The Department of Defense (DoD) owns over 1,000 underground storage tanks (USTs) of varying capacities. Although current federal regulations have deferred bulk USTs from the regular annual tightness test or a monthly monitoring test that smaller USTs must undergo, the State of California (where most DoD bulk USTs are located) has introduced regulatory guidance for monitoring. This project demonstrated the Low Range Differential Pressure (LRDP) leak detection system for bulk fuel USTs at the Navy's Point Loma fuel terminal in California and for smaller tanks at Hunter Army Air Field (AAF) in Georgia.

The LRDP is an automatic, computer-controlled, differential pressure (mass based) measurement system capable of detecting very small leaks in bulk fuel USTs. It relies on a closed reference tube that is the same height as the tank and is filled with fuel to the same level as the fuel in the tank. This significantly reduces the dynamic range required of the sensor, which allows the use of a dependable and robust, off-the-shelf differential pressure transducer, while maintaining the precision required for bulk UST leak detection.

A third-party evaluation of the LRDP system in a 122-feet-diameter, 2,100,000-gallon bulk UST at Point Loma was conducted over a wide range of temperatures and induced leak conditions. Results showed that a 24-hour test could detect leaks as small as 0.7 gallons per hour, with a 95 percent probability of detection and less than 5 percent probability of false alarm. An average of 12 monthly readings could detect a leak as small as 0.2 gallons per hour. Sensitivity improved further with smaller diameter tanks. At Hunter AAF, the LRDP met the 0.2 gallons per hour monthly monitoring requirement for a 50,000-gallon tank with an 8-hour test.

Installation and testing costs over a 10-year period are 3-12 times less than conventional tracer methods or other mass-based systems. Payback of the $50 thousand capital cost for the LRDP is 1-3 years. The technology also allows facility operators to avoid the costs of tank replacement and site cleanup. The LRDP leak detection system is the only system known to perform monthly and annual monitoring of bulk USTs and the only one that meets both Option 7 and Option 10 for California.

The 24-hour LRDP test can meet the monthly monitoring requirements for the full range of bulk USTs owned or operated by the DoD, and the results have been submitted for approval to the National Work Group on Leak Detection Evaluations, an oversight group sponsored by the Environmental Protection Agency. The LRDP system can be implemented in any size tank of any configuration and can be readily integrated into the DoD tank farm Fuel Automated System. The technology is not site-specific and its performance is not affected by site geology or topographic factors. (Project Completed - 2002)