Flammability Limit

Flammability Limit

Flammability limits, also called flammable limits, give the proportion of combustible gases in a mixture, between which limits this mixture is flammable. Gas mixtures consisting of combustible, oxidizing, and inert gases are only flammable under certain conditions. The lower flammable limit (LFL) describes the leanest mixture that still sustains a flame, i.e. the mixture with the smallest fraction of combustible gas, while the upper flammable limit (UFL) gives the richest flammable mixture.

There is a quantitative difference between flammability limits and explosive limits. In an explosive mixture, the fuel oxidizer mixture is closer to stoichiometric proportion. This difference has no practical application in safety engineering as the flammable vapor cloud is turbulent and the exact mixture of fuel and oxidizer varies greatly. Therefore, many references use the term flammability limit (LFL, UFL) and explosive limit (LEL, UEL) interchangeably.

Attaining the perfect combustible or explosive mixture between a fuel and air is important in internal combustion engines, for example in gasoline or diesel engines.

A deflagration is a propagation of a combustion zone at a velocity less than the speed of sound in the unreacted medium. A detonation is a propagation of a combustion zone at a velocity greater than the speed of sound in the unreacted medium. An explosion is the bursting or rupture of an enclosure or container due to the development of internal pressure from a deflagration or detonation as defined in NFPA 69.