EPA 510-K-92-810
REVIEW OF EFFECTIVENESS OF STATIC TANK TESTING
Jairus D. Flora
MIDWEST RESEARCH INSTITUTE
April 1, 1988
\
1. Introduction
This paper 1s a review of the document "Analysis of Static Tank Testing
as a Leak Detection Technique for Used Oil Tanks at Retail Outlets" (Ref.
No. 1). The paper deals with two Issues. It first estimates the standard
deviation of a stick reading and then uses that value 1n calculating an
average error associated with a static tank test under a variety of
scenarios. The error in the measured leak rate that results from the sticking
error is computed for two tank sizes (500 and 1000 gallons [gal.]) under a
variety of filling conditions and assuming a variety of forms of the static
tank test. The paper then recommends a particular form of the static tank
test based on consideration of the error in the estimated leak rate and the
practicalities of the test.
2. Summary of the Paper
The authors used a simulation to estimate the standard error of leak rate
estimates (called the "Effective Leak Rate Error") for a variety of con-
ditions. Table 3-4 in that report (see attachment for tables from the report)
contains these estimates together with a 95% confidence interval for the
errors.
The "effective leak rate errors" of Table 3-4 were arrived at by
considering the volume error induced by the error in the level measurement
resulting from manual sticking of the tank. Since the volume for a given
error in level depends on the cross sectional area of the tank, an average
error was calculated for the tank, assuming a cycle of filling and emptying
the tank. A simulation was done resulting in a number of volumes in the tank
and associated levels and cross sectional areas. The volume errors corre-
sponding to the assumed stick reading error (of 0.44 Inches (1n.) for the
difference of two readings) were calculated and averaged over the different
volumes. Simulations with larger rates of filling the tank resulted 1n a
smaller number of terms 1n the average, but for the most part, the levels in
the faster filling rate simulation would also be in the slower filling rate
scenarios, giving the same average error.
Once a standard error in terms of volume was determined for a tank size,
then it was scaled to reflect the length of the test. For example, consider
model tank 1, scenario 2.a. If the tank had an average volume error of 5.56
gal. corresponding to a static test, that 1s divided by the time of the test
to convert it to an error on the leak rate scale. Thus, an average error of
5 56 gal. is 0.93 gal./h for a 6-h test. If the static test protocol suggests
averaging a number of static tests, the leak rate error was reduced by
Recycled/Recyclable
Printed on paper that contains
at least 50% recycled fiber
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give the effective leak rate error (0.93/2.64 = 0.35) gal./h.
®£
of the Smum volume error that would occur when the tank is half full.
fled by deleting 2 scenarios and 2 tolumns (model tanks).
•HIP tahle nresents an interval stated to be a 95% confidence interval for
1f the chi-squared distribution had been used.
2?«iu2t fffl-Jflndlng put., and for different residence
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Table 1. Maximum Errors for Static Tank Tests
Capacity
Diameter
Length
Cross-sectional
area at midpoint
(square inches)
Volume error for
0.44 in. level error
(gal.)
Tank 1
500 gal.
48 in.
5.5 ft.
3168
6.034
Tank 2
1000 gal.
48 in.
11 ft.
6336
12.0685
Tank 3
2000 gal,
64 in.
12 ft.
9216
17.554
Test scenario
Daily 6 h 1.01
Daily 8 h 0.75
Daily 12 h 0.50
Ave of 7 daily tests
6 h 0.38
8 h 0.29
12 h 0.19
Ave of 30 daily tests
6 h 0.18
8 h 0.14
12 h 0.09
Ave of 4 weekly tests
6 h 0.50
8 h 0.38
12 h 0.25
24 h 0.13
36 h 0.08
48 h 0.06
Leak Rate Error (qal./hr)
2.01
1.51
1.01
0.76
0.57
0.38
0.37
0.28
0.18
1.01
0.75
0.50
0.25
0.17
0.13
2.93
2.19
1.46
1.11
0.83
0.55
0.53
0.40
0.27
1.
1.
