensive float vent valve installed in
the tank vent line that severely restricts product flow when 95% of tank
capacity is attained. A ball in the float vent valve closes the vent
line when the tank is 95% full and blocks the venting of air and vapor
This blockage causes the flow rate to decrease from a typical 400-450
gal/min to 3-5 gal/min. The reduced flow rate allows the delivery
operator to shut off the delivery, avoiding spillage. A completely
-45-
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automatic-shutoff system is not commercially available in this country
(91). However, such systems are claimed to be available in Europe. One
primary reason for the wider use of these systems in Europe is believed
to be the standardized designs of tanks and delivery trucks. The wide
variability in tank and truck design make such systems less used in this
country.
Regulations
Many states have regulations that call for overfill protection. For
example, California regulations (38) require that all underground storage
tanks be equipped with an overflow protection system that includes the
following provisions:
o 'A spill catchment basin that surrounds the fill pipe and prevents
the inflow of regulated substances into the subsurface;
o A level-sensing device that continuously monitors and indicates
the liquid level in the UST;
o An audible or visual alarm system triggered by a liquid-level
sensor to alert the operator of an impending overfill condition;
o An automatic shutoff device that stops the flow of delivered
product when a UST is full.
New York regulations (42) require product delivery operators to
determine if a tank has the capacity to receive additional petroleum.
Florida requires overfill protection in the form of an impervious manhole
that acts as a containment in case of overfill (40). Maryland relies on
strict operating procedures to prevent overfill (41).
TRANSFER SPILL PREVENTION
Proper operating practices that should be followed to prevent
transfer spills are well documented (34). These practices require:
o Tight connections between the hose and fill pipe;
o Periodic inspection of all transfer hoses;
o Inspection of tank ullage before product delivery to ensure
sufficient capacity;
o Proper identification of stored products and container capacities;
o Proper training of all operators who perform loading or unloading
operations.
-46-
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VAPOR RECOVERY SYSTEMS
Gasoline vapors and volatile organic compound (VOC) emissions from UST
systems may violate ambient air quality standards. These releases occur
during UST filling and vehicle refueling. Vapors are also emitted from
truck tanks as gasoline displaces the gasoline-enriched air in the tank
These vapors can be controlled by venting through charcoal filters in the
truck tank itself or back into the UST.
Two types of vapor recovery systems exist: Stage I vapor recovery and
Stage II vapor recovery. In a Stage I vapor recovery system at a gasoline
station, shown in Figure 6, vapors are vented to the top of the tank truck
during product transfer either from individual vent lines connected to
each tank, or from a single vent outlet valve in an interconnected tank
system. In Stage II vapor recovery systems, vapors from the gas
dispensing outlet nozzles are diverted back into tanks through vapor
return pipe lines. A Stage II recovery system is used where product
mixture cannot be tolerated.
LEAK DETECTION
Leak detection is an integral part of the regulatory requirements for
prevention of leaks. Methods and strategies for leak detection include-
inventory control, in-tank continuous leak monitoring, nonvolumetric
methods, leak effects monitoring, and tank integrity testing. Several
states and local jurisdictions have come up with specific requirements for
leak detection.
Inventory Control
Advocates of inventory control claim that it is -he simplest and most
economical method for detecting leaks. It is generally believed, however
that the technique has not worked well because recommended procedures are'
not always followed by practitioners.
An inventory accounting system that contains the following provisions
is believed to be very effective (66):
o A record of all sales and quantity of product otherwised
dispensed;
o Daily reconciliation between sales, use, receipts, and inventory
on hand.
The inventory accounting system also includes procedures to be
followed when products are received:
o
Gauging all tanks and checking them for water before and after
del ivery;
o Reinstalling all fill and gauge caps;
-47-
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o Calculating the amount of product received with proper accounting
for water level in the tank and comparing this with the amount
shown on the invoice.
Many of the manual procedures described above are unnecessary if an
in-tank automatic inventory system is used. These systems continuously
monitor the contents of tanks, record all deliveries, compare them with
metered deliveries, alert operators to product loss or leakage, and shut
off flow if the tank becomes too full.
Employees must be trained to look for evidence of leaks from both the
inventory control records and abnormal operation of pumping equipment.
Some of the more obvious signs of leaks are:
o Product level change in a tank during periods when product is not
dispensed. While this usually indicates a leaking tank, it might
also indicate unaccounted withdrawal, theft, or extreme
temperature change;
o An increase of water in the tank. Leakage of water into an
impaired tank is possible if surrounding ground is saturated.
However, an increase in water may also be due to a leaking gauge
or fill cap; both should be examined and, if necessary, made
watertight before concluding that the tank is leaking;
o Increasing differences between the amount of product received and
dispensed. These may also indicate a meter calibration problem
or theft;
o Large differences appearing consistently between amounts invoiced
and the tank gauges after deliveries. These may indicate a leak
in the remote fill line, which should be tested;
o A hesitation in delivery from a standard dispensing pump. This
may indicate a leak in the suction piping or foot valve, or, in
warm weather, vapor lock. Inventory control records may indicate
whether the cause is mechanical or whether product is actually
being lost;
o Meter spin without product delivery in a remote pumping system.
This may indicate a pipe leak;
o Gasoline odor in underground spaces adjacent to the station,
which may indicate leaks in either the tank or piping.
In-tank Continuous Leak Detection
Volumetric leak detection methods are the most vigorously pursued
in-tank continuous leak detection technologies by the UST industry. A
leak is quantified by measuring the changes in product level in the tank.
Each product level is then converted, with a knowledge of the containment
-49-
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geometry, to a corresponding liquid volume. The changes in volume are
then analyzed by various algorithms, and a determination of leak rate is
made. Currently, some 15 leak detection devices are available in the
market that claim to have the accuracy and precision that would be
acceptable to state and federal regulatory agencies. However, these
performance claims have not been fully validated, and these devices have
not yet been universally applied by the LIST community. A test program
initiated by the EPA in Edison, New Jersey, is presently underway to
establish the performance characteristics of these and other volumetric
leak detection devices (67).
Nonvolumetric Leak Detection Method
Nonvolumetric methods measure changes in a variable other than tank
product level. The most common approach is to monitor the presence of a
tracer gas (e.g., helium) or an acoustic signal in the tank. Changes in
these signal variables are evaluated, and after appropriate analysis, a
declaration of the tank integrity is made. Determination of a specific
leak rate based upon nonvolumetric observations, however, is difficult,
since a quantitative correlation between the measured variable and the
size of a leak is difficult to establish. Other deficiencies of the
method are:
o Potential for product contamination if tracer gas is not inert;
o Enhancement of small leaks and risk of explosion if tank has to
be pressurized;
o Long testing time.
Nonvolumetric methods can be used to detect leaks in double-walled
tanks and pipes by applying the method to both the interior of tanks and
pipes and the interstitial space between the double walls.
Detailed descriptions of available nonvolumetric leak detection
methods are given in (68).
Leak Effects Monitoring
Leak effects monitoring determines the presence of leaks by examining
the surrounding tank environment for evidence of product. Numerous
methods based on various types of instrumentation are available for
performing leak tests. These methods, while determining the presence of a
leak, do not provide a quantitative estimate of the leak rate. This
limitation is similar to that encountered with nonvolumetric methods.
