1、API PUBL*307 92 0732290 0510749 641 W4 An Engineering Assessment of Acoustic Methods of Leak Detection in Aboveground Storage Tanks HEALTH AND ENVIRONMENTAL AFFAIRS API PUBLICATION NUMBER 307 JANUARY 1992 American Petroleum Institute 1220 L Street, Northwest 11) Washington, D.C. 20005 An Engineering
2、 Assessment of Acoustic Methods of Leak Detection in Aboveground Storage Tanks API PUBLX307 92 m O732290 0530750 363 = Health and Environmental Affairs Department API PUBLICATION NUMBER 307 JANUARY 1992 PREPARED UNDER CONTRACT BY: ERIC G. ECKERT AND JOSEPH W. MARESCA, JR. VISTA RESEARCH, INC. MOUNTA
3、IN VIEW, CA American Petroleum Institute API PUBLX307 72 = 0732270 0530751i 2TT FOREWORD API PUBLICATIONS NECESSARILY ADDRESS PROBLEMS OF A GENERAL NATURE. WITH RESPEC TO PARTICULAR CIRCUMSTANCES, LOCAL, STATE, AND FEDERAL LAWS AND REGULATIONS SHOULD REWEWED. API IS NOT UNDERTAKING To MEET THE DUTIE
4、S OF EMPLOYERS, MANUFACTCJRERS, OR SUPPLIERS TO WARN AND PROPERLY TRAIN AND EQUIP THEIR EMPLOYEES, AND OTHERS EXPOSED, CONCERNING HEALTH AND SAFETY RISKS AND PRECAUTIONS, NOR UNDERTAKING THEIR OBLIGATIONS UNDER LOCAL, STATE, OR FEDERAL LAWS. NOTHING CONTAINED IN ANY API PUBLICATION IS TO BE CONSTRUE
5、D AS GRANTING ANY RIGHT, BY IMPLICATION OR OTHERWISE, FOR THE MANUFACTURE, SALE, OR USE OF ANY METHOD, APPARATUS, OR PRODUCT COVERED BY LETERS PATENT. NEITHER SHOULD A“G CONTAINED IN THE PUBLICATION BE CONSTRUED AS INSURING ANYONE AGAINST LIABILITY FOR INFRIGEMENT OF LE“ERS PATENT. Table of Contents
6、 Section 1: Introduction 1 Section 2: Background 2 Section 3: Summary of Results . 3 Section 4: Report Organization . 6 References 7 Appendix A: Detection of Leaks in the Floor of Aboveground Storage Tanks by Means of a Passive-Acoustic Sensing System A-1 Appendix B: Field Tests of Passive-Acoustic
7、Leak Detection Systems for Aboveground Storage Tanks When In Service . B-1 I API PUBLX307 92 W 0732290 0530753 072 Executive Summary Introduction Though a number of fm offer aboveground storage tank (AST) leak detection services based on passive acoustics, very little information has been published
8、concerning the performance of such systems or the nature of the acoustic leak signal. This document provides the results of an engineering assessment of passive-acoustic sensing methods for detecting srnail leaks in large ASTS. The assessment consisted of laboratory experiments, analyses of unpublis
9、hed data collected by industry in a 10-foot-diameter AST containing water, and field experiments at the Mobil Oil Refinery in Beaumont, Texas on a 114-foot-diameter AST containing a heavy naphtha petroleum product. Background The American Petroleum Institute (API) has completed two phases of a leak
10、detection project for aboveground storage tanks (ASTs). The purpose of Phase I was to assess different leak detection technologies to determine which had the greatest potential for field application. Because acoustic and volumetric methods were found to have significant operational and performance a
11、dvantages, they were the ones chosen for testing under Phase II of the project. The purpose of Phase II was to perform an engineering assessment of acoustic and volumetric methods for detecting small leaks in large ASTs. The principal objectives of Phase II were: to determine, in the case of acousti
12、c methods, the nature of the leak signal and the ambient noise in an AST; to determine, in the case of volumetric methods, the sources and magnitude of ambient noise associated with measurements in an AST; to perform field experiments on a large, full-scale AST; and to recommend ways to improve exis
13、ting AST leak detection methods. Conclusions can be used to detect small leaks in ASTs. The experiments have shown that a detectable leak signal does exist, but that the current approach to data acquisition and signal processing needs to be improved for the technology to achieve its full potential.
