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.
4.D.1.3 Classes of Flammability
Several systems are in use for classifying the flammability of materials. Some (e.g., Class I—flammable
|NFPA Flammability Ratinga||Flash Point (ºC)||Boiling Point (ºC)||Ignition Temperature (ºC)||Flammable Limits (% by volume)|
|Acetic acid (glacial)||2||39||118||463||4||19.9|
|Isopropyl alcohol||3||12||83||399||2||12.7 @ 200 (93)|
|Methyl ethyl ketone||3||−9||80||404||1.4 @ 200 (93)||11.4 @ 200 (93)|
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.