10. Extreme Fire Behavior

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There are many conflicts

between internationally

accepted definitions of thevarious events associatedwith rapid fire phenom-

ena. According to scien-tific definitions it is argu-

able that the variousevents can be attributed to

either flashover; backdraftor a range of differing ig-

nitions of the fire gases.

It is confusing for fire-

fighters to attempt to dif-ferentiate between the

various events. Whilst it isuseful to have a basic un-

derstanding of each event,the firefighter should con-

centrate more on eventcauses, outcomes and

counters (actions to pre-vent or suppress rapid fire

development).

The UK has around112,000 structure fires

every year and approxi-mately 50 of these demon-

strate backdrafts. How-

ever, around 600 of thesefires demonstrate other

unknown events associ-ated with abnormal rapid

fire development; thatsonce every 187 fires!

In the USA over 50 fire-

fighters were killed byrapid fire progress be-

tween 1990 and 2000. Afurther 50 died through

related phenomena. Thedeath rate is increasing

annually.

Flashover, Backdraft &Fire Gas Ignitions

Rapid Fire Phenomena - Three Basic Types

Flashover

Flashover is generally aheat induced development

of a compartment fire al-though added ventilation

may also speed up the

flashover process. Thebreaking of a window or

the opening of a door caninitiate a flashover.

Backdraft

Backdraft is an event thatmay occur where a fire

has been under ventilatedfor some period and is not

receiving enough air to

develop naturally. Anysudden inflow of air may

initiate the event.

Fire Gas Ignition

The term Fire Gas Igni-tion covers a wide range

of rapid fire phenomenawhere accumulations of

fire gases and smoke are

transported towards anignition source, or where

high energy heat is addedto a gas pre-mix.

Special points of interest:

Scientific definitions of events demonstratethere may be confusing crossovers from apractical firefighting viewpoint

It is important to appreciate the widerange of events that may occur

However, the firefighter needs to knowbasics - What might happen? - What ac-tions might cause an event? - What can bedone to prevent or alleviate an event?

1 Extreme Fire Phenomena - Types of Event

2 Hazard Recognition & Warning Indicators

3 Possible Effects of Tactical Venting Actions

4 Countering Actions used to Reduce Risks

5 What might Happen?

6 What Tactical Actions might Cause an Event?

7 What can be Done to Prevent or Alleviate Events

Learning Outcomes:

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3D FIREFIGHTING

Rapid ResponseFast Attack Tactics


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A repeated rising & lowering

cycle in the smoke layer

Pulsing smoke or

smoke seen push-

ing out of open-ings, appearing

under pressure

Heavily smoke

stained or cracked

window glass

Blue flames seen at the fires

bas, in the overhead or at exit

points

Flames seen in

the overhead, possiblyas firesnakes detach-

ing themselves from themain fire

Brief bursts of

flame seen in the gases

High velocity smoke exiting a

doorway or window

A sudden change in colour of

smoke, particularly darkening

A sudden change in heat condi-

tions, forcing crews to crouchlow

A sudden lowering of the smoke

layer

It is a stark fact that firefighters may betaught about the hazard warning signs

and fire behavior indicators of potentialevents associated with rapid develop-

ment, but rarely do they observe or acton these warnings at real fires! This fail-

ing has often cost multiple firefighters

their lives.

It is common for compartment & struc-

tural fires to demonstrate classic warn-ing signs for the potential of an event

but rarely do they actually progress to

flashover, backdraft or some form offire gas ignition.

This makes firefighters somewhat com-

placent and rarely do they communicatewhat they have seen to the Incident

Commander.

It is absolutely essential that any suchwarning signs or fire behavior indicators

are immediately communicated to theIC and where possible, an immediate

action/s should be taken to alleviateconditions whilst the building is evacu-

ated. Rapid Intervention Teams onstandby and sector/safety officers

should also be made aware of such indi-

cators.

If classic warning signs are observed

and any immediate tactical action takento counter such a situation are ineffec-

tive, a defensive firefighting approach

should be implemented until conditionsimprove and warning signs disappear.

It should be noted at this point that in

some instances, there may be no warn-ing signs whatsoever of an impending

event of rapid fire development. Someevents simply just happen without warn-

ing! Further still, some events may oc-

cur some way into the firefighting op-eration, even when general fire condi-

tions appear to be improving.

The above gravity current wheresmoke is seen to be exiting a doorway at

high velocity can be taken as a clearwarning of a fire that may be heading

towards an event of uncontrolled rapid

fire development. Entry into such anincident is highly dangerous.

lows the temperature to increase in thecompartment as the vent opening is not

large enough to allow sufficient heat to

escape. As more heat is being generatedby the air inflow feeding the fire than

can escape from the opening, a flash-over may result.

Therefore it can be seen that it is some-

times very hard to differentiate between

a flashover or a backdraft.

The basic scientific definitions of flash-over or backdraft suggest that flashover

is primarily heat induced whilst back-

draft is ventilation induced. However,it is also scientifically documented that

an inflow of air/oxygen into an underventilated fire compartment can cause a

situation leading to thermal runaway.

This is where ventilation (intentional or

as a result of fire development) causesan increase in the burning rate but al-

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Classic Warning Signs of an Impending Event

Hazard Recognition & Warning Indicators

Flashover or Backdraft?

The feeling or sound of air

rushing in to feed the fire, or the

reversal of smoke , causing it to

head back into an opening

EXTREM E FIRE BEH AVIOR


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3D FIREFIGHTING

sists our aims, it some-

times serves as the cata-

lyst for such an event. Ifnot applied correctly at

the right location and atthe right time, a tactical

venting action may actu-ally cause a fire to flash-

over or backdraft.

