Flashover and Backdraft

 

Reference: Adapted from CFBT–US (Ed Hartin)

Rapid fire progress presents a significant threat to firefighters during structural firefighting. If firefighters do not have a high level of situational awareness this hazard is increased. It is difficult to develop proficiency in recognizing fire behavior indictors and developing an understanding of fire dynamics from fireground experience or classroom study alone.

Extreme fire behavior phenomena may be classified on the basis of duration of increased heat release rate. Step events result in rapid fire development and sustained increases in heat release rate. Transient events result in an extremely rapid, but generally brief increase in heat release rate (i.e., deflagration).

Flashover

Flashover is the sudden transition from a developing to fully developed fire. This phenomenon involves a rapid transition to a state of total surface involvement of all combustible material within the compartment. If flashover occurs, the rate of heat release in the compartment as well as the temperature in the compartment increases rapidly. Flashover may occur as the fire develops in a compartment or additional air is provided to a ventilation-controlled fire (that has insufficient fuel in the gas phase and/or temperature to backdraft).

Indicators of flashover include an average upper layer temperature of 500 °C - 600 °C. More observable indicators include rapid flame spread and extension of flames out of compartment openings. Compartment windows may also fail due to rapid temperature increases on the inner surface of window glazing. Given adequate ventilation, flashover occurs as part of normal fire development as previously illustrated on the graph in Graph 2.

If ventilation is limited, the fire may become ventilation controlled prior to flashover. A subsequent increase in ventilation may result in flashover.

Backdraft

A backdraft involves deflagration (explosion) 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 progressed into a ventilation-controlled state with a high concentration of pyrolysis products and flammable products of combustion.
  • Oxygen concentration in the compartment is low, generally to the point where flaming combustion is limited.
  • In addition, there must be sufficient temperature to ignite the fuel when mixed with air (auto ignition temperature of fuel)

As shown in Figure 5, the energy release from a backdraft is extremely rapid and is generally transient, lasting only a short time. Backdraft generally results in a brief, but quite substantial release of energy. However, depending on the volume of fuel and location of ignition, this phenomenon may result in an extended release of energy. Like a ventilation induced flashover, the initiating event for a backdraft is a change in ventilation profile providing additional oxygen.

 

What then is the difference between these two events? The major difference is the speed with which the heat release rate increases (see Figures 4 and 5). Backdraft involves a deflagration while ventilation induced flashover does not. However, the distinction between these two phenomena is not completely clear with some events occurring in the field falling in the “gray area” illustrated below.

 

Nonetheless, it is important for firefighters to remember that increasing the oxygen supplied to a ventilation controlled fire will result in increased fire growth and heat release rate. This may occur relatively slowly or it may be explosive, depending on conditions within the compartment.