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
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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
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