United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati Ohio 45268
Research and Development
EPA/600/S2-91/044 Sept. 1991
EPA Project Summary
Volumetric Leak Detection in
Large Underground Storage
Tanks
James W. Starr, Richard F. Wise, and Joseph W. Maresca, Jr.
A set of experiments was conducted
to determine whether volumetric leak
detection systems presently used to
test underground storage tanks (USTs)
up to 38,000 L (10,000 gal) In capacity
could meet EPA's regulatory standards
(40 CFR Parts 280 and 281) for tank
tightness and automatic tank gauging
systems when used to test tanks up to
190,000 L (50,000 gal) In capacity. The
experiments, conducted on two partially
filled 190,000-L (50,000-gal) USTs at
Griffiss Air Force Base in upstate New
York during late August 1990, showed
that a system's performance in large
tanks depends primarily on the accu-
racy of the temperature compensation,
which is inversely proportional to the
volume of product in the tank. Errors
in temperature compensation that were
negligible in tests in small tanks were
important in large tanks. The experi-
ments suggest that volumetric systems
now capable of meeting regulatory stan-
dards when used to test 30,000- to
38,000-L (8,000- to 10,000-gal) tanks will
also meet these standards when used
to test 190,000-L (50,000-gal) tanks if
(1) the duration of the test is increased
from 1 or 2 h to 4 h or more to ensure
that the vertical gradients are accurately
measured and to reduce the ambient
and instrumentation noise, (2) the num-
ber of temperature sensors is increased
from 5 to 10 or more so that the accu-
racy of estimating the average ther-
mally induced volume change in the
layer of product surrounding each sen-
sor increases, (3) the waiting period
after any addition or removal of prod-
uct is increased from 6 to 24 h or longer
so that the horizontal and vertical tem-
perature gradients dissipate, (4) the av-
erage rate of change of temperature in
any one layer or in the tank as a whole
is small enough to allow accurate tem-
perature compensation, and (5) an ac-
curate experimental estimate of the
constants necessary for converting
level and temperature changes to vol-
ume is made. The experiments further
suggest that a multiple-test strategy Is
required in order for such systems to
meet the tank tightness regulatory stan-
dard.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that Is fully documented In a separate
report of the same title (see Project
Report ordering Information at back).
Introduction
The U.S. Environmental Protection
Agency (EPA) regulation for underground
storage tanks (USTs), published in the
Federal Register (40 CFR Parts 280 and
281) on September 23, 1988, specifies
the technical standards and a variety of
release detection options for minimizing
the environmental impact of tank leakage.
With several exceptions, the shop-as-
sembled tanks covered by the regulation
range in size from small (a few hundred
gallons in capacity) to very large, with no
clearly defined upper limit. (Requirements
for large field-erected tanks have not yet
been established). The number of large
tanks (defined here as those between
Printed on Recycled Paper
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57,000 and 190,000 L (15,000 and 50,000
gal) in capacity) represents a small but
important portion of the total tank popula-
tion. This number is increasing because
of the preference of tank owners/opera-
tors for a smaller number of larger tanks
to meet storage needs. Many large-vol-
ume storage facilities have tanks that are
nominally 190,000 L (50,000 gal) in ca-
pacity. Unfortunately, there is not enough
information to help owners and operators
of large tanks select a testing system that
will be In compliance with the regulations.
Furthermore, it is not known whether volu-
metric tests can achieve the same level of
performance in large tanks as they do in
smaller ones.
Objectives
The program of experiments conducted
at Griffiss Air Force Base was devised to
expand the understanding of large under-
ground storage tank behavior as it im-
pacts the performance of volumetric leak
detection testing. This report addresses
three important questions about testing
the larger underground storage tanks for
leaks. First, can the EPA regulatory stan-
dards be met when volumetric methods
are used to test tanks up to 190,000 L
(50,000 gal) in capacity? Second, what is
the precision required of the temperature
and level sensors and what'is the duration
of the data collection period in order for a
volumetric system to test larger tanks, par-
ticularly those that are partially filled? Third,
what are the important features of a volu-
metric system that meets or exceeds the
regulatory performance standards?
