In 1988, the EPA issued Underground Storage Tank (UST) regulations for fuel storage in order to implement monthly monitoring and annual tightness testing. Monthly monitoring requires that the leak detection system be certified for a leak rate of 0.2 gallons per hour (GPH), and annual tightness testing requires a certified leak rate of 0.1 GPH, both with a probability of detection of at least 95%, and a probability of false alarm of no more than 5%. The DoD owns and operates hundreds of underground storage tanks that are less than 100,000 gallons. These tanks, commonly operating at base fuel farms, must comply with EPA, state, and local regulations, and require that either an annual tightness test with monthly inventory reconciliation or monthly monitoring tests be conducted in accordance with published performance standards. Existing technologies either cannot meet performance standards for the large 50-K to 100-K gallon USTs or do not provide a cost-effective solution. The Portable Rapid Test (PRT) is designed and recommended for testing tanks containing lower-volatile, heavier-type fuels such as diesel, JP-5, JP-8, etc.
The PRT leak detection system comprises a tank fuel-level sensing unit, signal conditioning, and laptop computer. The tank level sensing unit has a vertical reference tube that spans the full height of the tank. This reference tube has a straight or shaped cross-sectional area that matches, with a constant ratio, the cross-sectional shape of the test fuel tank. The bottom section of the reference tube is about 8 to 12" and is detachable from the main upper reference tube. Attached to the bottom section of the reference tube is a sliding seal that is spring-loaded to be normally open. The sliding seal is designed so that, when resting on the bottom of the test tank, the weight of the reference tube will overcome the sliding seal spring force and the sliding seal will close the bottom of the reference tube so no additional fuel can enter into the reference tube. The sliding seal can be made from a multitude of resilient material configurations including sheet seals, o-rings, ball shaped, etc., and non-resilient material configurations including metal to metal, ceramic to metal, plastic to metal, etc.
A differential pressure transducer is attached to the side of the bottom section of the reference tube and is adjacent to the sliding seal. The pressure transducer measures the pressure difference between the height/mass of the fluid in the reference tube and the height/mass of the fluid in the test tank. To minimize fluid cavity areas that can potentially trap or contain vapor bubbles, the pressure transducer is mounted directly to the reference without the use of external tubing. To provide quick thermal stabilization, the pressure transducer body is also of a type that can be directly immersed in the test tank fuel without use of a containment vessel. The bottom section of the reference tube, the sliding seal, and the differential pressure transducer comprise a modular unit that can be attached to any length of upper reference tube so that the modular unit can be adapted to any tank configuration.
The electrical conductors are hermetically sealed to the differential pressure transducer and are contained in a fuel-resistant jacket. The conductors/ jacket are attached to the outside of the reference tube, run along the full length of the reference tube, and have approximately 20 to 30 feet of additional cable length available for routing to the electronics package outside the test tank. The electronics package consists of a signal conditioner, power supply, terminal block, and PCMCIA A/D card. The signal conditioner maintains an intrinsically safe 4- to 20-milliamp (ma) supply current to the differential pressure transducer. The pressure transducer modifies the magnitude of the supply current (i.e., the analog test signal) in relation to actual pressure differentials developed between the reference tube and the test tank. The analog test signal is then output from the signal conditioner and fed to a laptop computer for analog-to-digital conversion. Saved test data can also be easily downloaded into to a spreadsheet.
The system allows testing at EPA standard leak rates of 0.10 gal/hr and 0.20 gal/hr. This provides a significant improvement over current methods technologically, operationally, and in short- and long-term cost benefits.
This work was done by William Major and Leslie Karr of the Naval Facilities Engineering Command. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Physical Sciences category. NRL-0044
This Brief includes a Technical Support Package (TSP).
Portable, Rapid-Test Fuel Tank Leak Detection System
(reference NRL-0044) is currently available for download from the TSP library.
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