Backdraft
Backdraft
Backdraft
explosion: fuel, oxygen and heat. The fuel in a combustion engine explosion driving an
automobile is gasoline; the fuel in a backdraft explosion is smoke.
Fire protection engineers define the term explosion as an "effect" produced by a sudden
violent expansion of gases. Some "effects" of an explosion are shock waves, which can shatter
windows, blow down firefighters and collapse walls.
Differences
Firefighters sometimes confuse the terms backdraft and flashover. These two dangerous
violent events are different and knowing these differences helps us understand each one better.
There are four main differences. First of all, backdrafts do not happen often at fires. You may
experience only one or two during your entire career. Flashover - sudden full room
involvement in flame - happens often. You will probably see one at your next fire.
A second difference is that a backdraft is an explosion; a flashover is not. There will be
shock waves during a backdraft that will break the confining structure around the explosion.
Windows may break blasts of smoke and flame may blow out a doorway or a part of the
structure may collapse. Flashover is rapid fire development, but it stops short of an explosion's
speed of chemical reaction.
A third difference is the triggering or cause of a backdraft. Air sets off the backdraft
explosion. As firefighters enter a confined smoke filled area and bring fresh air with them,
sometimes a backdraft or smoke explosion happens. The trigger or cause of a flashover is heat,
not air. The theory of flashover is that heat, which is re-radiated back into a burning room from
the ceiling and upper walls, raises the gases and furnishings in the room to the auto-ignition
temperature and triggers a flashover.
The final difference between a backdraft and flashover is the stage of fire growth in which
they occur. There are three stages to a fire: the growth stage, the fully developed stage and the
decay stage. Backdraft explosions occur when there is smoke in a confined space that is during
the first and third stage of a fire. During the growth and decay stages, smoldering can take
place and generate explosive CO gas. Flashover, on the other hand, only occurs in the growth
stage of a fire and signals the end of the growth stage.
Investigation of Explosions
The term backdraft is used too frequently by the fire service. It is usually used to describe
any explosion that occurs at a fire; when actually a backdraft is only one type of explosion and
it does not happen that often. In fact, most explosions are not backdrafts. They are explosions
caused by leaking gas piping, meters or cylinders, or they are BLEVES of gas cylinders or
containers or they are explosions caused by flammable vapors leftover from an arsonist's
accelerant. Before a fire investigator declares an explosion at a fire to be a backdraft-smoke
explosion, a post-fire analysis must rule out all other possibilities. . If the gas piping is intact, if
there are no ruptured containers found and there are no traces of an accelerant residue, then the
explosion is a backdraft.
There are two important facts which firefighters should know about backdrafts or any other
type of explosion. One is that a room or fire area requires only 25 percent of its space to
contain the explosive mixture for the entire area to explode.
concentration of CO and heat are in one corner of a large, smoke filled room, the entire area
could explode when firefighters enter to search and allow fresh air to enter with them. The
other fact firefighters should know is that it does not take much explosive pressure in a
confined space for an explosion to cause destruction and death.
Glass shattering
0-5 psi
1-2 psi
2-3 psi
7-8 psi
15 psi
35 psi
50% fatalities
50 psi
99% fatalities
65 psi
psi
Quenching the superheated confined fire area is another safety and survival tactic
firefighters can use to prevent backdrafts. Before a room is entered, charged hose-line should
be positioned near the entrance. Firefighters in full protective equipment should immediately
discharge a hose stream into a fire area when it is opened up. This water can cool a potentially
explosive atmosphere. Before the air and searching firefighters enter a burning, confined,
potentially explosive fire area, the stream of a powerful water discharge might break up the
explosive atmosphere. This is not as effective as roof venting, but sometimes it is the only
alternative.
Flanking
When there can be no venting and the quenching of a quick dash of a hose stream is not
possible, firefighters can protect themselves from a backdraft explosion in some instances by
flanking a doorway to a burning room operating hose-lines. The officer in command can order
two hose-lines into position, one on each side of a door or window of a burning store, which is
suspected, of exploding. After the hose lines are charged with water and firefighters are in full
protective equipment, the front store glass door or window is broken. Both flanking hose lines,
safely out of the path of any potential explosive blast coming out of the opening, can be
directed into the burning store.
Lessons Learned
Warning signs of backdraft explosions must be taught to firefighters. They are, reversal of
air pulling smoke back into a smoke filled doorway, black smoke pushing out around a closed
door or window frames and glass windows, stained with smoke condensation and pulsating
from the pressure of the fire. These warning signs are important to know, but even more
important, firefighters must know that explosions happen fast, sometimes even too fast for
firefighters to take cover and protect themselves. The only real protection from the blast of a
backdraft or a flashover is full protective gear: helmets, hoods, gloves, boots, bunker pants,
coat and face mask. Protective fire gear may be hot, cumbersome and slow you down, but if
you are caught in an explosion, it will determine whether you survive the blast and how serious
your burns will be.
Questions:
1. Which one is not a difference between a backdraft and flashover?
A. Backdraft explosions do not happen as freqently
B. The trigger for a backdraft explosion is heat