It was a smoke explosion? Is that like a flashover or something?” I whispered to my partner at the incident critique. I’d heard of this phenomenon years ago as a cadet in the fire academy, but it certainly hadn’t been discussed in any detail, and the textbook barely broached the topic. Our battalion chief went on to describe this rare occurrence that nearly cost my crew our lives one morning in December 2007, when a smoke explosion caused a structural collapse in the three-story brick building next to an involved church.
The Play by Play
The church was fully involved on the battalion’s arrival; however, the brick structure next door appeared uninvolved, except as a possibly inhabited exposure. (It was not until after the fire was extinguished that we learned the structure was attached to and an annex of the church.)
Because the structure appeared residential on the second and third floors, we were ordered to search the building. Our crew of three broke through the first set of doors on the A-side, and we staged in the foyer just before a second set of wooden doors, waiting for a handline to take up the stairs and get started on the building’s search.
Inside, there was no smoke and no heat–nothing out of the ordinary, with the exception that there was no electric service. The first alarm companies–which comprised four engine companies, one quint and two ladder companies–were working to contain the fire defensively and protect the numerous exposures. Our second-alarm ladder company then arrived with three additional engine companies.
The battalion chief set up a unified command, with 20-minue PAR counts tracked by our 911 center. At this time, we were going on air as a precaution. However, an attentive officer on the D-side noticed visible fire through a third-floor window in the structure, and our search was suspended. We exited the foyer and stepped out to the sidewalk, where we took off our helmets to doff our facemasks as we came off air.
The collapse happened very quickly. While standing just outside the structure, I first felt a blast of air at my back that was strong enough to knock me forward. I was completely stunned, then realized that the building was collapsing, and my helmet was on the ground next to me.
We huddled down, shielding ourselves from the debris, most of which landed just a few feet from our position. A half-ton, roof-mounted HVAC unit landed just 4 feet from my head. I received minor bumps and cuts from the debris, most of which bounced off my air pack. One firefighter on our team required surgical repair for a shoulder injury. The third firefighter was uninjured.
A lieutenant positioned 50 feet above us on the end of a quint’s ladder had a bird’s eye view. As the explosion developed, he saw a wall collapse at the C-side, falling into the adjoining church. When this occurred, both flames and fresh air made contact with an area of accumulated smoke, causing a smoke explosion.
A shock wave went right over our heads, unbeknownst to those of us below. This was immediately followed by the structural collapse of the top two floors. The roof heaved up, then rippled as it fell in a wave-like motion.
As the collapse progressed, flames blew out the windows one-by-one until they blew out the windows over our heads. The collapse also created a large gust of wind that traveled down the stairs and out the front A-side door. This was the rush of air I had felt. The rest of the debris quickly followed as it spilled over our heads onto the streets and sidewalks.
Fortunately, the first floor walls held, so we escaped an LODD. Once we accounted for each other, brushed off the dust, counted our fingers and toes and reported back to command, we repositioned our aerial and set up for further defensive attacks. Later, the two buildings next to this one were condemned and torn down due to unrepairable structural damage from the shock wave and collapse.
Smoke Explosions Explained
So what is a smoke explosion? Typically, a smoke or fire gas explosion occurs when unburned pyrolysis products (gas phase fuel) accumulate and mix with air, forming a flammable mixture. It will remain static until it receives either an ignition source or the mixture climbs above 600 degrees C, when it will auto-ignite. Introduction of a source of ignition results in a violent explosion of the pre-mixed fuel gases and air. This phenomena generally occurs away from the fire, such as in our attached exposure. Smoke explosions differ from flashovers and backdrafts in that they already have oxygen to support oxidation, but need ignition. (For more on Smoke Explosions, see the sidebar “Smoke Is Fuel,” p. 54.)
Smoke explosions are classically determined after the fact by the degree of structural damage they cause and their lack of anticipation on the fireground. Little is known about these events, as information is generally gathered from untrained eyewitness reports, and the conditions are difficult to reproduce in laboratories on the scale we work on in real life.
Conclusion
This event reinforced to us the unpredictability of any fire scene and the adage that the most dangerous building is the one you don’t expect to be dangerous. But regardless, we must do everything we can to anticipate possible issues on the fireground, thereby “stacking the cards in our favor.” That way, when the unexpected happens, we’re as prepared as possible.
“It was a close one,” the chief said fittingly as he ended the critique, and we were all very happy to have survived a rare and sobering event.
Smoke Is Fuel
Conditions, indicators & mitigation strategies for smoke explosions
By Ed Hartin
Smoke explosions have resulted in three firefighter fatalities in the United States since 2005, two in Wyoming (see NIOSH Report F2005-13) and one last year in Los Angeles. However, many firefighters have not heard of this fire behavior phenomenon or simply misunderstand it.
For many years, the term “smoke explosion” was synonymous with backdraft. However, today it identifies a different, and in many respects more dangerous, extreme fire behavior phenomenon. Smoke (or fire gas) explosion is described in fire dynamics textbooks such as “Enclosure Fire Dynamics” (Karlsson and Quintiere), “An Introduction to Fire Dynamics” (Drysdale) and “Enclosure Fires” (Bengtsson). Of these, the text “Enclosure Fires” by Swedish Fire Protection Engineer Lars-Gà¶ran Bengtsson provides the best explanation of how conditions for a smoke explosion develop. But as mentioned, this phenomenon isn’t well known among firefighters and fire officers. In fact, many well-known fire service authors continue to use backdraft and smoke explosion interchangeably.
A smoke or fire gas explosion occurs when unburned pyrolysis products accumulate and mix with air, forming a flammable mixture and introduction of a source of ignition results in a violent explosion of the pre-mixed fuel gases and air. This phenomena generally occurs remote from the fire (as in an attached exposure) or after fire control.
Conditions Required
The risk of a smoke explosion is greatest in compartments or void spaces adjacent to, but not yet involved in, fire. Infiltration of smoke through void spaces or other conduits can result in a well-mixed volume of smoke (fuel) and air. Smoke explosions create a significant overpressure as the fuel and air are premixed. Several factors influence the violence of this type of explosion:
- The degree of confinement (more confinement results in increased overpressure);
- Mass of premixed fuel and air in the compartment (more premixed fuel results in a larger energy release); and
- How close the mixture is to a stoichiometric (the ideal mixture of fuel and air for complete combustion) concentration; the closer to an ideal mixture, the faster the deflagration.
Indicators
It’s very difficult to predict a smoke explosion; however, the following indicators point to the potential for this phenomenon to occur:
- Ventilation-controlled fire (inefficient combustion, producing substantial amounts of unburned pyrolysis products and flammable products of incomplete combustion);
- Relatively cool smoke (generally less than 600 degrees C or 1,112 degrees F);
- Presence of void spaces, particularly if they’re interconnected;
- Combustible structural elements; and
- Infiltration of significant amounts of smoke into uninvolved exposures.
Mitigation
As with recognizing the potential for a smoke explosion, mitigation can also be difficult. The gases are relatively cool, so application of water into the gas layer may have limited effect.
Tactical ventilation to remove the smoke is the only way to fully mitigate the hazard and establish a safe zone. However, use care not to create a source of ignition (such as the sparks created when using an abrasive blade on a rotary saw).
The best course of action is to prevent infiltration of smoke into uninvolved spaces using anti-ventilation (confinement) tactics. Anti-ventilation is the planned and systematic confinement of heat, smoke, and fire gases, and exclusion of fresh air (from the fire). In this case, anti-ventilation may involve pressurizing the uninvolved area to prevent the spread and accumulation of smoke.
