Fire Investigation Definition Series – Flashover

Posted: June 4, 2013 in Fire Investigation
Tags: , , ,

by: Jason A. Sutula

house fire

I was recently at a conference speaking with several individuals who were not fire investigators and did not work within the field of Fire Science or Fire Protection Engineering. The conversation eventually turned to the type of work that I do for a living, which led to several stories that I enjoy telling related to past fire investigations. Judging by each individual’s reaction at the end of the conversation, I could not help but wonder if I had instilled a new-found fear related to the speed at which a residential fire will spread throughout a modern family or living room.

In particular, the group I was with was surprised when I casually broke out my favorite metaphor that the polyurethane foam stuffed couches, love-seats, and easy chairs found in their respective living rooms were nothing more than large blocks of solidified gasoline. That statement certainly got their attention and opened their eyes to some understanding of why fire injuries and deaths continue to occur, and why the amount of time available to safely escape from a residential fire has decreased substantially over the last 30 years.

As I continued my conversation, I realized that I have developed a bad habit of using many “terms of the art” that other fire investigators would immediately understand, but people not associated with the field might have a hard time picking up on the meaning. I decided at that point that my next series of blog articles would cover some of these fire investigation terms in hopes of bringing further understanding. This article will kick off the fire investigation definition series and will begin with the phenomenon of “Flashover”.

One of the main fire phenomenon responsible for shortening the amount of time available to safely escape from a residential fire is the phenomenon of Flashover. In short, Flashover is a transition from a local fire in a compartment (e.g., your living room) into a fully developed fire that encompasses the entire room and all of its contents. NFPA 921: Guide for Fire & Explosion Investigations, 2011 Edition defines Flashover as, “A transition phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach ignition temperature more or less simultaneously and fire spreads rapidly throughout the space, resulting in full room involvement or total involvement of the compartment or enclosed space.” This definition can be broken into three basic parts: a compartment, a fire, and time.

For a fire to transition to Flashover, a fire must first be initiated in a compartment. Compartments are readily found in all structures and can include full rooms, cabinets, bureaus, closets, wall cavities, etc. A compartment provides a means by which to trap the energy released by the fire. The trapped energy will allow for the overall temperature of the gases in the compartment to increase and for the heat transfer through radiation to increase as well.

The fire itself is an important component. As a fire grows and spreads over the surface of a fuel package in the room (e.g., your living room couch), the rate of energy released by the fire increases exponentially. With the growth of the fire accelerating, the trapped hot gases in the compartment rise to a critical temperature. Additionally, the hot gases radiate energy toward the unburned fuel packages (e.g., other furniture, carpet, drapes, etc.), which raises the surface temperatures of those fuel packages closer and closer to a temperature where they will begin to burn. The critical gas temperature from the fire research literature that most scientists refer to for the onset of Flashover is approximately 1,112 °F (600 °C). Once the critical temperature and radiant energy levels are reached, all of the remaining fuel packages ignite throughout the entire compartment over a very short amount of time. The entire compartment is filled with fire and hot gases, which will expand rapidly through any opening from the compartment to adjacent rooms or to the outside of the building.

There are other variables that come into play with whether or not a particular fire in a compartment will achieve Flashover, such as the amount of oxygen available in the room (i.e., ventilation, window and door openings) and whether or not the fire growth is interrupted by fire fighting efforts or a sprinkler system activation. With enough of the final component, time, the transition will occur, and the after effects can be extremely dangerous to occupants trapped in the residence as well as fire service personnel. A future blog post will explore the dangers associated with Post-Flashover conditions.

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Comments
  1. Hola! I’ve been following your blog for a while now and finally got the bravery
    to go ahead and give you a shout out from Humble Texas! Just wanted to say keep up the great work!

    Like

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