1、API PUBLU323 94 0732290 O543762 551 An Engineering Evaluation of Volumetric Methods of Leak Detection in Aboveground Storage Tanks HEALTH AND ENVIRONMENTAL AFFAIRS API PUBLICATION NUMBER 323 JANUARY 1994 American Petroleum Institute 1220 L Street, Northwest rT Washington, D.C. 20005 A - API PUBLX323
2、 94 0732270 0543763 498 An Engineering Evaluation of Volumetric Methods of Leak Detection in Aboveground Storage Tanks Health and Environmental Affairs Department API PUBLICATION NUMBER 323 PREPARED UNDER CONTRACT BY: JAMES W. STARR, AND JOSEPH W. MARESCA, JR. VISTA RESEARCH, INC. MOUNTAIN VIEW, CAL
3、IFORNIA AUGUST 1993 American Petroleum Institute FOREWORD NI PUBLICATIONS NECESSARILY ADDRESS PROBLEMS OF A GENERAL NATURE. WITH RESPECT TO PARTICULAR CIRCUMSTANCES, LOCAL, STATE. AND FEDERAL LAWS AND REGULATIONS SHOULD BE REVIEWED. API IS NOT UNDERTmG n MEET THE DUTIES OF EMPLOYERS, MANUFAC- TURERS
4、, 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 CONSTRUED AS GRANTING ANY RIGHT, BY IM
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6、 instituie i API PUBL*323 94 O732290 0543765 260 ACKNOWLEDGMENTS We wish to express our gratitude to the members of the API Storage Tank Task Force and the Work Group for AST Monitoring for their cooperation, their technical support, and their assistance in coordinatuig this project. We would like t
7、o acknowledge the support and encouragement of the chairperson of the Work Group. Mr. James Seebold, and the API staf member monitoring the program, Ms. Dee Gavora. We especiaily acknowledge the help of Mr. John Collins, of Mobil Oil, who provided technical input to the research and was instrumental
8、 in coordinating the field tests at the Mobil Refinery in Beaumont, Rxas. Finally, we acknowledge the help of Monique Seiel and Christine Lawson of Vista Research in editing and typesetting this document iii API PUBL*323 94 0732290 05437bb LT7 ABSTRACT There are two approaches to detecting leaks in
9、an aboveground storage tank (AST) by means of the volumetric method. The first is the conventional approach in which measurements of the level and temperature of the product are made with a precision level sensor and a vertical array of temperature sensors. The second is a mass measurement approach
10、which employs a differen- tial pressure sensor to measure the level changes. In a tank with vertical walls, a differential pressure sensor inherently compensates for the level changes produced by thermal expansion and contraction of the product between the pressure port and the product surface. As p
11、art of Phase III of the American Petroleum Institutes (APIs) project to develop and evalu- ate the performance of different technologies for detecting leaks in the floor of ASTs, a con- trolled experiment was conducted in a 117-ft-diameter tank during late May and early June 1992. The purpose of thi
12、s experiment was to evaluate the performance of both approaches to volumetric testing. The tank contained a light fuel oil, and data were collected over a continuous 28-day period. The analytical and experimental results of this project suggest that a volumetric system can be used to detect small le
13、aks in ASTs. Analysis of the level temperature approach indicates that the largest source of volume fluctuations was thermal expansion of the product. It was found that effective compensation for this expansion could be achieved, and leak rates as small as 1.9 gavh could be reliably detected in a si
14、ngle 24-h test. Furthermore, extending the test period to 48 h would significantly improve leak detection performance, resulting in a detectable rate of about 1.0 gam. While in theory differential pressure systems should achieve a higher level of performance than the level temperature systems, this
15、was not the case. The setup of the differential pressure mea- surement system is extremely sensitive to air temperature changes, and to a lesser extent, the location of the bottom pressure reading. Regardless of the approach used, volumetric leak detection tests achieve their highest perform- ance w
16、hen the level of the product in the tank is low (approximately 3 ft), and the test duration is at least 24 h (48 h if possible), the test is begun and ended at night, and accurate temperature compensation is applied. When the test duration is significantly less than 24 h, it is not possible to accur
17、ately compensate for the effects of diurnal temperature changes. API PUBL+323 94 W 0732290 0543767 033 TABLE OF CONTENTS Executive Summary . ES- 1 Section 1 : Introduction 1 . 1 Section 2: Background 2-1 Section 3: Summary of Results . 3-1 Section 4: Conclusions and Recommendations 4-1 Section 5: Im
18、portant Features of a Volumetric Method with High Performance . 5-1 Section 6: Report Organization . 6-1 References R- 1 Appendix A: Leak Testing Aboveground Storage Tanks with Level and Tempera- ture Measurement Methods: Field Test Results . A- 1 Appendix B: Leak Testing Aboveground Storage Tanks w