14、As part of the field tests under Phase II, an algorithm based on radar beam-forming techniques was developed; this algorithm improved the detection of leaks. An example of the application of the algorithm to both impulsive and continuous leak signals is presented in this report. Both the beam-formin
15、g algorithm and the data collection strategy must be evaluated by means of further experiments designed to estimate the performance of a passive-acoustic system in the presence of real leaks in the floor of an AST. Section 3 of the body of this report consists of a short but detailed summary of the
16、technical results of this engineering assessment. A description of the expehents and analyses are presented in two professional papers, which are attached as appendices to the report. The analytical and experimental results of this project suggest that a passive-acoustic system 1 The results of die
17、volumetric study are provided in a separate API document entitled An Engineering Assessment of Volumetric Methods of Leak Detection in Aboveground Storage Tanks, by James W. Starr and Joseph W. Maresca, Jr. ES-1 API PUBL*307 92 W 0732290 0530754 TO9 1 Introduction This report summarizes Phase II of
18、a research program conducted by the American Petroleum Institute (MI) to evaluate the performance of technologies that can be used to detect leaks in the floors of aboveground storage tanks. During Phase I, an analytical assessment of the perfomance four leak detection technologies was investigated
19、i, 21. The four technologies included: (1) passive-acoustic sensing systems, (2) volumetric systems, especially differential pressure (or “mass“) measurement systems, (3) advanced inventory reconciliation methods, and (4) tracers methods. During Phase II, field tests were conducted on an aboveground
20、 storage tank to make an engineering assessment of the performance of two of these technologies, passive-acoustic sensing systems and volumetric detection systems. This report describes the engineering assessment of the acoustic systems that were examined; the engineering assessment of volumetric sy
21、stems is described in a separate report 3. The specific objectives of the Phase II research in the area of acoustics were to: assess the current state of AST leak detection technology determhe the nature of the leak signal and the ambient acoustic noise in an AST perform field experiments on a full-
22、scale AST recommend ways to improve existing AST detection systems The field tests were conducted at the Mobil Oil Refinery in Beaumont, Texas, on a 50,000-bbl, 114-fi-diameter AST containing a heavy naphtha petroleum product. The experiments focused on identisling and quanteing the acoustic leak si
23、gnal and its source mechanisms, and on formulating the strategies necessary to detect the leak signal. API PUBLx307 72 0732270 0510755 745 2 Background The choice of a particular strategy for the collection and processing of acoustic signals is strongly tied to the nature of the signai and the backg
24、round noise field in which the signal is immersed. The approach to AST acoustic leak detection adopted by the industry is based upon the success with which flaws and cracks in a variety of materiais have been identified through the use of acoustic emissions (AE) techniques, and the ease with which s
25、uch systems may be designed and operated. Though a number of hs offer AST leak detection services based upon passive acoustics, very little technical information has been published concerning the performance of such systems or the nature of the acoustic leak signal. While tank owners and operators c
26、ovet the operational features of the technology, there is a need to provide convincing evidence that the technology is effective. A first step toward providing this evidence is to review the few available test results provided by the leak detection industry and to perform a system analysis of the da
27、ta collection and processing approach being used. The assessment of passive acoustic leak detection technology presented in this work is based both on the industry-derived data and on laboratory and field experiments. The current method by which the presence of an AST leak is inferred is detection-
28、through-location. in order to locate a region of the AST floor that emits acoustic energy in excess of a measured, average level, an may of transducers is used to construct a sound-level map. Currently available acoustic leak detection systems require that the leak emit impulsive signals whose ampli
29、tude greatly exceeds the background noise level. The process of converting these impulsive signals into a sound-level map can be described as follows. For each element of the sensor array, an impulse arrival time is recorded when a preset threshold signai level is exceeded. Sets of impulse arrival t
30、imes then serve as input to a location algorithm that predicts the most likely origin of the signal. A large number of such location predictions are plotted on a diagram of the AST floor to produce the sound-level map. Regions of the map in which significant clustering of source locations is observe
31、d are interpreted as likely leak locations. Published results of field tests on full-scale ASTS, and unpublished results made avdable for review, offer linle convincing evidence that this approach to passive-acoustic leak detection perfoms adequately when applied to the AST leak detection problem. A
32、n analysis of the problems associated with the current generation of leak detection systems was performed in which two fundamental questions were addressed. First, are large-amplitude, impulsive signals (i.e., background noise) expected to dominate the acoustic signal in the case of real AST leaks?
