Fixed Gas Detection

The components of a gas detection system (sensor, control unit and alarm) can be supplied separately or combined into a single unit.

The position of the individual components should be considered carefully. Sensors should be positioned to detect any gas accumulation before it creates a serious hazard. Factors to consider are:

  • the process plant and equipment
  • the type of sensor
  • the properties and dispersion characteristics of the gas
  • the ventilation pattern
  • other safety issues, e.g. location of personnel or equipment protection

The number of sensors should also be considered. Failure, or removal for maintenance, of an individual sensor should 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.

The process plant and equipment should be assessed to identify the most likely sources of flammable gas. Examples are:

Leaks from pipework flanges, valves, damaged pipework, hoses; flammable vapour produced as part of processes such as drying coated products in a solvent evaporating oven. Positioning of sensors should consider the likely concentration gradients throughout the equipment.

Where there is the possibility of gas entering a confined space, the sensors should be positioned close to ingress points. An example is landfill gas entering a building - the sensors should be located close to drains or underground cable conduits.

The path of the gas or its dispersion characteristics will depend on the density of the gas and the ventilation patterns. The density can be used to determine at what height sensors should be positioned relative to the potential source. For example:

  • Methane (natural gas) is less dense than air so it will rise;
  • vaporised liquefied petroleum gas (LPG) is heavier than air so it will fall or remain at low level;
  • landfill gas (a mixture of methane, carbon dioxide)

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