Understanding Flammability and Flammable gases:

Flammable gases safety sign

Mixtures of dispersed combustible or flammable materials (such as gaseous or vaporised fuels, and some dusts) and air will burn only if the fuel concentration lies within well-defined lower and upper bounds determined experimentally, referred to as flammability limits or explosive limits. Combustion can range in violence from deflagration, through detonation, to explosion.

Flammability limits vary with temperature and pressure, but these are normally expressed in terms of volume percentage at 25 °C and atmospheric pressure. These limits are relevant both to producing and optimising explosion or combustion, as in an engine, or to preventing it, as in uncontrolled explosions of build-ups of combustible gas or dust. Attaining the best combustible or explosive mixture of a fuel and air (the stoichiometric proportion) is important in internal combustion engines such as gasoline or diesel engines.

Lower explosive limit (LEL)

Lower explosive limit (LEL): The lowest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). Many safety professionals consider this terms same as the lower flammable limit (LFL). At a concentration in air lower than the LEL, gas mixtures are “too lean” to burn. Methane gas has an LEL of 5.0%. If the atmosphere has less than 5.0% methane, an explosion cannot occur even if a source of ignition is present.

Ignition Curve of a flammable gas with LEL, UEL plotting
Ignition Curve of a flammable gas with LEL, UEL plotting

Upper explosive limit (UEL)

Upper explosive limit (UEL): Highest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). Concentrations higher than UFL or UEL are “too rich” to burn.

Hence, safety systems and gas monitoring systems are designed to monitor and keep concentration of flammable gases below their respective LEL levels, or below 100% LEL. Therefore, gas detectors or system for flammable gases are designed for monitoring 0% LEL to 100% LEL.