.47
.10
0.73
0.37
0.24
0.18
OTC73
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times of" the stick 1n the product. Table 5-3 of the referenced report
contains estimates of the standard deviations of the level reading on the
stick for different subsets of the data. (The title should be "Standard
Deviation of ... H rather than "Standard Error ") The authors used the
value of 0.31 1n. for all wood as the standard error 1n a single stick
reading. Since a static tank test requires a stick reading at the start and
the end of the test period, the standard error of a single period tank test is
2 times the standard deviation of a single reading or about 0.44 in. This
basic value of 0.44 1n. has been Used by the authors throughout the paper as
the standard error 1n height reading for a single static tank test.
3. Evaluation of the Paper
i.
In summary, the "Effective Leak Rate Errors" presented 1n Table 3-4 are
reasonable estimates for current practice. If special care is taken in the
readings these values can be reduced. Also, 1f two stick readings were taken
each time and averaged each time) the tank 1s stuck these errors would be
reduced, by about 3Q%. This appears a practical improvement.
The paper assumes that volume changes due to temperature changes can be
neglected. This assumption is questionable. Assume that the coefficient of
thermal expansion for the product is about 0.0004 per Fahrenheit degree. The
largest temperature change observed during the testing on US EPA's National
Survey of USTswould correspond to about 0.2 gal./h in the 500- or 1000-gal.
tanks, assuming that the same heat flow applied and that the specific heat of
the product is the same. During spring warming we have observed a long term
trend of product temperature in a half-full 2000-gal. tank rising about
0.05 degree Fahrenheit per hour, which would correspond to an Increase 1n
volume of about 0.02 gal./h. Comparing these volume changes to the estimated
leak rate errors, the smaller, O.;02 gal./h corresponds to about 25% of the
error estimated in the more precise static tank tests. These examples may be
extreme, but they suggest that there are conditions under which the effect of
temperature is not negligible.
Table 2 is a tabulation of temperature changes that would produce
specified volume changes. The volume changes included are small, so that they
will be comparable to a leak rate in terms of gallons per hour. Thus, the
temperatures in Table 2 should be thought of as changes per hour. Multiplying
these by the length of the test will give a temperature change over the test
period that could affect the measured leak rate by the amount indicated.
4. Improvements in Static Tank Tests
i
Generally, it 1s likely that1 the temperature effects will not be large
relative to the sticking error. However, 1t would be helpful to check this by
making a temperature measurement each time the tank is stuck. Referring to
Table 2, and considering the recommended 36 hour test period, if the
temperature at the beginning and end differs by less than 4 degrees
Fahrenheit, the temperature change per hour will be 0.1 degree Fahrenheit or
less and the temperature effect can be ignored. However, temperature changes
Induce a bias or systematic error that could cumulate over time. Expecting
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Table 2. Temperature Change Needed to Produce
Volume Changes
(Coefficient of Expansion 0.0004)
Product Volume
Volume
Change
0.2
0.1
0.05
0.043
0.021
200
2.50
1.25
0.63
0.54
0.26
300
1.67
0.83
0.42
0.36
0.18
400 500 750
Degrees Fahrenheit
1.25
0.63
0.31
0.27
0.13
1.00
0.50
0.25
0.22
0.11
0.67
0.33
0.17
0.14
0.07
1000
0.50
0.25
0.13
0.11
0.05
1500
0.33
0.17
0.08
0.07
0.04
2000
0.25
0.13
0.06
0.05
0.03
rrrr??
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operators "to do a temperature correction may be impractical, but asking that
the temperature be measured and the. results questioned 1f a large temperature
change 1s observed should be possible.
If the operator would take two stick readings and use the average level
each time the level is determined, a substantial reduction in the error would
be obtained. This would reduce the standard error of the difference from the
assumed 0.44 in. to 0.31 in.. This would reduce the errors of the leak rates
by about 3Qfc and would make the procedure better. (Requiring three readings
and an average each time would reduce the error from 0.44 in. to O.p4 in. and
the average of four readings each time would reduce it to 0.22 in..)
5. Implications of the Error Rates for Method Performance
If the "Effective Leak Rate Errors" are used as representing the error
associated with a static tank test, i the test performance can be calculated.
Since the errors result from reading errors on the stick, they will generally
follow a normal distribution closely. Assuming the normal distribution for
the errors, there are two approaches that can be taken in determining the
performance of the method. The first is to calculate the threshold leak rate
that would correspond to a IX false alarm rate and then determine the
probability of detection of specified leak rates, particularly 0.2 gal./h. The
second is to specify the decision threshold as 0.1 gal./h (midway between zero
and the performnce standard of 0.2 gal./h) and calculate the probabilities of
false alarm and of detection. A third consideration is that the procedure
will be repeated periodically, generally monthly, so that performance will
Improve over time.
