Combustible Gas and Explosive Limits

The primary risk associated with combustible gases and vapors is the possibility of explosions. Explosion, like fire, requires three elements: fuel, Oxygen, and an ignition source. Each combustible gas or vapor will ignite only within a specific range of fuel/Oxygen mixtures. Too little or too much gas will not ignite—these conditions are defined as the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL). Any amount of gas between the two limits is explosive. It is important to note that each gas has its own LEL and UEL, as shown in the chart below. The gas concentrations are shown by percent of total volume, with the balance as normal air.

Between these two limits explosions can occur under some conditions, with the maximum explosive energy available at approximately the midpoint. Note that these limits are sometimes referred to as LFL (Lower Flammable Limit) and UFL (Upper Flammable Limit). These limits are empirically determine, and various authorities sometimes quote slightly different figures, based on slightly different experimental procedures.

Common Combustible Gas LEL's and UEL's



Acetone (CH3)2CO 2.15% 13.0%
Acetylene C2H2 2.5% 100%
Benzene C6H6 1.2% 8.0%
Butadiene C4H6 1.1% 12.5%
Ethane C2H6 3.0% 15.5%
Ethyl Alcohol CH2H5OH 3.3% 19.0%
Ethyl Ether (C2H5)2O 1.7% 36.0%
Ethylene C2H4 2.7% 36.0%
Hexane C6H14 1.1% 7.5%
Hydrogen H2 4.0% 75.6
IsoButane C4H10 1.8% 8.5%
Isopropyl Alcohol (IPA) (CH3)2CHOH 2.0% 12.7%
Methane CH4 5.0% 15.0%
Methanol CH3OH 6.0% 36.0%
Pentane C5H12 1.5% 7.8%
Propylene C3H6 2.0% 11.1%
Toluene C7H8 1.2% 7.0%

Why is the LEL important in combustible gas detection?

In environments with combustible gas hazards, it is important to know long before the gas concentration reaches the LEL. Typical safety standards require that a gas detection unit give warnings at 10 - 20% of the LEL. Do not confuse the alarm level with the volume of gas required to reach the LEL. For example: Methane has an LEL of 5% by volume in air. For a gas detector to give an alarm at 10% of the LEL, it must trigger when it detects 0.5% by volume. The detector for this application would most likely be calibrated for the range from 0% to 5% gas by volume, but display the reading as 0 - 100% LEL.