API PUBL 322-1994

$24.05

An Engineering Evaluation of Acoustic Methods of Leak Detection in Aboveground Storage Tanks

Published By	Publication Date	Number of Pages
API	1994	70

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Category: API

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Description

The design of an aboveground storage tank (AST) leak detection system based upon passive-acoustic methods requires a detailed understanding of the acoustic leak signal and the ambient noise field against which the signal is measured. As part of Phase III of the American Petroleum Institute’s (API’s) project to develop and evaluate the performance of different technologies for detecting leaks in the floor of ASTs, a set of controlled experiments was conducted in a 40-ftdiameter tank during June 1992. Two sets of holes of various diameters, ranging from 0.5 to 3 mm, were drilled in the tank floor. These holes released product (water) into one of two backfill materials: native soil or sand. Two types of acoustic signals were generated and studied: (1) the continuous leak signal produced by turbulent flow through a hole in the floor of the tank, and (2) the impulsive leak signal produced by bubbles collapsing in the backfill beneath the tank floor.

The analytical and experimental results of this project suggest that a passive acoustic system can be used to detect small leaks in ASTs. The experiments have shown that the impulsive leak Signals identified through laboratory and field simulations are persistent and measurable within an AST. The experiments yielded two very significant findings, which must be addressed in the data collection and signal processing schemes used to detect leaks with the passive-acoustic method: (1) the multipath signals (leak-to-wall-to-sensor or leak-to-surface-to-sensor), which are associated with the direct path signal (leak-to-sensor), are very strong and may be stronger than the direct path signal, and (2) the time delays of the multipath signals relative to the direct path signal for each sensor in the wall array may be very different. Both phenomena are due to acoustic propagation in the highly reflective confines of a right circular cylinder AST.

During the experiments, a data collection procedure and a signal processing algorithm were used to separate the impulsive events associated with the leak from the strong multipath reflected Signals. All of the leaks that were generated in the floor of the 40-ft-diameter tank were successfully detected and located.

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