State Regulations
In compliance with federal mandates, many states have introduced leak
detection requirements in their UST regulations. A summary of such
requirements in California, Connecticut, and Florida, are given in Tables
6-8 (69). For example, the state of California allows local agencies to
-50-
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TABLE 6. ACCEPTABLE LEAK DETECTION REQUIREMENTS AND ALTERNATIVES FOR
EXISTING TANKS UNDER CALIFORNIA REGULATIONS
Required Tests and Their Schedules
Inventory
. Tank Vadoze Ground- reconciliation
Acceptable integrity zone water Soils using automatic . Pipeline
alternative testing monitoring monitoring sampling metering devices leak devices
1
2
3
4
5
Monthly
Daily or Semi- One-time
continuous annually
Annually Daily
Annually
Weekly
One-time
One-time
Daily
Continuous
-51-
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TABLE 7. REQUIRED TANK INTEGRITY* TESTING
SCHEDULE IN CONNECTICUT
Material of
construction
Required tank integrity testing
12 to 9 months
3 to 6 months 21 to 24 months prior to end of
after installation after installation life expectancy**
FRP
Yes
Yes
Yes
Cathodically
protected steel
No
Yes
Yes
*Alternative methods and schedules for leak detection at existing tanks may
be used only with prior written approval of the commissioner.
**Life Expectancy is determined by the warranty time provided by the tank
supplier.
TABLE 8. FLORIDA LEAK DETECTION REQUIREMENTS
FOR EXISTING TANKS
Year tank
installed
Year visual/odor* monitoring
wells must be installed
Prior to 1970
1970 to 1975
1976 to 1980
1981 to Sept. 1 , 1984
1986
1987
1988
1989
*Florida requires groundwater monitoring wells
but does not normally require laboratory analysis of
samples.
-52-
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adopt and.impose any one of the eight monitoring alternatives, five of
which are shown in Table 6, for detection of leaks in existing tanks.
The requirements of the three remaining alternatives are similar to
alternative number 5 listed in the table. Several other states also have
introduced leak detection requirements, generally less elaborate than
those in California, in their LIST system regulations. Tables 7 and 8
list the requirements in Connecticut and Florida, respectively.
DISCUSSION
Efforts to improve operating practices should focus upon three
areas: methods, equipment, and people. The methods that are currently
in vogue for overfill and transfer spill prevention, leak detection, and
monitoring, etc. are largely developed by manufacturers of equipment and
by industry organizations. Adequacy of these methods has not been fully
evaluated and established. For example, in the inventory control method,
which is widely used by gasoline station owners for potential leak
detection, compensation for product temperature variation and product
evaporation, accuracy and resolution of dip-stick measurements and
dispenser meters, etc., are not accurate enough to indicate leaks until
an appreciable fraction of the stored volume has leaked out. Similarly,
in nonvolumetric leak detection methods, there are presently no
procedures that can be used to correlate leak indicator readings to leak
rate. Methodologies to enhance the utility of these methods are needed.
In the area of equipment, two deficiencies are noted. First is the
need to standardize LIST system accessories and delivery and transfer
equipment. The second area is the improvement of volumetric leak
detection devices. Performance characteristics of available leak
detection devices must be known. In addition, leak detection devices for
large-volume tanks and for tanks that store chemicals other than gasoline
need to be developed.
Operator training is the third area that requires attention. It has
been noted (3), that nearly 50 percent of almost 2500 reported leaks from
various states resulted from structural failures caused by vehicle
impact, ruptures caused by excessive pressure during tank tightness
tests, ruptures due to improper excavation, etc. These failures could be
reduced by training operators. An ideal training program should include:
o indoctrination on the hazards of underground leaks;
o lessons on the proper use of tools and equipment;
o lessons on trouble-shooting and problem resolution;
o basic principles of safety and emergency procedures.
Training programs should include periodic refresher courses and
training updates.
-53-
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SECTION 8
CORRECTIVE ACTIONS
Corrective actions are taken to prevent or inhibit UST system failure
processes, to revamp and restore those UST components that can be
repaired, and to properly dispose of components and systems that are
irreparably damaged or that are targeted to be taken out of service.
Inspection, maintenance, and repair retrofitting, and closure comprise
corrective actions.
INSPECTION
Proper
out beforea
purpose is
components
to correct
already in
provisions
of access.
inspection of tanks and other UST system components is carried
, during, and after the system is installed and operated. The
to ascertain the structural defects of the UST system
, either existing or impending, and to suggest possible remedies
the defects. Inspection of tanks and components that are
place, however, is difficult, if not impossible, unless
have been made for inspection ports, manways, and other means
A quality inspection program should include methods for identifying-
excessive corrosion, erosion of interior parts due to abrasion by
particles suspended in moving fluids, structural fatigue or cracking,
deterioration of liners and accessories, and weakened or cracked welds and
joints (70). To ensure quality of the inspection program, formal
checklists should be prepared and used, and records of inspection
maintained. Frequency of inspection is usually recommended by the
manufacturers.
Structural Integrity Test Methods
Methods to test the structural integrity of tanks and other UST
components include (21,71):
o Radiographic inspection;
o Ultrasonic inspection;
o Magnetic particle inspection;
o Liquid (dye) penetrant inspection;
o Hydrostatic tests;
o Eddy current inspection;
o High voltage spark method.
These methods have been used, in varying degrees of success, in other
industries, e.g., boiler and pressure vessels industry. The types of
-54-
-------
defects measured, applications, advantages, and limitations of these
methods are summarized in Table 9. A brief description of these methods
follow.
Radiographic Inspection
Radiography is used to detect surface cracks, internal cracks, voids,
and defects in weldments. The technique is based on the differential
absorption of radiation—either shortwave electromagnetic radiation or
particulate radiation—directed toward the part that is inspected.
Variations in density, differences in thickness, internal flaws,
inclusions, defects, etc. that may be present in the part result in the
absorbance of different amounts of radiation. The unabsorbed radiation
passing through the part is recorded on a sheet or film or is viewed on a
fluorescent screen. After development, the film presents a
two-dimensional "shadow picture" of the object, which is analyzed to
determine the location, size and shape of flaws. X-rays and gamma rays
are widely used in radiography. Neutron radiography uses a stream of
neutrons rather than electromagnetic radiation, but the result is the s'ame
(72).
Ultrasonic Inspection
Ultrasonic inspection uses high-frequency sound waves to detect
flaws. The sound waves travel through the material and experience a loss
of energy (attenuation) depending upon the internal structure of the
part. The reflected beam is analyzed to determine the location and size
of flaws.
Magnetic Particle Inspection
Magnetic particle inspection is used to locate surface and subsurface
flaws in ferromagnetic materials. When a tank is magnetized, magnetic
discontinuities perpendicular to the magnetic field form a leakage field
at and above its surface. This field is detected by applying finely
divided ferromagnetic particles over the tank surface. These particles
are magnetically held by the leakage field to provide an indication of the
location, size, and shape of the flaws.
Liquid (Dye) Penetrant Inspection
Surface cracks can be detected by applying liquid penetrants, which
seep into any opening by capillary action, to the surface. The process is
well suited for detecting all types of surface cracks, porosity,
shrinkage, laminations, and other similar discontinuities. It is used
extensively to inspect wrought and cast products made of both ferrous and
nonferrous metals, powder metallurgy parts, ceramics, plastics, and glass.
