Flammability Limits of Gases in Air

Related Resources: thermodynamics

Flammability Limits of Gases in Air

The lower and upper limits of flammability indicate the percentage of combustible gas in air below which and above which flame will not propagate. When flame is initiated in mixtures having compositions within these limits, it will propagate and therefore the mixtures are flammable. A knowledge of flammable limits and their use in establishing safe practices in handling gaseous fuels is important, e.g., when purging equipment used in gas service, in controlling factory or mine atmospheres, or in handling liquefied gases.

Many factors enter into the experimental determination

Many factors enter into the experimental determination of flammable limits of gas mixtures, including the diameter and length of the tube or vessel used for the test, the temperature and pressure of the gases, and the direction of flame propagation—upward or downward. For these and other reasons, great care must be used in the application of the data. In monitoring closed spaces where small amounts of gases enter the atmosphere, often the maximum concentration of the combustible gas is limited to one fifth of the concentration of the gas at the lower limit of flammability of the gas-air mixture.

The calculation of flammable limits is accomplished by Le Chatelier’s modification of the mixture law, which is expressed in its simplest form as

L = \frac{100}{p_{1} / N_{1} + p_{2} / N_{2} + . . . + p_{n} / N_{n}}

where L is the volume percentage of fuel gas in a limited mixture of air and gas; p₁ , p₂, . . . , p_n are the volume percentages of each combustible gas present in the fuel gas, calculated on an air- and inert-free basis so that p₁ + p₂ + . . . + p_n 100; and N₁, N₂, . . . , N_n are the volume percentages of each combustible gas in a limit mixture of the individual gas and air. The foregoing relation may be applied to gases with inert content of 10 percent or less without introducing an absolute error of more than 1 or 2 percent in the calculated limits.

Sources:

Petrography of American Coals, U.S. BuMines Bull. 550. Lowry, ‘‘Chemistry of Coal Utilization,’’ Wiley. ASTM, ‘‘Standards on Gaseous Fuels, Coal and Coke.’’ Methods of Analyzing and Testing Coal and Coke, U.S. BuMines Bull. 638. Karr, ‘‘Analytical Methods for Coal and Coal Products,’’ Academic. Preprints, Division of Fuel Chemistry, American Chemical Society.

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