Week 4 Forum – Enclosure Fire Dynamics
Fire service professionals must achieve a solid theoretical knowledge of fire behavior, specifically enclosure fire behavior, to perform their duties effectively. In general, enclosure fire behavior is the study of the chemical and physical mechanisms controlling a fire within a compartment or room. Statistics and historical data prove that enclosure fires are the most dangerous to human life.
Once you complete the Week 4 readings, write a post to achieve the following:
1. Discuss the stages of an enclosure fire.
2. Define and discuss flashover, flameover, and backdraft as was stated in the Gorbett and Hopkins article.
3. List three components that control flashover, flameover and backdraft. That is three components for each.
4. List three indicators that a flashover, flameover, or backdraft may occur. That is three indicators for each.
There are four stages of an enclosure fire. The stages are ignition, growth, full room involvement and decay. An enclosure fire begins with ignition, which is the initial combustion when the three parts of the fire triangle are present, oxygen, fuel and heat. The next stage of an enclosure fire is growth. Growth occurs when the initial items burning begin to spread to other items in the enclosure causing the fire to grow and spread. The third stage of an enclosure fire is full room involvement, this takes place when all items within the enclosure have ignited and are burning. The final stage of a compartment fire is called decay.
As the contents of the enclosure are burned up and fuel or oxygen begins to be depleted in the enclosure the fire will begin to die out.
As we read in the Gorbett and Hopkins article the first definitions of Flashover from NFPA 1948 reference research from World War II. Flashover is defined as “A stage in the development of a contained fire in which all exposed surfaces reach ignition temperatures more or less simultaneously and fire spreads rapidly through the space.” (NFPA 921-2000) Gregory Gorbett and Professor Hopkins also list different and varying definitions of flashover throughout the article as they are referenced in different NFPA editions. The most practical definition of Flashover is described as “ A transitional phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach ignition temperatures 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” (NFPA 921 2004 edition p.11)
According to the Gorbett and Hopkins article the definition of Flameover has changed over the years the earliest definition of Flameover was defined as “A fire that spreads rapidly over the exposed linty surface of the cotton bales. In the cotton industry, the common term is flashover and has the same meaning.” The definition of flameover as described in the article also references varying definitions found in various publications over the years. All other definitions of flameover pertain to or reference “full room involvement” of combustible materials.
Backdraft also has various definitions according to the Gorbett and Hopkins article. One definition derived from the NFPA is “A deflagration resulting from the sudden introduction of air into a confined space containing oxygen-deficient products of incomplete combustion.” The article also provides definitions of backdraft as described by several other sources and authors.
Three components that control flashover are ambient temperature at the beginning of a fire, location of the fire within the compartment and heat release rate.
Three components that control backdraft are an underventilated compartment fire, sudden introduction of air and a gravity current carries fresh air into a compartment.
The three components that control flameover are an underventilated compartment fire, one or more of the fuels present in the layer accumulates to within its flammability range and ignition occurs at the location of the flammable mixture and the flame spreads until the local fuel and or oxygen is exhausted.
Three indicators that a flashover may occur are that the fire is in a ventilated compartment with oxygen available, all exposed combustible material is showing signs of combustion and there is a rapid buildup of heat in the compartment.
Three indicators that a flameover may occur are the upper layer begins to thicken, decreasing visibility, the upper layer temperature increases and there is turbulent mixing in the upper layer.
Three indicators that a backdraft may occur are no visible flames present in a room, hot doors and windows and window glass is discolored and may be cracked from heat.
The Current Knowledge & Training Regarding Backdraft, Flashover and Other Rapid Fire Progression Phenomena by Gregory E. Gorbett & Professor Ronald Hopkins
This week we are asked to discuss the stages of an enclosure fire, define each stage, the components, and indicators of the fire stages.
There are four stages of an enclosure fire, with the first being the incipient stage, which is the initial stage of the fire. There is enough heat/fuel/oxygen to support combustion. The second stage is the growth stage. If provided with enough fuel and oxygen, the fire will spread by direct flame contact or by the other fuel in the room, reaching its ignition temperature. The third stage is the fully developed stage. At this stage, the fire has consumed all the fuel within the compartment. In our reading, it stated that just because the fire is in the fully devolved stage, it does not mean that the fire flashed. The last stage is the decay stage, where the fire has either consumed all the fuel load or became an oxygen-deficient fire where there is no more oxygen to support further combustion.
Flashover is when all the fuel within the room reaches a temperature where the items ignite at the same time. Flashover occurs between the growth stage and the fully developed stage. Flame over or rollover is where heated gases rise to the ceiling and ignite. A rollover is usually a precursor to flashover. A backdraft is when oxygen suddenly is introduced into an oxygen-limited fire. The air mixes, and when added to an ignition source, it explodes.
The three components that would control flashover are to cool down the heated gas/smoke above. The old-timers always taught me never to apply water to smoke. But with all the research now, it is acceptable to try to cool down the gases. I believe if you apply water in the smoke and no water droplets come down that the temperature is around 900-1000 degrees. Another way to control a flashover is to get to the seat of the fire and extinguish the fire. Lastly, to control a flashover would be to coordinate ventilation to let out the hot gases; that way, the compartment would never reach flashover temps. Three ways to manage a backdraft would be to ventilate the compartment, practice good door control, and last is to not start any new flow paths in the building to limit oxygen.
The three indicators of a flashover are temperatures over 1100 degrees, windows breaking, and a rollover occurring. Three indicators of a backdraft are windows cracking because of the pressure, windows turning a greyish ellow color, and lastly, it would appear the fire is breathing.