Background
Leak detection systems used as volu-
metric tank tightness tests on tanks cur-
rently covered by the EPA regulation (i.e.,
those up to 190,000 L (50,000 gal) in
capacity) must be able to detect leaks as
small as 0.38 L/h (0.10 gal/h) with a prob-
ability of detection (PD) of at least 0.95
and a probability of false alarm (PFA) of
0.05. Leak detection systems used as
monthly tests, such as automatic tank
gauging systems, must be able to detect
leaks as small as 0.76 L/h (0.2 gal/h) with
a P0 of 0.95 and a PFA of 0.05. Most state
regulations, which require that a tank tight-
ness test be done annually, are more strin-
gent than the federal regulation. These
standards are based on the results of an
extensive experimental program conducted
by the American Petroleum Institute (API)
on 38,000-L (10,000-gal) tanks at retail
stations and by the EPA on 30,000-L
(8,000-gal) tanks at EPA's Underground
Storage Tank Test Apparatus in Edison,
NJ.
The EPA described the important fea-
tures of a generic volumetric tank tight-
ness system that would yield the required
level of performance in tests conducted in
overfilled and partially filled tanks. The
important features are the ones that com-
pensate for or minimize errors in the mea-
surement of volume changes not due to a
leak; these errors, which are due to ambi-
ent noise, are not associated with leaks
and thus occur in both leaking and
nonleaking tanks. Experiments on a
30,000-L (8,000-gal) tank showed that an
array of five or more equally spaced tem-
perature sensors, each weighted by the
volume of product in the layer surrounding
it, was sufficient to compensate for ther-
mally induced volume changes. This tem-
perature-sensing array was suitable
providing that adequate wajting periods
were observed after any addition of prod-
uct, whether this addition represented a
delivery to the tank or whether it consti-
tuted topping of the tank (as is required
when testing an overfilled tank). The addi-
tion of product to the tank produced
inhomogeneities in the temperature field
that were large enough to prevent an ac-
curate estimate of the mean rate of change
of temperature. As a means of minimizing
the effect of these thermal inhomo-
geneities, waiting periods of at |east 3 h
after topping and 4 to 6 h after a delivery
were recommended. Any addition of prod-
uct also changes the level of the liquid in
the tank and, therefore, the pressure that
is exerted on the tank walls. This change
in pressure causes the tank to deform;
observing a waiting period before the test
allows deformation to subside, thus elimi-
nating any errors that it might have pro-
duced. The waiting period may have to be
as long as 12 to 18 h, depending on the
properties of the tank and of the backfill
and native soil surrounding it. In tests
conducted in overfilled tanks, there is a
third source of potential error: the tem-
perature- and/or pressure-induced expan-
sion or contraction of any vapor trapped
in the tank. If the amount of trapped vapor
is significant, or if the range of tempera-
ture and/or pressure changes within the
trapped vapor is great, the error can have
a detrimental impact on the test. Unfortu-
nately, it is extremely difficult to identify
the presence of small volumes of trapped
vapor, and there is no satisfactory way to
compensate for volume changes due to
this phenomenon. A fourth source of po-
tential error is unique to tests conducted
in partially filled tanks: this is the error
produced by evaporation or condensation
at the liquid/vapor interface and the vapor/
tank-wall interface. It is believed that the
effects of evaporation and condensation
are usually small, but available data show
that under some air-temperature and at-
mospheric-pressure conditions they can
be large enough to produce false alarms
and missed detections. At the present time
there is no systematic way to determine
when these conditions will adversely af-
fect the test; however, any adverse ef-
fects can be minimized through the use of
a multiple-test strategy. The other sources
of error, which are produced by surface
and internal waves, can affect the test
results in both overfilled and underfilled
tanks. These can be minimized by proper
sampling of the level and temperature data.
