limit [LEL]) is the minimum concentration (percent by volume) of the fuel (vapor) in air at which a flame is propagated when an ignition source is present. The upper flammable limit (upper explosive limit [UEL]) is the maximum concentration (percent by volume) of the vapor in air above which a flame is not propagated. The flammable range (explosive range) consists of all concentrations between the LEL and the UEL. This range becomes wider with increasing temperature and in oxygen-rich atmospheres and also changes depending on the presence of other components. The limitations of the flammability range, however, provide little margin of safety from the practical point of view because, when a solvent is spilled in the presence of an energy source, the LEL is reached very quickly and a fire or explosion ensues before the UEL is reached.

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FIGURE 4.3 The fire triangle.

4.D.1.3 Classes of Flammability

Several systems are in use for classifying the flammability of materials. Some (e.g., Class I—flammable

TABLE 4.4 NFPA Fire Hazard Ratings, Flash Points (FP), Boiling Points (bp), Ignition Temperatures, and Flammable Limits of Some Common Laboratory Chemicals

NFPA Flammability Ratinga Flash Point (ºC) Boiling Point (ºC) Ignition Temperature (ºC) Flammable Limits (% by volume)
Lower Upper
Acetaldehyde 4 −39 21 175 4 60
Acetic acid (glacial) 2 39 118 463 4 19.9
Acetone 3 −20 56 465 2.5 12.8
Acetonitrile 3 6 82 524 3 16
Carbon disulfide 4 −30 46 90 1.3 50
Cyclohexane 3 −20 82 245 1.3 8
Diethylamine 3 −23 57 312 1.8 10.1
Diethyl ether 4 −45 35 180 1.9 36
Dimethyl sulfoxide 2 95 189 215 2.6 42
Ethyl alcohol 3 13 78 363 3.3 19
Heptane 3 −4 98 204 1.05 6.7
Hexane 3 −22 69 225 1.1 7.5
Hydrogen 4 −252 500 4 75
Isopropyl alcohol 3 12 83 399 2 12.7 @ 200 (93)
Methyl alcohol 3 11 64 464 6 36
Methyl ethyl ketone 3 −9 80 404 1.4 @ 200 (93) 11.4 @ 200 (93)
Pentane 4 <−40 36 260 1.5 7.8
Styrene 3 31 146 490 0.9 6.8
Tetrahydrofuran 3 −14 66 321 2 11.8
Toluene 3 4 111 480 1.1 7.1
p-Xylene 3 25 138 528 1.1 7

   a0, will not burn under typical fire conditions; 1, must be preheated to burn, liquids with FP ≥ 93.4 °C (200 °F); 2, ignitable when moderately heated, liquids with FP between 37.8 °C (100 °F) and 93.4 °C (200 °F); 3, ignitable at ambient temperature, liquids with FP < 22.8 °C (73 °F), bp ≥ 37.8 °C (100 °F) or FP between 22.8 °C and 37.8 °C (100 °F); 4, extremely flammable, readily dispersed in air, and burns readily, liquids with FP < 22.8 °C (73 °F), bp < 37.8 °C (100 °F).
SOURCE: Adapted with permission from Fire Guide to Hazardous Materials (13th Edition), Copyright © 2001, National Fire Protection Association.



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