Understanding LEL of Common Combustible Gases

by Lukman Nulhakiem

LEL-of-common-combustible-gasesThere are many common combustible gases found in chemical plant. These combustible gases have one similarity. They can be ignited when their concentration in air are within their flammability limits.

What are flammability limits? Flammability limits or commonly known as explosive limits are defined as concentration of combustible gas in mixture with air in which they can be ignited and exploded in the presence of source of ignition.

The lowest concentration of a combustible gas in mixture with air where it still can be ignited is called by Lower Explosive Limit (LEL) or Lower Flammable Limit (LFL). Whereas, the highest concentration of a combustible gas in mixture with air where it still can be ignited is called by Upper Explosive Limit (UEL) or Upper Flammable Limit (UFL).

Each combustible gas has its unique Lower Explosive Limit and Upper Explosive Limit. The information about LEL and UEL of a combustible gas can be found in its MSDS. Below are LEL of common combustible gases as well as UEL data. All units are stated in % volume.

Acetylene: LEL=2.5%, UEL=82%
Ammonia: LEL=15%, UEL=28%
Benzene: LEL=1.35%, UEL=6.65%
Ethane: LEL=3%, UEL=12%
Ethylene: LEL=2.7%, UEL=36%
Hydrogen: LEL=4%, UEL=75%
Methane: LEL=5%, UEL=15%
Propane: LEL=2.1%, UEL=10.1%
Toluene: LEL=1.27%, UEL=6.75% 

A collection of LEL and UEL data of common combustible gases can be found here.  

What is LEL of Common Combustible Gases data for?

Lower Explosive Limit (LEL) data of common combustible gases are very essential for safety. Let’s take an example. When we plan to do welding work on the top of a pressure vessel containing one of the above common combustible gases, the first thing that we have to do is to measure the combustible gas concentration inside the vessel.

The concentration should be lower than LEL of the combustible gas. Even it is recommended to set target concentration at 10% of LEL. It means that if the pressure vessel containing hydrogen, maximum hydrogen concentration inside it should be 0.4% before welding work is allowed.

Generally, LEL of common combustible gases is included into hot work permit form. Each combustible gas concentration is measured by combustible gas detector or gas chromatography and the results are recorded on the hot work permit form. Nitrogen substitution should be carried out if necessary.

In case of combustible gas purge, no works are allowed to be done during the gas purge. It is worried that the works produce fire or spark and ignite the purged gas, which is within its flammable limits. Middletown power plant explosion showed how dangerous natural gas purge operation that was carried out improperly.

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