LFL

The range of gas concentrations in air between which the mixture is flammable. This describes the range of concentrations between the LFL (Lower Flammable Limit) and the UFL (Upper Flammable Limit). Mixtures within this range are capable of being ignited.

From the diagram it can be seen that there is a flammable range which at 21% oxygen has an upper limit of approx 10.5% air/oil vapour and a lower limit of approx 1%. Within these limits the vapour may ignite if supplied with a source of ignition.

As the level of oxygen is reduced the flammable range becomes narrower. Ignition can not take place when the oxygen content is reduced to about 10%. For this reason the cargo tanks of oil tankers are inerted, that is the atmosphere is filled with a gas containing low levels of oxygen to eliminate the possibility of ignition.