Gard P&I Club reports that the US Coast Guard has found some gas detection equipment to be incorrectly calibrated. Here, Gard explains the importance of correctly calibrating gas detection equipment
During routine port state control (PSC) inspections, the US Coast Guard (USCG) found that fixed gas detection system sensors were outside the tolerances established by the manufacturer and failed subsequent calibration checks. Over the years, there have been many incidents in the shipping industry involving leakage of flammable and toxic gases into pump rooms, void spaces, enginerooms and other compartments.
A mixture of gas in the air beyond certain concentration levels can be life threatening for crew entering these spaces and pose a serious fire risk. The International Gas Carrier Code (IGC Code) states that gas detection equipment shall be installed to monitor the integrity of the cargo containment, cargo handling and ancillary systems, and should be tested in accordance with recognised standards. To ensure that the fixed gas detection system operates effectively, timely and accurately, calibration of the sensors is critical.
The type of span gas, or calibration gas, used depends on the type of sensor. There are three main types of sensors: infrared, electrotechnical and catalytic. The PSC deficiencies issued by the USCG for the fixed gas detection systems related to the improper calibration of catalytic sensors.
“The span gas should reflect the target gas which the crew is trying to measure”
The catalytic sensors are the only sensors which rely on oxygen to function correctly. When used in systems which are sampling from an inert atmosphere, the sample will have to be diluted with fresh air, i.e. minimum 10% oxygen, in accordance with IEC 60079-29-1, to support catalytic oxidation. If the oxygen concentration is below the specified limit, the crew cannot rely on the catalytic bead combustible sensor reading. It is therefore important that crew members understand the type of sensor used in the equipment and its limitations.
Crew members must also ensure that the span gas used for calibration of catalytic sensors is balanced with fresh air and not inert gas. Crew members are recommended to refer to the manufacturer’s instructions for the type of span gas to be used. Tests or calibrations conducted with the incorrect span gas can lead to the sensor operating outside established tolerances and cause it to show inaccurate readings.
When a sensor is calibrated using a specific gas, it would show a different reading for other flammable gasses. For example, if a sensor is calibrated using methane and is then exposed to pentane, then measurement of pentane gas would be lower than the actual content of pentane in the space. The span gas selected should therefore reflect the target gas which the crew is trying to measure in a space.
Ship’s crew should fully understand the manufacturer’s testing procedures and the permissible tolerances for maintaining and testing fixed gas detection systems. If, during testing, a sensor shows a reading outside the tolerances specified, the manufacturer’s instructions on correcting this should be followed. The USCG highlights in its safety alert that “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.”
It is important that regular training is carried out to ensure that crew members are familiar with these instructions. It is also recommended that the calibration and maintenance instructions are included in the vessel’s planned maintenance system (PMS) and are posted near the gas detection display panel as good practice. Finally, it should be highlighted that catalytic and combustible type gas sensors usually have a limited operating life, at the end of which they must be changed. Crew should therefore also be aware of the lifespan of the sensors.