For best performance, the instrumenta-
tion noise, or system noise, should be
less than the ambient noise. In small tanks,
whether partially filled or overfilled, this
can be achieved. However, as the volume
or the surface area of the product in the
tank increases, this becomes proportion-
ately more difficult.
Because the EPA regulations (and the
recommended "important features" of a
volumetric tank tightness test that are de-
scribed above) are based on experiments
conducted on 30,000-L (8,000-gal) tanks,
it is unclear whether a volumetric test that
meets the EPA standard when used to
test smaller tanks can achieve the same
level of performance when used to test
larger tanks. At the present time, there
are no experimental data that can be used
to make such an assessment, particularly
for tanks as large as 190,000 L (50,000
gal). Additional information about the mag-
nitude of the noise in large tanks would
be required before such an assessment
could be made. Based upon previous ex-
perimental and analytical work, we ex-
pected that four things would be necessary
in order for a volumetric leak detection
system to maintain the required level of
performance when used to test large tanks:
(1) more temperature sensors, (2) longer
waiting periods after topping or after deliv-
ery of product to the tank, (3) a longer test
duration, and (4) an increase in the preci-
sion requirements of the temperature and
level measurement systems.
Conclusions
In the present study, limited-scope field
experiments were conducted during late
August 1990 on two partially filled 190,000-
L (50,000-gal) underground storage tanks
located at Griffiss Air Force Base in up-
state New York. The purpose of the ex-
periments was to determine whether
volumetric systems intended for 30,000-
to 38,000-L (8,000- to 10,000-gal) tanks
could successfully be used with larger
tanks. The Griffiss tanks were 254.3 m
(77.5 ft) long and 320 cm (10.5 ft) in
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diameter. A level sensor with a precision
of 0.0005 cm (0.00025 in.) and an array
of 10 submerged thermistors spaced at
30-cm (12-in.) intervals, each thermistor
having a precision of 0.001 °C (0.002°F),
were used to collect the data. All experi-
ments were conducted in partially filled
tanks. Changes in product level were ef-
fected by shunting product from one tank
to another by means of a pump. The addi-
tion or removal of product produced
changes in the temperature of the product
that effected volume changes of several
liters per hour or more.
The following observations were made:
• Temperature fluctuations observed
throughout the tank after approxi-
mately 38,000 L (10,000 gal) of prod-
uct had been added or removed were
too great, unless at least 4 h had
elapsed, to permit an accurate leak
detection test. (The addition of
38,000 L (10,000 gal) simulated a
delivery of product to the tank.)
• Temperature fluctuations observed
after the addition of approximately
19 L (5 gal) of product 25°C warmer
or 8°C cooler than the in situ product
were too great, unless 2 to 3 h had
elapsed, to permit an accurate leak
detection test. (The addition of 19 L
(5 gal) simulated the topping of an
overfilled tank.)
• The average rate of change of tem-
perature between any two locations
along the long axis of the tank, as
measured by any two thermistors at
the same height, but distanced hori-
zontally, was less than 0.001 °C/h
beginning 4 h or more after product
additions or removals. Furthermore,
the mean temperature along the
centeriine of the tank was the same
at each height, i.e., the horizontal
gradient was negligible at the
centeriine.
• A horizontal gradient in the mean
temperature was observed in the
mid-region of the tank in the area
between the centeriine and the wall
of the tank. The rate of change of
temperature, as measured by three
thermistors spaced at horizontal in-
tervals of 30 cm (12 in.), was lower
in the area of the centeriine and
higher in the area of the tank wall,
an observation that is consistent with
the physical process of heat transfer
between the backfill surrounding the
tank and the product inside it. The
difference between the rate of
change of temperature measured at
the centeriine of the tank and at
locations off the centeriine was large
enough to suggest that a single ver-
tical array is not sufficient for tem-
perature compensation unless at
least 18 h has elapsed since any
product addition or removal. Addi-
tional horizontally oriented ther-
mistors must be incorporated into
the arrays to compensate for these
temperature changes, or the test
must not be started until the rate of
change of temperature has subsided.
