Smoke Explosion or Backdraft?
What is a smoke explosion? Is it the same thing as a backdraft or is it a completely different phenomenon? In one form or another I have encountered this question several times during the last week. In one case, I was asked to review a short article about an incident involving a smoke explosion that was submitted to FireRescue magazine. In another case, I was surfing the web and came across the following video titled large smoke explosion close call on firevideo.net. What happened in this incident? Was it a smoke explosion or a backdraft?
What’s in a Name?
For many years, the term smoke explosion was a synonym for backdraft. In fact, if you look up the definition of smoke explosion in the National Fire Protection Association (NFPA) 921 (2007) Guide for Fire and Explosion Investigation, it says “see backdraft“. However, smoke explosion is actually a different, and in many respects more dangerous extreme fire behavior phenomenon.
Smoke explosion is described in a number of fire dynamics texts including Enclosure Fire Dynamics (Karlsson and Quintiere) and An Introduction to Fire Dynamics (Drysdale). However, Enclosure Fires by Swedish Fire Protection Engineer Lars-Göran Bengtsson provides the most detailed explanation of this phenomenon. Paraphrasing this explanation:
A smoke or fire gas explosion occurs when unburned pyrolysis products and flammable products of combustion accumulate and mix with air, forming a flammable mixture and introduction of a source of ignition results in a violent explosion of the pre-mixed fuel gases and air. This phenomenon generally occurs remote from the fire (as in an attached exposure) or after fire control.
In some cases, the fire serves as a source of ignition as it extends into the void or compartment containing the flammable mixture of smoke(fuel) and air. This was the case in Evanston, Wyoming, where two firefighters died as the result of a smoke explosion in a two-story wood frame townhouse (see National Institute for Occupational Safety and Health (NIOSH) Report F2005-13). In other cases, firefighters may unintentionally provide the source of ignition. On 26 March 2008, a Los Angeles City firefighter was killed when he attempted to force entry into an electrical room filled with smoke from a manhole fire in the adjacent street. (see LAFD News and Information). Battalion Chief John Miller, Commanding Officer of the LAFD Arson/Counter-Terrorism Section reported:
This combustible smoke accumulated in the confined area of the electrical room. When Firefighter Lovrien attempted entry into the room, a spark was generated when the composite blade of the rotary saw struck the locking mechanism of the doorâ€¦ Investigators have concluded that unburned combustible gases, from a fire in the electrical vault located in the street at the front of the building, accumulated in the electrical room. These products of combustion reached its explosive limit and was ignited by a spark from the forcible entry attempts
Conditions Required for a Smoke Explosion
The risk of a smoke explosion is greatest in compartments or void spaces adjacent to, but not yet involved in fire. Infiltration of smoke through void spaces or other conduits can result in a well mixed volume of smoke (fuel) and air. Smoke explosion creates a significant overpressure as the fuel and air are premixed and ignition results in a very large energy release. Several factors influence the violence of this type of explosion:
- The degree of confinement (more confinement results in increased overpressure)
- Mass of premixed fuel and air within the flammable range (more premixed fuel results in a larger energy release)
- How close the mixture is to a stoichiometric concentration (the closer to an ideal mixture the faster the deflagration)
Potential Smoke Explosion Indicators
It is very difficult to predict a smoke explosion. However, the following indicators point to the potential for this phenomenon to occur:
- Ventilation controlled fire (inefficient combustion producing substantial amounts of unburned pyrolysis products and flammable products of incomplete combustion)
- Relatively cool (generally less than 600o C or 1112o F) smoke
- Presence of void spaces, particularly if they are interconnected
- Combustible structural elements
- Infiltration of significant amounts of smoke into uninvolved compartments in the fire building or into exposures
Smoke Explosion and Backdraft
A smoke explosion requires a relatively cool mixture of fuel (smoke) and air within its flammable range to come into contact with a source of ignition. On the other hand a backdraft requires introduction of air to an hot, extremely ventilation controlled fire where the concentration of gas phase fuel (smoke) is high and oxygen concentration is low. Both result in an explosion, but the initiating event and indicators that may be observed by firefighters and fire officers are considerably different.
Have another look at the video and see what you think: Smoke explosion or backdraft? Remember that both of these phenomena can occur in a building, a compartment, or even a small void space. Look closely at the building, smoke, air track, heat, and flame (B-SAHF) indicators. Check CFBT-US Resources more information on extreme fire behavior and reading the fire.
Ed Hartin, MS, EFO, MIFireE, CFO