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Flammability limit

Flammability limits, also called flammable limits, or "explosive 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) (lower explosive limit) describes the leanest mixture that is still flammable, i.e. the mixture with the smallest fraction of combustible gas, while the upper flammable limit (UFL) (upper explosive limit) gives the richest flammable mixture. Increasing the fraction of inert gases in a mixture raises the LFL and decreases UFL.

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.

Limits

Flammability limits of mixtures of several combustible gases can be calculated using Le Chatelier's mixing rule for combustible volume fractions $x_\left\{i\right\}$:

$LFL_\left\{mix\right\}=frac\left\{1\right\}\left\{sum frac\left\{x_\left\{i\left\{LFL_\left\{i\right\}$

and similar for $UFL_\left\{mix\right\}$.

Temperature and pressure also influences flammability limits. Higher temperature results in lower LFL and higher UFL, while greater pressure increases both values. The effect of pressure is very small at pressures below 10 millibar and difficult to predict, since it has hardly been studied.

Gases and vapours

Controlling gas and vapor concentrations outside the explosive limits is a major consideration in occupational safety and health. Methods used to control the concentration of a potentially explosive gas or vapor include use of sweep gas, an unreactive gas such as nitrogen or argon to dilute the explosive gas before coming in contact with air. Use of scrubbers or adsorption resins to remove explosive gases before release are also common. Gases can also be maintained safely at concentrations above the UEL, although a breach in the storage container can lead to explosive conditions or intense fires.

Dusts

Dusts also have upper and lower explosion limits, though the upper limits are hard to measure and of little practical importance. Lower explosive limits for many organic materials are in the range of 10&ndash;50 g/m³, which is much higher than the limits set for health reasons, as is the case for the LEL of many gases and vapours. Dust clouds of this concentration are hard to see through for more than a short distance, and normally only exist inside process equipment.

Explosion limits also depend on the particle size of the dust involved, and are not intrinsic properties of the material. In addition, a concentration above the LEL can be created suddenly from settled dust accumulations, so management by routine monitoring, as is done with gases and vapours, is of no value. The preferred method of managing combustible dust is by preventing accumulations of settled dust through process enclosure, ventilation, and surface cleaning. However, lower explosion limits may be relevant to plant design.

Examples

The explosive limits of some gases and vapors are given below. Concentrations are given in percent by volume of air.

ee also

*Flammability

References

*David R. Lide, Editor-in-Chief; "CRC Handbook of Chemistry and Physics, 72nd edition"; CRC Press; Boca Raton, Florida; 1991; ISBN 0-8493-0565-9

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