33、Secondly, if leak-generated impulsive signals exist, are they being acquired and processed correctly? 2 API PUBLX307 92 0732290 0530756 81 3 Summary of Results The nature of both the acoustic leak signal and its corresponding source mechanisms was investigated in a series of laboratory experiments.
34、A leak simulator was constructed in order to control the flow rate, backflU material, and pressure head above the leak. Time series of acoustic signals were recorded by a pair of transducers placed in close proximity to the leak. The results of these experiments showed that the acoustic leak signal
35、is comprised of both impulsive and continuous components. Turbulent flow, cavitation, and particulates in the backfill colliding with each other and with the tank floor were identified as the most likely source mechanisms for the production of continuous leak signals. The interaction between the lea
36、k flow field and air bubbles trapped within the backfLU material was the only source mechanism found to produce the large-amplitude, impulsive signals upon which the current leak detection technology is based. In addition, once the baca material became fully saturated, the production of impulses cea
37、sed. These results brought into question the persistence of impulsive leak signals in an operational AST, but also identified detectable, persistent signals that have not yet been exploited by the industry. The published results of field tests show a high degree of scatter in the data used to form s
38、ound-level maps. On the assumption that the backfill conditions present during these tests were appropriate for the production of impulsive leak signals, an analysis was made of the data collection and signal processing methods currently employed by the testing industry. The results of this study in
39、dicate that the manner in which data are acquired, i.e., collecting a set of impulse arrival times whenever a threshold exceedance occurs on any element of the sensor may, tends to produce inaccurate location estimates in proportion to the rate at which impulses are emitted from the leak. When only
40、impulse arrival times are collected, as opposed to continuous time series, the possibility exists that a given set of arrival times are not conelated with the emission of a single impulsive signal, but instead are correlated with two or more distinct events. The processing of these mixed-arrival tim
41、e sets by the location algorithm was suggested as a probable source of error. For a full-scale tank, it was shown that the inaccurate collection of impulsive signals would occu 50% of the time for a rate of impulse emission of only 12 s-. This analysis assumed that the noise was zero and that only d
42、etectable signals were present; the percentage of improperly collected signals would increase significantly if noise were included or the rate of impulse emission were increased. 3 API PUBL*307 92 0732290 0510757 718 The detectability of acoustic leak signals and alternative methods for the processi
43、ng of these signals were Investigated through the analysis of data obtained during field tests on 1 O- and 1 1443-diameter ASTs. One of the vendors provided continuous time series of impulsive leak signals recorded in a 10-fi-diameter AST by internal hydrophones and external resonant sensors. The pr
44、imary results of this analysis were that: (1) impulsive signais dominated the acoustic leak signal produced in a 10-fi-diameter test tank, and (2) the impulses were detected equaily well by external and internai sensors. The presence of impulsive leak signais in the test-tank data is consistent with
45、 the laboratory results cited above. The backfill material was well drained, thus allowing for the entrainment of air bubbles into the leak flow field. An extensive series of tests were conducted on a 114-fi-diameter AST located at the Mobil Oil Refmery in Beaumont, Texas. The primary goals of the B
46、eaumont experiment were to: (1) investigate the detectability of impulsive-vs.-continuous acoustic leak signais, (2) measure the ambient noise field against which the leak signals must be detected, and (3) obtain continuous time series on a variety of sensor mays so that improved detection algorithm
47、s could be tested. In order to gain a degree of control over the presence or absence of the leak signal, and over the source mechanisms that give rise to the leak signal, a pair of leak simulators were constructed for use in the AST. Both impulsive and continuous components of the simulated acoustic
48、 leak signal were found to be detectable in an AST of this dimension. The character of the impulsive leak signal produced by leakage into partially saturated backfs was such that currently used data collection and signal processing techniques would be unlikely to detect the leak in a reliable, convi
49、ncing manner. The ambient noise field was found to be strongest at frequencies below 10 kHz, thus masking a substantial portion of the continuous leak signai received by external sensors. Because the typical AST leak signai wiil most likely be influenced by a variety of source mechanisms, the possibility that both impulsive and continuous signals can be processed by the same detection algorithm was investigated. A leak detection algorithm based upon beam-forming techniques was applied to the impulsive and continuous leak signais collected during the Beaumont test. Good agreement bet
