Flammable mixtures Temperature, effects

Liquids present a different case. Some liquids will give off dangerous quantities of flammable vapors well below normal room temperature (the vapor pressure of a liquid is a measure of this effect) others do so at points only slightly above room temperature, and still others at much higher temperatures. It is apparent that the temperature at which a liquid evolves vapors which can form flammable mixtures with air is a measure of its hazard potential. This is indicated by the flash point... 

Effect of temperature on flammable limits. The higher the temperature at the moment of ignition, the more easily the combustion reaction will propagate. Therefore, the reference temperature (initial temperature) of the flammable mixture must be stated when flammable limits are quoted. There are not a lot of data for flammable limits under different conditions of initial temperature. The behavior of a particular mixture under different conditions of initial temperature usually must be determined by tests. 

Flammability limits are usually quoted for vapour/air mixtures at ambient temperature. However, flammable limits vary with temperature and with the oxygen content of the mixture. Increasing the temperature causes the limits to widen — for methane at 20°C the flammability range is 6.0-13.5%, while at 250°C it is 4.5-14.0%. Reference 111 gives more information on temperature effects. 

There may be superimposed effects on the temperature of the element (and hence V) arising from changes in the heat loss characteristics in different gas mixtures. These effects are particularly marked for flammable gases with thermal conductivities vastly different from that of air. The incorporation of the non-active element compensates for such effects in that this element... 

In view of the above adverse effects a safety factor should be applied where flammability is assessed using flash point. For pure liquids in containers the vapor should be considered potentially flammable if the liquid temperature is upward of at least 5°C below the reported flash point. For mixtures whose composition is less certain, such as petroleum mixtures, the safety factor should be about 15°C relative to the flash point [55]. Where combinations of adverse effects are identified the safety factors should be increased accordingly. A simple but very conservative approach is to assume that all liquids having a flash point <141°F may produce a flammable atmosphere under some ambient conditions, even where no mist or froth production is involved. A more practical approach is to assume that liquids handled in air at least 5-15°C below their closed cup flash points will not present ignition risks unless... 

Flammable gas or vapor-air mixtures Due to their low minimum ignition energies (<1 mj), avoidance of effective ignition sources in flammable gas or vapor-air mixtures is in principle possible only in exceptional cases. For hot surfaces a maximum permissible surface temperature Tmax must be specified, with the help of the minimum ignition temperature (MIT) of flammable gases, vapors, or liquids, so that the temperature of all surfaces is not exceeded (Table 23-8). 

Burgoyne and Thomas (17G) have passed mixture air through an iron-electrode arc prior to passing into a Bunsen tube. The effect of the minute iron oxide particles contained in the stream is to lower the lean limit of flammability of hydrogen. Sanger (33G) deals with such topics as the dependency of combustion on pressure, temperature, and the moisture content of powders, and gives a series of equations in support of his hypothesis. 

The basic test apparatus consists of a chamber into which a known concentration of vapor (gas) in air is introduced. After thorough mixing, ignition is attempted with a spark or a hot wire. A series of different concentrations are tested to establish the upper and lower concentration limits for flammability. Although normally run with fuel-air mixtures at ambient conditions, other oxidizing atmospheres, diluent effects and temperature and pressure variations can be studied. 

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