Hydrostatic Tests
Hydrostatic tests are performed by pressurizing the tank or piping to
a pressure higher than the design pressure. While under pressure, the
-55-
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tank is inspected for leaks.
tank may be leaking.
Eddy Current Inspection
A decrease in pressure indicates that the
Eddy currents are electrical currents induced within a conductor when
it moves through a nonuniform magnetic field. A test coil carrying
electrical current placed around the specimen creates the magnetic field.
Structural defects or variations within this specimen creates a nonuniform
magnetic field, and a corresponding eddy current is measured.
Eddy current inspection can be used to detect cracks, voids, and
inclusions. It can also be used to measure the thickness of a
nonconductive coating on a conductive metal, or the thickness of a
nonmagnetic metal coating on a magnetic metal.
High Voltage Spark Method
This method, also known as a hoiiday-detection method, is used to test
the integrity of coatings and linings. The method is based on a voltage
applied to the coating. The electrical resistance will be different
wherever a discontinuity (or "holiday") exists. The high-voltage (spark)
holiday detector is used for coatings with a thickness of 15 mils or
more. The low-voltage holiday detector is used for coatings with a
thickness of 20 mils or less.
Life Prediction
Inspection of underground tanks and other LIST components by visual,
sonar, or other nondestructive examination, is not always possible because
of the practical and technical limitations of such methods; nor do such
methods always give a reliable assessment of the physical s-tate and
integrity of underground tanks and other components. Predictive methods
based on theoretical or empirical models, therefore, become a useful tool
in leak prevention strategies. Such models can be used to complement and
supplement information gathered from physical inspections. Two predictive
models that have been proposed and used by the Petroleum Association for
Conservation of the Canadian Environment (PACE) are: the Soil
Aggressiveness Value (SAV) method (73) and Roger's Regression Analysis
(74).
Soil Aggressiveness Value (SAV) Method—
The SAV method is based on the premise that the age at which an
underground storage tank fails (leaks) is directly related to the soil
condition it is exposed to. The soil condition is assigned a numerical
value, viz., soil aggressiveness value (SAV). SAV is an aggregate number
determined on the basis of the points assigned to the following soil
properties at the site:
o Average values of soil resistivity, soil pH and soil moisture;
o Differential values, i.e., the ranges of resistivity and pH;
-57-
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o Presence of sulfides (which
surface of the tank).
promote bacterial action on the
Table 10 gives the points assigned to each of the above properties.
A relationship between SAV for the soil at a particular site and the
probable tank age at failure was established by PACE using actual
age-at-failure data and corresponding soil SAV data. This relationship,
illustrated in Figure 7, can be used as a decision tool for tank testing,
replacement, or retrofitting with additional protection. The use of this
graph is explained below.
Suppose an existing tank 25 years old is at a site with SAV = 10.
This ordered pair of SAV and age falls in region 1 of Figure 7, marked by
the b'oundary curve D. The most prudent decision is to immediately replace
the tank, because the actual tank failure data showed that 60 percent of
tanks that failed had an SAV/Age combination falling in region 1. If the
ordered pair of SAV and age of another tank falls in region 2, the
decision would be to replace or retrofit the tank with additional
protection based upon tests and inspection of the tank. The curve S that
marks the lower boundary of region 2 is drawn such that an assertion can
be made with a 95 percent confidence that failure data point for any tank,
most likely, will fall in regions 1 and 2 combined. Region 3 represents a
condition in which a tank is likely to fail at an age less than what is
normally considered an average useful life, viz., 17 years. Economics
dictate that additional tank protection should be provided to reduce the
failure probability and increase tank life to 17 years or more as desired
by the tank owner. Finally, region 4 represents a benign region where the
tank life is likely to be more than 17 years and thus no corrective action
is warranted. Based on the actual failure data, PACE (73) defined the
regions 1, 2, 3, and 4 as follows:
1.
2.
3.
4.
Region
1—180
I<69 and SAV>4
I<69 and SAV<4
Recommended Action
Replace tank
Test, and replace or retrofit
Retrofit
Benign, no corrective action warranted
where I = SAV x Age
The SAV method described above is conceptually attractive and has some
similarity to the well-established fatigue life prediction method used by
the manufacturing industry for design against fatigue failures. In the
fatigue design, the number of cycles to failure—a measure of the
"age"--is plotted against applied stress (Figure 8). A test data line is
drawn, line A in the figure, that gives the best fit of the data. (Note
that line A is the one-to-one equivalent to curve S in the SAV/Age
graph.) A factor of safety of 20 on number cycles and 2 on applied stress
is used to obtain line B which is used for safe fatigue design.
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TABLE 10. BASIS FOR THE EVALUATION OF UNDERGROUND
ENVIRONMENTS (SAV SYSTEMS)
I. BASIC CHARACTERISTICS
Soil Resistivity
Soil pH
Soil Moisture
less than 300
300 - 1,000
1,000 - 2,000
2,000 - 5,000
5,000 - 10,000
10,000 - 25,000
greater than 25,000
less than 3
3-5
5 - 6.5
6.5 - 7.5
7.5 - 9
greater than 9
Saturated
Damp
Dry
II. DIFFERENTIAL CHARACTERISTICS
Resistivity
(ratio of
extremes)
Soil pH
(difference
in pH value)
III. SULFIDES
greater than 1:10
greater than 1: 5
greater than 1: 3
less than 1: 3
3
1.5 - 3
0-1.5
Positive
Negative
POINTS
12
10
8
6
3
1
0
8
6
4
2
1
0
3
2
0
3
2
1
0
2
1
0
4
0
Based on (41 ).
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180
10
15 20
TANK AGE (TA)
25
30
Region 1 Replace
2 Test, and replace or retrofit
3 Retrofit
4 Benign, no corrective action
warranted
Figure 7. Tank evaluation graph. (Adapted from (73).)
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10'
10°
10 =
Best-fit curve of test data
B) Design curve with factors of safety: 2 on
stress, 20 on cycles
10
J l i t i i 111
10
10'
103 1Q4
Number of Cycles
10=
10°
Figure 8. Construction of design fatigue curve.
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The SAV method proposed by PACE differs from that described above for
fatigue design in one fundamental way. The SAV method allows the UST
system to continue to operate, albeit with tests and additional
protection, above the most probable failure curve S (in region 2), whereas
the time-honored approach for design against fatigue imposes a factor of
safety that brings the design stress and applied number of cycles far
below the failure regime, i.e., below curve B. This fundamental
difference in the approach used for the SAV method needs to be evaluated
before it can be recommended for use in UST system design.
Roger's Regression Analysis—
As the name suggests, this is an empirical method based on statistical
analysis of the age-to-leak data correlated to measureable characteristics
of the'tank environment (74). The correlation equation for mean age-to-
leak is given by
L = 5.75 R-05 T--017
exp
(.12P - .42 M - .265)
where
L =
R =
T =
P =
M =
S =
mean-age-to-leak, years
soil resistivity, ohm-cm
tank size, Imperial gallons
soil pH
a factor related to
1 = saturated, 0.5
a factor related to
moisture content in the soil
= damp, and 0 = dry
sulfides content in
1 = strongly present, 0.5 = trace, and
the
0 =
soil;
no sul fides
of
range of
certainly close to
It is claimed that approximately 75 percent of the total variability
in the dependent variable L is explained, with a high degree of
statistical significance, by the full set of independent variables
included in the model. Roger's equation, exercised with four sets
values for independent variables, results in a mean age-to-leak
13.5-16 years, with an average of 14.9 years. This is
reported mean ages-to-leak of 17-19 years.
