Backdraft – How to recognise it and respond safely. Part 1

7/30/2019 Backdraft How to recognise it and respond safely. Part 1

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Backdraft How to recognise it and respond safely. Part 1There is still some confusion about what exactly a backdraft is.

The IFE Dictionary of Fire Technology uses the following definition:

"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."

The NFPA definition is:"The explosive or rapid burning of heated gases that occurs when oxygen is introduced into a

building that has not been properly ventilated and has a depleted supply of oxygen due to fire."

Basically there has been an accumulation of smoke gases in a compartment (containing unburnt

fuel particles) due to a lack of ventilation. Heat will be present in the form of smouldering

contents. The missing ingredient is sufficient oxygen to allow for combustion.

The ignition mechanism can take a number of forms.

a) Sometimes the inrush of air is sufficient to stir up embers or to gradually rekindle thesmouldering contents.

b) Where the smoke temperature is very high it is possible for the escaping gases tospontaneously ignite once diluted with the fresh air outside. The resulting fireball can

propagate back into the burning compartment and trigger the backdraft. In the mid nineteen

seventies, Swedish Fire Engineer Krister Giselsson used the termed Hot Rich Flashover to

describe this ignition mechanism. Perhaps a better description is Hot Rich Combustion. No

matter what it is called it is very important for firefighters to be aware of this trigger event.

This photo shows hot rich smoke leaving the compartment above the auto ignition temperature.

At the point where the fuel in the smoke has diluted to the UFL, auto ignition occurs. Photo

courtesy Tim Watkins.

A very simple way of describing backdraft is:

+ = =FUEL + HEAT OXYGEN BACKDRAFT


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INDICATORS.

Before we can attempt to safely deal with fire phenomena such as backdraft and flashover we

must be able to identify the warning signs. A vital part of the dynamic risk assessment is being

able to determine the various stages of fire development, and likely changes. The skill ofreading the fire is essential in ensuring the correct firefighting strategy and tactics are

deployed.

To simplify the process of reading the fire I use the acronym SAHF. A Smoke, Air, Heat,

Flame (SAHF). This assessment should be made by the OIC to determine the firefighting tactics

and also by tasked teams to assess the risk in their area of operations. As more information

becomes available it should be relayed to the Rapid Intervention Team (RIT) and the OIC so that

a more accurate profile of the fire can be developed and maintained.

1. SMOKEColour and density:Smoke colour can vary with the products and the ventilation available. There are some general

principles that can be used in that initial size up.

Dark smoke often indicates rich conditions due to restricted air supply.

Where flaming or smouldering combustion is occurring the carbon in the product is released in

the smoke and a very dark colour is the result. Where the temperature is low, or oxygen levels

are too low to support flaming combustion, the product breaks down (pyrolysis) with out active

flaming and most of the carbon remains on the material. This produces a lighter coloured

smoke. It is important to realise that as the fire develops heat will be transferred to neighbouring

compartments, which can result in pyrolysis and an accumulating of fuel laden white smoke.

As the fire progresses the smoke layer will lower and the density of the smoke layer will

increase.

This photo clearly shows the

lighter pyrolysis products on

the top left-hand opening (no

communication with the other

compartments). The top right

hand compartment is in

communication with the basefire and the dark smoke

indicates a rich (ventilation

controlled) combustion

process.

Photo courtesy Wayne Atkins


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Volume and location.

While the location of the smoke can be a valuable aid in determining the location of the fire, it

can also be deceptive in large or complex structures. Likewise the volume of the smoke

discharging can be misleading as some products can produce surprisingly high volumes ofsmoke from a relatively small fire.

Height of neutral plane. (The separation of the heated layer and the cooler lower layer). The

height of the neutral plane gives an indication of the stage of fire development and the volume of

unburnt fuel in the room. A very low neutral plane with dense smoke is a good indicator of the

potential for a backdraft. Sudden lifting of the neutral plane can indicate that high ventilation has

occurred. Sudden lowering can indicate that a confined fire may have broken out into the

remainder of the structure.

