In order to benefit the public and society by ensuring the safe use of gas and minimizing accidents caused by gas leaks, installing gas alarms can provide timely warning information and take appropriate actions before the accumulated gas reaches a hazardous concentration. The concentration setting for gas leak alarms is closely related to the lower explosive limit (LEL). In some domestic standards, the requirement for setting the gas leak alarm concentration is 20% of the LEL, while others require 25% of the LEL. Many urban gas systems in China follow the standard of 20% of the LEL, as follows:

1. "Gas Engineering Project Specification" GB 55009-2021

4.2.17: In gas plant stations, where the concentration of flammable gases may reach 20% of the lower explosive limit within the gas facility area or inside buildings, fixed flammable gas concentration alarm devices should be installed.

6.1.5: For residential users in high-rise buildings using gas, the following regulations should be adhered to:

1) Gas supply should be through a piped system.

2) In locations where the building height exceeds 100 meters, gas-using areas should have gas leak alarm devices installed, and emergency automatic cutoff devices should be placed at the gas inlet.

Corresponding text explanation (excerpt):

Gas, being a highly flammable and explosive medium, poses significant hazards at fire scenes. By installing an emergency automatic cutoff valve at the gas inlet, its control system remains in a closed state during power outages or gas supply interruptions. This enables remote control to quickly shut off the gas source, preventing the spread of accidents, reducing their impact, and avoiding secondary disasters. Emergency automatic cutoff valves are ideally placed outdoors.

Gas leak alarm devices should be installed at every gas utilization site in accordance with the specified standards. The minimum alarm concentration for gas leak alarm devices is set at 20% of the lower explosive limit of flammable gases. Once a certain concentration of gas leak is detected, it should immediately notify management for accident investigation, maintenance, and timely detection of potential hazards, thereby preventing more significant accidents.

2. "Design Code for Compressed Natural Gas Supply Stations" GB 51102-2016

10.2.5: The design of flammable gas detection alarm systems should comply with the following regulations:

1) Flammable gas detection alarm systems should be installed in places where flammable gases are produced, used, or generated, and they should comply with the relevant provisions of the current national standards "Technical Regulations for Urban Gas Alarm Control Systems" CJJ/T146 and "Design Specification for Flammable Gas and Toxic Gas Detection and Alarm in Petrochemical Industry" GB 50493.

2) The alarm concentration for flammable gas detection should be 20% of the lower explosive limit of natural gas (volume percentage).

3) Fixed-type flammable gas detectors should be used, and places with flammable gas detectors should be equipped with audio-visual alarms.

4) Alarm controllers should be placed in monitored rooms with personnel and should be connected to the automatic control system.

3. "Design Code for Liquefied Petroleum Gas Supply Engineering" GB 51142-2015

12.3.4: Liquefied petroleum gas supply stations should be equipped with flammable gas detection alarm systems and video surveillance systems.

12.3.5: In areas of liquefied petroleum gas supply stations with explosion hazards, a flammable gas leakage alarm control system should be installed, and it should comply with the following regulations:

1) The selection and installation of flammable gas detectors and alarm controllers should comply with the relevant provisions of the current national standards "Design Specification for Flammable Gas and Toxic Gas Detection and Alarm in Petrochemical Industry" GB 50493 and "Technical Regulations for Urban Gas Alarm Control Systems" CJJ/T 146.

2) Bottle group gasification stations and bottled liquefied petroleum gas supply stations may use portable flammable gas leakage alarm devices. The alarm setting value of flammable gas detectors should be determined based on 20% of the lower explosive limit of flammable gases.

3) The flammable gas alarm controller should be interlocked with the control system.

4) Indicating and alarm devices for the flammable gas alarm control system should be placed in locations with personnel on duty, such as the duty room or instrument room.

4. "Safety Specification for Flammable Gas and Toxic Gas Detection Alarm Systems in Petroleum and Natural Gas Engineering" SY/T 6503-2022

6.6.2: The alarm setting values should conform to the following regulations:

a) For fixed-type flammable gas detectors, the first-level alarm setting value should be less than or equal to 20% LEL (Lower Explosive Limit), preferably 10% LEL. The second-level alarm setting value should be greater than the first-level alarm setting value and less than or equal to 40% LEL. For portable flammable gas detectors, the first-level alarm setting value should be less than or equal to 10% LEL, and the second-level alarm setting value should be less than or equal to 20% LEL.