Some limitations of the method, however, should be noted. Regression
equations, by their very mathematical construction can only be used to
explain the data within the range of the independent variables that have
been considered, and not outside of their range. Thus, caution should be
exercised in the use of the method. A second issue is that the method
does not allow the changes in the values of the independent variables that
are likely to occur during the life of the UST system, e.g., changes in
soil resistivity and moisture content, during the life of the tank
system. The third consideration is that the method assumes that tank
failure occurs only as a consequence of the external soil properties
without any influence of internal conditions.
Even with the limitations noted above, life prediction methods are
useful tools to establish test, maintenance, and repair schedules and
programs. However, additional work is required to improve the accuracy
and range of applicability of such methods. The UST system failure data
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base must be improved, both in its content and sample size, to develop
better statistical models that incorporate a wider range of factors that
contribute to failures.
MAINTENANCE AND REPAIR
Maintenance and repair are routine tasks required to keep an UST
system in working order. Equipment manufacturers usually recommend
maintenance tasks and their required frequencies. Maintenance helps
increase mean time between failures and the operating life of UST
components and systems.
Repairs must be carried out once impairment of tanks or other
components of UST systems are detected. Generally, repairs can be carried
out only by qualified professionals responsible for maintaining the
equipment.
There are presently no standard maintenance or repair practices or
programs available that tank owners can follow and implement. However
many states are now introducing regulations that mandate proper inspection
and maintenance repair programs. Implementation of such state
requirements would require a pool of qualified and trained inspectors,
testers, and maintenance personnel. Personnel must be trained to
recognize impending failures and failure warnings, and to respond with
appropriate corrective actions. Input from designers, manufacturers
installers, and suppliers is required to develop training manuals with
procedures and practices that can be easily implemented by the owners and
operators of UST systems.
The need for certification or licensing of inspectors, testers, and
maintenance personnel must also be evaluated. The issues to be resolved
are:
1. What is an acceptable level of competency required for these tasks?
2. Is certification or licensing necessarily the most effective way to
impart this competency and to develop a competent labor pool?
3. Should the certifcation or licensing, if determined to be valid, be at
the national, state, or local level?
4. What are the organizational, economic, and institutional issues that
must be resolved for a certification or licensing program to work
effectively?
RETROFITTING
Retrofitting extends the useful life of an existing UST system. The
decision to retrofit depends upon the nature and degree of the system or
component impairment, physical condition of the site and its surroundings,
and the anticipated performance improvements that result from
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retrofltti'ng. Two retrofitting ideas that merit consideration are tank
relining, and retrofitting unprotected steel tanks and piping systems with
cathodic protection.
Relining of Tanks
The relining of the interior of a tank is an acceptable retrofitting
method provided: (1) the tank has never before been so restored; and
(2) the metal thickness of the tank is adequate to ensure the structural
integrity of the tank. The steps involved in relining include opening the
tanks, preparing and inspecting the tank interior, selecting a lining
material, applying the lining, pretesting before closing, and tank closing
and final tightness testing (75).
Opening of Tanks
Appropriate safety procedures, including tank isolation, product
removal, removal of flammable vapors (gas freeing), and testing of
flammable vapor concentrations, should be implemented before opening and
entering a tank. If there is no manhole in the tank, an opening with
minimum dimensions of 18 in x 18 in should be cut through the top of the
tank. Cutting through welded seams should be avoided.
Preparation of Tank Interior
The sludge accumulations on the bottom of the tank must be removed and
the interior surface of the tank prepared for inspection. If the wall is
badly deteriorated, the tank cannot be lined and returned to service. The
following defects are considered as limiting conditions:
o
o
An open seam or split longer than 3 in;
A perforation larger than 1-1/2 in, or a perforation larger than
2-1/2 in below an opening;
0 Five or more perforations, none larger than 1/2 in in any 1
area;
0 20 or more perforations none larger than 1/2 in in a 500
area.
0 A crack or fissure within 6 in of any seam weld.
To enable visual inspection of the defects, the interior surface is
abrasively cleaned to render it free of scale, rust, or foreign matter.
Perforations and seams are hammered with a brass ballpeen to obtain
structurally sound edges.
The tank surface must be cleaned of all dirt, grease, moisture, scale,
rust, and foreign matter. Abrasive blasting should be performed as per
SSPC specifications for white metal blast cleaning. The State of New York
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recommends sandblasting to SSPC-SP6 commercial blasting (2) Surface
preparation specifications vary depending upon the type of application
Before sandblasting, all perforations should be plugged with boiler plugs
or hydraulic cement.
Selection of Lining Material
Lining materials and adhesives should be compatible with the tank
material and stored products. Table 4 in Section 5 listed the
compatibility of 18 lining materials with six chemical products commonly
stored in UST systems. Advantages and limitations of seven lining
materials are listed in Table 11. While lining material suppliers are
believed to know the type and properties of adhesives used, and specific
combinations of liners, adhesives, and tank materials, such information
does not appear to be readily available in the open literature.
Application of Lining
Lining material can be applied by brushing, rolling, or spraying
(76). Brushing and, to a lesser degree, rolling, have the advantage of
working a coating into a rough or irregular surface. Spraying, however
is by far the most common application method. The latter includes
conventional air spraying, high pressure hydraulic airless spraying and
electrostatic spraying.
After the coating is cured, it is inspected for thickness and
integrity. Dry gauges, such as magnetic and semi destructive scratch
gauges, and a wet gauge known as the comb-type gauge (2) are used to
measure coating thickness and porosity. Other instruments that can be
used include surface temperature thermometers, sling psychrometers for
calculating dewpoint and its relation to the surface being coated, surface
profile comparators for blast-cleaned steel surfaces, and moisture meters
for concrete and masonry surfaces.
Tank Closing
If an opening has been cut into the tank, guidelines to closing it are
as follows (2):
o A 1/4 in thick steel cover plate, rolled to the contour of the
tank, should be made to overlap the hole at least two inches on
each side (e.g., plate should measure at least 26 in by 26 in if
manhole as cut 22 in by 22 in);
o The cover should be used as a template to locate 3/4 in diameter
holes not exceeding 5-in centers, 1 in from the edge of the cover;
o The cover plate should be sandblasted to white metal on both sides
and the entire inside surface coated with coating material to act
as a gasket;
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TABLE 11. ADVANTAGES AND LIMITATIONS OF COMMON LINING MATERIALS
Lining
Material
Advantages
Vinyls
Chlorinated
Rubbers
Epoxy, Coal
Al kyl s
Polyesters
Si licone
Zinc rich
Insoluble in oils and grease
Resistant to water and salt
Fire resistant
Good abrasion resistance
Low toxicity
Tough and flexible
Excellent resistance to water
Resistant to alkalis, acids
Good abrasion resistance
Excellent adhesion to concrete
and steel
Excellent resistance to salt and
fresh water
Good acid and alkali resistance
Relatively low cost
Excellent primers for rusted and
pitted steel
Good resistance to weathering
Relatively low cost
Excellent resistance to acids
and organic solvents
Good abrasion and abuse resistance
Can resist temperatures up to 760°C
Can be combined with other coatings
to improve properties
Resistant to weathering and mild
chemical fumes
Resistant to abrasion and
temperatures up to 370°C
Eliminates pitting corrosion
Limitations
Will not adhere to base steel]
Pinholes in dried film
more prevalent
Degraded by heat (60°C)
Difficult to spray
Embrittles in cold weather
Will not cure below 10°C
Not suitable for alkaline
surfaces
Hard and inflexible
Swells and softens by strong
alkalis
Only moderate chemical fume
resistance
Difficult to apply
Requires clean steel
Must be top coated
surfaces
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o Before the coating on the cover cures, the cover should be
fastened to the tank with at least 1/2-in diameter bolts. The
bolt shafts should be placed through the holes from the inside of
the tank and held in place by spring clips, then fastened with
local washers and nuts;
o After being bolted to the tank, the cover plate and surrounding
tank surface should be properly sandblasted, coated with material,
and allowed to cure before backfilling the hole.
