The Department of Defense (DoD) owns more than 4,000 aboveground storage tanks (AST) of varying capacities. Current federal regulations deferred ASTs from the requirement to perform monthly monitoring and/or annual precision testing because no online leak detection system was available, but some states require DoD facilities to remove their tanks from service to install double bottoms and perform interstitial monitoring. This project demonstrated the Low Range Differential Pressure (LRDP) leak-detection system, which had previously been successfully demonstrated on bulk underground storage tanks, on ASTs.
The LRDP is an automatic, computer-controlled, mass-based measurement system capable of detecting very small leaks in bulk fuel ASTs. It relies on a closed reference tube that is the same height as the tank and filled with fuel to the same level as the fuel in the tank. If a leak occurs, a difference between the height of the liquid inside the reference tube and the bulk liquid develops, which is detected by a differential pressure (DP) sensor. Because of the short dynamic range required of the sensor, a dependable and robust, off-the-shelf DP transducer can be used, while maintaining the precision required for leak detection. The only difference in comparison to the underground tank application for which the LRDP was earlier validated is that a temperature sensor must be attached to the exterior wall to compensate for diurnal expansion and contraction, and tests should begin and end at night to minimize the effects of these temperature variations.
The system was first tested on a 54-foot-diameter, fixed-roof tank to ensure that it could be used to perform accurate leak-detection tests in a tank with a floating pan. A third-party evaluation of the LRDP system, which was similar to and compliant with the Environmental Protection Agency's standard test procedure for bulk underground tanks, was then conducted over a wide range of temperatures and induced leak conditions in a 164.5-foot-diameter, 6,470,000-gallon bulk AST located at the Fleet Industrial Service Center, Pearl Harbor, Hawaii. Results showed that a single 20-hour test could detect a leak of 0.932 gallons per hour with a 95 percent probability of detection and 5 percent probability of false alarm. By averaging multiple tests, the LRDP could detect leaks as small as 0.2 gallons per hour in a 108-foot-diameter tank with less than 5 percent probability of false alarm. Sensitivity improved further with smaller diameter tanks.
The costs of installation and testing using the LRDP over a 10-year period are 3-12 times less than conventional tracer methods or other mass-based systems. Therefore, significant savings can be realized across the DoD. Payback of the $50,000 capital cost for the LRDP is 1-3 years. The technology also allows facility operators to avoid out-of-compliance fines, the costs of removing the tank from service for modifications (such as installing a double bottom) or premature replacement, and the cost of remediation for cleanup of undetected fuel spills. The 24-hour LRDP test can meet the monthly monitoring requirements for the full range of ASTs owned or operated by the DoD and can be readily integrated into the DoD tank farm Fuel Automated System. (Project Completed - 2004)