Let L denote the leak rate to, be detected, let C denote the critical
measured value for declaring the tank to be leaking, and let R denote the
measured rate. Then the probability;of a false alarm 1s given by
P(FA) » P(R < C|0),
where the value after the vertical bar denotes the true leak rate (zero if a
false alarm occurs). Note that, following the usual convention, leak rates
are represented as negative numbers since they correspond to volume reductions
or losses from the tank. Using the assumption of.the normal distribution for
the measuring errors, and using s to denote the standard error of the measure-
ment, we have
P(FA) * P( R/s < C/s|0)
- D (C/s),
where D denotes the standard normal distribution value. The probability of
detecting a leak of size L is the probability that the measured leak rate, R,
is less than the threshold, C, when the true leak rate is L. This probability
1s given by
P(D) » P(R < C|L)
= P[(R-L)/s < (C-L)/s|L]
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= D {(C-L)/s}.
Taking weekly 36-hour static tests, averaged over four weeks each month,
the estimated standard error is 0.08 gal./h for a 500-gal. tank and
0.16 gal./h for a 1000-gal. tank. We will use these values to illustrate the
results.
First we set the probability of a false alarm to be 0.01.
corresponds to the tabled value of -2.326 from the normal distribution.
1ng for the threshold, C, gives
This
Solv-
or
C/s = - 2.326 ,
C = -2.326 s.
Substituting the values of the standard error, the threshold for
declaring a leak would be a measured leak rate of -0.186 gal./h or more for a
500-gal. tank and -0.372 gal./h or more for a 1000-gal. tank, with the nega-
tive sign denoting that the rate represents a volume loss (flow out of the
tank). Substituting these values for C one can determine the probability of
detecting a leak of -0.2 gal./h in a single month as
P(D|L=-.2) = D {(C + .2)/s}
, D {0.175} for the 500-gal. tank
*• D £-1.075} for the 1000-gal. tank.
The tabulated values of the normal distribution give the probability of
detection of a leak rate of -0.2 gal./h as 0.57 for the 500-gal, tank and 0.14
for the 1000-gal. tank.
If we set C = -0.1 gal./h as the threshold for declaring a leak, then the
symmetry of the normal distribution will make the probability of the two error
types (false alarm and missed detection of a leak rate of -0.2 gal./h)
equal. Substituting Into the formulas gives
P(FA) = 0.11
P(D) = 0.89
for a 500-gal. tank and
P(FA) = 0.27
P(D) = 0.73
for a 1000-gal. tank.
6. Modifications to Improve Performance
If the average of two stick readings were used throughout, the standard
error of level measurement would be reduced from 0.44 in. to 0.31 in.. This
would have the effect of changing the thresholds, C, for a probability of
false alarm of 0.01 to -0.133 gal./h for a 500-gal. tank and to -0.263 for a
1000 gal. tank. The probabilities of detecting a leak of -0.2 gal./h would
then become 0.88 and 0.29, respectively.
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If 'the fixed threshold of -0.1 gal./h were used with this reduced
standard error, the probability of a false alarm for a 500-gal. tank would be
reduced to 0.04 and the probability of detecting a leak of -0.2 gal./h would
increase to 0.96. The corresponding numbers for a 1000-gal. tank would be a
false alarm rate of 0.19 and a probability of detection of 0.81.
If the duration of the test -Is extended, an improvement in the size of
the leak that can be detected can be obtained. Extending the time of the leak
test decreases the leak rate that can be detected in proportion to the length
of the test. Extending the time from 36 to 48 hours reduces the size of the
detectable leak by a factor of 36/48 = 0.75. Extending the time from 36 to 56
hours would reduce the size of the detectable leak by 36/56 = 0.643.