After closing the tank and before backfilling, tightness testing is
recommended.
Retrofitting with Cathodic Protection
Retrofitting of UST systems with cathodic protection is another way
to extend the useful life of old installations. However, the design and
layout of the UST system may prevent this option. For example, if tanks
are buried very close together, there may not be enough space to position
the anode between them. In such a situation, unless major changes in the
layout of the tanks are made, retrofitting with cathodic protection will
not be effective. Recommended practices for retrofitting by cathodic
protection issued by NACE suggests that suitability of this option has to
be evaluated on a case by case basis.
TANK SYSTEM CLOSURE
Proper closure of underground storage tank facilities is necessary to
prevent the environmental hazard posed by abandoned leaking USTs.
There are few state regulations that address tank system closure.
For example, Florida regulations (40) require owners to dispose of the
tank as per API (77), within 90 days of discovery. California requires
that property deeds include notification of abandoned tanking (38).
(8):
Underground tanks can be taken out of service by one of three methods
o Temporary closure, in which the tank and piping system are emptied
and sealed;
o Abandonment in place, in which the tank and piping system are
emptied, filled with inert material, and sealed;
o Removal, in which the tank and piping system are emptied and
removed from the ground.
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Temporary Closure
Underground tanks may be considered temporarily out of service if
they are in good condition and idle, but will be returned to service
within a defined period of time (depending on state and local regulations)
or will be abandoned in place or removed within 90 days.
For this procedure all stored product should be removed from the tank
and piping system unless the stored liquid is flammable, in which case a
sufficient quantity (approximately 4 in) of product should be left in the
tank to ensure a saturated vapor space. This is done as a safety
procedure to keep the vapor space above the upper explosive limit. All
fill gauge pipe, and draw-off lines are then capped, however, keeping the
vent lines open. All power to the system must then be turned off and the
area secured against tampering.
Permanent Closure
Tanks can be closed permanently by abandonment in place or by
removal. The decision to choose either of these options depends on the
tank's age, condition, salvage value, and reuse potential. Other factors
that determine the option for permanent closure, include (77):
o Tank location and proximity to other structures and tanks;
o Cost of available labor and equipment;
o Proximity of disposal site;
o Use of the site after closure.
Abandonment in place—
The procedure for abandonment should include the following steps:
o Remove all product;
o Remove fill tube and disconnect fill gauge and all product lines;
o Plug all pipes and lines except the vent line;
o Punch a large hole in top of tank and fill tank with inert
material such as sand, gravel, or concrete;
o Keep records of tank location, date of abandonment, and method use,
Removal is the permanent closure procedure. The recommended steps
are as follows (77):
o Drain and flush the fluid in the pipes into the tank;
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o Remove all liquid and flammable/explosive vapors from the tank;
o Test for flammable/explosive vapors;
o Disconnect and cap all plumbing and controls that are not to be
used further;
o. Temporarily plug all tanks openings;
o Remove tank from ground;
o Plug or cap all holes except for a 1/8-in hole for venting;
o Transport the tank from site.
API provides guidelines for both storing and selling used tanks for
reuse or as scrap (77). These are summarized below:
o Store used tanks in secure areas to which the public has no access;
o Store gas-free tanks with all openings plugged, with one plug
havng a 1/8-in vent hole to allow the tank interior to remain
clean and to prevent the tank from being subjected to extreme
pressure differentials due to temperature changes;
o Record the tank's former content, gas-freeing technique, and date;
o Handle tanks that contain flammable liquids carefully, even if
gas-free.
Tanks that contained flammable or hazardous liquid should have
warnings on both the tank and the bill of sale stating that the tank
should not be used for storing drinking water or food.
DISCUSSION
The methods and procedures described in the proceeding paragraphs if
applied rigorously can substantially improve the useful life of UST
systems, reduce unexpected and costly failures, and reduce the potential
for fines and damages resulting from noncompliance of applicable
regulations or losses resulting from lawsuits.
It is not clear, however, how much of these procedures are adopted
and applied by practitioners in UST system operation. The need exists to
disseminate effective methods to the practitioners, and to train them in
their application.
-69-
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Retrofitting of existing tank systems with cathodic protection is one
area that warrants special attention. Attempts made in this study to
evaluate the degree of application and effectiveness of retrofitting LIST
systems with cathodic protection have resulted only in fragmentary
information. The general consensus among those experts, manufacturers and
potential users of the system who were consulted in this study appears to
be that retrofitting is effective if it is properly designed, installed,
inspected, and maintained according to the guidelines set forth in NACE
Standard RP-02-5 and API Publications 1615 and 1632. However, no
definitive data exist as to the extent of the actual use of the
retrofitting concept and the degree of success in extending the life of
UST system as a result of such use. A need to generate this information
exists.
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Product Storage Systems at Retail Outlets. No. 231-070. American
Petroleum Institute, Washington, DC, 1984. 28 pp.
73. Petroleum Association for Conservation of the Canadian Environment.
Underground Tank Systems: Review of State of the Art and
Guidelines. PACE Report 82-3. Ottawa, Canada, 1983. 69 pp.
74 Rogers, W. Tank Integrity Program. In: Proceedings of the
Washington Conference on Underground Storage Tanks, Arlington, VA,
1985.
75
76.
77.
American Petroleum Institute. Recommended Practice for Interior
Lining of Existing Steel Underground Storage Tanks. API Publication
1631. Washington, DC, 1983. 6 pp.
Tator, K.B. Protective Coatings. Chemical Engineering, Deskbook
Issue. McGrawHill, New York, NY, 1972.
American Petroleum Institute. Recommended Practices for Abandonment
and Removal of Used Underground Service Station Tanks. API Bulletin
1604. Washington, DC, 1981. 4 pp.
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APPENDIX A .
CORROSION PREVENTION
CORROSION PROCESSES
Engineering alloys have an inherent tendency to revert to their more
stable oxide forms when exposed to the environment. This reversion
process induces corrosion which may be defined as the process of metal .
deterioration that occurs as a consequence of chemical or electrochemical
reactions with the surrounding environment. Corrosion can occur in both
external and internal surfaces of LIST system components.
Most of the destructive effects of the corrosion process in steel USTs
and piping are the result of electrochemical reactions. Such reactions
take place on metal surface areas with differing electrical potentials
(anodes and cathodes) that are electrically connected with an
electrolyte. Anodes and cathodes exist on the surface of almost all
engineering alloys because of inherent chemical or structural
nonhomogeneity, surface discontinuities, inclusions, heterogeneities, and
surface contamination incurred during fabrication, handling, and
installation.
