Fixed gas detection systems discrepancies on liquefied gas carriers

USCG advice about common discrepancies with fixed gas detection systems
USCG advice about common discrepancies with fixed gas detection systems

The US Coast Guard has issued advice about common discrepancies associated with fixed gas detection systems onboard liquefied gas carriers specifically, which are a critical safety measure on liquefied gas carriers.

The International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) requires the activation of alarms at specified vapor concentrations.

During inspection on three separate LNG carriers in Boston, Port State Control Officers (PSCOs) discovered issues with the fixed flammable gas detection system that resulted in the issuance of deficiencies and delay of cargo operations. See USCG Marine Safety Alert 02-20.

While witnessing tests, multiple sensors measured outside the tolerances published by the manufacturer and subsequent calibration checks failed.

In each case, crew members were not following established procedures as specified in their vessel’s SMS. Crews responsible for maintaining and testing the fixed gas detection system should understand testing procedures and allowable tolerances says the USCG.

Although the examined gas carriers’ SMSs and manufacturers’ instructions specified procedures for testing the fixed gas detection systems and stated the acceptable parameters for the sensors, many of the sensor tests observed by the PSCOs had readings outside of the established tolerances.

Sensors operating outside of established tolerances pose a significant safety threat and could be grounds for vessel control actions, such as delayed departure from port, delayed cargo operations, or detention.

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.

There are three main types of sensors: catalytic, infrared, and electrochemical.

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.

Recommendations to liquefied gas carrier owners
USCG strongly recommends that liquefied gas carrier owners and operators ensure the following:
– Testing and calibration of fixed gas detection systems are completed per the vessel’s SMS and manufacturer’s instructions.
– Sensors are operating within established parameters as required by the vessel’s procedures and manufacturer’s instructions.
– The appropriate span gas for the type of sensor is correctly applied to the sensor, as established by the manufacturer and vessel’s SMS.
– The crewmember responsible for maintaining the gas detection system has adequate training, is fully knowledgeable on the system, and is proficient in conducting system tests.
– Meanwhile, investigating officers, inspection personnel, servicing technicians, and shipboard personnel are encouraged to maintain an acute awareness of these issues and initiate corrective actions as needed.

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