If both of these modifications (2 stick readings and longer tests) are
performed, the performance of the method is improved. For example, with a 48-
hour test in a 500-gallon tank,, the standard error is reduced to 0.042
gaWh. This, 1n turn, implies a' threshold of 0.099 gal./h for declaring a
leak with a 1% false alarm rate and means that a leak rate of 0.197 gal./h
could be detected with probability 99%. Table 3 summarizes the performance
that would be expected from this modification for tank sizes of 500, 1000, and
2000 gallons and for tests with a duration of 36, 48, and 56 hours. Note that
a test of 56 hours could be done by not adding product to the tank from Friday
evening until Monday morning. (With a tank used to hold used oil, this may be
a reasonable time period, as the oil is usually accumulated in a drum and
dumped periodically.) It should be emphasized that the detectable leaks In
Table 3 are the result of a monthly average of 4 weekly tests and that
duplicate stick readings are used at the beginning and end of each test.
Table 4 is a tabulation of! performance information for a variety of
standard (steel) tank sizes. The tank lists the nominal volume of the tank as
well as its dimensions (length and diameter) in inches. Using the change In
level that corresponds to the 1% false alarm rate at the threshold
faDDroximately 0.75 in., assuming duplicate stick readings at the beginning
and end of each static test), the average volume change that corresponds to
the threshold for a single test is tabulated. This average was estimated as
85% of the maximum volume that would occur when the tank was half full. The
next column in the table is the average volume changes that would be detected
with 99% probability for a single test. The last two columns are the 1% false
alara ttSioTds Sd the volumes detectable with 99% .probability for the
monthly test that averages 4 weekly tests.
Table 5 is a tabulation of performance similar to Table 4. In Table 5
the volume changes detectable with 99% probably in the monthly (4 week
average) tests have been converted to leak rates for three test durations:
36? 48, and 56 hours. As can be ;seen, only the 500 andI 550 gallon tanks can
achieve detection of 0.2 gal./h leak rates with probability of 99% and a false
alarm rate of 1%.
As a result of somewhat disappointing performance of the static tank test
for tank sizes larger than 550 gallons, another modification to the test was
evaluated. Rather than require owners and operators to perform the static
test every week and average the results monthly, one could require that the
test be done with the tank at a! specific percent of capacity, for example,
95% This has several advantages. First, it ensures that the test checks
8
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TABLE 3
STANDARD ERRORS, THRESHOLDS, AND DETECTABLE LEAK RATE (gal/h)
FOR 4 WEEKLY TESTS AVERAGED3
500 gal. Tank
Standard Error
Threshold at 1%
Leak Detectable at 99%
1,000 gal. Tank
Standard Error
Threshold at 1%
Leak Detectable at 99%
2,000 gal. Tank
Standard Error
Threshold at IX
Leak Detectable at 99%
TEST
36
0.057
0.132
0.264
0.107
0.249
0.498
0.146
0.340
0.679
DURATION
48
0.042
0.099
0.197
0.082
0.191
0.382
0.109
0.254
0.507
(HOURS)
56
0.036
0.085
0.170
0.069
0.160
0.320
0.094
0.219
0.437
a Assuming duplicate stick readings.
OTC73A
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TABLE 4
TANK SIZES AND THRESHOLD VOLUMES
NOMINAL
VOLUME (gal.)
500
550
1,000
1,000 •
2,000
3,000
4,000
5,000
6,000
8,000
10,000
12,000
TANK
DIMENSIONS
DIAM. LENGTH
(IN.) (IN.)
48 66
48 72
48 128
64 73
64 144
64 216
64 288
96 ,. 160
96 192
96 256
96 320
96 384
AVERAGE IX
FALSE ALARM
VOL. CHANGE
THRESHOLD (gal.)
(SINGLE TEST)
8.7
9.5
17.0
12.9
25.4
38.2
50.9
42.4
50.9
67.8
84.8
101.7
AVERAGE VOL.
DETECTED
WITH 99*
PROB. (gal.)
(SINGLE TEST)
17.5
19.1
33.9
25.8
50.9
76.3
101.7
84.8
101.7
135.6
169.6
203.5
AVERAGE 1%
FALSE ALARM
VOL. CHANGE
THRESHOLD (gal.)
(MONTHLY)*1
4.4
4.8
8.5
6.4
12.7
19.1
25.4
21.2
25.4
33.9
42.4
50.9
AVERAGE VOL.
DETECTED
WITH 99X
PROB. (gal.)
(MONTHLY)4
8.7
9.5
17.0
12.9
25.4
38.2
50.9
42.4
50.9
67.8
84.8
101.7
a
4 test average.