The corrosion can be either general or localized, with localized
corrosion being far more destructive because of its intensification of
electrolytic cell activity.
General Corrosion (1-6)
When a metallic surface is wetted by moisture or water an electrical
potential is created between anodic (+) and cathodic (-) sites located a
short distance from each other on the surface. The moisture or water,
which contains equal concentrations of positively charged hydrogen ions
(H+) and negatively charged hydroxyl ions (OH~), permits the transfer
of ions between the anodic and cathodic sites, in a manner similar to an
electrolytic cell, resulting in corrosion of the anodic cell. As the ion
transfer process proceeds, oxidation occurs at the anodes, and hydrogen
gas, which inhibits the corrosion process, accumulates at the cathodes.
However, the hydrogen combines with the oxygen to form water, and the
electrochemical reactions and microcorrosion processes at the individual
anodic cells are continued. When a large number of such microcorrosion
cells form on the metal surface, uniform metal loss or general corrosion
occurs.
The severity of corrosion depends upon the magnitude of the electrical
potential differences, which are greatly influenced by the chemical,
structural, and surface characteristics of the metal surface and water or
moisture content, chemical composition, conductivity, pH, and temperature
of the soil.
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Localized Corrosion (1-5)
Localized corrosion includes pitting, crevice corrosion, bimetallic
qalvanic corrosion, and stray-current corrosion. These occur in a manner
similar to general corrosion but result in more site-specific destruction
of metal.
Pitting occurs in minute locations on metal surfaces where protective
oxide films or coatings have broken down. This breakdown is followed by
the formation of electrolytic cells, the anodes being the minute areas of
exposed metal and the cathodes, the larger surrounding area of the
protected metal. The electrical potential difference induces a flow of
current resulting in rapid corrosion of the anodes. Pitting processes are
accelerated in the presence of chloride ions, particularly when combined
with such depolarizers as oxygen or oxidizing salts, e.g., ferrous
chloride. Once an electrical potential has been established, the solution
within the pit usually becomes increasingly acidic and corrosive, even
though the surrounding material may be neutral or alkaline.
Concentration cells or crevice corrosion—
Crevice corrosion is often associated with conditions where moist,
stagnant fluid areas are in contact with the tank's metal surfaces. The.
bottom of a tank pit is an ideal site for crevice corrosion. The most
important factors in initiating crevice corrosion processes are variations
in oxygen and metal ion content, pH, and temperature of the electrically
conductive environment (electrolyte) in contact with the tank.
Bimetallic/galvanic corrosion—
The coupling of two dissimilar metals placed in an electrolyte results
in bimetallic or galvanic corrosion. The magnitude of the corrosion
current depends upon the differences in electrical potential of the
dissimilar metals.
In the galvanic series of various metals and alloys (Table Al ) metals
at the top of the list are more reactive (anodic) and have a greater
tendency to corrode than those at the bottom of the list (7). Coupling of
metals far removed from each other in this series will result in
accelerated corrosion of the anodic metal based on the increased
electrical potential or reactivity differential of the metals. For
example, a pipe, made of the more reactive (anodic) mild steel, connected
to a valve made of the less reactive (cathodic) bronze results in an
electrical potential difference that allows an electrochemical reaction
and corrosion to occur on the steel pipe. Therefore, when dissimilar
metals are placed in contact with each other, they should be as close as
possible in the galvanic series. Such galvanic corrosion is greatly
accelerated if the area of the cathode is larger than that of the reactive
anode.
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TABLE Al . THE GALVANIC SERIES OF METALS AND ALLOYS (7)
Corroded End (Anodic, or Least Noble)
Magnesium
Zinc
Galvanized steel or galvanized wrought iron
Aluminum
Cadmium
Mild Steel
Wrought iron
Cast iron
13 percent Chromium stainless
18-8 stainless type 304
Lead
Tin
Naval brass
Nickel (active)
Inconel (active)
Yellow brass
Aluminum Bronze
Red brass
Copper
Silicon bronze
Nickel (passive)
18-8-3 stainless type 316
Silver
Graphite
Gold
Platinum
Protected end (Cathodic or Most Noble)
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Stray-current corrosion (1)—
Stray-current corrosion occurs in buried metallic objects when direct
current generated from outside sources (e.g., machinery, electrified
railways, or transit systems) travels through the electrolyte, e.g., soil,
and enters and leaves the object, e.g., USTs and piping. The area where
the current leaves suffers corrosion.
Other localized corrosion mechanisms include intergranular corrosion,
which occurs between the grains of metals and alloys due to electrical .
potential differences set up at the different grain boundaries, and
stress-corrosion cracking, which occurs when stress is applied to a metal
causing electrical potential differences at the grain boundaries. These
two types of corrosion are less common in carbon steel structures.
However, welded austenitic stainless steel components are particularly
susceptible to these corrosion processes when subjected to a
chloride-ion-bearing environment.
FACTORS THAT AFFECT EXTERNAL CORROSION IN USTS (7,10,12)
Soil Resistivity
Of all the factors affecting corrosion, in USTs, soil resistivity is
probably the most important. Soil resistivity (ohm-cm), a measure of the
resistance of soil to the flow of electric current, determines the
potential rate of corrosion of underground tanks and piping. It is a
function of moisture content and the ionized salts present in the soil,
well as of temperature. The lower the resistivity of the soil, the
greater the probability of corrosion. The general relationship between
corrosivity, resistivity "J -—"' ~u
Soil Type and Variation
as
and soil characteristics is shown in Table A2.
Variations in soil type and composition promote corrosion of USTs and
piping. Factors include moisture content, acidity, bacterial content, and
the presence of adjacent structures.
Moisture Level— . .
High moisture content in soil decreases soil resistivity and provides
an electrically conductive environment for both general and localized
corrosion.
High acidity of the electrically conductive environment increases the
conductivity and therefore the ion transfer and corrosion rate of carbon
steel UST systems. Higher acidity values (pH 4) are particularly
corrosive, while basic values (pH 9.5) are relatively noncorrosive to
steel.
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TABLE A2. SOIL CORROSIVITY VS. SOIL RESISTIVITY (10)
Corrosivity
Soil Characteristics
Resistivity
(Ohm-cm)
Very Low
Low
Medium
.High
Very high
Well drained gravel
Well drained sand & gravel
Poorly drained sand & gravel
Poorly drained fine sand
and silt
Poorly drained clay
10,000 & higher
5,000 - 10,000
2,000 - 5,000
1 ,000 - 2,000
Less than 1,000
Bacterial Action—
The metabolic activity of certain microorganisms can alter the
resistance of metal surface films and create electrolytic concentration
cells leading to crevice corrosion. Bacteria found in many soils consume
the hydrogen generated in steel corrosion processes. Hydrogen also
combines with sulfates in the soil to form hydrogen sulfides. The
reduction of hydrogen on the corroded metal surfaces accelerates
corrosion.
Adjacent Underground Metal Structures (2)~
Corrosion of underground tanks and piping may occur when new
structures/piping are installed near existing USTs or piping. Older
structures usually contain protective layers of corrosion products (rust),
which represent oxidized metallic ions, making them cathodic to newer
tanks or replacement piping. The system behave as an electrical cell --
with the older tank acting as the cathode, the newer structure as the
anode, and the moist soil between them as the electrolyte. Current
flowing through the system carries metal ions away from the newer
structure. If the surface area of the old structure is much larger than
that of the new structure, the latter will corrode even more rapidly.