• The area near the bottom of the
tank exhibited the largest vertical gra-
dient and the greatest rate of change
of temperature. In this region, a spac-
ing of 30 cm i(12 in.) between ther-
mistors was 'not adequate, in the
experiments conducted, for correct
temperature compensation unless at
least 10 h had elapsed since the
last addition or removal of product.
• The temperature fluctuations that fol-
lowed changes in product level in
these experiments were so large that
changes due to deformation were
masked. As a consequence, none
of the classical exponential level
changes associated with tank defor-
mation was observed in these ex-
periments.
' Analysis of the level data revealed
the presence of surface waves
(seiches) having periods of 2 to 10 s
and peak-to-peak amplitudes of 1 to
2 L. These waves were consistent
with the fundamental and first har-
monic of waves propagating along
the long and short axes of the tank.
Analysis of the temperature data re-
vealed the presence of internal
waves having periods of 3 to 30
min. These subsurface waves were
large enough to produce periodic sur-
face waves.
Experimental estimates of the height-
to-volume conversion factor were
within 5% of the theoretical estimates
made with a tank chart.
When 10 h had elapsed after a prod-
uct addition or removal, and ther-
mistors spaced at 30-cm (12-in.)
intervals were used for thermal com-
pensation, the residual volume
changes in each of three tests on
nonleaking tanks were 0.36, 0.67,
and -0.22 L/h (0.095, 0.177, and
0.058 gal/h) respectively. When 2 h
had elapsed after topping and 20 h
after a product addition or removal,
residual volume changes in each of
two tests were 0.036 and -0.043 L/h
(0.0095 and -0.011 gal/h) respec-
tively. These residual volume
changes can be attributed to inad-
equate temperature compensation.
(There was insufficient coverage near
the surface of the product, at the
bottom of the tank, and in the area
between the centeriine and the tank
wall, all locations where large tem-
perature gradients were present.)
The effects of evaporation and con-
densation within the tank could not
be quantified, but their contribution
to the residual volume changes ap-
pears to be smaller than the errors
in temperature compensation.
The data collected during these experi-
ments, combined with theoretical analy-
sis, were sufficient for the researchers to
address each of the technical objectives
of this study. A summary of the key con-
clusions of this research project are pro-
vided below.
Volumetric leak detection systems
can meet EPA's performance standards
for testing tanks up to 190,000 L (50,000
gal) In capacity. The experiments on
190,000-L (50,000-gal) tanks suggest that
volumetric leak detection systems can
meet the EPA standards for both volumet-
ric tank testing and automatic tank gaug-
ing. That is, they can detect leaks of 0.38
L/h (0.10 gal/h) with a PD of 0.95 and a
PF/> of 0.05, which is the performance re-
quired of tank tightness tests. (By defini-
tion they can also meet the requirement
for automatic tank gauging systems, which
is the ability to detect a leak of 0.76 L/h
(0.20 gal/h) with a Pp of 0.95 and a PFA of
0.05.) However, achieving this goal is very
difficult and probably requires a multiple-
test strategy.
A 0.76-L/h (0.2-gal/h) leak was detect-
able with the thermistor array used in these
experiments, but a smaller leak of 0.38 L/
h (0.1 gal/h) could not be reliably de-
tected. However, leaks as small as 0.38
L/h (0.1 gal/h) were detectable if the wait-
ing period after any addition or removal of
product from the tank was at least 18 h.
After some analysis, this was explained
by the fact that there was an insufficient
number of thermistors at the bottom of the
tank, near the surface of the product, and
between the centeriine and the walls of
the tank, areas where either the rate of
change of temperature or the gradient in
the rate of change of temperature was
greater than in other parts of the tank.
