【Y2025】What is the criterion for determining whether a combustible gas is lighter or heavier than air?

“Facilities involving leaks of flammable and toxic gases that fail to install detection and alarm devices in accordance with **standards** represent a serious potential hazard for production safety accidents! “Alarm systems for leaks of flammable and toxic gases are not in operation or are in an abnormal state, with alarms remaining unaddressed for extended periods. ”It is a key aspect of the special campaign to address the issue of equipment operating with defects! This underscores the importance of combustible and toxic gas detection and alarm systems; constant attention is required to maintain their sensitive ‘sense of smell’ – carelessness is not acceptable!

GB/ T 50493-2019 sets clearer requirements for alarm systems. I. The settings for gas detectors are more clearly defined. 1. For single-component gas media that are both flammable gases and toxic gases, a toxic gas detector should be installed. 2. For multi-component mixtures containing both combustible gases and toxic gases, it is possible that both the concentration of the combustible gas and that of the toxic gas could reach the alarm set points in the event of a leak; therefore, separate combustible gas detectors and toxic gas detectors should be installed.

II. Gas detectors are equipped with functions for remote signal transmission as well as on-site audible and visual alarms. 1. The secondary alarm signals for combustible gases, as well as the fault signals from the alarm control units of combustible gas and toxic gas detection systems, should be sent to the fire control room. 2. An audible and visual alarm for combustible gases and toxic gases should be installed in the control room operation area ； The area alarms on-site should have audible and visual alarm functions.

III. Reduced coverage area of gas detectors: 1. When the source of emission is located in an outdoor area or within a device area in an open-type factory building, the horizontal distance between a combustible gas detector and any source of emission within its coverage area should not exceed 10 m, while the horizontal distance between a toxic gas detector and any source of emission within its coverage area should not exceed 4 m. 2. 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 the 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 emission within its coverage area should not exceed 2 m. 3. When the source of flammable or toxic gases that are lighter than air is located in a closed or semi-open facility with poor local ventilation, in addition to installing detectors above the source of emission, detectors for flammable or toxic gases should also be placed at the highest points in the facility where gases tend to accumulate. 4. Detectors shall be installed within the fire dike of liquid storage tanks that produce flammable gases, such as liquefied hydrocarbons, Class A B liquids, and Class A B liquids. The horizontal distance between a combustible gas detector and any source of release within its coverage area should not exceed 10 m, while the horizontal distance between a toxic gas detector and any source of release within its coverage area should not exceed 4 m.

IV. More detailed requirements for the installation height of gas detectors: 1. When detecting flammable gases or toxic gases that are heavier than air (with a molecular weight ratio of 1.2 or higher), the installation height of the detector should be between 0.3 m and 0.6 m above the floor (or floor level). 2. When detecting flammable gases or toxic gases that are lighter than air (with a molecular weight ratio of less than or equal to 0.8), the installation height of the detector should be within 2 meters above the source of emission. 3. When detecting flammable or toxic gases that are slightly heavier than air (with a molecular weight ratio of 1.0 or greater and less than 1.2), the installation height of the detector should be between 0.5 m and 1 m below the source of emission. 4. When detecting flammable gases or toxic gases that are slightly lighter than air (with a molecular weight ratio of 0.8–1.0), the installation height of the detector should be 0.5 m to 1 m above the source of emission. 5. The installation height of the environmental oxygen detector should be between 1.5 m and 2.0 m above the floor or floor slab.

V. The alarm thresholds for toxic gas detectors are more clearly defined: the first-level alarm threshold for toxic gases should be less than or equal to 100% of the OEL, while the second-level alarm threshold should be less than or equal to 200% of the 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.

1. Identify risks and properly plan the installation of gas detectors. Specialists in various areas such as process engineering, equipment, instrumentation, and safety are brought together, along with the design team, to analyze the types of flammable and toxic gases involved in the raw materials. Potential leakage points are identified, with particular attention paid to the locations around sources of release such as the dynamic seals of gas compressors and liquid pumps, liquid and gas sampling ports, liquid (gas) discharge and vent ports, as well as frequently removed flanges and valves that are often operated. This approach helps to avoid situations where detectors are not installed or are installed in the wrong type.

2. Perform accurate calculations to set reasonable alarm thresholds for toxic gas detectors. Alarm signals at different levels should be distinctly different; depending on the toxicity of the toxic gas, at least two levels of alarm should be provided. Taking carbon monoxide as an example, according to the tables, carbon monoxide does not have a MAC value; therefore, PC-TWA of 20 mg/m³ is used as the reference value. At standard atmospheric pressure and a temperature of 20°C, the value in Cppm is calculated using the formula: Cppm = (22.4 × 298 / 273 × C in mg/m³) / molecular weight. Thus, the corresponding values are 17.5 ppm for Category 1 and 35 ppm for Category 2.

3. Respond proactively and strengthen the management of detector alarm signals. Establish a ledger for the detectors, verify it against the layout plan, and draw layout diagrams showing the locations of flammable and toxic gas detectors throughout the plant and in each unit, to facilitate management and prompt response to alarms. Personnel in the fire control room must strengthen the monitoring and management of secondary alarm signals for combustible gases as well as fault signals from the alarm control units; such signals should not be ignored or left unaddressed. Both the control room operation area and the area alarms should be equipped with acoustic and visual alarms to prevent the alarms from going unnoticed due to a noisy environment. It is strictly prohibited for company employees to shut down the alarms on their own due to reasons such as frequent alarms. Management of the alarm system must be strengthened, and a responsibility system for the first person to handle a situation should be established to detect and address potential hazards as soon as possible.

4. Take proactive measures and handle gas detector alarms scientifically. Organize training for relevant personnel to ensure proper filling out of alarm records for flammable and toxic gases. Strictly follow the abnormality handling procedures: upon receiving an alarm from the GDS system, the personnel in the control room identify the instrument tag number and exact location of the alarm, promptly record it, and notify relevant personnel to determine the cause of the alarm. Upon receiving the notification, the on-site personnel or instrument operators identify the cause of the alarm, resolve the fault, and then inform the control room; they sign in the confirmation column to ensure a closed-loop management process.