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API REPORT 17 Document Information:
Title
Review of Gas Analysis Techniques for Application to Combustible Gas Detectors
American Petroleum Institute
Publication Date:
Mar 31, 1980
Scope:
The capability of detecting combustible gases is directly related to
the safety of offshore
drilling and production operations of the oil and gas industry. Of
particular concern is the
presence of methane in combustible concentrations. The hazard posed by
a given concentration of
methane is measured in terms of the lower explosive limit (LEL), which
is the lowest concentration
of a gas that will, when mixed with air, propagate a flame. The LEL is
not an absolute value;
changes in temperature, oxygen content of the air, and purity of the
gas mixture will affect the
LEL. For methane, the LEL is considered to be 5.1 percent by volume. A
detector for offshore
platforms should produce a warning signal at 25 percent LEL and should
produce a "shut-down" at
60-75 percent LEL.
Approximately 10 years ago, the United States Geological Survey (USGS)
required that combustible
gas detectors be installed in all enclosed areas containing gas-fueled
and gas-handling equipment.
As a result of the short notice given by USGS, little preliminary work
was performed to collect
data on the operational environment for detectors installed on
offshore platforms. The work
performed for this project has isolated several environmental
variables that must be considered in
selecting the detector best-suited for that site and in installing the
sensor.
Another factor that bears on the performance of a detector has become
apparent during the project
work. The human-detector interaction at an installation - including
maintenance of, and response
to, the detector - directly affects the utility of the detector
regardless of its inherent value.
The project work also has revealed an obstacle to selection of the
best detector for a particular site. While there is considerable activity in the manufacture and use of combustible gas detectors,
there is little freely available scientific information about
commercial methods of detecting methane concentrations. In particular, manufacturers are reluctant to discuss details of their
products. Much of the information that is available has resulted from efforts of the Bureau of Mines. While this information is valuable, it has not been related directly to the environment of
offshore platforms.
The project work involved acquisition of information, including site
visits, literature search, and
survey of manufacturers. Particular attention was given to specific
problems encountered in using
methane detectors on offshore production platforms. Discounting human
interface problems common to
all instrumentation and safety systems, these problems include:
(1) Short life time of the detector head
(2) In accessibility of heads for calibration
(3) Signal returning to "safe" indication when LEL is appreciably
exceeded
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