The upper portion of the layer closest to
the surface was influenced by large
changes in the temperature of the vapor,
so that the implicit assumption that the
rate of change of temperature varied lin-
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early through the layer was violated. The
thermistor closest to the surface was not
physically centered in the layer, and the
product surrounding it was not equally dis-
tributed above and below it. When the
thermistor was too far away from the sur-
face (e.g., 25 cm (10 in.)), the contribution
from the surface was not properly included
in the average. Any error in measuring
the average rate of change of tempera-
ture was magnified by the volume of prod-
uct in the layer. The error in measuring
the average rate of change in tempera-
ture was even greater in the layer of prod-
uct at the bottom of the tank. (In
measurements made at the bottom, how-
ever, the magnitude of the error decreased
with time; in measurements made near
the surface it did not.) Although the tem-
perature sensor is indeed centered in this
bottom layer, the curvature of the tank is
such that the volume of the product above
the sensor is significantly greater than that
below it. The average rate of change of
temperature and the gradient in the rate
of change of temperature were significantly
greater here than in any other layer. While
errors of similar magnitude in measuring
the average rate of change of tempera-
ture are present in small tanks, the vol-
ume of each layer in a small tank is
significantly less than in a large tank, par-
ticularly near the bottom. The largest
source of error in measuring the average
rate of change of temperature was due to
horizontal gradients in the rate of change
of temperature between the centerline of
the tank and the wall of the tank. Better
temperature compensation would have
been achieved if additional temperature
sensors had been located near the bot-
tom of the tank and near the product sur-
face, ensuring a better estimate of the
rate of change of temperature in the layer
surrounding each thermistor, or if a longer
waiting period had been used, thus mini-
mizing the rate of change of temperature
in these layers and throughout the tank as
a whole.
The Important features of a generic
volumetric leak detection system that
can meet the EPA performance stan-
dards have been Identified. The experi-
ments on 190,000-L (50,000-gal) tanks
allowed us to identify the key feature that
a volumetric leak detection system used
on larger tanks must possess in order to
meet the EPA standard. K must have bet-
ter temperature compensation than what
is deemed sufficient for a 30,000- to
38,000-L (8,000- to 10,000-gal) tank, and
it must have longer waiting periods and a
longer test duration. All the other features
required for accurate detection of leaks in
small tanks are applicable equally to the
detection of leaks in large tanks.
Five things are necessary for success-
ful temperature compensation in tanks as
large as 190,000-L (50,000-gal). First, a
test must not be started until the horizon-
tal gradients in the rate of change of tem-
perature between the centerline and the
tank walls have dissipated. Second, the
number of temperature sensors must be
sufficient that the volume of product in the
layer around each sensor is not too great;
the smaller the volume in each layer, the
less likely it is that a temperature mea-
surement error, when summed with mea-
surements from the other layers, will
adversely affect the test. Third, the dura-
tion of the test must be bng enough that
(1) the fluctuations in volume observed 6
h or more after any product additions or
removals can be averaged, and (2) the
precision of the temperature and level is
sufficient to detect a leak with a specified
performance. Fourth, a test should not
begin unless the average rate of change
of temperature in the tank as a whole or
in any one layer is small enough to allow
accurate temperature compensation. Fifth,
an accurate experimental estimate of the
constants necessary for converting level
and temperature changes to volume is
required: these constants include the co-
efficient of thermal expansion, the volume
of product in the tank or in each layer,
and the height-to-volume conversion fac-
tor.
How long must the waiting period be?
In testing tanks up to 190,000 L (50,000
gal) in capacity, the length of the required
waiting period is controlled by the gradi-
ent in the rate of change of temperature
between the centerline and the wall of the
tank. This waiting period must be suffi-
ciently long that the temperature fluctua-
tions and tank deformation associated with
a product addition or removal will have
time to subside. A waiting period of 24 h
or bnger may be required.
The data suggest that the minimum du-
ration of a test should be at least 4 h, bng
enough that an average of the ambient
volume fluctuations can be made. Whether
4 h is sufficient depends on the resolution
and precision of the temperature and level
instrumentation.
