Extreme Fire Behavior: Backdraft

A backdraft involves deflagration or rapid combustion of hot pyrolysis products and flammable products of combustion upon mixing with air. Several conditions are necessary in order for a backdraft to occur within a compartment. The fire must have...


The gas laws outlined in the preceding section provide a foundation for understanding why smoke moves the way that it does under fire conditions. However, there is another piece to the puzzle. Ventilation under fire conditions involves both smoke and fresh air. In general the smoke is hot and fresh air is cooler. What impact does this have? In fluid dynamics, the term gravity current is is used to describe a primarily horizontal flow in a gravitational field that is driven by a density difference. Air track describes the movement of smoke and fresh air in a compartment fire. Air track is influenced by both pressure caused by heating of gases and the difference in density between hot smoke and cooler air as illustrated in Figure 3.

Smoke and air mix along the boundary of the gravity current. If the concentration of the smoke (fuel) air mixture reaches the flammable range and is above its ignition temperature it will auto ignite, resulting in a backdraft. The volume of smoke that is within its flammable range and the extent to which it is confined are major factors influencing the violence of this combustion reaction.

Case Study Method

In the previous article Extreme Fire Behavior - Flashover, 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. When using case studies as an element of fire behavior training, the following questions serve as a good starting point for your analysis:

  1. Was extreme fire behavior involved in this incident? If so, what type of event happened?
  2. How did the fire develop and what factors influenced the occurrence of the extreme fire behavior phenomenon?
  3. What clues were present that may have indicated potential for rapid fire development?
  4. Compare and contrast these the case studies with other cases or events in your own experience. What aspects of these incidents were similar? Which were different?

In addressing the first two questions, what happened and what were the contributing factors it might be useful to examine the fire development curve presented in Fire Development in a Compartment. What stage (incipient, growth, fully developed, or decay) and what burning regime (fuel controlled or ventilation controlled) is the fire in immediately prior to changes in fire behavior. Additional questions can focus on incident specific issues such as the effectiveness of tactical operations.

Case Study

This incident involved an early morning fire in a two-story, mid-terraced house (townhouse). This fire resulted in the deaths of a child and two firefighters in Blaina, Gwent, in Wales (UK). Data for this case was obtained from the Fatal Accident Investigation report conducted by the British Fire Brigade Union (FBU, 1996) and analysis of the incident by Paul Grimwood (1998; personal communication February, 2006). In that this case is situated outside the United States, a brief explanation of deployment, resources, and tactics is likely in order. This area of Wales is served by retained duty (paid on call) firefighters operating from several fire stations each with a pump (engine company). Staffing varies, but on the morning of the incident the first arriving company, pump B031 (Blaina Station 3, Engine 1) was staffed with a sub-officer (Lieutenant), apparatus operator, and four firefighters. In the UK tactical operations are not unlike those used in the US (fire attack, primary search, etc.). However, 19 mm (3/4") and 25 mm (1") hosereels (booster lines) are commonly used for initial attack on contents fires. While deployment and tactical differences are interesting and a great starting point for discussion, don't be distracted from the significant fire behavior lessons presented by this incident.

 

Construction: The house was built on a concrete slab with concrete block walls with brick veneer on Sides A and C. A concrete block wall also separated the stairwell from the living room and the kitchen from the living room. The second floor was supported by a mix of 75 mm x 175 mm (3" x 7") and 50 mm x 150 mm (2" x 7") joists with weyroc (tongue and groove particleboard) flooring (thickness not specified). All other internal partitions were fabricated with 50 mm x 100 mm (2" x 4") studs. Interior finish was plasterboard (unspecified thickness). The ceiling had a skim coat of Artex (textured plaster). Windows on floors one and two were 780 mm x 870 mm (36.7" x 34.25") and constructed of unplasticised polyvinyl chloride (UPVC) and glass. Interior doors were lightweight wood with "egg crate" internal separation.