Extreme Fire Behavior: Smoke Explosion

In the case of both backdraft and smoke explosion, smoke is the fuel. However, the other sides of the fire triangle are quite different.


Other Incidents Involving Smoke Explosions

Given the relative infrequency of this phenomenon it is useful to examine several other incidents involving smoke explosions. The first occurred in Edinburgh, Scotland, February of 2002 and the second in Gresham, Oregon in February of 2005.

In February 2002, the Lothian & Borders Fire Brigade responded to Sleigh Drive for an apartment fire with persons reported. On arrival, the officer on the first pump observed fire showing from one window on floor one of an ordinary constructed apartment building (brick veneer over concrete block). Crews extended hoselines to attack the fire and support search in the apartment above. The crew working above the fire reported finding a gap between the floor and exterior wall that was permitting smoke to pass from the fire apartment to the upper floors. Shortly after the crews working on the upper floors exited the building to replace air cylinders in their SCBA, an explosion rocked the building. The force of the explosion injured several personnel, falling debris struck several firefighters outside the building, and the crew working in the involved unit received burns due to a dramatic increase in fire intensity and loss of water supply in their hoseline.

In this incident, smoke traveled from the involved unit to the apartment above through a gap between the floor and exterior wall and eventually reached its flammable range. Fire extending to floor two through a void space ignited the flammable fuel/air mixture resulting in a smoke explosion. The violence of the explosion caused failure of a substantial portion of the brick veneer and part of the block wall on floor 2 as well as a partial collapse of floor two.

In the case study as well as the incident on Sleigh Drive, the conditions required for a smoke explosion developed due to normal fire progression and the structural characteristics of the building involved. Conditions in the next incident developed quite differently.

Gresham Fire and Emergency Services responded to a fire in a vacant dwelling in mid-February 2002. The officer of the first arriving company observed dark gray smoke showing from the attic of a two and one-half story, wood frame structure. As additional companies arrived, companies deployed 1-3/4-inch handlines to control the fire on floors 1 and 2 in support of a quick primary search. After the search was completed and all clear, the Incident Commander withdrew companies from the building and ordered an indirect attack to control the increasing volume of fire in the attic. Shortly after initiating the indirect attack from the gable ends, the building was rocked by a violent explosion that blew out a large section of the roof in the center of the structure (see Figure 2).

After its initial effect (large volume of steam production), the indirect attack became less effective. Application of water in the indirect attack cooled the hot smoke in the attic, but also introduced a large volume of air, which mixed with the smoke creating a flammable mixture. Fire burning up from floor two provided the source of ignition, igniting the fuel/air mixture. Companies were withdrawn outside the collapse zone and shifted to a defensive strategy.

Study and Discussion Questions

Use the information presented in the case to answer the following questions.

  1. Was extreme fire behavior involved in this incident? If so, what type of event happened? Use the fire development curve illustrated in figure 9 to work out your answer. Consider the fire burning in the void spaces. Where on the fire development curve do you think it was? What happened after the explosion? (Figure 9: Fire development curve)

References

  • Karlsson, B. & Quintiere, J. (2000). Enclosure fire dynamics. Boca Raton, LA: CRC Press.
  • National Institute for Occupational Safety and Health (NIOSH) (2000) Death in the line of duty, Report F2005-13. Retrieved March 12, 2006 from http://www.cdc.gov/niosh/fire/pdfs/face200513.pdf
  • Gollogly & McGill (n.d.). Case study: Sleigh drive Edinburgh presentation and trainer's notes. Hertfordshire, United Kingdom: Hertfordshire Fire and Rescue Service.