FLASHOVER
VOLUNTEERS CORNER
RECENTLY THERE has been a rash of incidents across North America involving the fire phenomenon of flashover. Unfortunately, the fire service has lost and the fire has won in most cases.
In order to discuss this event in a fire’s development, some definitions should be reviewed.
Flash Point: The temperature at which a liquid gives off enough vapor to form an ignitable air-vapor mixture. This does not involve flashover and should not be confused with it.
Ignition Temperature: The temperature to which any material (fuel) must be heated in air to initiate combustion.
Rollover: The burning of tire gases at the ceiling level as the fire builds during the initial stages, rolling across the ceiling.
Hackdraft: A rapid flash fire caused by introduction of oxygen (air) into a tight, unvented structure that has accumulated combustion gases under high heat and pressure. A hackdraft is an explosive force accompanied by a wall of flames from ceiling to floor. The energy forces given off are sufficient to cause structural damage.
Flashover: The point when the gases distilling from the room’s contents (furnishings) at ignition temperature mix in proper proportions with the airborne oxygen (air) and reach ignition temperature, causing the immediate involvement of the entire room’s contents in a free-burning fire.
The definition of flashover indicates the physical happenings of a flashover. The combustible materials in the room are heated and begin destructive distillation. giving off flammable gases. This includes such things as paneling, vinyl wallpaper, rugs, drapes, upholstered furniture, and the numerous ordinary combustibles so commonly found in homes and businesses. With this tremendous fuel load, as the heat builds it will eventually reach the ignition temperature of the gases; however, by this time everything in the room is at its ignition temperature, and not only do the gases ignite, but the entire area bursts into flames and subsequently continues to burn —in essence, a “created hell” with fire in the free-burning stage until extinguishing streams can be applied.
The critical question asked by firefighters is, “What are the signs to watch tor so we are not trapped in this possibly lethal environment?” Of course, the obvious is that the room or area will be getting very hot —rapidly. This can be difficult to detect with the new protective envelopes so necessary in firefighting. A more readily detectable circumstance is a very rapid inversion of the smoke layer from the ceiling to the floor. In other words, the smoke you were working under now suddenly drops to the floor level, engulfing you.
Now that the firefighters are in a very dangerous situation, what procedures can you follow? Get down as low as you can and get out as quickly as possible; perhaps there will be time before ignition. Use doors, windows, or any available safe exit. If the firefighters have a charged hoseline when the smoke rapidly drops, it should be immediately discharged against the ceiling. This will cool the area, create a steam blanket, and possibly reduce the chance of ignition. Because of the high heat, there will be a large increase in the volume of superheated air as the water quickly turns to steam. This could result in painful steam burns, which are the lesser of the evils when compared to entrapment in a hellish flashover.
Probably of most importance to firefighters is how to develop procedures to reduce the chances of a flashover. A department may wish to use the new heat-sensitive tapes that can be applied to protective clothing. Although visibility may be impaired, this is one means of discerning temperature buildup.
The best procedure to follow would be one of adequate ventilation. If the heat and ignitable fire gases are exhausted from the area, there is little fuel and ambient temperature rise. Ventilation can be by vertical or horizontal means as necessary for the area. (Remember, hackdraft conditions warrant vertical ventilation.) Mechanical ventilation can also be used, pushing the products out—positive pressure —before the attack, or exhausting the area by pulling the products in coordination with the attacking handline. The bottom line is ventilate and remove the problem.
A standard operating procedure outlining flashover, what to do if it is expected or encountered, and how to alleviate it should be written by every fire department.
