Fire Investigation Ignition Source Analysis: Hot Surface Ignition Temperatures by Lee K. McCarthy, P.E., IAAI-CFI, CFEI

There are several advantages for a Certified Fire Investigator (CFI) to have an engineering background when tasked with determining the origin and cause of a fire. One of these advantages is the additional knowledge and understanding of ignition. After a fire origin has been determined and the first fuel has been identified, a good fire investigator will complete a thorough analysis to identify a competent ignition source(s). NFPA 921, Guide for Fire & Explosion Investigations, outlines an overall methodology to evaluate potential ignition sources in Chapter 19 Fire Cause Analysis, of the 2014 Edition:

19.4.2 Ignition Source Analysis. The investigator should evaluate all potential ignition sources in the area of origin to determine if they are competent. A competent ignition source will have sufficient energy and be capable of transferring that energy to the fuel long enough to raise the fuel to its ignition temperature.

The analysis to determine whether or not a “potential” ignition source is actually a “competent” ignition source for a particular fuel is a complicated process that often requires understanding of engineering principles. This usually cannot be determined by simply comparing published ignition temperatures for a particular fuel (i.e. wood, gasoline, etc.) to the expected temperatures of the potential ignition source. Fire investigators that reach conclusions about ignition sources by merely looking up these published temperature values may face significant scrutiny if the opposing experts have a more thorough knowledge of the ignition process.

For example, evaluating the hot surface ignition temperature (HSIT) of a spilled liquid fuel (e.g. gasoline, diesel fuel, brake fluid, etc.) is a complex phenomenon that depends on a multitude of variables including, but not limited to, residence time on the surface; surface properties such as geometry, size, and roughness; environmental factors such as air temperature, humidity, and airflow; and the physical properties of the fluid itself, such as the initial liquid temperature, flash point, auto ignition temperature (AIT), vapor pressure, and chemical composition.  It has been established that the HSIT is often much higher than the published AIT.  However, fire investigators will often cite the AIT of liquids like gasoline when evaluating if a hot surface was a competent ignition source. This difference in ignition temperatures is largely due to the fact that when liquid fuel contacts a hot surface, heat is transferred to the liquid which cools the surface and induces a buoyant, convective flow driving the flammable vapors away from the surface; thereby reducing both the temperature of the ignition source and limiting the time the fuel is in contact with the ignition source (i.e. residence time).

Hot surface ignition is discussed in Chapter 27 Motor Vehicle Fires of the 2014 Edition of NFPA 921. The complexity and confusion associated with this topic among fire investigators is illustrated by the fact that there has been an incorrectly cited HSIT value in NFPA 921 since it was added in the 2011 Edition. The value currently listed, 670°F, is based on the AIT rather than the HSIT of 1355°F. Clearly this could result in an improper and erroneous determination of fire cause, if a hot surface can be shown to be above 670°F. Public input has been submitted this year, and the values will hopefully be corrected to more accurately reflect HSITs in future editions.

While numerous researchers have studied ignition from hot surfaces experimentally, there is not yet a fundamental theory that would clearly determine if a particular hot surface was a competent ignition source for a known liquid spill due to the number of variables involved in specific scenarios as described herein. In order to adequately evaluate potential hot surface ignition sources, a fire investigator/engineer with knowledge and experience in evaluating these scenarios should complete a thorough analysis, likely to include experimentation.

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