19、ith Mass-Measurement Methods: Field Test Results . B- 1 API PUBL8323 94 W 0732290 0543768 T7T EXECUTIVE SUMMARY INTRODUCTION There are two approaches to detecting leaks from an aboveground storage tank (AST) by means of the volumetric method. The first is the conventional approach in which measureme
20、nts of the level and temperature of the product are made with a precision level sensor and a vertical array of closely spaced, precision temperature sensors. The second is a mass-measurement approach, which employs a differential-pressure sensor to measure the level changes. In a tank with verti- ca
21、l walls, a differential-pressure sensor inherently compensates for the level changes produced by the thermal expansion and contraction of the product betweeen the pressure port, which is located near the bottom of the tank, and the product surface. Because of the possibility of large horizontal grad
22、ients in the rate of change of temperature of the product in an AST (gradients which cannot be accurately measured with a single vertical array) the mass-measurement approach should, in theory, have a performance advantage over the conventional approach. As part of Phase III of the American Petroleu
23、m Institutes (APIs) project to develop and evalu- ate the performance, in actual operational environments, of different technologies for detecting leaks in the floor of ASTS, a controlled experiment was conducted in a 117-ft-diameter tank at Mobils refinery in Beaumont, Texas, during late May and ea
24、rly June 1992. The purpose of this experiment was to evaluate the performance of both approaches to volumetric testing. The tank contained a light fuel oil, and data were collected over a continuous 28-day period. Two vertical arrays of thermistors were placed at two locations inside the tank to det
25、ermine the magnitude of the horizontal gradients in the rate of change of product temperature. Temperature measure- ments of the tanks exterior shell were also made. BACKGROUND The API has completed three phases of a leak detection project for ASTs. The purpose of Phase I was to assess different lea
26、k detection technologies in order to determine which had the greatest potential for field application. Phase II addressed in detail two of the methods studied in Phase I: passive-acoustic and volumetric methods. The results of the volumetric experiments indicated that, in order for a test to achieve
27、 sufficient compensation for the temperature-induced changes in the product and in the wall needed for high performance, the product should be at lower levels and test duration should be approximately 24,48 or 72 hours. 1 Phase III also included an engineering evaluation of passive-acoustic methods
28、of leak detection for ASTs. The results of the acoustic study are provided in a separate API document entitled An Engineering Evaluation ofAcous- tic Methods of Leak Detection for Aboveground Storage Tanks, by Eric G. Eckert and Joseph W. Maresca, Jr. ES-1 API PUBLr323 99 = 0732290 0543769 906 The o
29、bjectives of Phase HI, which addressed both volumetric and passive-acoustic leak detection technologies, were: to determine, in the case of acoustic methods, the nature of the acoustic leak signal resulting from realistic leaks in the floor of an operational AST; to determine, in the case of volumet
30、ric systems, if differential pressure (mass-measurement) systems have significant advantages over the con- ventional level and temperature measurement systems; to characterize the ambient noise encountered under a wide range of test conditions for both detection technologies; to evaluate data collec
31、tion and signal processing techniques that would allow the detection of the leak signal against the ambient noise; to identify any operational issues for implementation of methods based on either technology; to demonstrate the capabilities and, if possible, make an estimate of the performance, of bo
32、th technologies through field tests; and to identify, in the case of both volumetric and passive-acoustic technolo- gies, those features of a leak detection test that are necessary for achiev- ing high performance. CONCLUSIONS The analytical and experimental results of this project suggest that a vo
33、lumetric system can be used to detect small leaks in ASTS. Analysis of the float-based system indicated that the largest source of volume fluctuations was thermal expansion of the product. During this project it was found that effective compensation for this expansion, as well as compensation for th
34、e thermal expansion of the tank walls, could be achieved. Analysis of the test results suggested that leak rates as small as 1.9 gavh could be detected in a single 24-h test at a probability of detection (PD) of 95% and a probability of false alarm (PFA) of 5%. Furthermore, test results suggest that