The spacing between thermistors in a
190,000-L (50,000-gal) tank may need to
be as small as 15 cm (6 in.) to detect
leaks as small as 0.38 L/h (0.1 gal/h),
particularly near the bottom and top of the
tank where more dense coverage will re-
sult in a more accurate estimate of the
rate of change of temperature. Since er-
rors in temperature compensation increase
as the average rate of change of tem-
perature increases, the most direct way to
avoid errors is to wait until the average
rate of change of temperature has dimin-
ished before starting a test.
The precision of the Instrumentation
used to measure temperature and level
changes establishes the minimum du-
ration of a test. In order for a volumetric
leak detection system to meet the EPA
standards, the length of a test must be
appropriate for the precision of the
system's instrumentatbn for temperature
and level measurement. Given a certain
level of precision, the optimum duration of
a test can be calculated. As part of the
experiments, calculations were made to
estimate the minimum duration of a test
conducted on a 182,000-L (48,000-gal)
tank as a function of the precision of the
temperature and level sensors, ft was as-
sumed in the calculations that the resolu-
tion of the sensors was 2 to 3 times smaller
than the most extreme level change that
occurred over the duration of the test. The
calculations indicated that the test dura-
tion must be at least 2 h in the case of a
level sensor having a precision of 0.0005
cm (0.00025 in.) and a temperature sen-
sor having a precision of 0.001°C
(0.002°F). When the instrumentation is less
precise, the test duration must be com-
mensurately longer. For example, if the
level sensor had a precision of 0.0025 cm
(0.001 in.), the test would have to be at
least 4 h long.
Recommendations
The recommendations developed from
this research project are based on a lim-
ited set of data. The recommendations for
controlling the key sources of noise might
be further refined if additional experiments
were conducted. We believe, however, that
additional data would not have any sub-
stantial impact on the general nature of
the recommendations made here, and that
further refinements to these recommenda-
tions would not materially change the ef-
fort or cost involved in developing or
modifying a method of testing large tanks.
Although the experiments were limited to
tests conducted on partially filled tanks,
many of the conclusions are applicable to
the use of volumetric systems in overfilled
tanks as well. The recommendations that
emerged from this research project fall
under two headings.
Temperature Compensation
The single most important cause of er-
rors in testing large tanks with volumetric
leak detection systems appears to be in-
accurate temperature compensation. Two
things are necessary for successful tem-
perature compensation. First, the number
L.
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of temperature sensors must be sufficient
that the volume of product in the layer
around each one is not too great; the
smaller the volume in each layer, the less
likely it is that a measurement error, when
averaged with measurements from the
other layers, will adversely affect the test.
Second, a test must not be started if the
average rate of change of temperature of
the product in the tank as a whole, or
even in a single layer, is great enough to
prevent the system from detecting a leak
of given size.
The following procedure is recom-
mended for compensating for the thermal
expansion or contraction of the product:
• Place the lowest temperature sen-
sor approximately 8 cm (3 in.) from
the bottom of the tank and the up-
permost sensor approximately 8 cm
(3 in.) below the surface.
• Space the temperature sensors at
intervals of 15 to 30 cm (6 to 12 in.)
or less along the vertical axis of the
tank; space the sensors at intervals
of 15 cm (6 in.) or less in the bottom
46 cm (18 in.) of the tank and in the
15 to 30 cm (6 to 12 in.) of product
located immediately beneath the sur-
face. (A 30-cm (12-in.) spacing can
be used if the rate of change of
temperature between adjacent lay-
ers of product throughout the entire
tank is nearly identical.)
• Partition the tank into layers, each
. of which is centered about a tem-
"• perature sensor. Then calculate the
volume of product in each layer.
• Wait at least 24 h for horizontal gra-
dients in the rate of change of tem-
perature to dissipate. (These
horizontal gradients occur between
the centerline and the wall of the
' tank.) Alternatively, measure these
horizontal gradients directly, and do
not attempt to compensate for tem-
perature until they have dissipated.
