Flash point (FP) is the temperature, as determined by test, at which a liquid (or solid) emits sufficient vapor to form combustible mixtures with air. The FP may be estimated from vapor pressure data for pure species. The FP of mixtures are best determined using any of several test methods. An extensive listing of FP data is given in NFPA 325M.
The autoignition temperature (AIT) is the lowest temperature at which a mixture of a flammable gas and air will spontaneously ignite. The AIT, determined at one atmosphere pressure, for many gases and vapors is reported in NFPA 325M. Flammable gases in closed vessels that may become pressurized may exhibit AITs lower than reported in NFPA 325M.
The ignition temperature of a dust cloud is designated Tc and is the test temperature at which a dust cloud will ignite. Dust cloud suspensions in test systems are short lived due to settling. Values of Tc are useful in making relative ratings of dust ignition hazard potential. A standard test method for measuring Tc is presently in development by ASTM Committee E-27.
The ignition temperature of a dust layer is designated Ti (Bartknecht 1989; Nagy and Verakis 1983) Ti is the test temperature that leads to sufficient self heating of the dust layer sample to cause ignition. Dusts accumulated in layers and exposed to hot surroundings or surfaces can ignite at temperatures well below the Tc values. This fact arises from slow oxidation reactions that the dust may undergo when heated. The insulating properties of dust layers may retard the cooling necessary to prevent self-heating to the point of incandescence and ignition. Methods for measuring TI for both surface layer ignition (Miron and Lazzara 1988) and ignition due to uniform heating are employed.
Dust and vapor clouds maybe readily ignited if exposed to electric discharges that exceed the minimum ignition energy (MIE) (ASTM E 582) for the combustible mixture. The energies of sparks that are capable of igniting gas mixtures are usually very much smaller than those required to ignite dust suspensions.
A number of solid surfaces can act as ignition catalysts in process equipment. Catalytic surfaces, which act to lower the activation energy for key elementary combustion reactions, can result in apparent AITs much lower than for “clean” surface test chambers. Ferric oxide and iron sulfide have been shown to lower AIT values of a number of organic compounds (Bodurtha 1980).