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Hostile Fire Events: Flashover

Hostile Fire Events: Flashover

In this “Kitchen Table Debrief” I will discuss some familiar types of instantaneous and near instantaneous combustion (also known as explosions and near-explosions), their warning signs, and some techniques for surviving these events, where they exist.

On the fireground, the major antagonists are our long-time adversaries: flashbacks and overdrafts… NO NO! I mean flashovers and backdrafts. These two hostile events are often indistinguishable at the time of the event, but are the results of very different circumstances.

I’ll also talk a little about smoke, smoke explosions, and the laws of thermodynamics.

Some Basics

In order to have a serious discussion about fire behavior, and the dangers of “Things That Go BOOM!” (otherwise known as hostile fire events), we need to have some basic information under our belt. Let’s start with some concepts and definitions:

Not everything that goes “BOOM” is an explosion. Not everything that is an explosion goes “BOOM”, either.

Explosion: NFPA 921 (Guide for Fire and Explosion Investigations) defines an explosion as: “…the sudden conversion of potential energy to kinetic energy with the production and release of gasses under pressure. …Although an explosion is almost always accompanied by the production of a loud noise, the noise itself is not an essential element in the definition of an explosion. The generation and violent escape of gasses are the primary criteria of an explosion. The ignition of a flammable vapor/air mixture in a can, which bursts the can or even only pops off the lid, is considered an explosion.”

Combustion: Technically, according to the IFSTA manual (“Essentials of Firefighting IV Edition”), combustion is defined as a form of oxidation. Examples of oxidation are: rust (slow oxidation), fire (rapid oxidation), flashover (near-instantaneous oxidation) and explosion – like a backdraft (instantaneous oxidation).

Thermodynamics: There are only three laws, believe it or not, that define thermodynamics. These laws can be explained in convoluted ways, to try to impress people, or they can be explained is simple ways so that everyone understands them. Let’s go for the simple way:

1st Law: Energy can be changed from one form to another, but it cannot be created nor destroyed. The terms “energy” and “matter” are interchangeable for this law.

“So what?” you say. Well, here’s what we in the Fire Service already know: When something (matter) burns, it produces heat (energy), light (energy), and smoke (more matter in the form of carbon monoxide – CO, hydrogen cyanide – HCN, other toxic and non-toxic gasses, soot and ash). Keep that in mind because I talk about it repeatedly!

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