In today’s society firefighters are responding to fires during the later stage of the fully developed stage or at beginning of the decaying stage of fire. This is to no fault of our own, in which multiple factors go into this such as time of day, traffic, faulty fire alarms systems or lack thereof, etc., but understanding the risk of responding to fires as this stage and the vital role we play by knowing fire behavior and knowing the effects of what happens at each stage of fire growth. In this stage, we as firefighters are at an inherent risk of responding to confined fires deprived of oxygen and without proper ventilation and coordination, we are adding to the problem rather than making it better. One of the biggest risk we run into is called a backdraft. A backdraft, also known as a smoke explosion NFPA 921: Guide for Fire & Explosion Investigations, 2011 Edition as, “A deflagration resulting from the sudden introduction of air into a confined space containing oxygen-deficient products of incomplete combustion.”
So let’s talk now about the science of a backdraft. In decaying stage burning is incomplete because of the lack of oxygen feeding the fire, causing the fire to smolder or burn out. Nonetheless, the heat released from the fully developed state still remains and all flammable products of combustion are waiting for an introduction to oxygen so they can instantaneously combust in the room. We as trained firefighters can prevent this act from happening through proper ventilation. By doing so properly and in a timely manner, the upper areas in the room will be cleared from smoke and unburned gases which is done at the highest point possible. Coincidentally, if this is improperly done, the same unburned gases and other products of combustion are now at a higher risk to instantaneously combust once oxygen is introduced to the room causing a devastating effect on the structure and those in and around it. It is to be known that it only takes 25% of the space in a room or fire area to cause a backdraft to occur.
With this being said there are certain warning signs that can be seen to indicate if a backdraft appears emanate:
• Black smoke becoming dense, greyish yellow
• Confinement and excessive heat
• Little or no visible flame
• Smoke leaving the building in puffs and being drawn back in
• Smoke stained and/or rattling windows
• Muffled sounds
• Sudden, rapid movement of air and smoke inward when an opening is made
Now that we have discussed the science of a backdraft, let us now discuss how we can safely manage this type of situation.
Vertical ventilation is possibly the best and most effective method of venting a structure showing signs of a backdraft. This will allow for the superheated gases to escape without introducing an excessive amount of oxygen into the structure. Should an explosion still occur during vertical vent, the force will be directed up and away from the vent crew that created the opening (roof cuts, skylight pops, etc.). Should vertical ventilation not be possible for us, horizontal ventilation is a consideration. This could be done using pike poles 6 feet or greater and positioning yourself near the corner of the building and moving all companies out of the “hot zone” or collapse zone.
Another method would be to quench the gases. This is done before we enter the room where the superheated gases were present. A few short blasts of water to the ceiling (also known as penciling) will provide enough cooling to stop an explosion from occurring. By doing this, we are controlling the heat release rate (HHR) and if we can control the heat release rate we can control the fire.
If vertical vent and quenching not be an option, we could consider flanking the fire. To flank the fire, we should position engine company crews at the sides of the doorway and cool the room (compartment) with their hose streams. When doing this, make sure to not cross hose streams and take note of the windows in the room, if possible. A psi pressure of a mere 0.5 will blow windows out.
Let’s all take this and answer these questions next time we are working on the fire ground.
What do you see upon arrival as the initial Incident Commander or first due Engine Company?
What significance do these observations tell us about the fire conditions?
What's is the smoke telling us?
What would be your actions?
Until next time; work hard, stay safe and live inspired.
About the Author
NICHOLAS J. HIGGINS is a firefighter with 14 years of service all within departments in Piscataway, NJ. Nick has held the ranks of Lieutenant and Captain as well as being a township elected District Fire Commissioner for 1 term (3 years) in Piscataway, NJ. He is also a NJ State certified level 2 fire instructor. He holds a B.S. in Accounting from Kean University working in Corporate Taxation and is the founder/contributor of the Firehouse Tribune website.