Recently, the US Coast Guard informed about common discrepancies associated with fixed gas detection systems onboard liquefied gas carriers, identified during PSC controls. In this regard, the Gard P&I Club discussed two main reasons for these deficiencies, both of which relate to the calibration of fixed gas detection sensors: Using inappropriate span gas on the sensors, and not following proper calibration procedures.
-Using appropriate span gas
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 sensors.
The PSC deficiencies by the USCG for the fixed gas detection systems, related to the improper calibration of catalytic sensors.
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. In addition, they must 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 causing 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.
-Following proper testing procedures
Ship’s crew should fully understand the manufacturer’s testing procedures and 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 their 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”.
As such, regular training is essential to ensure that crew members are familiar with these instructions.
Meanwhile, calibration and maintenance instructions should be in the vessel’s planned maintenance system (PMS) and to be 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.
Recommendations
To ensure safety of fixed gas detection systems, the Club advised operators and crews to consider the following:
- Crew should know the type and limitations of the sensors used.
- Labels can be put on the equipment itself to note key dates, such as when a sensor is to be changed.
- There should be procedures onboard covering at least the following:
- Risks of improper calibration
- Testing and calibration frequency
- The type of span gas to be used
- Testing, calibration and maintenance procedures
- Training of crew in not only using the equipment but also testing, calibrating and maintaining it
- A proper inventory should be maintained of spare sensors and span gas bottles.
- Audits and inspections by shore management should include an assessment of the gas detection systems covering areas like crew awareness and calibration.
