Issue Date: August 23, 1999

Vista Research, Inc. and Naval Facilities Engineering Service Center

LRDP-48 (V1.1)

Bulk Underground Storage Tank Leak Detection Method (50,000 Gallons or Greater)

Certification

Leak rate is proportional to product surface area (PSA).
For tanks with PSA of 6,082 ft², leak rate is 0.749 gph with PD = 95% and PFA = 0.012%.
For other tank sizes, leak rate equals [(PSA in ft² ÷ 6,082 ft²) x 0.749 gph].

Example:
For a tank with PSA = 10,000 ft²; leak rate = [(10,000 ft² ÷ 6,082 ft²) x 0.749 gph] = 1.232 gph.
Calculated minimum detectable leak rate is 0.376 gph with PD = 95% and PFA = 5%.
Leak rate may not be scaled below 0.2 gph.
Leak Threshold

Leak threshold is proportional to product surface area (PSA).
For tanks with PSA of 6,082 ft², leak threshold is 0.563 gph.
For other tank sizes, leak threshold equals [(PSA in ft² ÷ 6,082 ft²) x 0.563 gph].

Example:
For a tank with PSA = 10,000 ft²; leak threshold = [(10,000 ft² ÷ 6,082 ft²) x 0.563 gph] = 0.926 gph.
A tank system should not be declared tight if the test result indicates a loss or gain that equals or exceeds the calculated leak threshold.
Applicability Gasoline, diesel, aviation fuel.
Other liquids may be tested after consultation with the manufacturer.
Tank Capacity Use limited to single field-constructed vertical tanks larger than 50,000 gallons. Maximum product surface area (PSA) is 15,205 ft² (approximately 139 ft diameter). Performance not sensitive to product level.
Waiting Time Minimum of 24 hours after delivery or dispensing.
Valve leaks and pump drain-back may mask a leak.
Allow sufficient waiting time to minimize these effects.
Waiting times during evaluation ranged from 16.08 to 115.8 hours.
Test Period Minimum of 48 hours.
There must be no dispensing or delivery during test.
Temperature Measurement not required by this system.
Water Sensor None.
Water leaks are measured as increase in mass inside tank.
Calibration Differential pressure sensor must be checked regularly in accordance with manufacturer's instructions.
Comments Tests only portion of tank containing product.
As product level is lowered, leak rate in a leaking tank decreases (due to lower head pressure).
Consistent testing at low levels could allow a leak to remain undetected.
Evaluated in a nominal 600,000 gallon, vertical underground tank with product surface area (PSA) of 6,082 ft².
Not evaluated as a stand alone system.
Significant bias of 0.078 gph was detected during the evaluation.
Evaluator believes this bias was the result of product inflow into the tank from valve leaks and product drain back from the pump return line and thus evaluator did not use this bias in calculating above results.
Performance of the system can be improved by combining results of 2 or more tests.
If this option is used, it is important to determine the number of tests, their timing and the number of passing results necessary to confirm a tank is tight.
The LRDP-24-5 (V1.1) combines the results of 5 tests and is one evaluated option to improve the performance of this system.
  • Vista Research, Inc.
    755 North Mary Ave.
    Sunnyvale, CA 94085
    Tel: (408) 830-3300
  • Evaluator: Ken Wilcox Associates
    Tel: (816) 443-2494
    Date of Evaluation: 01/29/99

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