This sequence shows the gradual lowering of the neutral plane. They were taking approximately

60 seconds apart. Photo courtesy Tim Watkins.

Pulsations

Smoke seen pulsing out of small openings can indicate a ventilation-controlled fire. This is due

to variations in pressure due to limited oxygen supply. As the oxygen level decreases so does the

combustion process, which decreases the temperature and leads to a contraction of the smoke

gases. This causes air to be drawn in and the fire starts to increase and the pressure rises. As the

fire starts to grow again there is an increase in the volume of smoke which is forced out through

gaps. This continues until the air is consumed. The cycle then starts again. This pulsation cycle

is a good backdraft indicator.

2. AIR TRACKThe air track is the movement of air toward the firebase, and the movement of the super heated

combustion products out of the compartment.

Velocity and Direction

When an opening is created the heated gases will flow out of the top of the opening and cool air

will flow in through the bottom of the opening. A total and sudden inward movement of the air

track could indicate a potential backdraft event. In some cases this will be followed by an out

rush, and seconds later the backdraft.

Flow Turbulent or smoothIf the air track is slow and laminar (smooth) it could indicate that the fire is in the early stages

and most likely still fuel controlled. If the air track is fast and turbulent (often the neutral plane


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Photo courtesy Wayne Atkins.

When reading the fire it is essential not to look at only one indicator and draw a conclusion.

They must be read in conjunction to ensure accurate assessment. It is important for crews to

communicate the indicators to the Incident Commander so that a complete profile of the fire can

be achieved as soon as possible. Once the fire profile is established, any changes in the indicators

should be communicated and tactics amended as required.

The effect of opening up a ventilation controlled compartment fire can be extremely dangerous

and must be planned, communicated, controlled, and coordinated. Some Fire Services are not

using tools such as PPV to it full advantage because they are not sure exactly how and when to

use it. Ventilation techniques cannot be utilised in a random or routine manner. Fire tactics that

rely on set practices of opening up are prone to fail. The thinking fire commander will use a

Reddish orange flames can be

seen in the upper area due to

the fact that the oxygen

concentration is lower.

The air track supplies fresh

air to the base of the fire

producing a yellow flame.


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combination of ventilation techniques and tools (including PPV) in a planned and timed manner.

Often this will be performed in conjunction with Antiventilation, which is the practice of

keeping all, or part of the structure closed as part of the tactical plan. If the compartment is still

intact, as would be encountered in a potential backdraft scenario, this can be very effective in

containing the fire until other tactical benchmarks are achieved. Ventilation, including PPV, can

only be safely utilised once the dynamic risk assessment has been performed, a tactical plan

developed, and the necessary support is in position.

Swedish Firefighters have mastered the skill of reading the fire since the introduction ofcompartment fire behaviour training in the mid 1980s. Photo courtesy Ian Roberts.

In Part 2, I will discuss in more detail Tactical Ventilation, entry procedures and other response

strategies

Note on terminology.

Sometimes scientific terms do not serve the needs of firefighters in emergency situations. Thismay be because the are too long or are not meaningful to firefighters.

Many of the definitions used in this paper are designed to simplify the task of communicating the

indicators in the urgent and dangerous situations encountered during firefighting operations.

Some of these terms originated in Sweden and some have been adapted to suit UK and

Australian Firefighters. They have proven very effective in aiding rapid and clear

communication of fire conditions in all countries concerned.

The opinions expressed in this paper are my own and do not necessarily reflect those of my

employer.

For more information contact Shan Raffel GIFireE [email protected]


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Shan Raffel is a professional firefighter in Brisbane, Australia. He has served as an operational

firefighter for 19 years and a Station Officer for 13 of those years. He has studied realistic

compartment fire behaviour training (CFBT) in Sweden and the UKand has pioneered the

acceptance of CFBT in Australia. In 1999 he worked with a Swedish Fire Engineer on the design

of compartment fire fighting simulation for the new QFRS training academy.

Backdraft – How to recognise it and respond safely. Part 1

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