Document Rj2D4GgRKD5YEwnDE1RY0gmGz

FLAMMABLE AND COMBUSTIBLE LIQUIDS vapor-air mixture will ignite under de fined conditions without an external source of ignition Vapors and gases will spontaneously ig nite at a lower temperature in oxygen than in air, and their auto-ignition temperature may be influenced by the presence of cata lytic substances. Flammable limits Flammable liquids have a minimum concentration of vapor in air below which propagation of flame does not occur on contact with a source of ignition There is also a maximum proportion of vapor or gas in air above which propaga tion of flame does not occur The extremes of vapor or gas concentra tion with air which, if ignited, will just propagate flame are known as the "lower and upper flammable limits" and are usu ally expressed in terms of percentage by volume of gas or vapor in air These limits are also commonly referred to as, respec tively, the lower and upper explosive limits A mixture with less than approximately 10 per cent of gasoline vapor is too lean, and propagation of flame will not occur on contact with a source of ignition Similar ly, if there is more than approximately 8 per cent of gasoline vapor, the mixture will be too nch Other gases such as hydrogen, acetylene, and ethylene have a wider range of flammable limits Flammable range is the difference be tween the lower and upper flammable lim its, expressed in terms of percentage of vapor or gas m air by volume, and is also often referred to as the "explosive range" For example, the limits of the flammable range of gasolme are generally taken as 14 to 7 6 per cent which is relatively narrow Thus, a mixture of 1 4 per cent gasoline vapor and 08 8 per cent aur is flammable, as are all the intermediate mixtures up to and including 7 8 per cent gasoline vapor and 924 per cent air The range is there fore the difference between the limits, or 6 2 per cent Rate of diffusion indicates the tendency of one gas or vapor to disperse into or mu with another gas or vapor This rate de pends on the density of the vapor or gas as compared with that of air, which is given a value of 1 Whether a vapor or gas is lighter or heavier than air determines to a large extent the method of solving ventila tion problems Vapor pressure xs the pressure measured in pounds per square inch absolute (psia) exerted by a volatile liquid, as determined by the "Standard Method of Test for Vapor Pressure of Petroleum Products (Reid Meth od)," ASTM D323-58 Volatility is the tendency or ability of a liquid to vaporize Such liquids as alcohol and gasolme, because of their well known tendency to evaporate rapidly, are called volatile liquids Oxygen deficiency designates an atmos phere having less than the percentage of oxygen found in normal air Normal air contains about 21 per cent oxygen at at mospheric pressure 'When the oxygen concentration in air is reduced to approximately 18 per cent, many individuals become dizzy, experience a buzz ing in the ears, and have a rapid heart beat In addition to tests for toxicity,* the oxy gen content of the atmosphere of a tank or similarly confined space should be deter mined by pre-entry and subsequent tests made with instruments approved for the purpose by the U S Bureau of Mines No one should enter or remain m a tank or enclosed space that tests show has less than 18 per cent oxygen in its atmosphere at any time unless he wears approved re spiratory protective equipment, such as a fresh-air hose mask or self-contained or self generating breathing apparatus Self-contained compressed air breathing apparatus, approved by the U S Bureau of Mines, have proved satisfactory in oxygen deficient atmospheres They are especially useful where it is difficult to run a hose line Propagation of flame is the spread of flame through the body of the flammable vapor-air mixture independently of the source of ignition A vapor-air mixture be low the lower flammable limit may bum at the point of ignition without propagat ing (spreading) away from ignition source *U S Bureau of Mmes Mine Cases and Methods for Their Detection Circular No 83 .32-3