To this point, we have examined the extreme fire behavior phenomena of flashover and backdraft. Flashover, transition of a compartment fire from growth to fully developed stage, is a heat driven event that depends on the presence of adequate fuel and oxygen. A backdraft is quite different. If ventilation is limited (as it usually is), the available air supply will limit the rate of combustion. As the fire consumes atmospheric oxygen and temperature within the compartment rises, backdraft conditions may develop. Introduction of air into a compartment filled with extremely hot, fuel rich smoke may result in a backdraft.
Many old texts dealing with basic fire behavior or ventilation used the terms smoke explosion and backdraft interchangeably. However, smoke explosion or fire gas explosion and backdraft are quite different phenomena. In the case of both backdraft and smoke explosion, smoke is the fuel. However, the other sides of the fire triangle are quite different. A backdraft requires a high concentration of fuel gas/vapor, low concentration of air, and temperature above the ignition temperature of flammable products of combustion and pyrolysis produces. On the other hand, a smoke explosion requires a mixture of fuel (smoke) and air within the flammable range but will be below the ignition temperature of flammable products of combustion and pyrolysis products (see Figure 1: Explosive/Flammable Range). If the fuel/air mixture had reached its ignition temperature, it would already have ignited.
If a source of ignition is present, the fuel/air mixture will ignite explosively as illustrated in Figure 2. In this incident, a smoke explosion occurred while companies were making an indirect attack on an attic fire in this vacant structure (more on this in a bit). As shown in the "after" photo, the overpressure from the smoke explosion blew out a section of the roof. The firefighters working on the ladder were below and to the side of the opening and were not injured.
Factors that influence the violence of a smoke explosion include the extent to which the structure confines the fuel/air mixture and how close the concentration of fuel and air is to a stoichiometric mixture (ideal for complete combustion). The more confined and closer the concentration is to stoichiometric, the greater the violence of the explosion.
Smoke from an underventilated fire can flow through leakage in a structure to collect in concealed spaces or other compartments within the building. Remember, smoke is fuel! If smoke is present, even if cool and well away from involved compartments there is potential for a smoke explosion. Karlsson and Quintiere (2000) observe that this phenomenon is "seldom observed in enclosure fires". However, while infrequent, the conditions required for a smoke explosion can develop within a structure and present a significant threat to firefighters as illustrated by the following case study.
Case Study Method
In previous articles on extreme fire behavior, the case study method was presented as an excellent approach for developing your knowledge and understanding of fire behavior. Just to review how to approach a case study: Read the questions to be answered first, this provides you with a framework for understanding the information presented. Second, read the case to get an overall understanding of the incident. Last, examine the incident in detail to answer the questions posed at the start of the case. For additional information on the case study method, please refer to Extreme Fire Behavior: Backdraft and Extreme Fire Behavior: Flashover.
This incident involved a fire in a tri-level townhouse that resulted in the deaths of a firefighter and company officer. The mid-afternoon fire occurred in the unit located on Side Delta (see Figure 3). This case study will focus on fire behavior related aspects of this incident. However, this case provides an opportunity to learn a number of other important lessons (see the NIOSH and NIST reports for additional information on the incident).