Gas detector protection

Another means of protecting against the explosion hazard in flammable atmospheres is to employ gas detectors. The principle is that the gas detectors detect flammable gas/air mixtures at a low percentage, typically 10%, of the LEL and cause an alarm to sound. If the concentration reaches 25% or so of the LEL, the system automatically initiates a shutdown of the process. This type of technique is commonly used in process plants such as oil and gas refineries, LEG storage tank farms, chlorine doping rooms in water treatment plants, and on vehicles such as electric fork lift trucks operating in Zone 1 and 2 areas. 

An example of gas detector protection is found in unattended cold stores and cooling plants which use ammonia as the refrigerant. There is practically no risk in attended plants because a few parts per million of ammonia in air is easily detectable by the pungent smell, so the attendant is aware that there is a leak and can take remedial action before the concentration becomes dangerous. The lower explosive limit is comparatively high at 16% this concentration is only likely under abnormal conditions, such as a blown compressor head gasket, and is intolerable for the eyes and respiratory system. 

Gas detectors can also be usefully employed in other unattended Zone 2 plant areas where it is desirable to trigger an alarm in the event of a large gas leak which, if not detected and remedied, might spread beyond the zonal boundaries and reach a source of ignition in an adjoining safe area. 

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