35、 exten- sion of the test period to 48 h would significantly improve leak detection performance, resulting in a detectable rate of about 1.0 gaVh. This high level of performance was achieved in tests begun and ended at night because the horizontal gradients in the rate of change of product tem- perat
36、ure were negligible during the night. Both estimates could have been improved with more extensive measurement of the vertical temperature profile of the product, particularly in the upper layers of the product where the greatest rates of temperature change persistently occurred. Some degradation of
37、the performance estimates probably occurred as a result of non-uniform inflow of product from neighboring tanks through leaking isolation valves. This inflow condi- tion was present during the entire 28-day data collection period. ES-2 API PUBL*323 94 O732290 0543770 628 H While in theory differenti
38、al pressure systems should achieve a higher level of performance than temperature and level systems, this was not the case in the field tests conducted as part of this project. We found that the setup of the differential pressure measurement system is extremely sensitive to air temperature changes a
39、nd, to a lesser extent, the location of the bottom pressure reading. In principle, these setup problems can be eliminated by careful design; in practice, how- ever, as shown by these tests, they are sometimes difficult to avoid. Regardless of the approach used, volumemc leak detection tests achieve
40、their highest performance when the level of product in the tank is low (approximately 3 ft), the test duration is at least 24 h (48 if possible), the test is begun and ended at night, and accurate temperature compensation is made for the thermal expan- sion and contraction of the instrumentation, th
41、e tank shell and the product. When the test dura- tion is significantly less than 24 h, it is not possible to accurately compensate for the effects of diurnal temperature changes. This document presents the results of these volumetric experiments in two technical papers, which are attached as append
42、ices. The first provides a description of the capabilities of a level- and-temperature leak detection system for use in ASTs. This paper quantifies the sources of ambient noise, describes those features of a leak detection system that are crucial for high performance, and estimates the performance o
43、f the volumetric method of testing. The second describes the capabilities of a differential-pressure leak detection system for use in ASTs. This paper focuses on the temperature compensation requirements necessary to achieve high perform- ance with this type of measurement system. ES-3 API PUBL*323
44、94 W 0732290 0543773 564 W 1 INTRODUCTION This report is one of two that summarize Phase III of a research program conducted by the American Petroleum Institute (API) to evaluate the performance of different technologies that can be used to detect leaks in the floors of aboveground storage tanks (AS
45、Ts). During Phase I, an analytical assessment of the performance of four leak detection technologies was investigated (Vista Research, Inc., 1989; Maresca and Starr, 1990). The four technologies included: (1) passive-acoustic sensing systems, (2) volumetric systems, especially differential-pressure
46、(or “mass“) measurement systems, (3) enhanced inventory reconciliation methods, and (4) tracer methods. During Phase II, field tests were conducted on a 114-ft-diameter AST containing a heavy naphtha for the purpose of making an engineering assessment of the performance of two of these technologies,
47、 passive-acoustic sensing systems and volumetric detection systems. The results of the Phase II research program are described in two API final reports and three profes- sional papers (Vista Research, Inc., 199 1, 1992; Eckert and Maresca, 199 1, 1992). During Phase III, additional field tests were
48、conducted on a pair of ASTs in order to test acoustic and volumetric leak detection strategies that emerged from the Phase II study, and to further evaluate the current state of leak detection technology. To evaluate the performance of the volumetric method, volumetric tests were conducted in a 117-
49、ft-diameter tank containing a light fuel oil. A nearly continuous time series of level and temperature data was collected over a 28-day period. The acoustic tests were conducted in a 40-ft-diameter AST, which contained water and was espe- cially configured to assess the nature of the acoustic signai produced by a hole in the floor of the tank. This report describes the results of the Phase III volumetric tests; the results of the acoustic tests are described in a separate report (Vista Research, Inc., 1993), which consists of brief over- view of the work and two detail
