How to Select a Gas Detector?

Fire & Gas System
1

For Selecting a Gas Detector for specific location or industry depends on many factors.

The Engineer has to consider:

  • The gas to be measured, is it noxious, explosive or both?

  • Have the potential sources of leakage been determined in conjunction with the Process Engineer and as part of the Hazard and Operability (HAZOP) or Control Systems Hazards and Operability (CHAZOP) studies? It is necessary to ensure that there are no areas which are unmonitored especially small pockets at both high and low levels.

  • The choice of Positioning Strategy. Are point detectors only required or is a hybrid approach (a combination of both types of point and Line of Sight Infrared needed to achieve the necessary coverage?

  • Do you require point (spot), area or fence monitoring?

  • The number of sensors, ensure that failure, or maintenance removal of an individual sensor does not compromise the safety of the area being monitored. Duplication (or triplication) of sensors and control apparatus may be required for continuous monitoring and to prevent false alarms.1 From the UKHSE

  • What Measurement Technique is required? For Toxic Gases in lower concentrations Electrochemical types are used. Catalytic gas detectors are used for point detection or can be positioned in a grid to ensure wider coverage. Infrared (IR) gas detectors are used for both point and line of sight applications. They are low maintenance devices.

  • What are the various advantages and disadvantages associated with the different types of gas detectors?

  • Is an aspirated system required? Aspiration systems are used where it is not possible to install gas detectors directly or natural diffusion as a sampling method is assessed to be slow. In many cases a faster response is needed, and the sample is transported to the sensor using a sampling pump. This is called aspirated or extractive sampling.

  • What voting strategy is required for the F&G system, 1001, 1002, 2003 etc. These voting strategies are used to ensure that false alarms do not cause false alerts or shutdown.

  • What Alert and Shutdown Levels are required? This is generally determined by the operator in line with the associated safety case. A common setting is Low alarm = 20% LEL, High alarm = 40% LEL, High-High alarm = 60% LEL. Although some operators are more conservative and set LEL Low = 10% and LEL High =20%.

  • There are also Infrared Cloud Cameras available, this again enhances the overall safety of the plant.

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