Safety Parameters for Flammable Gases and Vapours

Combustion can occur only if the mixture of fuel and oxygen lies within a certain range. This is described by the lower and upper explosion limits (LEL and UEL). In older references theses Umits are referred to as the lower and upper limits of flammability (LFL and UFL) (vid. [4]). They represent the volume ratio of fuel vapour in air. Below the lower explosion limit the mixture is too lean, above the upper limit it is too rich for combustion to occur. The explosion Umits are not fixed values. They depend on whether we deal with a mixture with air or with oxygen. Furthermore they are influenced by (vid. [4, 5])  

Additionally they depend, as already mentioned, on the boundary conditions of their measurement, as illustrated by Table 2.1. In general the most flammable mixture is close to but not exactly equal to the stoichiometric one [5]. 

In Eqs. (2.1) and (2.2) Cst is the stoichiometric concentration (volume percent of fuel in air). In case of a stoichiometric equation of combustion of the form 

However, Example 2.2 shows that the differences between calculated and measured values are considerable. Hence, whenever possible measured values are 

This applies as well for the pressure dependence of the explosion limits. The following logarithmic relationship is given for the pressure dependence of the UEL (vid. [6]) 

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