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Flashover
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A flashover is the near simultaneous ignition of all combustible material in an enclosed area when the majority of surfaces in a space are heated to the temperature at which the flammable gases that are being produced from the combustible materials in the space are hot enough to ignite. Flashover normally occurs at 500 °C (930 °F) or 1,100°F for ordinary combustibles, and an incident heat flux at floor level of 1.8 Btu/s*ft2 (20 kW/m2).
An example of flashover is a situation in which a piece of furniture is ignited in a domestic room.
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Encyclopedia
A flashover is the near simultaneous ignition of all combustible material in an enclosed area when the majority of surfaces in a space are heated to the temperature at which the flammable gases that are being produced from the combustible materials in the space are hot enough to ignite. Flashover normally occurs at 500 °C (930 °F) or 1,100°F for ordinary combustibles, and an incident heat flux at floor level of 1.8 Btu/s*ft2 (20 kW/m2).
An example of flashover is a situation in which a piece of furniture is ignited in a domestic room. The fire involving the initial piece of furniture can produce a layer of hot smoke which spreads across the ceiling in the room. The hot buoyant smoke layer grows in depth, as it is bounded by the walls of the room. The radiated heat from this layer heats the surfaces of the combustible materials in the room, causing them to give off flammable gases via pyrolysis. When the surface temperatures become high enough, these gases ignite. The resulting flame spread across all combustible surfaces in the room is very rapid, and can engulf the entire space in fire within a few seconds.
Different types of flashover
The original Swedish terminology related to the term 'flashover' has been altered in its translation to conform with current European and North American accepted scientific definitions as follows:
- A lean flashover (accepted scientific terminology is rollover) is the ignition of the gas layer under the ceiling, leading to total involvement of the compartment. The fuel/air ratio is at the bottom region of the flammability range.
- A rich flashover occurs when the flammable gases are ignited while at the upper region of the flammability range. This can happen in rooms where the fire subsided because of lack of oxygen. The ignition source can be a smouldering object, or the stirring up of embers by the air track. The internationally accepted scientific definition of such an event is known as 'backdraft'.
- A delayed flashover occurs when the colder gray smoke cloud ignites after congregating outside of its room of origin. The results can be very unpredictable, and if the ignition occurs at the ideal mixture, the result can be a violent smoke gas explosion. The internationally accepted scientific terms for this process are either smoke explosion or fire gas ignition depending on the severity of the combustion process.
- A hot rich flashover occurs when the hot smoke with flammable gas ratio above the upper limit of flammability range and temperature higher than the ignition temperature leaves the compartment. Upon dilution with air it spontaneously ignites and the resultant flame can propagate back into the compartment, resulting in an event similar to a rich flashover. The internationally accepted definition of this process is known as auto-ignition which is another form of fire gas ignition.
Dangers of flashovers
Flashover is one of the most-feared phenomena among firefighters. Firefighters are taught to recognize flashovers and avoid backdrafts. For example, they have certain routines for opening a closed door in a building on fire, such as sitting beside the door instead of in front of it, and to be ready to fight shooting flames.
Despite superior protective gear, a firefighter has less than 5 seconds to evacuate a flashover environment if wearing proper NFPA approved gear.
Flashover indicators
The following are some of the signs that firefighters are looking for, when they attempt to determine whether a flashover is likely to occur.
- The fire is in a ventilated compartment, so there is no shortage oxygen in the room.
- The neutral plane is moving down towards the floor. In this situation, a flashover is plausible.
- All exposed combustible materials are showing signs of pyrolysis.
- "Rollover" or tongues of fire appear (known as "Angel Fingers" to firefighters).
- There is a rapid build up of heat. This is generally the best indication of a flashover.
Firefighters memorize a chant while in training "Thick dark smoke, high heat, rollover, free burning." So they memorize the four signs of a flashover.
The color of the smoke is often considered too, but there is no connection between the color of the smoke and the risk of flashovers. Traditionally black, dense smoke was considered particularly dangerous, but history shows this to be an unreliable indicator. For example, there was a fire in a rubber mattress factory in London in 1975, which produced white smoke. The white smoke wasn't considered dangerous, so firefighters decided to ventilate, which caused a smoke explosion and killed two firefighters. The white smoke from the pyrolysis of the rubber turned out to be extremely flammable.
See also
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