Flashover
Encyclopedia
A flashover is the near simultaneous ignition of most of the directly exposed combustible material in an enclosed area. When certain organic materials are heated they undergo thermal decomposition
and release flammable gases. Flashover occurs when the majority of the exposed surfaces in a space are heated to their autoignition temperature
and emit flammable gases (see also flash point
). 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/ft²*s (20 kW/m²).
An example of flashover is when 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 directly exposed combustible materials in the room, causing them to give off flammable gases via pyrolysis
. When the [surface] temperatures of the evolved gases becomes high enough, these gases will ignite, throughout their extent.
s. Firefighters are taught to recognize rollovers [flashovers] and avoid backdrafts. For example, they have certain routines for opening a closed door in a building on fire, such as positioning adjacent to [sitting beside] the door instead of in front of it, and to be ready to fight any advent of shooting flames.
A firefighter has about 2 seconds to evacuate a flashover environment, even if wearing proper NFPA
approved gear.
Firefighters memorize a chant to help remember these during training: "Thick dark smoke, high heat, rollover, free burning."
The color of the smoke is often considered, as well, 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.
Thermal decomposition
Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes....
and release flammable gases. Flashover occurs when the majority of the exposed surfaces in a space are heated to their autoignition temperature
Autoignition temperature
The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion...
and emit flammable gases (see also flash point
Flash point
The flash point of a volatile material is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a flash point requires an ignition source...
). 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/ft²*s (20 kW/m²).
An example of flashover is when a piece of furniture
Furniture
Furniture is the mass noun for the movable objects intended to support various human activities such as seating and sleeping in beds, to hold objects at a convenient height for work using horizontal surfaces above the ground, or to store things...
is ignited in a domestic room. The fire involving the initial piece of furniture can produce a layer of hot smoke
Smoke
Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...
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
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
from this layer heats the surfaces of the directly exposed combustible materials in the room, causing them to give off flammable gases via pyrolysis
Pyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
. When the [surface] temperatures of the evolved gases becomes high enough, these gases will ignite, throughout their extent.
Types of gaseous 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 (sometimes called rolloverRollover (fire)Rollover is a stage of a structure fire when fire gases in a room or other enclosed area ignite. Since heated fire gases, the product of pyrolysis, rise to the ceiling, this is where a rollover phenomenon is most often witnessed...
) 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 (ie: lean). - A rich flashover occurs when the flammable gases are ignited while at the upper region of the flammability range (ie:rich). 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. Such an event is known as 'backdraftBackdraftA backdraft is an explosive event at a fire resulting from rapid re-introduction of oxygen to combustion in an oxygen-starved environment, for example, the breaking of a window or opening of a door to an enclosed space. Backdrafts present a serious threat to firefighters, even those with a high...
'. - A delayed flashover occurs when the colder gray smoke cloud ignites after congregating outside of its room of origin. This results in a volatile situation, and if the ignition occurs at the ideal mixture, the result can be a violent smoke gas explosion. This is referred to as 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 (non-cooiing?) air it can spontaneously ignite, and the resultant flame can propagate back into the compartment, resulting in an event similar to a rich flashover. The common 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 firefighterFirefighter
Firefighters are rescuers extensively trained primarily to put out hazardous fires that threaten civilian populations and property, to rescue people from car incidents, collapsed and burning buildings and other such situations...
s. Firefighters are taught to recognize rollovers [flashovers] and avoid backdrafts. For example, they have certain routines for opening a closed door in a building on fire, such as positioning adjacent to [sitting beside] the door instead of in front of it, and to be ready to fight any advent of shooting flames.
A firefighter has about 2 seconds to evacuate a flashover environment, even if wearing proper NFPA
National Fire Protection Association
The National Fire Protection Association is a United States trade association that creates and maintains private, copywrited, standards and codes for usage and adoption by local governments...
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 of oxygen in the room.
- The neutral plane is moving down towards the floor. In this situation, a flashover is plausible.
- All directly exposed combustible materials are showing signs of pyrolysisPyrolysisPyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
.
- "RolloverRollover (fire)Rollover is a stage of a structure fire when fire gases in a room or other enclosed area ignite. Since heated fire gases, the product of pyrolysis, rise to the ceiling, this is where a rollover phenomenon is most often witnessed...
" or tongues of fire appear (known as "Angel Fingers" to firefighters).
- There is a rapid buildup of heat given off by the rapidly burning (ie: deflagrating) gases. This is generally the best indication of a flashover.
Firefighters memorize a chant to help remember these during training: "Thick dark smoke, high heat, rollover, free burning."
The color of the smoke is often considered, as well, 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
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
mattress
Mattress
A mattress is a manufactured product to sleep or lie on, consisting of resilient materials and covered with an outer fabric or ticking. In the developed world it is typically part of a bed set and is placed upon a foundation....
factory
Factory
A factory or manufacturing plant is an industrial building where laborers manufacture goods or supervise machines processing one product into another. Most modern factories have large warehouses or warehouse-like facilities that contain heavy equipment used for assembly line production...
in London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
in 1975 which produced white smoke. The white smoke wasn't considered dangerous, so firefighters decided to ventilate
Ventilation (firefighting)
In firefighting, ventilation is an important part of structural firefighting tactics, and involves the expulsion of heat and smoke from fire building, permitting the firefighters more easily and safely find trapped individuals and attack the fire...
, which caused a smoke explosion and killed two firefighters. The white smoke from the pyrolysis of the rubber turned out to be extremely flammable.
External links
- Living Room Flashover Video
- Rapid Fire Progress & Flashover related fire development
- Realistic hot fire training to deal safely with flashover and backdraft
- Flashover / Backdraft training
- Photo and video of a flashover in a living room (Forschungsstelle für Brandschutztechnik (University of Karlsruhe)
- Flashover training Croatia
- Flashover during house fire in Baltimore, MD. Video taken January 2010