Overview
The LRDP was initially developed for the world's largest USTs, which are owned and operated by the U.S. Navy [2]. The Red Hill tanks, buried over 100 feet deep in the hills above Honolulu, are 100 feet in diameter and 250 feet high, and each contains 12.5 million gallons of fuel. The LRDP has also been used for testing some of the bulk USTs owned by the Department of Defense (DoD) [1]. The LRDP has been integrated into the DoD Fuels Automated System (FAS), making it compatible with all the DoD's bulk fuel storage facilities.
The LRDP consists of three integrated components:
- an in-tank sensor unit for making measurements
- a local controller to implement a test and analyze the data from the test
- a host computer to initiate a test and to report and archive the results of the test
The LRDP can, with only minor hardware and software modifications, also conduct leak detection tests on ASTs.
How the LRDP Works
The key component of the LRDP is the vertical "reference" tube, which spans the full usable height of the tank (see diagram). The fuel in the tank is allowed to enter or leave the reference tube through a valve located at the bottom of the tube. When the tank is to be tested, the valve is closed, isolating the fuel in the tube from the fuel in the rest of the tank.
The level of fuel in the reference tube mimics that in the tank in every way except for the level changes due to a leak. A differential-pressure sensor, which is housed in a sealed container at the bottom of the tube (which is itself at the bottom of the tank) then detects very small changes in pressure between the fuel in the tank and the fuel in the tube, with the LRDP converting pressure changes to the equivalent level changes. Thus, when the valve is closed, the differential-pressure sensor directly senses, and the LRDP quantifies, the level changes due to a leak (if a leak is present).
The LRDP achieves a very high level of performance against small leaks because of (1) its high precision and (2) its inherent method of compensating for the thermal expansion and contraction of the fuel in the tank. In addition, the LRDP compensates for evaporation and condensation within the tank.
The LRDP not only delivers high performance but is rugged and fieldworthy.
Most importantly, because of its unique, patented design, the LRDP eliminates the two factors responsible for the poor performance of other mass-based measurement systems—thermal drift of the pressure sensors and thermally induced vertical movement of the in-tank sensor unit. All of the sensors are mounted in a sealed container at the bottom of the tank, where temperature changes are too small to affect sensor performance; and the in-tank sensor is held in place by a bellows-type mounting system that prevents changes in tank geometry from affecting the position of the sensor.
The LRDP was developed by Vista Research with technical support from the U.S. Naval Facilities Engineering Service Center (NFESC) and funding from the Pollution Abatement Ashore Program managed by the Naval Facilities Engineering Command and sponsored by the Environmental Protection, Safety and Occupational Health Division (N45) of the Chief of Naval Operations. The LRDP was demonstrated and validated under the Department of Defense's Environmental Security Technology Certification Program (ESTCP).
Benefits
Fully Automated
The LRDP is a fully automated, computer- controlled system that is easily installed and operated. An operator initiates a test from the host computer, and the embedded controller located at the tank does the rest.
Rugged and Fieldworthy
The LRDP's unique, patented, "reference" tube design ensures that it achieves a high level of precision and accuracy with an off-the-shelf, industrial-grade pressure sensor.
Mobile Option
The LRDP can be permanently installed for on-line monitoring and periodic tightness testing, or it can be transported to a site for one-time tightness testing. The former is normally the preferable configuration because of the regulatory requirements for monthly monitoring of tank integrity. The LRDP, once integrated into a facility, can test all of the tanks in a fuel farm or bulk storage terminal.
Certified Performance
In a series of third-party evaluations conducted according to the standard EPA and ASTM procedures for evaluating detection systems for bulk storage tanks [3,4], Ken Wilcox Associates, Inc. (KWA), evaluated 16 different ways of conducting a test with the LRDP, using various combinations of test length, testing frequency, and averaging of test results. Depending on test length, the LRDP can detect a leak as small as 0.38 gallons per hour in tanks up to 140 feet in diameter [5]. (The KWA evaluations were certified by the independent National Work Group on Leak Detection Evaluations.)
Precision
The LRDP has a precision of 0.0002 inches when making a single level measurement, and enough sensitivity to measure volume changes as small as 0.03 gallons per hour in a 100-foot- diameter tank. This high degree of precision is obtained because the differential-pressure sensor does not have to operate over the entire height of the tank; it only has to operate over the difference between the level in the tube and the level in the tank (perhaps 0.8 feet in a 20-foot-high tank, for example). This configuration increases the precision of the measurement made by the DP sensor, in such a 20-foot-high tank, by a factor of at least 240 over a system that does not use a reference tube.
Mobile LRDP
The LRDP can be permanently installed for on-line monitoring and periodic tightness testing, or it can be transported to a site for one-time tightness testing. The former is normally the preferable configuration because of the regulatory requirements for monthly monitoring of tank integrity. The LRDP, once integrated into a facility, can test all of the tanks in a fuel farm or bulk storage terminal.
