By: Eddie Crombie
Over the last two articles we have examined event we call flashover and heat release rate, the dynamic measure of energy that partially defines why the traditional fire behavior curve is changing. The next step in expanding our understanding of the modern fire ground requires us to delve into the realm of extreme fire phenomena. Extreme fire phenomena are those events that are best defined as “characteristics of fire behavior that typically precludes direct fire attack.” Smoke explosion and backdraft are the two most common types of extreme fire phenomena we will discuss. According to the summary published by the NFPA covering firefighter deaths from 1977-2009, we see about 40 firefighter fatalities every 10 years due to “rapid fire progress”.1 One conclusion that can be drawn from this study is that we are not skilled in predicting that these events will occur.
Before we can determine the signs that identify these deadly events we must understand what influences extreme fire phenomena. Referring to figure 1 there are three areas that affect the conditions needed for a backdraft or smoke explosion to take place. First is fuel, specifically the physical and chemical characteristics along with its availability. The second is the ventilation profile. Where is the nearest opening? How large is it? Is it above or below the fire? Together the fuel and air directly affect the heat release rate. Heat release rate, as we know, is the single most important factor in determining survivability of a fire for both the occupants and firefighters. Third is the geometry of the compartment; physical characteristics such as size, height, and shape and the general configuration of ventilation openings and fuel location. The compartment and its arrangement will affect if the fire is fuel or ventilation controlled. All extreme fire phenomena occur during a ventilation controlled fire.
Rapid progression, backdrafts and smoke explosions are instantaneous events and normally are not survivable if you are within the compartment when they take place. Therefore we must be able to observe the overall conditions before we extend ourselves beyond the survivability threshold. Most text books will give you specific signs to observe for each type of event. This does not lend itself well to the street firefighter because extreme fire events are difficult to differentiate on the scene due to the dynamic nature of the fire ground. Additionally distinguishing if a backdraft or smoke explosion is about occur is a matter of semantics, since either event is likely to seriously injure or kill a firefighter. Recognizing the general signs that an extreme phenomenon may take place improves firefighter situational awareness; we can define the kind of event after the fact. The following signs are your crystal ball for these events:
-High velocity smoke discharging through similar sized openings. This will be seen from the exterior and shows a fire that is well involved with enough fuel to rapidly transition.
-A sudden change in the color of smoke, specifically darkening. Darker smoke is rich in unburned fuel and oxygen starved. All that is need is a vent point ignition to cut you off from your primary egress.
-A sudden increase in heat conditions shows the fire is still growing and has not progressed into the decay stage. Many describe this sign as “heat that suddenly drives you to the floor”. If you experience this it’s time to go!
-The sudden lowering of the smoke layer means either the ventilation profile has changed or the atmosphere at the ceiling has suddenly became hotter than the smoke, pushing these cooler gases to the floor. In either case conditions will change suddenly.
-A repeated raising and lowering cycle of the smoke layer is signs of a very unstable environment. The fire is actually starving for oxygen creating an unbalance in the thermal layer. This is best seen through a doorway.
-The high-pressure pulsing of smoke is similar to the previous example but will we seen from a window or other smaller opening. If you see this through eaves or brick take extreme caution.
-Heavily stained or cracked window glass shows a hot, oxygen starved compartment. Ventilate these from the side, preferably the corner.
-Flames “licking” through the smoke and detaching from the main body of fire shows that smoke is reaching it’s ignition temperature and flammable range. This translates to rapid fire progression possibly cutting off egress routes.
-Doors forced open by rushing air or the feeling or sound of air rushing into the fire. This happens when a large fire suddenly transitions consuming all available oxygen.
-The reversal of smoke, causing it to head back into an opening. Much like the pulsing of smoke this fire is large, starving and consuming any oxygen it can.
All of these signs are telling us the conditions inside the container are deteriorating and, most importantly, very unstable. You must be able to recognize these signs and use your situational awareness to orient yourself and make a quick decision.
By generalizing backdrafts and smoke explosions into an all encompassing category of extreme fire phenomena we are able to identify and avoid these deadly conditions more effectively. Using our knowledge of what influences these events along with observable warning signs helps us decide on what actions can safely mitigate the situation. To take our discussion to the next level we need to specifically explore the rare phenomena of backdraft and the lesser understood smoke explosion to obtain a complete understanding of the topic and decrease the number of firefighters killed by these extreme events.
1 Fahy, R. (2010, June) U.S. Fire Service Fatalities in Structure Fires 1977-2009. Retrieved on February 06, 2012 from NFPA website: http://www.nfpa.org/assets/files//PDF/OS.FatalitiesInstructures.pdf