【2026 Instrumentation Technology】Can on-site alarms for flammable gases and toxic gases be omitted?

Unlike the integrated audio-visual alarms built into the detectors and those located in the control room, the area alarm serves the purpose of ensuring that personnel anywhere within the area affected by the alarm can hear and see the warning signals clearly when the gas leakage reaches the second-level emergency alert threshold, thereby allowing them to stop working immediately and evacuate the dangerous area.

This post was last edited by The one on 2026-5-15 11:12. Since the detector has its own alarm, is it then unnecessary to set up area alarms? The answers can actually be found in the explanatory notes for the standard \"Flammable and Toxic Gases in Petrochemical Industries\" (GB 50493-2019). It is only permissible to omit a zone alarm if all three conditions are met simultaneously: the number of detectors in the alarm zone is less than 10, the ambient noise level is below 85 dBA, and the detectors in that area are equipped with integrated audio-visual alarms. None of these conditions may be missing.

Considering the actual conditions in most chemical plant areas, it is often difficult to meet all three of these requirements at the same time; therefore, it is standard practice to install area alarms, with their absence being the exception.

One thing that requires special attention is that when the noise level at the site exceeds 85 dBA, area alarms must be installed.

The noise level in chemical plant areas is usually high; in places such as compressor rooms and pump houses, it is common for the noise level to exceed 85 dBA. Many companies tend to overlook this, thinking that the built-in alarm of the detector is sufficient; however, in high-noise environments, the low-power alarm sound cannot travel far and is not audible clearly.

Additionally, in relatively enclosed areas where personnel enter and exit for inspections and where there is a risk of the accumulation of flammable or toxic gases, such as compressor rooms, pump houses, and analysis booths, area alarms should be installed in prominent locations at the entrances and exits to alert inspectors to be cautious upon entering.

The following response is for reference: \"Flammable and Toxic Gases in Petrochemical Industries\" (GB 50493-2019) 3.0.4 An audible and visual alarm system for flammable and toxic gases should be installed in the control room operation area； On-site area alarms should be installed based on the area occupied by the equipment, the layout of the devices and structures, the physical and chemical properties of the emission sources, as well as the characteristics of air flow at the site. Such alarms must have both audible and visual warning functions. Explanation of the provision: 3.0.4 Installing audible and visual alarms for combustible gases and toxic gases in the control room operation area helps the operators there to detect such hazards promptly and take appropriate actions. To alert on-site staff, on-site area alarms are typically installed at the main entrances and exits of the production site as well as in areas with high noise levels【where the noise exceeds 85 dB(A)】. In relatively enclosed areas such as compressor rooms, pump rooms, silos, analysis booths, and laboratory facilities where inspections are carried out and where the accumulation of flammable or toxic gases may occur, audio-visual alarms should be installed in prominent locations at the entrances and exits. Its purpose is to remind inspection personnel to be cautious when entering these areas.

In terms of sound level, the alarm signal level of area alarms should be above 110 dBA, while the total sound pressure at a distance of 1 meter from the alarm should not exceed 120 dBA; this is to prevent hearing damage due to being too close to the alarm during wiring or debugging. There is also a practical requirement: the alarm sound level of the on-site alarm should be 15 dBA higher than the background noise level. In other words, when making a selection, it is necessary to determine the noise level at the site to ensure that the alarm sound can be clearly heard even in a noisy environment.

Regarding the optical alarm, the optical component of the audible and visual alarm should employ a pulse-based alerting method, with the pulse flashing frequency ranging from 60 times per minute to 120 times per minute. The effective luminous intensity of light alarms designed for outdoor use is generally not less than 300 cd; this brightness is roughly equivalent to that produced by 300 ordinary candles burning together, and it is sufficient to penetrate the rain, fog, and darkness in chemical plant areas ； The effective luminous intensity of the optical alarms used in the factory buildings is generally not less than 150 cd. Different types of alarms should also be distinguished by color; typically, fire alarms use red, gas alarms use blue, and accident alarms use yellow.

Management recommendations for area alarms: First, establish a standardized ledger for managing alarm zones. Based on the actual conditions of the facility, alarm zones are established according to the plants or units in the production and storage & transportation facilities. Information such as the list of detectors in each zone, the setup of area alarms, and the hierarchy of activation signals is specified, ensuring clear zoning and complete records.