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INTERNAL CORROSION FACTORS
Incompatibility of materials of construction with the stored products
resulting from either improper design or product contamination is the
primary cause of internal corrosion. UST systems at service stations are
commonly made of carbon steel because of its compatibility with gasoline.
Product contamination can occur due to several reasons (12). Consider
a gasoline station, for example: gasoline is a hygroscopic product.
Small amounts of water as well as oxygen are usually introduced during use
and fill operations. Condensation can also add water to the system. In
addition, the introduction of other contaminants (dirt and scale).and
bacteria lead to the formation of precipitates and sludge, which settle on
bottom surfaces in crevice areas and provide the environments that are
conducive to localized corrosion.
Mechanical factors also affect the rate of corrosion of tank
interiors. These factors include:
o Slope of the tank;
o Continual striking of the tank bottom by the measuring dipstick;
o Frequency of filling and emptying the tank;
o Length of the drop-tube fill connection;
o Dents and irregularities caused by installation.
Since water, which is corrosive to steel, has a higher density than
gasoline, it sinks to the tank bottom. If the tank is sloped, water will
accumulate in the sloped area as well as along the bottom and thus
contribute to internal corrosion. Severe internal corrosion can also
occur at welded joints of laps, butts, and offsets, where dissimilar
metals are in contact with each other, and immediately below any submerged
drop tube or dipping point. Internal corrosion is also often found
directly under the fill pipe, since this area is repeatedly struck by the
measuring dipstick. Such impact breaks down any protective film that may
have developed, and accelerates pitting in the area.
CORROSION PREVENTION
Corrosion prevention is critical for decreasing failure rates of UST
systems. UST systems preengineered with both external and internal
corrosion protection are marketed by several companies. Generally,
external corrosion processes are considered less predictable than internal
corrosion. Therefore, more attention has been paid by the industry to
prevent external corrosion.
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External Corrosion Protection
The corrosion of external surfaces of steel tanks can be controlled bv
a number of methods, some of which are used in combination to provide the
necessary protection. Such corrosion control systems are often
' fabr1cation' Sorae «f the more common
o
o
0
Protective coatings
Cathodic protection
Electrical isolation/cladding
Protective coatings-
Coatings isolate the external surfaces of tanks and piping from the
coat1ngs must have the
o High dielectric resistance;
o Resistance to moisture and penetration;
o Good adhesion to metallic surface;
o Resistance to mechanical damage during handling, storage, and
installation;
o Resistance to cathodic bonding;
o Ease of repair;
o Retention of physical properties with time.
The most common coatings applied by the tank manufacturer are eooxv-
and urethane-based coatings. These coatings are designed to withstand
environmental and abrasive conditions and are usually 15 to 20 mil thick
when dry and cured.
Coal-tar epoxy a widely used coating, cures by the chemical action of
a resin and a catalyst. This coating is durable, requires no primer
resists gasoline, has excel 1 ent adhesion properties, resists gouging and
scratching, and may be applied cold by spray or brush. However, this type
of coating is also costly and requires excellent surface preparation and
immediate application after mixing.
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For optimum corrosion protection from any protective coating product,
manufacturers' recommendations for curing time/temperature, required
thicknesses, application, and surface preparation methods should be
followed. Steel surfaces are best prepared by sandblasting according to
Steel Structures Painting Council (SSPC) specifications for commercial
blasting (15). Sandblasting to this specification produces a clean
surface with a good metal profile for adhesion.
After application, the thickness of the coating is determined by using
nondestructive magnetic film thickness testers. The coatings are also
electrically tested for the presence of pinholes and other defects, which
are remedied before shipment. Extra care must be taken in handling and
shipping coated tanks to avoid damage to the coatings.
Despite al.l efforts to ensure total integrity of the coatings, some
pinholes or ruptures in the coatings may go undetected by inspection. The
presence of these holidays is extremely dangerous as any defective area
becomes an anodic focal point for intensive electrolytic cell corrosion.
Cathodic protection—
One of the most widely used techniques for external corrosion control
is cathodic protection, a technique that makes the entire tank surface the
cathode of an electrochemical cell. Corrosion processes are not
eliminated, but are transferred from the metal surface to an external
anode. Two types of cathodic protection systems are: sacrificial or
galvanic anode systems and impressed-current systems.
The following factors should be considered when designing cathodic
protection for UST systems.
o Soil resistivity;
o Present and future current requirements;
o Life of the cathodic protection system in relationship to the
intended life of the tank system;
o Presence of stray currents;
o Condition of the tank systems to be protected (new or old, coated
or uncoated);
o Reliability of cathodic protection system components.
The following information should be obtained before designing cathodic
protection system (32):
o Site plan and layout;
o Construction dates;
o Neighboring buried metallic
structures, including
location, ownership, and
corrosion control;
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o Accessibility of structure;
o Feasibil ity of electrical iso-
lation from adjacent structures;
o Electrical continuity of the
sy s tern.
o Pipes,:fittings, and other
appurtenances;
o Pumps and power supply;
o Protective coatings;
o Possible sources of electrical
interference;
o Special environmental conditions;
The most important guideline on cathodic protection systems is NACE
Standard RP-02-85, "Control of External Corrosion in Metallic Buried,
Partially Buried, or Submerged Liquid Storage Systems" (14). This document,
along with other standards cited therein, provide guidelines for the design,
installation, maintenance, and monitoring of cathodic protection. An adequate
cathodic protection system is one that is designed, installed, and maintained
by competent corrosion engineers using these guidelines.
Sacrificial or galvanic anode method—The sacrificial or galvanic anode
method utilizes a metal anode that is significantly more reactive (higher on
the galvanic list) than the tank material being protected. For steel tanks,
magnesium or zinc anodes are commonly employed. The anodes are electrically
connected to the LIST; a galvanic corrosion cell develops; and the active metal
anode sacrificially corrodes, while the UST becomes cathodic and is
protected. The galvanic cell induces a current flow from the sacrificial or
galvanic anode to the cathodic LIST; the current then returns to the
sacrificial anode through a metallic conductor (Figure Al ). Once this
galvanic corrosion cell has been established, it minimizes the potential for
general or localized external corrosion processes to proceed by preventing the
competing electrochemical reaction to occur.
The low driving voltages and low current outputs (usually less than 0.10
amp/anode) of sacrificial anodes generally limit them to protecting
well-coated structures. New installations involving coated tanks or
distribution piping are particularly amenable to sacrificial cathodic
protection.
New USTs with attached sacrificial anode cathodic protection systems are
available from tank manufacturers. These "preengineered" tanks are designed
specifically to meet standards of industrial groups such as the Steel Tank
Institute or tank companies. Preengineered cathodic protection systems for
new tanks are developed to satisfy requirements for most soil situations. In
some instances, such as in locations with low soil resistivity, the life of
these systems may not be as long as expected.
Tanks with preengineered cathodic protection systems must be carefully
handled during transportation and installation to protect against coating
damage or rupture of anode packages. Anode wires, test leads, tank coatings,
and tank isolation bushings should be inspected for obvious damage before
final installation. A regular monitoring program is necessary after
installation to determine that corrosion protection is being maintained.
