Ignition of Gases, Vapors, and Liquids by Hot Surfaces

Gases, vapors, liquid sprays, aerosols and other forms of ignitable fluids dispersed into the atmosphere, under certain circumstances, may encounter a hot surface. When investigating a fire, it may be necessary to determine in such cases if the hot surface was a competent ignition source. The paper reviews the available experimental data and findings on this topic and gives appropriate advice. It is shown that, unlike the autoignition temperature (AIT), which is only slightly dependent on test conditions, the hot-surface ignition temperature (HSIT) is highly dependent on the test environment conditions. The primary variable affecting the outcome is the degree of 'enclosedness.' If the degree of enclosedness is not extreme, a standard recommendation is that the hot-surface ignition temperature might be assumed to be 200 degrees C higher than the AIT. But for conditions of significant enclosedness, the actual ignition temperature is more influenced by the fuel's volatility (which is related to its flash point) than its AIT value. Higher volatility fuels are harder, not easier, to ignite from a hot surface. Since gasoline is the most volatile of the common automotive-use ignitable liquids, it turns out to be the one which is the hardest to ignite by a hot surface. Nonetheless, in some cases, vehicular engine compartment temperatures can become high enough for gasoline to get ignited. When conducting HSIT tests, it is important to be cognizant of the probabilistic nature of the ignition problem.