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The Difference between Flashover and Backdraft

Posted by Shan Raffel
Shan Raffel
“Shan has pioneered and championed a global paradigm shift in fire fighting tech
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on Friday, 09 March 2012
in Industry Insights
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:

  1. Rapid transition
  2. 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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“Shan has pioneered and championed a global paradigm shift in fire fighting techniques that are now accepted as standard practice. His expertise in this area is what drives QLFA seamlessly through the industry”

Comments

Sebastian Jacobs
Sebastian Jacobs
Keen Australian based fire fighter
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Sebastian Jacobs Friday, 09 March 2012

Shan,
As you know, Lasse Bengtsson made a great work a few years ago, which resulted in the book "Enclosure fires". He looked a various definitions of flashover and backdraft and there are some great explanation in the book. I can strongly recommend this book (I know for sure Shan's got a copy...).
It can be found here (assuming the technology works...): https://www.msb.se/sv/Produkter--tjanster/Publikationer/Publikationer-fran-SRV/Enclosure-fires/
Keep up the good work!

Sebastian Jacobs
Sebastian Jacobs
Keen Australian based fire fighter
User is currently offline
Sebastian Jacobs Friday, 09 March 2012

One of the great things about my visit to Revinge was the explanation given by Stefan, Lasse et al describing flashover as a period. For me the "flashover period" describes things a whole lot better that most other definitions. Always a pleasure learning from you guys !

jc

Sebastian Jacobs
Sebastian Jacobs
Keen Australian based fire fighter
User is currently offline
Sebastian Jacobs Friday, 09 March 2012

Thank you, Shan. This is a very good and concise description of these two critical situations.

Sebastian Jacobs
Sebastian Jacobs
Keen Australian based fire fighter
User is currently offline
Sebastian Jacobs Friday, 09 March 2012

Shan you are very didactic in concepts. For you successes...!!!

Sebastian Jacobs
Sebastian Jacobs
Keen Australian based fire fighter
User is currently offline
Sebastian Jacobs Saturday, 24 March 2012

Thanks guys.
I can certainly recommend the book by Lasse Bengston. I would go as far as saying it is essential reading for anyone who wants to develop a good understanding of fire behaviour.
If you want to go beyond the basics and delve deeper into the science of firefighting, I recommend that you follow the following link from Vyto Babrauskas, Ph.D. (AKA Dr Fire) http://www.doctorfire.com/FireServiceBooks.pdf

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