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PAVEMENT
SOIL ELECTROLYTE
INSULATED COPPER WIRE
^_ J-
r
« 1;:
CURRENT |
* 6
^ 1!
I-
SACRIFICIAL
ANODE
ANODE
BACKFILL
Figure Al. Sacrificial anode cathodic protection.
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Where sacrificial anodes
confirmed by a qualified
and one year thereafter.
inspection intervals can
have been installed, their proper operation should be
person within six and twelve months of installation,
If these tests confirm proper operation, subsequent
be extended to five years. However, if underground
work is performed at the protected site, cathodic protection should be
remonitored six to twelve weeks after work completion and one year thereafter
before again extending the inspection interval.
There are several advantages to sacrificial anode cathodic protection
systems, including:
o Mo requirement for an external power supply;
o Relatively easy installation;
o Low operating costs;
o Minimal maintenance costs after installation;
o Rare interference problems (stray currents) on foreign structures.
Disadvantages of sacrificial anode cathodic protection systems are:
o Limited driving potential preventing protection of large
structures;
o Subject to soil resistivity limitations,
Impressed-current method—The impressed-current method utilizes an
anode made of relatively inert electrically conductive materials that are
subjected to a direct current from a rectifier powered by an AC power
source. The system works on exactly the same principle as a sacrificial
anode system, except for this external power source. Impressed-current
cathodic protection is often the most economical way to control corrosion
of existing buried steel petroleum storage tanks and distribution piping
systems. Figure A2 illustrates the impressed-current cathodic protection
system.
Because the electric current flow is induced by an external power
source, impressed-current anodes are typically made of relatively inert
electrically conductive materials. This ensures efficient flow of current
and minimal corrosion of the anode. Materials commonly used include
graphite, high-silicon cast iron, platinized niobium, tantalum, or
titanium. Anodes can be located in remote ground beds, in deep wells, or
distributed closely around the structure. Wherever possible, anodes
should be installed in carbonaceous backfill, which provides good
electrical contact and reduces the total circuit resistance by lowering
anode-to-soil resistance.
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A.C.POWER
CURRENT '::
*•
IMPRESSED
CURRENT ANODE
ANODE BACKFILL
Figure A2. Impressed current cathodic protection.
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Proper^ Installation of the impressed-current system is critical to the
performance of the system. The negative lead of the rectifier must be
attached to the UST. All connections and wire splices should be
waterproofed and covered with electrical insulating material. Backfill
should be free of sharp stones so as to prevent damage to wire
insulation. A permanent soil-access manhole should be provided so that
the cathodic protection system can be monitored and tested. Anchor
straps, if used, should be installed so that the tank coating is not
damaged. After installation of the impressed-current protection system,
voltage on the UST system must be measured with a reference electrode on
the soil surface as close as possible to the UST.
Monthly checks of rectifiers are necessary to verify that they are
operating properly. Structure-to-soil and structure-to-structure
potentials of an impressed-current system must also be tested routinely to
ensure continued satisfactory operation.,
Some advantages of impressed-current cathodic protection system are:
o Electrical potential limited only by power supply;
o High current output capable of protecting other underground steel
structures at a low operating cost;
o Flexible current output control;
o Applicability to almost any soil resistivity;
o Ability to protect large, bare-steel structures.
The following disadvantages should, however, be noted:
o Possibility of electrical interference (stray currents) with other
structures;
o Potential for switching off the current and eliminating protection
if not equipped with a fail-safe device;
o Requirement of monitoring and maintenance on a regular schedule.
Electrical isolation/cladding—
Electrical isolation improves corrosion prevention provided by
cathodic protection method. This method involves installing devices to
isolate metal components in an UST system. Nonconductive dielectric
fittings, bushings, unions, etc. are usually used as isolating devices.
Use of an electricity-resistive envelope also isolates the tank system.
Electrical isolation devices are rated for temperature, dielectric
strength, and compatibility with the stored product. The tank system must
be installed so that these devices remain physically separated from all
foreign underground metallic structures. Isolating devices may also
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require protection from voltage surges caused by lightning or alternating
current from overhead high-tension wires. Guidelines for protection from
such damage have been developed.
Another method for electrical isolation of steel tanks is an external
cladding of fiber-reinforced plastic material, which acts as an insulator
and eliminates electrolytic activity.
Internal Corrosion Protection
The corrosion control measures commonly used to protect the internal
surfaces of steel tanks include:
o Coatings/linings
o Galvanic protection (sacrificial
anodes)
o Striker plates below fill lines
o Avoidance of dissimilar
metal weld joints
o Use of double-welded butt
joints
To be effective, internal coatings/linings must be chemically
compatible with products.
Galvanic protection may also be provided internally by the
installation of zinc strips in a manner similar to magnesium anodes
applied externally. These anodes are usually installed near the bottom of
the tank, where corrosion occurs due to the accumulation of water and
other corrosive contaminants.
Striker or wear plates provide valuable protection against dipsticks
puncturing protective oxide films and blast erosion occurring under the
fill tube. Wear plates should be installed under each opening. Striker
plates in steel tanks are normally flat, 1/4 in thick and 12 in square.
The plates should be sandblasted to ensure that they are anodic to the
tanks.
Coupling of dissimilar metals, which leads to galvanic corrosion, is
for the most part controlled by the tank manufacturer. However,
dissimilar metals can be accidentally introduced during installation or
when in service. Such a situation must be carefully avoided. Initial
installation instructions and operation and maintenance procedures should
clearly specify the metallurgical requirements necessary to prevent
galvanic corrosion.
The joints of a steel tank may be butt welded or lap welded. However,
double-welded butt joints are less susceptible to corrosion and are
preferred. They are stronger than lap joints, which may be susceptible to
the concentration-cell or crevice-attack corrosion mechanisms described
earlier.
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REFERENCES.
C0mpi'ation- "» HoUenback Press,
2. Jpblett, W. R. Jr., and G. V. Amoruso. A Panoramic View of
WO? 7pp'.
"'
- d0llr1 "^ S
-. London,
N'Clie' C°mpany' Inc' Corrosion In Action. New York,
6. Ecology and Environment, Inc. and Whitman, Requardt and
-
7. New York State Department of Environmental Conservation, Division of
Water Bureau of Water Resources. Technology for the Storage of
Hazardous Liquids: A State-of-the-Art Revle^. Albany! SY?!
Cathod1c Protection of Underground
'
9' rNoni^alJS?°rat°? f Corrosi'on Engineers. Recommended Practice for
Control of External Corrosion of Underground or Submerged Metallic
Piping Systems. NACE RP-01-69. Houston, TX, 1983 MetalMc
10. U.S. Department of Agriculture. Control of Underground Corrosion
Design Note 12. Soil Conservation Service, Washington, DC° 1971
11. State of Maryland Regulation. Title of Department of Natural
12. Petroleum Association for Conservation of the Canadian Environment
state of the
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14. National Association of Corrosion Engineers. Recommended Practice for
Control of External Corrosion on Metallic, Buried, Partially Buried,
or Submerged Liquid Storage Systems, NACE RP-02-85. Houston, TX,
1985. 16pp.
15. Steel Structures Painting Council. Specifications for Commercial
Blast Cleaning. SSPC-SP-6.
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» us, owtwauoiTPwmHa OFFICE; tM7 . 743-121/40715
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