Kitchen Table Debriefs - Hostile Fire Events (Part II)
Edward M. Raposo
Any explosion in a burning structure is usually considered a hostile fire event. Firefighters and spectators alike hear a BOOM! and see a fire event and we all whisper, under our breath, “backdraft!” Thanks to Hollywood.
In reality, very few of these explosions are actually backdrafts. Again, I reference one of my virtual mentors- Retired Deputy Chief Vincent Dunn, FDNY. In his video, “BACKDRAFTS and SMOKE EXPLOSIONS” (Fire Engineering Books and Videos) he explains that most fireground explosions are: gasoline, natural gas, or propane ruptures (piped into the house or apartment for heat or food preparation); aerosol ruptures (aerosol cans of various sizes and uses fit this category); or arson flammable liquid explosions. He goes on to say: “Only after you have eliminated those possibilities, do you turn to a backdraft.” As Chief Dunn points out, the determination of an event to have been a backdraft is not made until after an investigation has been completed.
Backdraft or Smoke Explosion
“Did you check that door for heat, Tim?”
In the 1991 movie, “Backdraft”, this phrase is uttered by the Fire Lieutenant moments before a probationary Firefighter forces an unchecked door with disastrous results. He introduces oxygen into a superheated room and an explosive backdraft ensues. In the Fire Service, this phrase has become a joking precursor to an anticipated disastrous event.
In reality, there is nothing lighthearted, or amusing about a backdraft.
The higher the room temperature in the container/compartment gets, the less oxygen it needs to support flaming combustion. To a point. In post-flashover temperatures, and with today’s combustibles, comprised mostly of plastics or similar compounds, these temperatures are usually well over 2000 degrees F (1093’C), smoldering combustion can continue in nearly 0% oxygen!
Think about that! Oxygen levels in the room are nearly 0%, and smoldering (non-flaming) combustion continues, slowly (very slowly) driving the temperature higher… Since combustion is in less than perfect conditions, you get more gasses, soot, and turbulent smoke. This means more carbon monoxide (CO) in the smoke, and that in turn means more fuel in the smoke! CO gas has an explosive range of 12% to 74%. In other words, when the air has between 12% and 74% of CO gas, it can explode.
Even though the fire is only smoldering, pyrolysis continues and more flammable gasses, particulates and aerosols are released into the smoke cocktail. So what, exactly, is a backdraft or smoke explosion? According to NFPA 921 (Guide for Fire and Explosion Investigations), a backdraft is “an explosion resulting from the sudden introduction of air (i.e., oxygen) into a confined space containing oxygen-deficient superheated products of incomplete combustion.” In essence, oxygen is again added to the fire triangle/tetrahedron.
The NFPA 921 Guide is the definitive document on fire and explosion investigations and considered a required reference to fire investigators (NFPA 1031). This guide makes no distinction between backdrafts and smoke explosions. It also describes explosions as the “…sudden conversion of potential energy (chemical or mechanical) to kinetic energy with the production and release of gas(es) under pressure.”
The guide also defines the types of explosions as: Mechanical (ruptured gas line or oxygen tank failure), BLEVE (boiling liquid expanding vapor explosion), Chemical (reactions involving gases/vapors/ or dusts mixing with air), Combustion (deflagrations and detonations), Electrical (high-energy electrical arcs that generate sufficient heat to cause an explosion), and Nuclear (caused by fusion or fission).
Backdrafts fall under the “Combustion” type of explosion. The difference between a deflagration and a detonation is the speed of the reaction. In a deflagration, the velocity of the reaction is less than the speed of sound. In a detonation, the velocity of the reaction is greater than the speed of sound. Backdraft or smoke explosion velocities are subsonic, therefore considered deflagrations.