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LEL and UEL Explained (Explosive Gas)

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The lower explosive limit (LEL) is the minimum concentration of a specific combustible gas required to fire combustion when in contact with oxygen (air). If the concentration of the gas is below the LEL value, the mix between the gas itself and the air is too weak to spark. The upper explosive limit (UEL) is the maximum level of concentration of the gas that will burn when mixed with oxygen; when the gas concentration is above the UEL value for the gas/vapor, the mix is too “fat” to ignite or explode. 

LEL AND UEL: WHY ARE IMPORTANT?

LEL UEL for combustible gas

The range between the lower and the upper explosive limit (LEL / UEL %) is defined as the flammable range of a specific explosive and flammable gas.
Flammable range meaning (LEL)

Examples of LEL for common gases:

  • LEL for Hydrogen: 4.0
  • LEL for Methane: 5.0

The risk of explosion of combustible gases has to be managed carefully in any production site handling gases.

To fire an explosion, three conditions should occur at the same time:

  1. The presence of combustible gas, the fueling element, in a specific concentration
  2. Presence of oxygen
  3. The existence of a sparking element (that ignites the two elements)

The proportion of fuel and the oxygen needed to generate an explosion depends on the type of combustible gas. Gases will ignite only when mixed with air within a specific concentration range. If the gas is mixed with oxygen with too low or too high concentrations, the gas will not ignite and explode.

The lower and the upper explosion values (LEL and UEL) define the required level of concentration by type of gas.

Explosions will occur for gas concentrations within the LEL and the UEL value, not above or below, and the maximum explosive power will be for concentration at the midpoint of the flammable range.

LEL UEL CHART

(Note: LEL / UEL values are based on room temperature and atmospheric pressure, ignition fired by a tube of 2-inch diameter.

As the temperature, the pressure and the ignition increase, the explosive limits by gas vary.

The values are determined empirically and may change depending on the source of the information). The lower and the upper explosive limits by gas are:

LEL Gas LEL % UEL %
Acetone 2.6 13.0
Acetylene 2.5 100.0
Acrylonitrile 3.0 17
Allene 1.5 11.5
Ammonia 15.0 28.0
Benzene 1.3 7.9
1,3-Butadiene 2.0 12.0
Butane 1.8 8.4
n-Butanol 1.7 12.0
1-Butene 1.6 10.0
Cis-2-Butene 1.7 9.7
Trans-2-Butene 1.7 9.7
Butyl Acetate 1.4 8.0
Carbon Monoxide 12.5 74.0
Carbonyl Sulfide 12.0 29.0
Chlorotrifluoroethylene 8.4 38.7
Cumene 0.9 6.5
Cyanogen 6.6 32.0
Cyclohexane 1.3 7.8
Cyclopropane 2.4 10.4
Deuterium 4.9 75.0
Diborane 0.8 88.0
Dichlorosilane 4.1 98.8
Diethylbenzene 0.8

1,1-Difluoro-1-Chloroethane 9.0 14.8
1,1-Difluoroethane 5.1 17.1
1,1-Difluoroethylene 5.5 21.3
Dimethylamine 2.8 14.4
Dimethyl Ether 3.4 27.0
2,2-Dimethylpropane 1.4 7.5
Ethane 3.0 12.4
Ethanol 3.3 19.0