【Y2025】Calculation of alarm setpoints for toxic gases

The first-level alarm setting for toxic gases should be less than or equal to 100% OEL, and the second-level alarm setting should be less than or equal to 200% OEL. For toxic gases with multiple OELs, the MAC, PC-TWA, and PC-STEL values should be used in that order of priority; for toxic gases for which no OEL is specified, 10% of the IDLH value can be used. When the measurement range of existing detectors is insufficient to meet the measurement requirements, the first-level alarm setting for toxic gases shall not exceed 5% IDLH, and the second-level alarm setting for toxic gases shall not exceed 10% IDLH.

Toxic gas detectors only measure the concentration of toxic and harmful gases in the air. If the concentration of toxic gases exceeds the normal level in the air and poses a threat to human safety, a toxic gas alarm will trigger automatically. Warn people to take action in a timely manner to prevent accidents. A toxic gas alarm generally consists of four parts: the circuit section, the sensor section, and the operation section.

This post was last edited by zhoudingshengs on 2025-6-11 08:43. The main functions of toxic gas detectors are as follows: A. Leak detection: Detection and alarm of leaks of harmful gases or liquids (vapors) in the equipment’s pipelines, as well as detection of leaks that occur during the operation of those pipelines. B. Detection before entry: When workers enter the isolation room for handling hazardous substances, or go into sewers, cable trenches, or equipment in hazardous areas to carry out operations, it is necessary to detect harmful gases or liquid vapors. C. Maintenance and testing: After equipment repair or replacement, it is particularly important to detect any residual harmful gases or liquids (vapors), especially before a fire occurs. D. Emergency testing: When abnormal situations occur at the production site or accidents need to be dealt with, harmful gases or liquids (vapors) should be tested to ensure safety and hygiene. E. Inspection: During safety and hygiene checks, harmful gas or liquid vapors should be detected.

In the workplace, it is common to encounter situations where the settings of toxic gas detection alarms do not meet the requirements, and there are mainly two such scenarios. 1. The alarm threshold values are set unreasonably; it is common for employers to set the alarm levels in ppm, and after conversion, these values often deviate significantly from the established exposure limits. 2. In locations where many flammable and toxic substances are present simultaneously, and where substances that are both flammable and toxic exist, flammable gas detection alarms are often the only devices installed.

By comparing the \"Code for Installation of Toxic Gas Detection and Alarm Devices in Workplaces\" (GBZ/T223-2009), which is familiar to occupational health professionals, with the \"Design Standard for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industries\" (GB/T50493-2019), which is known to safety professionals, it can be seen that both standards give priority to toxic gas detection; in other words, safety requirements are not given precedence over those related to occupational health. Relatively speaking, it is just that the provisions of GB/T 50493 are more comprehensive and detailed, while the requirements of GBZ/T 223 are stricter.

The alarm threshold levels can be set at three levels: pre-warning, alarm, and high alarm; the alarm signals for these different levels should be distinctly distinguishable from each other. Employers should, based on the toxicity of the toxic gases and the conditions at the site, set at least two levels: an alarm value and a high-level warning value, or two levels: a pre-warning value and an alarm value.

The predicted value is 1/2 of the MAC specified in GBZ2.1 or 1/2 of PC-STEL; for substances without a PC-STEL value, it is 1/2 of the excess factor value. The forecast indicates a possible release of toxic gases at that location; it is necessary to inspect the relevant equipment and implement effective preventive control measures.

The alarm value is the MAC or PC-STEL value specified in GBZ2.1; for substances without a PC-STEL value, it is the excess factor value. The alarm indicates that the concentration of toxic gases in the workplace has reached or exceeded the **occupational health standards; it is necessary to immediately locate the source of release and take appropriate measures such as preventing further release, improving ventilation, and providing protection for workers.