Observations on Two Fires in Which the Spill of Flammable Liquids Led to Deflagration and/or Flash Fire in a Stratified Heavy Vapor/Air Mixture
Giovanni Cocchi, P.E., Ph.D.
ARSON Fire, Safety and Environmental Investigations S.r.l., Italy
Presented at the International Symposium on Fire Investigation Science and Technology, 2014
The most common kind of diffuse phase explosions investigated by fire investigators involve methane or LPG, either accidentally or willingly allowed to accumulate in the confined volume of the room(s) of a building. Many volatile ignitable liquids can form a flammable vapor/air mixture when spilled on the floor and let evaporate for enough time. Upon ignition, flash fire or deflagration will take place, possibly followed by the development of a compartment fire. This work reviews the relevant literature about evaporation of volatile ignitable liquids, heavy gas dispersion and propagation of flame front in stratified heavy vapor/air mixtures. Subsequently, two cases in which the author was asked to provide its technical opinions will be discussed. The first one is a massive semi-confined deflagration in a large storage room that eventually vented through the weakest brick wall of the building. The building was not equipped with natural gas lines and no LPG can was retrieved at the scene. Subsequent fire debris analysis demonstrated the use of volatile accelerants. The second one is a flash fire that was caught on CCTV camera. Around four minutes before, a person was taped while pouring a liquid…After the flash fire, the puddles of liquid of what was intended to be a trailer were observed to burn as pool fires, until smoke obscured the camera. The flame front of the flash fire shows all the relevant features that other authors have previously shown to be peculiars of flash fires in a stratified heavy vapor/air mixture. Fire investigator should consider that when an explosion or a flash fire take place before fire development and natural gas or LPG sources can be legitimately ruled out, use of highly volatile accelerants should be regarded as a legitimate hypothesis and tested according to the scientific method of NFPA 921-2014 .
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