The Vista HT-100 is a dual pressure volumetric leak detection system for testing underground bulk and hydrant fuel piping (it is certified for use on lines with a capacity of 3400 gallons or more). Its high performance is due to a novel method of compensating for the thermally induced changes in fuel volume that occur during a test. In 1999, Aviation Week & Space Technology selected the HT-100 for one of its Annual Technology Innovation Awards.

The HT-100 is a computer- controlled, fully automatic system. All data are acquired and processed electronically. As shown in the schematic diagram at right, the HT-100 sensor unit consists of (1) a measurement cylinder (Tank 1); (2) a storage cylinder (Tank 2); (3) a differential pressure sensor to measure level changes in the measurement cylinder electronically; (4) a pump for transferring fuel from the measurement cylinder or storage cylinder to the line in order to increase line pressure to a specified level; and (5) two pressure relief valves and a bypass valve for removing fuel from the line and adding it to the measurement cylinder or storage cylinder in order to decrease the pressure in the line to a specified level. When combined, the pump and the pressure relief valves can be used to maintain a constant pressure in the line at a specified level. The size of the cylinders is determined by the capacity of the largest pipeline segment that will be tested at a given site.

The storage cylinder is normally used to set or change the overall pressure in the line. (Pressure can also be set or changed with the pressure management system used to operate the line or by adding or removing fuel from a nearby tank, tanker truck, or pipeline.) The measurement cylinder is normally used to maintain constant pressure during a test and to make the volume measurements required during the test. A test can be conducted with the measurement cylinder alone if there is no need for the extra storage capacity that Tank 2 provides. The pump and pressure-relief-valve system are used to adjust and maintain a constant pressure in the line. As shown by the valves on the "ladder," the system can be operated at three pressures: a high pressure, a low pressure, and atmospheric pressure (by means of the bypass valve). The high- and low-pressure relief valves can be set to operate at any desired pressure. Finally, a pressure relief valve and an overfill alarm in each cylinder ensure safe operation of the system.

The LT-100 is independently certified and is listed by the NWGLDEas complying with all EPA protocols.

When the volume of fuel is less than 3400 gallons—roughly 1300 feet of 8- inch-diameter pipe—the LT-100 can reliably detect leaks as small as 0.1 gallon per hour. When the volume is greater than 3400 gallons it can detect a leak equivalent in size to 0.0021% of the total volume per hour. Most importantly, through its algorithm for temperature compensation, the LT-100 overcomes some of the major deficiencies of conventional pressure tests.

The LT-100 is a computer- controlled, fully automatic system. All data are acquired and processed electronically. As shown in the schematic diagram at right, the LT-100 sensor unit consists of (1) a measurement cylinder (Tank 1); (2) a storage cylinder (Tank 2); (3) a differential pressure sensor to measure level changes in the measurement cylinder electronically; (4) a pump for transferring fuel from the measurement cylinder or storage cylinder to the line in order to increase line pressure to a specified level; and (5) two pressure relief valves and a bypass valve for removing fuel from the line and adding it to the measurement cylinder or storage cylinder in order to decrease the pressure in the line to a specified level. When combined, the pump and the pressure relief valves can be used to maintain a constant pressure in the line at a specified level. The size of the cylinders is determined by the capacity of the largest pipeline segment that will be tested at a given site.

The storage cylinder is normally used to set or change the overall pressure in the line. (Pressure can also be set or changed with the pressure management system used to operate the line or by adding or removing fuel from a nearby tank, tanker truck, or pipeline.) The measurement cylinder is normally used to maintain constant pressure during a test and to make the volume measurements required during the test. A test can be conducted with the measurement cylinder alone if there is no need for the extra storage capacity that Tank 2 provides. The pump and pressure-relief-valve system are used to adjust and maintain a constant pressure in the line. As shown by the valves on the "ladder," the system can be operated at three pressures: a high pressure, a low pressure, and atmospheric pressure (by means of the bypass valve). The high- and low-pressure relief valves can be set to operate at any desired pressure. Finally, a pressure relief valve and an overfill alarm in each cylinder ensure safe operation of the system.

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).

Leak detection methods— techniques that determine whether underground piping systems are in fact leaking—have been around commercially for some time, and the number of methods is increasing annually. Reliable leak location methods are equally important, yet scarcer in the marketplace. Many users of leak detection systems face a dilemma if a leak is actually detected—how to accurately and reliably determine the location of the leak while minimizing both the disruption of service and the need for excavating buried piping.

The acoustic signal produced by a leak can be masked by background noise, and signal-processing techniques must be applied that allow the detection of the signal amid the noise. Vista Research, long recognized as a leader in signal processing, specializes in separating meaningful signals from the background noise that has been the bane of other acoustic methods. Vista's PALS system uses a unique approach to signal processing that is based on coherence analysis. The PALS, which is gaining great commercial recognition, has regularly outperformed other acoustic methods by a wide margin.

The PALS is a portable passive-acoustic system comprised of three acoustic sensors, three small pre-amplifiers, and a portable computer equipped with a data acquisition card. Each sensor measures the acoustic signal generated by the turbulent flow through a hole in the pipeline. The diagram at right shows how leaks are located. The acoustic signal generated by any leak that might be present between the "Ref" (reference) and "Pos" (position) sensors is picked up and transmitted to the computer via the pre-amplifiers; the speed at which the acoustic signal is propagating through the pipeline is measured in the region between the "Vel" (velocity) and Ref sensors, which—unlike Ref and Pos—do not bracket the leak. For the leak to be correctly located, the distances between the sensors must be known, since the measurement made by the PALS determines the location of the leak relative to the reference sensor. A leak location measurement can be made in as little as 2 to 5 minutes.