[2025: Understand at a Glance] A quick guide to the proper installation of combustible gas and toxic gas detectors
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The attached illustration shows the proper installation of combustible gas and toxic gas detectors in one view.
The following responses are not included in the attachment and are provided for reference only: The placement of detection points should take into account the dispersion characteristics of the leaked gas (vapor) at the scene after a leak occurs. To ensure timely detection and alarm in the event of a leak, the arrangement of detection points must consider, in different leakage scenarios, the distribution of potential leak sources in various locations where leaks might occur. The detectors should be positioned in such a way that they can detect the presence of flammable and toxic gases emitted from the leak site before their concentrations reach levels that could pose a threat to the safety of production facilities and personnel, thereby triggering an alarm.
To determine whether the leaking gas is heavier than air, the ratio of the molecular weight of the leaking gas to that of ambient air should be used as a basis, and the following criteria should apply: 1 If the ratio is 1.2 or greater, then the leaking gas is heavier than air; 2 If the ratio is 1.0 or greater but less than 1.2, then the leaking gas is slightly heavier than air; 3 If the ratio is between 0.8 and 1.0, then the leaking gas is slightly lighter than air; 4 If the ratio is 0.8 or less, then the leaking gas is lighter than air.
When the source of release is located in an area with an open or exposed structure, the horizontal distance between the combustible gas detector and any source of release within its coverage area should not exceed 10 m. The horizontal distance between a toxic gas detector and any source of emission within its coverage area should not exceed 4 m.
If the source of release is located in a closed factory building or a semi-open factory building with poor local ventilation, the horizontal distance between a combustible gas detector and any source of release within its coverage area should not exceed 5 m; the horizontal distance between a toxic gas detector and any source of release within its coverage area should not exceed 2 m.
Sources of flammable or toxic gases that are lighter than air are located in enclosed or semi-open facilities with poor local ventilation. In addition to installing detectors above the source of release, combustible gas or toxic gas detectors should also be placed at the highest points within the plant where gases are likely to accumulate.
When detecting flammable gases or toxic gases that are heavier than air, the installation height of the detector should be between 0.3 m and 0.6 m above the floor (or floor slab). When detecting flammable gases or toxic gases that are lighter than air, the detector should be installed within 2.0 m above the source of emission. When detecting flammable or toxic gases that are slightly heavier than air, the installation height of the detector should be between 0.5 m and 1.0 m below the source of emission; when detecting flammable or toxic gases that are slightly lighter than air, the installation height of the detector should be between 0.5 m and 1.0 m above the source of emission.
Flammable gases: Class 1 ≤ 25% LEL, Class 2 ≤ 50% LEL. Toxic gases: Level 1 ≤ 100% OEL, Level 2 ≤ 200% OEL. Oxygen: Peroxide setting at 23.5 vol%, hypoxia at 19.5 vol%.
The detection and alarm signals for flammable gases and toxic gases should be sent to manned on-site control rooms, central control rooms, etc., for display and alarm； The secondary alarm signals for combustible gases, as well as the fault signals from the alarm control units of the combustible gas and toxic gas detection systems, should be sent to the fire control room. The control room operation area should be equipped with audible and visual alarms for combustible gases and toxic gases ； On-site area alarms should be installed based on the area occupied by the equipment, the layout of the devices and structures, the physicochemical properties of the emission sources, as well as the characteristics of air flow at the site. Such alarms must have both acoustic and visual warning functions.
(1) For flammable gases of light hydrocarbons, catalytic combustion or infrared gas detectors are recommended; for vapor of heavy hydrocarbons, photoionization detectors are suitable； (2) For hydrogen detection, catalytic combustion type, electrochemical type, and heat conduction type detectors are recommended ； (3) For organic toxic gases, semiconductor-type or photoionization-type detectors should be used ； (4) For the detection of inorganic toxic gases, electrochemical detectors are recommended ；