Also, the use of the 25% lower explosive limit for alarm is found in cases like the following:

1. "Technical Standards for Automobile Refueling, Gas Filling, and Hydrogen Filling Stations" GB 50156-2021

13.4.1: In open-air areas with LPG equipment, LNG equipment in refueling and gas filling stations, as well as rooms, containers, and shelters with CNG equipment, hydrogen equipment, and liquid hydrogen equipment in combined refueling, gas filling, and hydrogen filling stations, flammable gas detectors should be installed.

13.4.2: The first-level alarm setting value for flammable gas detectors should be less than or equal to 25% of the lower explosive limit of flammable gases.

13.4.6: The selection and installation of flammable gas detectors and alarms should comply with the relevant provisions of the current national standard "Design Specification for Flammable Gas and Toxic Gas Detection Alarm in Petrochemical Industry" GB/T 50493.

2. "Technical Specifications for Gas-Based Heating, Cooling, and Power Combined Supply Engineering" GB 51131-2016

10.0.6: In station buildings where there are gas equipment and pipeline connections, flammable gas detection alarm devices should be installed. These installations should comply with the relevant provisions of the current industry standard "Technical Regulations for Urban Gas Alarm Control Systems" CJJ/T 146 and should adhere to the following regulations:

1) When the concentration of flammable gas reaches 25% of the lower explosive limit, an alarm should be triggered, and accident exhaust fans should be interlocked for activation.

2) When the concentration of flammable gas reaches 50% of the lower explosive limit, the gas emergency automatic shut-off valve should be interlocked for closure.

3) Automatic alarms should include audible and visual signals at the local and main control panel. The technical specifications for gas-based heating, cooling, and power combined supply engineering CJJ145-2010 require compliance with these requirements.

3. "Building Design Fire Protection Code" GB 50016-2014 (2018)

This standard does not specify specific alarm concentration values in its text. However, there are two instances in its explanatory notes where it mentions 25% of the lower explosive limit.

Fire Protection Handbookof The National Fire Protection Association (NFPA) in the United States, states that the warning response concentration for flammable vapors and gases is 20% of their lower explosive limit. When the concentration reaches 50% of the lower explosive limit, operations should be stopped, and inerting should be initiated. In the guidance standards for flammable gas detection alarm design in China, it is stated as less than or equal to 25%. Therefore, the selection of 20% or 25% in the above-mentioned standard is based on corresponding regulatory guidelines.

4. "Design Standard for Combustible Gas and Toxic Gas Detection Alarm in Petrochemical Industry" GB/T 50493-2019

5.5.2: The alarm value settings should comply with the following regulations:

1) The first-level alarm setting value for combustible gases should be less than or equal to 25% LEL.

2) The second-level alarm setting value for combustible gases should be less than or equal to 50% LEL.

3) The first-level alarm setting value for toxic gases should be less than or equal to 100% OEL, and the second-level alarm setting value for toxic gases should be less than or equal to 200% OEL. When the existing detectors' measurement range cannot meet the measurement requirements, the first-level alarm setting value for toxic gases should not exceed 5% IDLH, and the second-level alarm setting value for toxic gases should not exceed 10% IDLH.

4) The peroxide alarm setting value for ambient oxygen should be 23.5% VOL, and the alarm setting value for oxygen deficiency should be 19.5% VOL.

5) The first-level alarm setting value for linear combustible gas measurement should be 1 LEL·m, and the second-level alarm setting value should be 2 LEL·m.

Explanatory Notes (Excerpt):

5.5.2: In order to ensure the safety of on-site personnel and in conjunction with the improvement of domestic safety management levels, and in accordance with the practices of international petrochemical enterprises, during the engineering design process, the alarm setting values for combustible gases and toxic gas leakage may be set more stringent than the requirements of this clause. The setting values for various alarm levels should be lower than the values specified in the standards.

5. "Technical Regulations for the Operation, Maintenance, and Emergency Repair of Urban Gas Facilities" CJJ51-2016

4.7.4: Before entering indoor work areas, a check for gas leaks should be conducted first. If a gas leak is detected, measures should be taken to reduce the indoor gas concentration. When it is confirmed that the concentration of flammable gas is lower than 20% of the lower explosive limit, maintenance work can proceed.

Note: The above excerpt only includes requirements for the detection of combustible gas leak concentrations and not for toxic gas concentrations.