A constant-flow fog pattern may alsoserve to push fire gases around and up-

set thermal balance. Ar-

guably, the most effective

approach is normally toconfine the fire wherever

possible and practical andapply pulses or bursts

of water fog into the 3Dgas layers in a controlled

manner, observing the

effects after each application and adjust-ing the bursts to suit conditions.

The three main approaches that are

commonly used to prevent or counter

any potential for rapid fire developmentare -

Tactical Ventilation & PPV

Fire Confinement

3D Water-fog tactics

It is important to realise that althoughthe first of these three options often as-

It can be seen that there are several

types of event that might lead to an igni-tion of the fire gases accumulating in

different parts of the structure. Theseignitions may sometimes be explosive

but in other instances any such ignitionmight result in a slow rolling flame

front or brief burst of flame pockets

where the gas air mix is not uniformthroughout the entire smoke layer.

A fire may cause smoke accumulationsto build up in the fire compartment, in

adjacent compartments and possibly in

compartments or voids sited quite someway from the fire itself. If these gases

are heated sufficiently they could auto-ignite on reaching air. If they are cool

but already existing in a pre mixed statewith air, any introduction to a flame

source could ignite the gas accumula-

tion, possibly explosively.

If a fire smoulders in a sofa, or under-

neath a mattress or rubbish bag, or per-haps down the back of a pile of foam

backed carpet rolled up in a cupboard,the room or space containing the fire

may become filled with highly flamma-ble smoke. If the fire is uncovered, or

disturbed, prior to clearing the smoke

from the compartment, a violent explo-sion may occur. This type of event has

resulted in the deaths and serious inju-ries of multiples of firefighters.

On occasions a flammable smoke layer

will accumulate at high level in a largevolume structure, or within a suspended

ceiling. Its presence may not even benoticed by firefighters dealing with the

base fire. If a burning ember floats onconvection into the gas layer above,

again an ignition of the smoke may oc-

cur.

If a fuel rich smoke layer exists at high

level it can be forced to move in the di-rection of an air supply and fire source

at a lower level. This could occur wherea constant flow fog pattern is used that

might push the gases around in the firecompartment. Or a ceiling might col-

lapse, causing a large movement of

gases to transport towards an ignitionsource. In either situation the gases may

ignite.

In some instances it is arguable that an

inflow of air is needed before a fire gas

accumulation becomes ignitable and thismight be defined as a backdraft. The

major consideration in defining an event

is to look at the final movement thatcaused the ignition - either air to fire; airto smoke; fire to smoke; smoke to fire;

smoke to air.

killed multiples of firefighters. It is cer-tain that wood products or wall panels

will give off white smoke when heated

and this smoke is highly flammable. Ifallowed to accumulate in a compartment

the hazards of entering that compart-ment should be fully appreciated and

addressed in the risk versus gain assess-ment.

A review of previous fire reports andexperiences suggests that although

smoke colour is often misleading as an

indicator of an impending event of rapidfire development or extreme fire behav-

ior, the existence of white smoke in anygreat amount may be taken as an addi-

tional indicator or warning sign.

There have been several fires where

large quantities of white smoke wereobserved just prior to an event that

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Tactics used to Prevent or Counter Rapid Fire Development

Smoke Explosion, Backdraft or Fire Gas Ignition?

White Smoke Explosions

Recognise the 3D hazard and

use techniques that are

effective on the 3D risk without

upsetting thermal balance


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ganization. It might include the purpose of the organization, its mission,

founding date, and a brief history. You could also include a brief list of

the types of products, services, or programs your organization offers, the

geographic area covered (for example, western U.S. or European mar-

kets), and a profile of the types of customers or members served.

It would also be useful to include a contact name for readers who want

more information about the organization.

Fire2000.com

Suite 17

Bn43 5GS

United Kingdom

STANDARD OPERATING GUIDELINESOG4242/2006

may be devastating and is termed highpressure backdraft.

A slightly different effect may occurwhere a fire is subjected to a constant or

gusting wind. Just as blowing on a bar-becue increase the burning rate, an exte-

rior wind will raise the heat release of aroom fire. Such effects are particularly

common in high rise buildings. If a win-dow should fail whilst firefighters are

occupying a compartment then the exte-rior wind blowing in might create a

blowtorching effect as the flames in-crease dramatically.

The dynamic pressures sometimes exist-ing in stairshafts (natural stack effects)

may actually cause windows to besucked inwards or blown outwards

within a few seconds of entering a com-partment, allowing an exterior wind to

create havoc on the fire floor! It is es-

Severe instances of rapid fire develop-ment are common in situations where an

exterior wind is blowing into a fire com-partment opening. It might be that

smoke and heat is prevented from exit-ing a fire compartment by an external

wind and this then accumulates insidethe structure. This build-up of exterior

air, smoke and fire gases actually causes

internal pressures within the fire com-partment to far exceed normally ex-

pected values.

A sudden decompression and resultingignition may occur where an internal

door is opened by firefighters makingtheir approach on the fire. This ignition

sential that Incident Commanders takethe wind strength and direction into ac-

count when doing a 360oand selectingan entry point. If the wind is likely to

create difficulties during advancementthen always have it on tour back wher-

ever possible. If it is not possible to se-lect an entry point with the wind to your

back then serious consideration mightbe given to a defensive attack on any

particular fire.

HowVentilation ParametersInf luenceRapidFireProgress

Phone: +44 (0)7843 476824

Fax: +44 (0)870 762 7515

Email: [email protected]

Rapid Response

Fast Attack Tactics

3D Firefighting

High Pressure Backdrafts& Blowtorching Fires

10. Extreme Fire Behavior

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