OTC73A
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TABLE 5
TANK SIZE, THRESHOLDS, AND DETECTABLE LEAK RATES
TANK
SIZE
NOHIAL
VOLUME (gal
500
550
1,000
1,000
2,000
3,000
4,000
5,000
6,000
8,000
10,000
12,000
DIMENSIONS
DIAM.
1.) dn.]
48
48
48
64
64
64
64
96
96
96
96
96
LENGTH
i (1n.)
66
72
128
73
144
216
288
160
192
256
320
384
AVERAGE IX
FALSE ALARM
LEAK RATES DETECTABLE .
WITH 99* PROBABILITY (gal./h)b
VOL. CHANGE TEST DURATION (HOURS)
THRESHOLDa(qa1.) 36 48 56
4.4
4.8
8.5
6.4
12.7
19.1
25.4
21.2
25.4
33.9
42.4
50.9
0.243
0.265
0.471
0.358
0.706
1.060
1.413
1.177
1.413
1.884
2.355
2.826
0.182
0.199
0.353
0.269
0.530
0.795
1.060
0.883
1.060
1.413
1.766
2.119
0.156
0.170
. 0.303
0.230
0.454
0.681
0.908
0.757
0.908
1.211
1.514
1.817
a
b
4 Weeks Average (gal.)
Value from last column of Table 4 divided by test duration.
OTC73A
11
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nearly all of the tank for possible holes. Second, 1t will result in a much
smaller surface area for the product during the test, and, as a result, the
level change will correspond to a smaller volume change, increasing the
performance of the test. Third, it should reduce the burden of testing on the
Swne? and operator, since they will now be required to test only each time the
tank reaches 95% capacity, or, at most once a month.
Tables 6A-6C contain information about the static tank test when
performed at 95* of the capacity, of the tanks. The three standard tank
diameters (for steel tanks) of 48, 64, and 96 in. are Included. In addition,
the standard tank lengths and nominal volumes are included for tanks from 500
tra to 12,000 gal. For each tank diameter, the distance from the bottom of the
tank that corresponds to 95* of the capacity has been calculated. Thus, an
Imlr or operator would be required to allow the used oil tank to reach this
level and then test. Following the,test, the oil could be pumped out.
Assuming that duplicate stick Readings are used at the beginning and end
of each test, the standard error of the difference in height is 0.31 in. The
SatS tink test indicates a possible leak if the product height drops more
than 3/4 inch from start to finish (the actual number is 0.31 x 2.326 - 0.721
in for a 1* false alarm rate, but three-fourths of an inch is a practical
field number). In the first section of the table, for each tank diameter the
standard error of the height differences, the height difference threshold, and
the difference detectable with probability of 99* have been converted to
gal. TMs conversion results from multiplying the height times the cross
sectional area of the tank at the 95* capacity level and expressing the
resulting volume in gal.
Tables 6A-6C also present the volume change that can be detected with 99*
probability in terms of the equivalent leak rate for tests of different
durations. The durations range from 12 hours to 56 hours, corresponding to
durations from overnight to over a weekend from Friday evening until Monday
morning. The results are not restricted to used oil, but are valid for any
product.
The results in Tables 6A-6C do not include any temperature correction.
The convention followed is that an increase in volume requires use of water-
finding paste and action if water is found. A decrease in volume signals a
leak. Temperature changes could cause a-false alarm or a missed detection.
However, incorporating a temperature correction would substantially complicate
the static tank test.
Calculations show that the cross sectional area of a *** at. 95* of
capacity is about 59* of the maximum cross sectional area of the tank As a
result, requiring that the static tank test be performed when a tank is filled
to 95* of capacity reduces the volume error by only a factor of 0.59 (i.e. to
about 60* of the maximum volume error when the tank is half furl). As can be
seen in Table 6, a leak rate of 0.2 gal./h is detectable with 99* probability
onlj for tanks of about 550 gal. or less. Thus, the performance of the single
static test at 95* capacity is about the same as the average performance of
the average of four weekly tests of the same duration (see Table 5). It
should be noted that the weekly tests use an average error that is about 85*
of the maximum error when the tank is half full. The test at 95% capacity has
a fixed error size. ;
12
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TANK
DIAMETER
TABLE 6A
PERFORMANCE OF MONTHLY STATIC TANK TEST AT 95X
OF CAPACITY FOR VARIOUS TANKS AND TEST DURATIONS
TANK LENGTH Mn.)
