During an examination of a liquefied natural gas (LNG) carrier whose
cargo tanks contained ethylene vapors, CO (Carbon Monoxide) gas alarms
were received which were traced to an eight-inch crack on a cargo vapor
line.
It was noted that the molecular weight of Ethylene (28
g/mol) was identical to the molecular weight of CO, which accounted for
the CO PPM readings. Coast Guard personnel contacted the manufacturer
who confirmed that gases such as methane, propane, ethylene and
mercaptan, could actuate the CO sensor without ever coming into the
range of the LEL limits.
As a result of these events the Coast Guard “strongly
reminds all surveyors, marine inspectors, port state control examiners,
and any other persons utilizing portable gas monitors and detectors
while working onboard on Liquefied Gas Carriers to remain acutely aware
that the ethylene gas vapors can exhibit cross-sensitivity. This issue
is not limited to the monitors that the Coast Guard uses but also those
made by other manufacturers. Everyone using a monitor must be aware
that if the CO alarm goes off it may be an indication of dangerous gases
or chemical vapors and not the presence of CO. When the alarm sounds
users must take corrective action to minimize exposure risks.”
Courtesy: USCG
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