Hostile Fire Events: Flashover
A flashover produces an impressive fireball, depending on the size of the compartment. This fireball, impressive though it is, is not an explosion, though it is sometimes labeled as one by some observers.
In fact, many times, even those with many years of experience are not sure if the hostile event that occurred was a flashover or a backdraft until further studies are made. There are several reasons for this, but mainly because the end results of both events are similar, even though the origins are different.
A flashover occurs because of heat build-up, pure and simple. Some would argue that there needs to be some level of oxygen starvation, too, which results in the rapid combustion and fire-ball seen in so many flashover videos. The container in the photo above has an open wall feeding fresh oxygen to the combustion.
From my observations, I believe the oxygen starvation of a super-heated compartment, followed by the oxygen contribution introduced by firefighters making an entrance, is what leads to the fireball often seen. The absence of oxygen starvation in many of these demonstrations is why, I believe, flashovers occur so rapidly. The absence of the oxygen depravation period seems to be why these demonstrations lack the fireball effect, but do experience the ceiling-to-floor fully engulfed flames.
In the past, if a flashover had occurred, it would have been within the first 20 minutes of the fire. With the increase in plastic furnishings and decorations in the modern home, and the tighter construction for heat conservation, heat rises more quickly in today’s buildings and can cause potential flashovers in about 10 minutes or less.
Many of these open-compartment demonstration fires, like in these photos I included here, result in flashovers within three or four minutes. YouTube has other videos of drills and presentations where you can observe similar tests, too. Some of these demonstration fires flash in even less time. It simply “got hot enough” inside the compartment/container, and radiant heat feedback did the rest.
Flashover only occurs at the tail end of phase one of a fire, as the fire transitions from “free burning” to the “fully developed” stage. Fire Service legend, Chief Vincent Dunn, (et al.) refers to this as the transition from a content fire to a structure fire.
There are a few signs that a container may flash: intense heat coming from the compartment, heat pushing smoke (making it appear boiling or turbulent), an intense increase in the heat in the compartment driving you to the floor (flat to the floor for a little relief), and rollover – also called flameover or vent-point ignition. Rollover is the term to describe the phenomenon of igniting gasses at the ceiling level, usually near the doorway or vent point (thus the term vent-point ignition).
They appear as tongues/snakes of fire, or bursts of flame. The temperature is hot enough to ignite the gasses, but not quite hot enough to sustain the burning of the fuel. Seeing this is a sign that the room may very well flashover.
But keep this in mind: The presence of rollover generally precedes a flashover, but does not always mean a flashover will occur within the compartment. If the compartment is particularly large, or the ceiling is high, the compartment may not flash. But then again, it might. This is when we go back to it being the fire’s choice, again…