Abstract
Stages of fire behaviou from inception to decay have been described. Fire events namely flameover, flashover and backdraft have been described. Their causes and procedures to prevent and protect against them have been discussed.
Stages of Fire Behavior
The development of compartment fires is usually divided into 4 major stages, namely, the incipient or ignition stage, the growth stage, the fully developed stage and the decay stage. The graph in figure 1 represents these stages on a heat release rate vs. time diagram. The descriptions below are mainly as explained in ‘Fire development and fire behavior indicators’.
Figure 1: Stages of development of compartment fires
Incipient/ Ignition stage
This is the stage that begins with ignition when requirements of heat, fuel, and oxygen are met. At this stage, fire is largely fuel controlled. Radiant heat warms adjacent fuel and a plume of hot gases and flame rises from the fire and mixes with the cooler air within the room. Overall temperature of the room increases. As this plume reaches the ceiling, hot gases begin to spread horizontally across the ceiling. When the layer of hot gases becomes more clearly defined and increases in volume the fire has moved beyond its incipient stage.
Growth stage
In presence of adequate oxygen, the fire proceeds to growth stage. As the heat release rate increases, fire continues to grow through flame spread or by ignition of other fuel. 2 temperature layers are developed – a hot layer near the ceiling and a cold layer, towards the floor. As the fire moves further into the growth stage, the dominant heat transfer mechanism shifts from convection to radiation.
Fully developed stage
In presence of adequate fuel and oxygen, the fire continues to grow and flashover occurs, whence the fire is said to be in fully developed stage. When flashover occurs, there is a rapid transition to a state of total surface involvement of all combustible material within the compartment.
Decay
When the average temperature in the compartment falls to approximately 80% of the peak value, the decay stage is said to have begun, as fuel limitation sets in.
Events of Fire Development and Their Causes
3 distinct, but fatal events that might occur during the development of fire are flameover, flashover and backdraft. The definitions and causes of these events are presented below based on an extensive and authoritative literature survey by Gorbett & Hopkins (2007).
Flameover or rollover
“The condition where unburned fuel (pyrolysate) from the originating fire has accumulated in the ceiling layer to a sufficient concentration (i.e., at or above the lower flammable limit) that it ignites and burns” is called a flameover. It can even occur without ignition and prior to the ignition of other fuels separate from the origin.
For flame over to occur, unburned fuel must be accumulated in the upper layer which may be the case with an underventilated compartment fire. Secondly, fresh air entrained from the compartment to the bottom of the upper layer must be mixed with this fuel to heat it up to the auto ignition temperature.
Flash over
“A transitional phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach ignition temperature more or less simultaneously and
fire spreads rapidly throughout the space resulting in full room involvement or total involvement of the compartment or enclosed area” is termed a flashover.
It is generally characterized by a ceiling temperature of 500o-600o C or the heat flux to the floor of 15-20 kW/m2. When flashover occurs, burning gases will push out openings in the compartment (such as a door leading to another room) at a substantial velocity. For flashover to occur, a minimum heat release rate is required, which can happen only when there is a sufficient supply of both fuel and oxygen.
Back draft
“Backdraft is initiated by air being admitted into a non-ideal pre-mix of gases and combustion products, but bringing it within the ideal range of flammability. When an opening is suddenly introduced, the inflowing air forms a gravity current and begins to mix with unburned pyrolysis products, creating a combustible mixture of gases in some part of the enclosure. Any ignition sources, such as a glowing ember, can ignite this combustible mixture, resulting in an extremely rapid burning of gases/pyrolysis products forced out through the opening and causes a fireball outside the enclosure”.
Procedures to Prevent and Protect against Rapid Fire Progress
Flameover happens in a fuel-controlled fire. Flashover can happen in both fuel-controlled and ventilation controlled fires. Back draft is caused in ventilation controlled fires when suddenly air is introduced. Possible causes of rapid fire progress and the tactics to counter them have been explained and summarized by Grimwood(2007).
Extreme care is required with regard to vent openings. Incorrect location or mistimed vent opening or inappropriate entry point/procedure for gaining access to structure may lead to a fatal event. Vent openings must not be created without confining the fire or laying a charged primary attack hose-line. A super-heated fuel rich smoke layer needs cooling before venting. A heavy pre-mix layer of smoke with a suppressed fire needs removing (tactical venting) before overhaul, or disturbing hot spots. 3D Tactical Door Entry Procedures, confinement of the fire to room of origin by closing doors, getting sufficient water on the fire as quickly as possible, getting water into the gas layers as quickly as possible, tactical Ventilation (under strict protocols) and anti-ventilation may also be important fire tactics in case of vent controlled fires.
If rollover occurs, firefighters must take immediate countering actions, or consider moving away to safety. In case of an unshielded fire, a direct hit at the base of the flames could be useful although the dormant gaseous phase at the ceiling would still have to be tackled. If the fire's base is shielded then tactics such 3D water-fog applications, penciling and surface 'painting' to cool the wall linings must be used to tackle gaseous fire development directly.
References
1. Gorbett, G. E., & Hopkins, R. (2007, June 4). The Current Knowledge & Training Regarding Backdraft, Flashover, and Other Rapid Fire Progression Phenomena. Presentation at the National Fire Protection Association World Safety Conference . Boston.
2. Grimwood, P. (2007). Flashover and related phenomena. Retrieved June 14, 2012, from firetactics.com: http://www.firetactics.com/FLASHOVER_answer.htm
3. Hartin, E. (n.d.). Fire Development and Fire Behavior Indicators. Retrieved June 14, 2012, from http://cfbt-us.com: cfbt-us.com/pdfs/FBIandFireDevelopment.pdf
