According to The Secret List, two Cleveland (Ohio) firefighters were injured on Thursday while operating at a dwelling fire. Firefighters say the fire took off quickly through the dwelling, and the two firefighters were injured when there was a smoke explosion as they opened an attic door. The firefighters, who were conducting a floor-by-floor search of the house at the time, suffered second-degree burns and were taken to MetroHealth Medical Center. Visit http://firefighterclosecalls.com/fullstory.php?83566 to see photos.

FireRescue offers the following article about smoke explosion indicators and mitigation strategies. Look for more information about smoke explosions in a future issue.

SEEING THROUGH THE SMOKE

Conditions, indicators & mitigation strategies for smoke explosions

By Ed Hartin, MS, EFO, MIFireE, CFO

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-Göran 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.

Note: There are conflicting reports as to whether the Ohio incident was a smoke explosion or backdraft. While smoke explosion and backdraft are often confused, there are fairly straightforward differences between these two extreme fire behavior phenomena. A smoke explosion involves ignition of pre-mixed fuel (smoke) and air that is within its flammable range and does not require mixing with air (increased ventilation) for ignition and deflagration. A backdraft, on the other hand, requires a higher concentration of fuel that requires mixing with air (increased ventilation) in order for it to ignite and deflagration to occur. While the explanation is simple, it may be considerably more difficult to differentiate these two phenomena on the fireground as both involve explosive combustion. For additional information on extreme fire behavior phenomena, see my recent blog post Extreme Fire Behavior: An Organizational Scheme (http://cfbt-us.com/wordpress/?p=399).

Ed Hartin, MS. EFO, MIFireE, CFO is a battalion chief with Gresham (Ore.) Fire & Emergency Services and owner of CFBT-US, LLC, a training company specializing in compartment fire behavior training. Hartin is one of the co-authors of “3D Firefighting: Training, Techniques, and Tactics” and has presented on compartment fire behavior and firefighting tactics throughout the United States and internationally. For additional information, e-mail me at ed.hartin@cfbt-us.com.