66 ~~^I12
NOMINAL VOLUME (qal.)
500
550
1,000
48
(43.4)a
Standard Error 2.515 2.743 4.877
1* Threshold (gal) 5.849 6.381 11.344
99*DrtSlonSal) 11-698 12.762 22.688
.eak
Detectable with 99% Probability fgal/h);
12-hour test
24-hour test
36-hour test
48-hour test
56-hour test
0.975
0.487
0.325
0.244
0.209
1.063
0.532
0.354
0.266
0.228
1.891
0.945
0.630
0.473
0.405
• • '•
value 1n parenthesis 1s the distance from tank bottom for 95% capacity,
OTC73A
13
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TABLE 68
PERFORMANCE OF MONTHLY STATIC TANK TEST AT95*
OF CAPACITY FOR VARIOUS TANKS AND TEST DURATIONS
TANK
DIAMETER (1n.)
TANK LENGTH (in.)
73 144 216 288
NOMINAL VOLUME (gaT-1
1,000 2,000 3,000 4,000
64
(57.8)a
Standard Error 3.709 7.316 10.973
1% Threshold (gal) 8.626 17.016 25.524
99% Detection (gal) 17.252 34.032 51.048
14.631
34.032
68.064
Leak Rates Detectable with 99% Probability (gal/h):
12-hour test
24-hour test
36-hour test
48-hour test
56-hour test
1.438
0.719
0.479
0.359
0.308
2.836
1.418
0.945
0.709
0.608
4.254
2.127
1.418
1.063
0.912
5.672
2-836
1.891
1.418
1.215
a Value in parenthesis is the distance from tank bottom for 95% capacity.
OTC73A
14
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TABLE 6C
PERFORMANCE OF MONTHLY STATIC TANK TEST AT 95X
OF CAPACITY FOR VARIOUS TANKS AND TEST DURATIONS
TANK
DIAMETER (1n.)
TANK LENGTH (1n.)
160 192 256
NOMINAL VOLUME (gal.)
320
384
96
(86.7)a
Standard Error
1% Threshold (gal)
99% Detection (gal)
5,000 6,000 8,000 10,000 12,000
12.19
28.36
56.72
14.63
34.03
68.06
19.51
45.38
90.75
24.39
56.72
113.44
Leak Rates Detectable with 99% Probability (gal/h):
29.26
68.06
136.13
12-hour test
24-hour test
36-hour test
48-hour test
56-hour test
4.727
2.363
1.576
1.182
1.013
5.672
2.836
1.891
1.418
1.215
7.563
3.781
2.521
1.891
1.621
9.453
4.727
3.151
2.363
2.026
11.344
5.672
3.781
2.836
2.431
a Value 1n parenthesis is the distance from tank bottom for 95% capacity.
OTC73A
15
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One'can obtain somewhat better1 performance by averaging the four weekly
tests. However, the single test at 95* capacity may be preferred by some in
that It requires less testing for about the same preformance. It may be
easier to comply with than the averaging of four weekly tests.
One could improve the performance by increasing the level of product in
the tank when the test is to be done. Requiring the test at 99* of capacity
would reduce the size of the detectable leak rates by a factor of 0.6 for the
numbers in Table 6. This would require product levels of at least 46.6 in. in
a 48-in. diameter tank, 61.9 1n. 1n.a 64-1n. diameter tank, and 9,5.8 in. in a
96-ln diameter tank. From a practical standpoint, requiring any higher
levels would almost amount to requiring that the product be brought into the
fill nine for testing. This would probably require monitoring to ensure that
tempeJatuWchanges did not influence the test and to ensure that expansion
did not cause an overflow. '
In summary, either the weekly static test averaged over four weeks with a
duration of 48i hours or longer or a single test of 56 hours duration when the
oroduct levels is at least 95* of capacity will give a detection level of
about 0.2 gll./h in small (550 gal.[ or less) tanks. Operators could be given
the ootion of which they prefer to use. Static tank tests in larger tanks do
not reach the 0.2gal.% leak rate detectable with 99* probability for tests
of reliable duration. Requiring that the test be done at 95* capacity and
averaged over 4 weeks would extend the size of tanks up to 1,000 gal.,.and
almost UP to 2,000 gal. if a 56-hour test is used. Requiring the test level
to be 99* of capacity would result in about a 40* improvement (reduce the
detectable leak by a factor of 0.6);, but may not be practical.
