The United States Coast Guard issued a Marine Safety Alert after they discovered three (3) separate Liquefied Natural Gas (LNG) carriers with issues with their fixed flammable gas detection systems that resulted in the issuance of deficiencies and delay of cargo operations. While witnessing tests, multiple sensors measured outside the tolerances established by the manufacturer and subsequent calibration checks failed. In each case, crew members were NOT following established procedures as specified in their Safety Management System (SMS). Another common discrepancy observed was the use of improper span gas on catalytic sensors. To prove proper operation of the fixed gas detection system, the span gas used to test the sensors or complete calibration checks must be appropriate for the type of sensor.
Catalytic sensors rely on the presence of oxygen to function correctly. Absent the means to introduce oxygen into the sample, if the composition of the span gas used on a catalytic sensor does not include oxygen and is balanced with an inert gas, the sensor will not operate correctly. Tests or calibrations conducted with the incorrect span gas can cause the sensor to operate outside established tolerances.
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