A flammable gas is any gas capable of igniting and burning when mixed with air within a specific concentration range known as the flammable range. The lower boundary of this range is the lower flammable limit (LFL), below which the mixture is too lean to ignite, and the upper boundary is the upper flammable limit (UFL), above which the mixture is too rich to ignite.
The precise definition of a flammable gas varies by regulatory authority.
- OSHA defines it broadly as any gas that has a flammable range with air at 20°C and 101.3 kPa.
- NFPA 55 uses a more specific quantitative threshold: a gas that ignites at 13% concentration or less, or has a flammable range wider than 12%.
- The U.S. Department of Transportation (49 CFR 173.115) classifies any gas that forms a flammable mixture with air at 13% or less by volume, or has a flammable range of at least 12%.
- OSHA’s PSM defines it with a complex definition beyond the scope here
These differences matter in practice, and can be frustrating— compliance with one standard does not guarantee compliance with others, and engineers must identify which authority governs their specific application.
Key Points
- The flammable range between LFL and UFL defines the concentration window where ignition is possible; gases with wider ranges or lower LFLs present greater risk.
- OSHA, NFPA 55, and DOT each define flammable gas using slightly different quantitative thresholds — a substance that qualifies under one definition may not qualify under another.
- Under IEC 60079, flammable gases are assigned to explosion groups (IIA, IIB, IIC) based on their MESG and MIC ratio, which determine the required protection level of electrical equipment in hazardous areas.
- Common process industry examples include hydrogen (Group IIC), ethylene (Group IIB), and propane (Group IIA).
See Also: hazardous areas, NFPA hazard ratings, flammable liquid, flashpoint, MESG
Hydrogen meets the flammable gas definition under OSHA, NFPA 55, and DOT. With a lower flammable limit of 4% and an upper limit of 75%, it has one of the widest flammable ranges of any common industrial gas. This, combined with its Group IIC classification under IEC 60079, makes it one of the most demanding flammable gases to manage in hazardous area design.
See Also: Hazardous Areas, NFPA Hazard Ratings, Flammable Liquid, Flashpoint, MESG
Cited Sources
- OSHA 29 CFR 1910.1200, Appendix B — Hazard Communication Standard, Physical Hazard Criteria
- NFPA 55, Compressed Gases and Cryogenic Fluids Code
- DOT 49 CFR 173.115 — Definition of Flammable Gas (Division 2.1)