7. Suggested Operating Protocol
The following is suggested at a practical operating procedure for using a
static tank test as a leak detection method for used oil tanks.
1. Take a stick reading on a tank and write down the depth of the product.
2. Wipe off the stick, take a secpnd stick reading, and write down the depth
of the product.
3. Add these two depths and divide by two to find an average depth.
4 Convert this average depth of product to gallons using the tank chart.
Record the volume and the time of the stick readings.
5. After the set period (e.g., 48 hours) has elapsed, stick the tank again
and record the depth of the product.
6. Wipe off the stick, stick the ;tank a second time, and record the depth of
the product.
7. Add these two depths and divide by two to obtain an average reading.
8. Convert this depth to volume by using the tank chart and record the
volume and time at the end of the static test period.
16
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9. If the depth at the end of the period 1s more then 3/4 inch greater than
that at the start period, wipe off the stick, apply water-finding paste
to the lower end, and re-stick the tank.
10. If the water-finding paste Indicates that water is present, the increase
in level could be cause by water coming into the tank. Unless an
alternative source for the water is identified, this should be regarded
as evidence that the tanks may have a hole in it, allowing water to come
in.
11 If the depth at the end of the period is more than 3/4 inch less than
that at the start of the period, this 1s evidence that product has been
lost from the tank, and the tank may be leaking.
12 If the depth at the beginning and end of the static tank test period does
not change by more than 3/4 in. 1n either direction, subtract the volume
at the end of the period from that recorded at the start of the period.
13. At the end of each four-week period, add the four differences in volumes
corresponding to the weekly static tests. Divide this sum by four.
14. For a 500-gal. tank (nominal size) and a 48-hour test, if the average
volume change indicates a loss of more than 4.75 gal. per weekly test,
the tanks fails the static tank test.
The next to last column in Table 4 provides volume thresholds for other
tank sizes. Table 3 gives the leak rate detectable with 99% probability for
some tank sizes and test durations. These should be used 1n determining the
appropriate duration of the static tank test.
The above protocol may be modified to improve the performance of the
static tank test by requiring the level 1n the tank to be at 95% of tank
capacity. The modification required ensures that the depth of the product In
the tank is high enough before beginning the static test.
If the diameter of the tank is 48 in., the product level must be at least
43.3 1n.
If the diameter of the tank is 64 in., the product level in the tank must
be at lest 57.75 in.
If the diameter of the tank 1s 96 in., the product level must be at least
86.65 in.
After ensuring that the required level of product has been met, refer to
Table 6 to determine an appropriate test duration for the tank size and the
required detectable leak rate.
When the minimum product level has been determined at the test duration
set, proceed as before 1n steps 1 through 11 above.
With this modification (testing at 95% of capacity), there is no need to
17
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<*.
loss of product and a positive sign a gain of product).
8. Effect of Monthly Tests
ind is? J
Sonth to determine whether there was evidence of a leak.
leak eacl! , 55h for n months is P raised to the n-th power.
Tf the error in failing to detect a leak results from random errors in
the leak woSld be 0^ The nSmber of monthly tests required is given by
n = log(0.01)/log(P),
Teak for each of 7 consecutive months.
perpetually.
18
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Reference "
Shareef, G. S. and Dicker-man, J. C., "Analysis of Static Tank Testing as a
Leak Detection Technique for Used Oil Tanks at Retail Outlets." American
Petroleum Institute, February 1987.
19
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Attachment
MiS'-VEST RESEARCH KG
0
1 ,
TABLE 3-1. MODEL TANK PARAMETERS
Model
Tank No.
1
2
3
4
Nominal
Tank Capacity
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Attachment MIDWEST RESEARCH KG P. 4/4
TABLE S-.3. STANDARD ERROR OF KAKVAL GAUGING RESULTS
' FOR DIFFERENT SUBDIVISIONS OF. THE DATA
Standard Error
Daea S«c - (Inehea)
All dtea 0*^0
Fib*rsl«ai> 0.25
Wood 0.31
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