If the compensated volume rate ex-
ceeds the threshold, continue to test
until the measured volume rate
ceases to decrease and remains
constant.
• Using real-time measurements, wait
for the rate of change of tempera-
ture to diminish sufficiently that the
maximum potential error in measur-
ing the average rate of temperature
for each test is small. The accept-
able rate of temperature change de-
pends on the number of thermistors,
the precision of each thermistor, and
the degree of compensation that can
be achieved with the array of ther-
mistors. A very conservative ap-
proach is to incorporate the follow-
ing analysis tests.
- Do not begin a test if the rate of
change of temperature is great
enough in any one layer to pro-
duce a volume change that will
exceed the detection threshold.
(When using a threshold of 0.19
L/h (0.05 gal/h) in a tank contain-
ing JP-4 fuel, this would limit the
rate of change in temperature to
less than 0.008°C in the largest
layers of ,a 190,000-L (50,000-
gal) tank divided into ten layers.)
- Do not begin a test if the aver-
age rate pf change of tempera-
ture throughout the tank is great
enough to produce volume
changes that exceed the thresh-
old based on an average level of
compensation to be achieved.
(When using a threshold of 0.05
gal/h in a: tank containing JP-4
fuel, this would limit the rate of
change in temperature to less
than 0.019°C throughout a
190,000-L'(50,000-gal) tank if on
average the method is able to
compensate for 95% of the tem-
perature changes.)
• Use the most precise temperature
and level measurement systems
available and calibrate them fre-
quently and properly, ft is recom-
mended that temperature sensors
have a precision of 0.001 °C and the
level sensors have a precision of
0.00025 cm (0.0001 in.).
• Check that all sensors function prop-
erly during a test. If a sensor mal-
functions, the test should be
repeated.
• Make sure the test is at least 4 h
long so that ambient fluctuations will
be properly averaged and will not
affect the test., Longer tests may be
required depending on the resolu-
tion and precision of the level and
temperature sensors.
• Measure the coefficient of thermal
expansion experimentally.
• Determine the height-to-volume con-
version factor used to convert level
measurements to volume measure-
ments experimentally.
• Use a multiplexes! strategy.
Whether this temperature compensation
procedure is sufficiently adequate for a
volumetric leak detection system to meet
the EPA's regulatory standard for a tank
tightness test (or a monthly monitoring
test) will not be known until an actual
performance evaluation is conducted on a
system that incorporates some or all of
these procedures.
Evaluating a Volumetric Leak
Detection System
Volumetric leak detection systems that
will be used on large tanks should be
experimentally evaluated according to the
EPA's standard test procedure for evalu-
ating volumetric tank tightness tests. This
includes the performance of the system in
terms of probability of detection and prob-
ability of false alarm. The primary features
that should be examined are the method
of temperature compensation, the waiting
periods, and the duration of the test. The
results of the present study suggest that,
when a volumetric leak detection system
is used to test larger tanks, longer waiting
periods, a longer test duration and better
temperature compensation are required if
leaks are to be detected with reliability.
Unfortunately, none of the existing facili-
ties specializing in evaluations is equipped
with 190,000-L (50,000-gal) tanks. There-
fore, systems must be evaluated at large-
volume storage facilities that are
operational, and the EPA's standard test
procedure should be modified to accom-
modate this type of evaluation.
The full report was submitted in fulfill-
ment of Contract No. 68-03-3409 by Vista
Research, Inc., under the sponsorship of
the U.S. Environmental Protection Agency.
'U.S. Government Printing Office: 1992 — 648-080/60042
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James W. Starr, Richard F. Wise, and Joseph W. Maresca, Jr., are with Vista Research,
Inc., Mountain View, CA 94042.
Robert W. Hlllger is the EPA Project Off her, (see below).
The complete report, entitled "Volumetric Leak Detection in Large Underground Storage
Tanks," (Order No. PB91-227942/AS; Cost:$23.00, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield,VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Edison, NJ 08837
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-91/044
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