United States Environmental Protection Agency Industrial Environmental Research Laboratory Cincinnati OH 45268 Research and Development EPA-600/S2-81-073 July 1981 Project Summary Evaluation of the Walkthrough Survey Method for Detection of Volatile Organic Compound Leaks Robert C Weber and Kenneth Mims During 1978 and 1979 the Emis- sion Standards and Engineering Divi- sion of EPA's Office of Air Quality Planning and Standards conducted a fugitive volatile organic compound (VOC) emission sampling program in organic chemical manufacturing plants and petroleum refineries. As a part of their sampling program, sev- eral "walkthrough surveys," also called "unit area surveys," were con- ducted. The assistance of EPA's In- dustrial Environmental Research Lab- oratory-Cincinnati was requested in the analysis of the walkthrough survey data. Fourteen walkthrough surveys were analyzed, from four plants. The analy- sis reported here focuses on the vari- ability and reproducibility of the survey method. One indicator of variability which was studied was the coefficient of variation (CV). The CV's ranged from 55% to 408%, for each pair of walkthrough surveys. Further, the linear correlation coefficients for each set of surveys ranged from 0.046 to 0.98. No attempt was made to evaluate the sources of the variability. This Project Summary was developed by EPA's Industrial Environmental Research Laboratory. Cincinnati. OH, to announce key findings of the re- search project that is fully docu- mented in a separate report of the same title (see Project Report ordering information at back). Introduction During 1978 and 1979, the Emission Standards and Engineering Division of EPA's Office of Air Quality Planning and Standards conducted a fugitive volatile organic compound (VOC) emission sampling program in organic chemical manufacturing plants and petroleum refineries. The data were used in the development of background information documents for regulations to control VOC emissions resulting from leaks in process equipment. As a part of this sampling program, several "walk- through surveys," also called "unitarea surveys," were conducted. A unit area survey involves measuring the ambient VOC concentration within approximately 1 meter of all ground level equipment within a processing area. These measure- ments are performed with a portable VOC detection instrument utilizing a strip chart recorder. An elevated reading on the strip chart is assumed to be indi- cative of a leak. Each individual piece of equipment located in the area where the elevated VOC concentration was found is then checked to determine the sources of the VOC emissions. The purpose of the walkthrough sur- veys conducted by EPA was to deter- mine if this approach represents a ------- viable technique for the detection of leaks, within a regulatory context. The assistance of EPA's Industrial Environ- mental Research Laboratory in Cincin- nati was requested in the analysis of the walkthrough survey data. The purpose of this report is to document the tech- niques of data analysis and to present the results. Fourteen walkthrough sur- veys were analyzed, from 'four plants. The analysis reported here focuses on the variability and reproducibility of re- peated surveys, and does not attempt to correlate ambient VOC concentrations with specifically located VOC sources. The walkthrough surveys were con- ducted at four plants: two chloromethane units, one ethylene unit, and a benzene- toluene-xylene (BTX) unit. The instru- ment used in all cases was the Century Systems Corporation Organic Vapor Analyzer (OVA), Model 108, which was equipped with a strip chart recorder. The instrument measures organic vapor concentrations in ppmv. For each unit or section of a unit, a walkthrough path was developed. This path was intended to pass within a meter of major pieces of equipment at ground level, e.g., pump rows, compressors. Two surveys were conducted sequentially, usually within a few minutes of each other. Conclusions As stated before, the data analysis focused on the variability and repro- ducibility of the measurement technique. Therefore, both the coefficient of vari- ation (CV) and the linear correlation coefficients (R) were evaluated, for each pair of walkthrough surveys (i.e., the sequential surveys for each path). First, however, a technique was developed to facilitate direct comparison of the two chart recordings. The first step was to trace a contin- uous curve through the discontinuous marks made on the strip chart by the OVA recorder. The "smooth" curve on the strip chart was then divided into cells which contained the location designations A, B, C, etc. This was done by finding the midpoints of the interval between two locations, e.g., B to C, and C to D. These two midpoints, then, form the cell boundary for the cell which contains "C." These are shown in the charts as long, darker horizontal lines. Once the cells were identified, the maximum value for each cell was read directly from the chart. Although the actual units are unimportant for the analysis reported here, the instrument read-out is in ppmv. It should be noted that the charts have a logarithmic scale. The charts for a pair of walkthroughs have been designated as Survey A and Survey B according to the order in which they were executed. The mean differ- ence between Survey A and Survey B was studied as one indicator of vari- ability. The absolute value of the differ- ence was used since only the magni- tude of the difference is important, and not the fact that values from Survey A are higher (or lower) than Survey B. The following statistics were calcu- lated for each pair of walkthrough surveys: x= mean difference = £|(A-B)| s = standard deviation of the mean difference CV = coefficient of variation = j_x 100 x R = linear correlation coefficient (be- tween the Survey A and Survey B values) The coefficient of variation provides one way to evaluate the variability of data sets which have widely varying means. Because the means observed were widely different, a simple analysis of the standard deviations for each walkthrough survey may not yield meaningful results. To reduce the effect of the differing means, the coefficient of variation was selected. The CV's ranged from 85% to 408%. Further there does not appear to be ai trend relative to plant or type of pr duction facility. The sample linear correlation coe ficients ranged from 0.046 to 0.98. value near 1 indicates a strong line; relationship in which the value fro Survey B increases when the value fro Survey A increases. A value of R close • zero results from data that display strictly random effect, which implie little or no relationship. However, sine R isa measureof the linearrelationshi a value of R near zero really implies lack of linearity and not necessarily lack of association. Recommendations Based on the results reported here, appears that the walkthrough surve technique is highly variable and is ni reproducible in many cases, even whe repeated within minutes. In sever. instances, there is essentially no co relation between the pair of surveys, c indicated both by the linear correlatio coefficients (R) and by plots of the dat; Thus, there is no indication that th walkthrough survey method can b used as the basis of a leak detectio program for regulatory purposes. To b used for such purposes a maximut local ambient VOC concentration whic triggers remedial action would have t be established. This analysis shows the on repeated surveys a local concentre tion is usually not repeated on tw passes by the same location, nor are th concentrations linearly proportional. The EPA authors are Robert C. Weber and Kenneth Mims with the Industrial Environmental Research Laboratory, Cincinnati, OH 45268 Robert C. Weber is the EPA Project Officer (see below). The complete report, entitled "Evaluation of the Walkthrough Survey Method for Detection of Volatile Organic Compound Leaks," (Order No. PB 81 -199 382; Cost: $8,00, subject to change/ will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Industrial Environmental Research Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 t, US GOVERNMENT PRINTING OFFICE 1981 -757-012/722 ------- United States Center for Environmental Research Environmental Protection Information Agency Cincinnati OH 45268 Official Business Penalty for Private Use $300 RETURN POSTAGE GUARANTEED Postage and Fees Paid Environmental Protection Agency EPA 335 utm L^ Third-Class Bulk Rate * \ fc ^ L 0 1 ? 0 7 0 6 * LlbrtAKY ^EGin'x V U.S. 6.PA PSf) S DfAKHOPr! ST CHiCAi.n IL 60t>oa ------- |