I am often asked to explain the difference between flashover and backdraft, both of which I will define using the International Standards Organisation (ISO)
The key elements in all definitions are:
Leads to a fully developed compartment fire
Flashover is defined as: "The rapid transition to a state of total surface involvement in a fire of combustible materials within an enclosure."
Most fires start relatively small as some form of heat energy is applied to the object which leads to the ignition temperature being reached. The flame radiates back onto the object and this increases the rate of combustion. The radiation also heats up neighbouring items and they will begin to pyrolyse (break down into fuel and passive agents). The heated fire gases accumulate in the ceiling area forming an over pressure area. As the temperature increases the unburnt fuel in the smoke layer approaches its’ Auto Ignition Temperature (AIT) . When this fuel ignites the fire gas combustion rolls across the ceiling releasing an enormous amount of radiant energy which rapidly ignites the remaining combustibles in the room.
In a nutshell, flashover occurs when there is a good supply of air that allows the accumulated unburnt fuel to heat up to its’ Auto Ignition Temperature (AIT).
Fuel & Air + AIT = Flashover
Backdraught is defined as: "An explosion, of greater or lesser degree, caused by the inrush of fresh air from any source or cause, into a burning building, where combustion has been taking place in a shortage of air."
Well insulated rooms with limited air supply can limit the development of a fire. The fire will grow until the air is consumed and smouldering may continue for some time. The room may stay quite hot and the combustible contents will continue to pyrolyse (or breakdown) allowing the accumulation of large quantities of unburnt fuel.
If an opening is created air will flow into the compartment and add the missing ingrediant. It is possible for a sudden and explosive propagation of flame through the compartment and out through the openings. There will be pockets of gas remaining that are too rich to ignite immediately these will burn progressively as air is drawn into the compartment. The combustion process could continue for some time after the initial explosion and the heat generated could lead to a fully involved fire.
In a nutshell, backdraft occurs when air is added to a room with high temperatures and large quantities of unburnt fuel.
Fuel and Heat + Air = Backdraft
So how does this knowledge affect our fire attack strategy and hose lay tactics? Before we talk about that we must understand the 3rd critical fire development event, fire gas ignition.
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