By: Eddie Crombie
On March 28, 1994 the Fire Department of New York responded to reports of heavy sparks and smoke coming from a chimney in an apartment located at 62 Watts Street1 Upon arrival companies found light smoke showing and prepared for an offensive attack. Crews positioned themselves outside the apartment door to mount an attack. When the apartment door was forced open a blow torch of fire rushed out and extended up the entire staircase killing 3 members of the FDNY. What occurred that day was the rare, extreme event we know as a backdraft. To give ourselves the best chance on the fire ground we must understand this event, know what signs help us predict a backdraft, and be able to safely mitigate this turbulent environment.
A backdraft can be defined as an unsustained explosion of hot pyrolysis and flammable products of combustion upon mixing with air. This occurs during a ventilation controlled fire where oxygen levels are extremely low and flaming combustion is limited. If you recall the previous article about ventilated controlled flashover, you may notice similarities between these two phenomena. Both triggered with an increase in ventilation. However, there is a difference in the speed and level of the heat release rate (Fig 1). Backdraft is an explosion that may not transition into a fully developed fire. A flashover is a transitioning event that always results in sustained fire.
Photographer Glen Ellman of the Fort Worth Fire Department captured these series of photos that explicitly demonstrates the nature of backdrafts.
As the crews forced entry there was a slight inward tract of air followed quickly by a push of dark, silver smoke. Also note the ignition of the pyrolysis gases in this oxygen starved compartment.
As the heat release rate increases the hot gases expand. The growing flame front pushes the fuel rich gases out through the ventilation opening. (Note the thick black smoke to the left.) This excess, fuel rich gas ignites into a explosive fire ball after exiting the compartment in the right photo. This is an excellent example of why it is important to stay low when making entry.
As quickly as it developed, the explosion consumed most of the unburnt pyrolysis gases showing that backdrafts often do not sustain and transition into a fully developed fire. Normally they simply diminish into a manageable fire.
Next, we need to examine what conditions produce these dangerous events. The slide shown in Fig. 2 shows the contributing factors present prior to a backdraft occurring2
The fire at 62 Watts Street had all these conditions making conditions perfect for a backdraft. First, like most residential fires, there was an excessive amount of fuel within the apartment. The furnishings and personal belongings alone creates a massive fuel load that we normally take for granted. Second, originally built in 1800, this building underwent numerous renovations including extensively insulating and sealing any openings to increase it’s emergency efficiency. This essentially eliminated any natural air movement causing the fire to have an inadequate air supply. Third, the cause of the fire was a plastic bag left on the stove. This smoldering, incomplete combustion created a hot, fuel rich environment. This volatile powder keg of conditions only needed a spark in the form of increased ventilation to set it off which it received when companies forced the door open.
Once open, crews noted seeing signs of extreme fire behavior just prior to the backdraft much like the Fort Worth incident. They reported seeing a pulsing air track consisting of an inward rush of air followed by an outward flow of smoke. This single pulsation was followed by a large volume of flame that rushed out the doorway and up the stairwell. This intense fire lasted nearly 7 minutes. Upon further investigation the NIST was unable to recreate this event with their computer models. This was truly a perfect storm.
Finally, now that we understand and can identify the signs of a backdraft we need to know how to safely manage this situation. Remember this:
Vent – Quench – Flank
Venting vertically is perhaps the most effective method. When conditions permit, vertical ventilation lets the superheated, volatile gases escape without introducing excessive amounts of oxygen. Even if an explosion occurs the force will be directed upward away from the vent crew and attack company. If vertical ventilation is not an option horizontal can be preformed using a long pike pole and positioning yourself to the side near the corner of the building. Also place all companies outside the collapse zone.
Next we need to quench the gases. Before entering a superheated room where a backdraft is expected quenching the compartment with short blasts near the ceiling (penciling) can provide enough cooling to stop an explosion from happening. Remember if we control the heat release rate we can control the severity of the fire.
Lastly, when vertical ventilation and quenching is not an option, flank the fire. Position crews to the side of the doorway so they can flank and cool the compartment with their hose streams. This protects the firefighters from the possible explosive pressure wave. Note the location of windows because a mere 0.5 psi will break glass.
Like flashover, backdraft is a dynamic event that has the capability of injuring or killing us. To be efficient and effective as a company we need to understand the conditions needed for a backdraft to occur, recognize the signs, and safely mitigate the situation before we over commit our personnel.