Explosions are either detonations or deflagrations; the difference depends on the speed of the shock wave emanating from the explosion. In front of a flame front moving in a pipe is a pressure or shock wave. If the pressure wave moves faster than the speed of sound in the unreacted medium the explosion is a detonation; if it moves at a speed less than the speed of sound it is a deflagration.
A fixed temperature above which a flammable mixture is capable of extracting enough energy from the environment to self-ignite.
Vapor-air mixtures will only ignite and burn over a well-specified range of compositions. The mixture will not burn when the composition is lower than the lower flammable limit (LFL); the mixture is too lean for combustion to occur. The mixture will not burn when the composition is higher than the upper flammable limit (UFL); the mixture is too rich for combustion to occur. A mixture is flammable only when the composition is between the LFL and the UFL. Typical units are volume percent fuel as a percentage of fuel and air. Lower explosive limit (LEL) and upper explosive limit (UEL) are used interchangeably with LFL and UFL.
Lower flammability limits in air are deceased approximately 8 percent by a temperature increase of 100 °C. Upper flammability limits are increased 8 percent by a temperature increase of 100 °C.
Pressure has only a slight effect on LFL. Lower limits are essentially constant down to about 5 kPa generally, below which pressure flame does not propagate. The effect of higher pressures on LFL and correspondingly on MOC is slight. On the other hand elevated pressure greatly increases the UFL.