Gas Name% LEL / LFL % UEL / UFL NFPA ClassFlash PointMin. Ignition energy (mJ)Autoignition temperature
Acetaldehyde457IA−39 °C0.37175° C
Acetic acid (glacial)419.9II39° C to 43° C463° C
Acetic anhydrideII54??C
Acetone2.6–312.8–13IB−17 °C1.15 @ 4.5%465° C, 485° C
AcetonitrileIB2° C524° C
Acetyl chloride7.319IB5° C390° C
Acetylene2.5100IAFlammable gas0.017 @ 8.5% (in pure oxygen 0.0002 @ 40%)305° C
Acrolein2.831IB−26 °C0.13
Acrylonitrile317IB0° C0.16 @ 9.0%
Allyl chloride2.911.1IB−32 °C0.77
Ammonia1528IIIB11° C680651° C
Arsine4.5–5.178IAFlammable gas
Benzene1.27.8IB−11 °C0.2 @ 4.7%560° C
1,3-Butadiene212IA−85 °C0.13 @ 5.2%
Butane, n-butane1.68.4IA−60 °C0.25 @ 4.7%420–500 °C
n-Butyl acetate, butyl acetate1–1.78–15IB24° C370° C
Butyl alcohol, butanol111IC29° C
n-Butanol1.411.2IC35° C340° C
n-Butyl chloride, 1-chlorobutane1.810.1IB−6 °C1.24
n-Butyl mercaptan1.410.2IB2° C225° C
Butyl methyl ketone, 2-hexanone18IC25° C423° C
Butylene, 1-butylene, 1-butene1.989.65IA−80 °C
Carbon disulfide150IB−30 °C0.009 @ 7.8%90° C
Carbon monoxide1275IA−191 °C Flammable gas609° C
Chlorine monoxideIAFlammable gas
1-Chloro-1,1-difluoroethane6.217.9IA−65 °C Flammable gas
Cyanogen6.0–6.632–42.6IAFlammable gas
Cyclobutane1.811.1IA−63.9 °C426.7° C
Cyclohexane1.37.8–8IB−18 °C to -20 °C0.22 @ 3.8%245° C
Cyclohexanol19IIIA68° C300° C
Cyclohexanone1–1.19–9.4II43.9–44 °C420° C
Cyclopentadiene[12]IB0° C0.67640° C
Cyclopentane1.5–29.4IB−37 to −38.9 °C0.54361° C
Cyclopropane2.410.4IA−94.4 °C0.17 @ 6.3%498° C
Decane0.85.4II46.1° C210° C
Diborane0.888IA−90 °C Flammable gas38° C
o-Dichlorobenzene, 1,2-dichlorobenzene29IIIA65° C648° C
1,1-Dichloroethane611IB14° C
1,2-Dichloroethane616IB13° C413° C
1,1-Dichloroethene6.515.5IA−10 °C Flammable gas
Dichlorofluoromethane54.7Non flammable,[18] -36.1° C552° C
Dichloromethane, methylene chloride1666Non flammable
Dichlorosilane4–4.796IA−28 °C0.015
Diesel fuel0.67.5IIIA>62° C210° C
Diethanolamine213IB169° C
Diethylamine1.810.1IB−23 to −26 °C312° C
Diethyl disulfide1.2II38.9° C
Diethyl ether1.9–236–48IA−45 °C0.19 @ 5.1%160–170 °C
Diethyl sulfideIB−10 °C
1,1-Difluoroethane3.718IA−81.1 °C
1,1-Difluoroethylene5.521.3−126.1 °C
Diisobutyl ketone1649° C
Diisopropyl ether121IB−28 °C
Dimethylamine2.814.4IAFlammable gas
Dimethyl sulfideIA−49 °C
Dimethyl sulfoxide2.6–342IIIB88–95 °C215° C
1,4-Dioxane222IB12° C
Epichlorohydrin42131° C
Ethane312–12.4IAFlammable gas -135° C515° C
Ethanol, ethyl alcohol3–3.319IB12.8° C365° C
2-Ethoxyethanol31843° C
2-Ethoxyethyl acetate2856° C
Ethyl acetate212IA−4 °C460° C
Ethylamine3.514IA−17 °C
Ethylbenzene17.115–20 °C
Ethylene2.736IA0.07490° C
Ethylene glycol322111° C
Ethylene oxide3100IA−20 °C
Ethyl chloride3.815.4IA−50 °C
Ethyl mercaptanIA
Fuel oil No.10.75
Furan214IA−36 °C
Gasoline (100 octane)1.47.6IB< −40 °C (−40 °F)246–280 °C
Glycerol319199° C
Heptane, n-heptane1.056.7−4 °C0.24 @ 3.4%204–215 °C
Hexane, n-hexane1.17.5−22 °C0.24 @ 3.8%225° C, 233° C
Hydrogen4/18.375/59IAFlammable gas0.016 @ 28% (in pure oxygen 0.0012)500–571 °C
Hydrogen sulfide4.346IAFlammable gas0.068
Isobutane1.89.6IAFlammable gas462° C
Isobutyl alcohol21128° C
Isophorone1484° C
Isopropyl alcohol, isopropanol212IB12398–399 °C; 425 °C
Isopropyl chlorideIA
Kerosene Jet A-10.6–0.74.9–5II>38° C as jet fuel210° C
Lithium hydrideIA
Methane (natural gas)515IAFlammable gas0.21 @ 8.5%580° C
Methyl acetate316−10 °C
Methyl alcohol, methanol6–6.736IB11° C385° C; 455° C
MethylamineIA8° C
Methyl chloride10.717.4IA−46 °C
Methyl etherIA−41 °C
Methyl ethyl etherIA
Methyl ethyl ketone1.810IB−6 °C505–515 °C
Methyl formateIA
Methyl mercaptan3.921.8IA−53 °C
Mineral spirits0.76.538–43 °C258° C
Morpholine1.810.8IC31–37.7 °C310° C
Naphthalene0.95.9IIIA79–87 °C540° C
Neohexane1.197.58−29 °C425° C
Nickel tetracarbonyl2344° C60° C
Nitrobenzene29IIIA88° C
Nitromethane7.322.235° C379° C
Octane1713° C
Pentane1.57.8IA−40 to −49 °Cas 2-Pentane 0.18 @ 4.4%260° C
n-Pentane1.47.8IA0.28 @ 3.3%
iso-Pentane1.329.16IA420° C
Propane2.19.5–10.1IAFlammable gas0.25 @ 5.2% (in pure oxygen 0.0021)480° C
Propyl acetate2813° C
Propylene211.1IA−108 °C0.28458° C
Propylene oxide2.936IA
Pyridine21220° C
Silane1.598IA<21° C
Styrene1.16.1IB31–32.2 °C490° C
Tetrahydrofuran212IB−14 °C321° C
Toluene1.2–1.276.75–7.1IB4.4° C0.24 @ 4.1%480° C; 535° C
Triethylborane−20 °C−20 °C
TrimethylamineIAFlammable gas
Turpentine0.8IC35° C
Vegetable oilIIIB327° C
Vinyl acetate2.613.4−8 °C
Vinyl chloride3.633
Xylenes0.9–1.06.7–7.0IC27–32 °C0.2
m-Xylene1.17IC25° C525° C
o-XyleneIC17° C
p-Xylene16IC27.2° C530° C
LPG1.88.4-60° C405° C