Ex-pert Electrical Technologies

Dynamic elaboration on the Art & Science of Hazardous Area Protection

Explosive Limits

LOWER EXPLOSIVE LIMIT (LEL

LOWER EXPLOSIVE LIMIT (LEL)—is the lowest percentage by volume of gas (or vapor) in a gas-air mixture that will form an ignitable concentration. Below that concentration there is insufficient gas or vapor in the mixture and the gas-air mixture is too lean to be ignited.

UPPER EXPLOSIVE LIMIT (UEL)

UPPER EXPLOSIVE LIMIT (UEL)—is the highest percentage by   volume   of   gas   or   vapor   in   a   gas-air   mixture that will form an ignitable concentration. Above that concentration there is too much gas or vapor in the mixture and the gas-air mixture is too rich to ignite.

explosive_limits

 

An interesting example:

Methane (CH4) is highly flammable and when mixed with air it forms a mixture which can easily be ignited. Flammable mixture is one which will burn itself once ignited (Even after removal of the ignition source).
The Lower Explosive Limit of Methane is about 5% (By Volume: Meaning volume of Methane divided by total volume of mixture of air and methane: 5 ml of methane in 100 ml of Mixture) .
The Upper Explosive Limit is about 15%.
That means methane will not explode below 5% and above 15%. (It may burn around the source of ignition or heat)

 

Explosive limit slide

More information at The working of a Fire Triangle

 

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