Limits of inflammation

Jones, G. W. and Kennedy, R. E.. Limits of Inflammability of Natural Gases Containing High Percentages of Carbon Dioxide and Nitrogen, Bureau of Mines Report of Investigation, 3216, 1933. 

The value obtained will allow direct comparison with the limits of inflammability that are always given in percentage form and enable us to deduce whether the substance presents the risk of building an explosive mixture with air. Note that if the chosen unit is the millibar, the value P ap is to be divided by ten in order to obtain the concentration. This expression will be used in the determination of II code (infammability index (see para 1.5.5). 

LEL is the most important of the two limits. It is mostly useful when inflammable substances are handled in confined spaces (reservoirs, painting cabins, ovens etc). Detaiis of limits of inflammability are kept by chemical substance manufacturers who are required to mention them on safety sheets that have to be put at clients disposal. When compared with the equilibrium concentration determined as indicated before, LEL aiiows determination of whether a working environment presents a risk of explosion in the presence of a source of ignition. 

The object of these comments is, first of all, to draw attention to the fact that experimental limits of inflammability present a big experimental error and have to be handled with caution. The methods enable an estimation of the relevance of these experimental values to be made, and when these are not sufficiently reliable or are unknown - an estimation based on calculation models is made. 

A brief study of the available data related to limits of inflammability in Part Two shows that these parameters are subject to high experimental uncertainty. For a large number of substances, the experimental values are widely dispersed. When they are submitted to quality estimation using statistical tools, in many cases they reveal that it is impossible to use them with confidence. The examples of difficulties raised by the statistical analysis of the LEL data can be multiplied. 

These considerations, aiong with the fact that this parameter is hardly mentioned in any availabie table, have led several authors to research models that incorporate estimations of limits of inflammability. 

Two methods for estimating lower limits of inflammability will be considered. The first involves data on flashpoints which will be analysed in the next paragraph the other, which is also used for the estimation of DEL, is due to HiladoL in which the stoichiometric concentration of a substance has only to be multiplied by 0.5 to 0.55 to estimate its LEL approximately (and by about 3.2 for the UEL). Hilado has perfected this approach. For the present writer LEL and UEL are estimated with the help of the following equations ... 

It is only in a few cases that the limits of inflammability of a vapour mixture can be calculated, thanks to the limits of inflammability of the components in the pure state of a mixture. In fact, Le Chatelier s law only applies to mixtures of saturated aliphatic hydrocarbons and to two or three other mixtures of inorganic substances (CO, H2) alone or with methane. 

As a consequence, operating conditions that enable one to work in concentration ranges that do not overlap the range set by the limits of inflammability are often sought in industrial synthesis. 

The exponential dependence of the evolution of heat upon temp, besides rendering eq (1) nonlinear and hence difficult to solve, accounts for the existence of concepts such as ignition, ignition temp, induction period, limits of inflammability and, indeed, of the concept of "explosion itself, which. can be defined no more rigorously than by the requirement that the reaction rate become sufficiently high. This fundamental point is implied in every theoretical treatment of thermal expln, but it has been brought out explicitly in Russian scientific literature (Refs 5, 6, 9, 11 14). 

In the absence of decomposition catalysts, it has been heated above 500° F with very little decomposition. It is completely insensitive to shock, friction or electrical discharge. Hydrazine vapor has a lower limit of inflammability of 4.67% by volume in air. The upper limit is 100% since the vapor may be exploded without the addition of air (Ref 10)... 

Refs 1) H.F. Coward, Limits of Inflammability of Gases and Vapors, B of M Bull 279 (Revised 1939) 2) L. Do lie, Inflammability Limits of Vapors from Solids, Magasin CTO Paris (1953) CA 48, 13222 (1954) 3) G. Dixon-Lewis G.L. Isles, Seventh Symposium (Inti) on Combustion (1960) 4) B. Lewis ... 

Despite intensive theoretical and experimental research on the combustion wave over the past decade, a universally accepted concept of the phenomenon has not been achieved. This is particularly true with respect to the nonsteady state phenomena of limits of inflammability and ignition. 

Table 50—Lower Limits of Inflammability of Certain Dusts...

It is a matter of common knowledge that a room may smell quite strongly of coal gas without its being dangerous to strike a match within it. When this observation is pushed to its logical conclusion it is evident that a certain minimum quantity of the coal gas must be present for its inflammation to be self-supporting. This minimum quantity is termed the lower limit of inflammation of the combustible gas, and is influenced by two factors 2... 

Consideration will show that there must also be a higher limit of inflammation of the combustible gas, for if its proportion over that of the oxygen be largely increased, the excess will function as a diluent, absorb heat, and tend to retard flame propagation. 

Since gaseous combustion is a reciprocal phenomenon, it follows that the amount of oxygen present in this latter case is the minimum quantity supporting combustion, and may be termed the lower oxygen limit of inflammation. 

Amongst the earliest experiments carried out with a view to the quantitative determination of the limits of inflammability of combustible gases were those of Davy with fire damp, wfliich is mainly methane, CH4. Owing to the importance of this gas m connection with gob fires and explosions in coal mines, several other workers have also investigated it. The value of the results, however, is restricted by the fact that firedamp, like most natural products, is subject to very considerable variation in composition.3 Even Davy recognised that it was not pure methane indeed, perfectly pure methane is not easy to prepare in quantity. The gas, as obtained from sodium acetate, may contain as much as 8 per cent, of hydrogen, as well as ethylene.4 No doubt this variation m composition is one contributory cause of the very varied results listed in the table on p. 93. 

The limits of inflammation of organic substances, such as ether, alcohol, and acetone, in air have been determined by White and Price, Trans Ghent Soc., 1919, 115, 1462. 

As a first approximation, therefore, it would appear probable that the lower limit of inflammation should vary inversely as the calorific value of the gas that is to say, if L is the proportion of the combustible gas necessary to form a lower-limit mixture, and C its calorific value,... 

The value obtained for the lower limit of inflammation of methane when mixed with air is 5-6. The calorific value of methane is 189T. Substituting m the above equation a value of 1059 is obtained for k, and the relative values for L for other gases can then be calculated. 

The agreement between the observed and calculated values of L is very striking, and seems to point to a definite and dominating relationship between the calorific values of the combustibles named in the table —the paraffin hydrocarbons—and their lower limits of inflammation when mixed with air. 

INFLUENCE OF DIAMETER OF TUBE UPON THE LIMITS OF INFLAMMABILITY. 

Widely differing results have been obtained for the limits of inflammability of hydrogen-air mixtures, as is evident fiom the following table ... 

The following results have been obtained by different investigators (see p. 100) for the lower and upper limits of inflammability of mixtures of carbon monoxide and air. The wide divergence which characterises the published data for hydrogen is not so evident, for the results agree much more closely. Undoubtedly the most reliable are those of Coward and his co-workers. 

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