RECOGNITION AND COMBAT OF
BACKDRAFT AND FLASHOVER CONDITIONS
ROLLOVER
The fire condition know as rollover is a dramatic display of unburned, superheated gases, that gather at the ceiling or upper levels of a fire area.
During a fire, it’s not the materials in the room that are burning, but
the flammable gases that are given off by the materials. This process,
called pyrolysis, is caused from the materials being preheated, or from
exposure to the radiant heat of an already burning fire. The gases rise to the ceiling and are pushed by expansion or thermal currents from the fire area. When these concentrated gases reach an area where sufficient ambient air mixes with them, the gases will reach their upper flammable limits, and ignite. The now burning gases rapidly expand and roll along the ceiling. This may be commonly observed above the heads of firefighters as they
proceed towards the seat of a fire.
Keep in mind that the visible burning is in fact the gases, and not the ceiling materials themselves. While it’s important to closely note the progress of the rollover to prevent the fire from extending and cutting you off from your point of entry, it’s not productive to attempt to fight the rollover outright. The rollover condition will continue until the main body or seat of the fire is extinguished. In other words, “To kill the dragon, you don’t cut
off his tail, you must cut off his head”. Proceed under the rollover and attack the seat of the fire.
FLASHOVER
The fire condition known as flashover, the simultaneous ignition of all
combustible materials in an enclosed area, occurs during the free burning phase of a fire. It’s the result of the room and it’s contents being gradually heated to their ignition temperature, (the temperature at which a material will ignite and burn without ignition source). When the ignition temperature is reached, the entire room and contents will ignite from wall to wall, floor to ceiling. This ignition is almost instantaneous. If firefighters are not familiar with, or don’t recognize the buildup of this condition, and are
caught within a flashover, the results will be devastating.
The warning signs of a pending flashover are:
- Extremely high heat within the fire room, extending possibly all the way to the floor.
- Heavy smoke banking down towards the floor.
- Room contents may show signs of melting or warping, and may be emitting smoke or vapors.
Great care should be taken to watch for the visible signs of flashover, due to the effectiveness of current personal protective equipment. Firefighters are so well protected, that the high heat conditions proceeding a flashover are not detected until the situation becomes highly dangerous, or flashes over.
Preventing Flashover
The chance of flashover occurrence can be successfully prevented or lessened, but should be done in a careful and deliberant manner. This is accomplished by directing a hose stream towards the ceiling area, and the rooms contents, cooling them to well below their ignition temperature.
This should only be done if there is the possibility of a flashover occurring, as indiscriminate application of the hose stream is an unnecessary waste of water, and caused excessive water damage. Use of the hose stream to prevent flashover should be done by using the following methods:
- Apply the hose stream in a semi-fog pattern to the contents of the room, circling the nozzle in a rapid, clockwise motion, to cool the contents below their ignition temperature.
- Direct the hose stream in a straight stream pattern towards the ceiling, in 3 to 5 second bursts.
The use of the straight steam pattern is critical, as it relates directly to the tactic of staying low in a fire. The entire purpose of staying low is to remain in the relatively cool air near the floor, below the heat and toxic conditions extending down from the ceiling. If the thermal layers are disturbed by the fog patterns tendency to push the products of combustion, or by excessive use of the straight stream, will cause the high heat, smoke, and the steam to descend upon the interior teams, subjecting them to punishing heat
conditions, and further decreasing the already minimal visibility.
BACKDRAFT
The fire condition known as backdraft, also known as smoke explosion, begins to form well into the free burning stage of a fire. It results from a free burning fire consuming nearly all the available oxygen within a confined area. This smoldering, oxygen starved fire will be reduced to glowing embers resembling the coals found in backyard grills, and continues to fill the confined area with flammable, superheated gases. If a sudden supply of fresh air is introduced into the confined area, either by improper or uncoordinated
ventilation, there will be a violent ignition of the gases, and may contain the force of a substantial bomb.
The warning signs of a potential backdraft are:
- Smoke that is pressurized exiting through small openings.
- Smoke that exits the building in puffs or intervals.
- Black smoke changes color to a grayish-yellow color, similar to the color of hay.
- Confined area of excessively high heat, with little of no visible flames.
- Black, oily, tar like substance staining the windows.
PREVENTING BACKDRAFT
The method preventing the occurrence of a backdraft is careful execution of ventilation tactics.
Vertical ventilation should be performed directly, or as near as possible, over the seat of the fire. The products of combustion exiting up through the opening will remove the high heat and flammable gases from the house, without providing a significant amount of oxygen rich air to the fire. Do not open doors or windows to supplement this ventilation, until the backdraft potential has been reduced.
Another option, such as used in backdraft situations involving aircraft, is to use a piercing type nozzle or cellar nozzle to apply a fire stream to the fire area, while still preventing a supply of fresh air to the fire. The conversion of water to steam will absorb a tremendous amount of the heat, and the steam has a smothering effect upon the fire through oxygen displacement. *(see note below)Once the backdraft hazard has been satisfactorily removed, you can resume
normal interior firefighting tactics.
* Although this oxygen displacement and steaming effect would indeed be hazardous to possible life hazards within the structure, it is highly unlikely that the extreme conditions encountered during a backdraft situation are survivable.
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