The Department of Defense (DoD) owns over 10,000 fuel storage tanks and associated piping networks that must be periodically monitored and tested for leaks. Contamination from such leaks must be remediated, often at high cost to DoD. Studies have indicated that most leaks occur in pipelines rather than the tanks themselves, and although pipeline leak detection systems such as tracer, cable and vapor monitoring are presently available, there is still a need for a low-cost and rapid means of locating leaks. An acoustic system can locate leaks with significant performance, operational and cost advantages such as real-time leak location and detection verification. This project demonstrated a prototype passive-acoustic, on-line, portable monitoring system on bulk and hydrant pipeline systems at Air Force, Army, and Navy installations and at the test facility of the Environmental Protection Agency in Edison, NJ.

The Passive Acoustic Leak-Location System (PALS) is made up of three acoustic sensors, a signal conditioning system, a data acquisition and signal processing system, and a simple user interface for initiating a test and displaying results. The sensors are attached by clamps or epoxy to the outer wall of the pipeline. A reference and velocity acoustic sensor pair, located on one side of the suspected leak, estimates the velocity of propagation of sound in the pipeline contents by using a coherence function algorithm. Position and reference sensors bracket the leak. Knowing the velocity of propagation allows acoustic measurements from this second pair of sensors to be used to estimate the distance between the leak and the reference sensor. The system can be used in real time (i.e., tens of seconds to several minutes) and can provide immediate operational feedback not possible with existing technologies.

The PALS system located artificially induced leaks at all four demonstration sites to within one percent of the sensor separation distance, or better, at sensor separation distances ranging from 125 to 700 feet. The system operated well under aircraft operational noise conditions. However, there was some loss in sensitivity when complex piping geometry (e.g., bends or elbows) produced many acoustic reflections.

There is more than a 15 to 1 cost advantage in using PALS instead of excavation and more than a 4 to 1 cost advantage when compared to liquid-tracer methods. The cost of PALS may be as low as $0.22 per linear foot. The payback on capital equipment purchases is less than one year. Other benefits associated with the implementation of a PALS system are: (1) it will increase facility readiness and reduce revenue lost by quickly returning the facility to full operational status; (2) the service life for older pipelines can be significantly extended with rapid detection and location of leaks; and (3) a dramatic reduction in excavation costs will be realized with the high-precision leak location capabilities of passive acoustics. There are also significant savings in cleanup cost avoidance. The Army Environmental Center estimates the average cleanup cost associated with tank and piping leaks to be $193 thousand, even higher if the groundwater is contaminated. Future cost avoidance at 659 Navy sites is estimated at $890 million. (Project Completed - 2003)