Fire Investigation Definition Series – Backdraft

Posted: June 12, 2013 in Fire Investigation
Tags: , , ,

by: Jason A. Sutula

firefighters

The movie Backdraft opened in 1991 while I was still in high school. I will admit that there was something about that movie that peaked my interest in fire and fire investigation. I cannot give the movie full credit for why I eventually chose the fields of Fire Protection Engineering and Fire Investigation, but it definitely deserves some credit as an influence in my life. I have vivid memories of watching the fire move throughout the movie as if it was alive and with a very devious personality. Only a few short years later, during my education in the Department of Fire Protection Engineering, did I discover that many of those wonderful scenes were far from the reality of fire behavior and were just the product of some amazingly creative camera work.

Backdraft does do a great job of reiterating just how dangerous a fire can be, both to occupants of a structure involved in a fire as well as the fire department personnel who risk life and limb to perform rescue and extinguishment operations. Unfortunately, the movie does not do a great job of portraying or explaining just what a “backdraft” is.

The term backdraft is defined in NFPA 921: Guide for Fire & Explosion Investigations, 2011 Edition as, “A deflagration resulting from the sudden introduction of air into a confined space containing oxygen-deficient products of incomplete combustion.” We can examine this definition by starting with the basic components of a fire. We need something to burn (a fuel), a supply of oxygen (air), and enough heat energy to allow for the combustion process to begin and then continue. If you are missing one of these basic components, you will have no fire.

A backdraft, then, is nothing more than a specialized type of fire. To produce conditions that will allow for a backdraft to develop, you need a confined space (in a residential fire, typically this will be a room) to hold all of the components of the fire. If a fire starts in our hypothetical room and cannot get enough oxygen (imagine a fire burning in a room with the windows and doors completely closed), unburned fuel will escape the fire and fill the room. Once a door or window is then opened to this room, colder fresh air containing oxygen will flow into the room through the bottom of the opening while the hotter combustion products from the fire will flow out the top of the opening. Where these two flows meet, a flammable mixture of fuel and oxygen will develop in the room. The mixed zone will grow and spread into the room and eventually reach the area where the small fire was originally located. At that place in the room, all three conditions for fire are present, and the result is an extremely rapid ignition and spread of flame from the point of origin out toward the opening of the room.

The speed at which the burning flame travels is much more rapid than the speed at which an individual can react to or move away from the fully engulfing flames. Fire service personnel tasked with rescue operations are particularly vulnerable to backdrafts as they open doors within structures to look for possible victims. A tragic example of this phenomenon was reported in a case study report put out by the Building and Fire Research Laboratory (BFRL) at the National Institute of Standards and Technology (NIST) where the lives of two District of Columbia firefighters were claimed by fire in a townhouse at 3146 Cherry Road NE, Washington D.C. The report clearly shows the dangers associated with under-ventilated fires and also demonstrates that these types of fires can be modeled with computational fluid dynamics to better understand the hazard. For more information, the NIST report can be found by following this link: http://fire.nist.gov/CDPUBS/NISTIR_6510/6510.htm

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