"Every fire sends out signals that can assist the firefighter in determining the stage of fire development, and most importantly the changes that are likely to occur. This skill is essential to ensure the correct firefighting strategy and tactics are employed.

Being able to “read a fire” is the mark of a firefighter who is able to make decisions based on knowledge and skill, not guess work or luck."

Shan Raffel

*NEW - Fire behaviour risk assessment chart.

Reading the Fire

Every fire sends out signals that can assist the firefighter in determining the stage of fire development, and most importantly the changes that are likely to occur. This skill is essential to ensure the correct firefighting strategy and tactics are utilised.

Being able to “read a fire” is the mark of a firefighter who is able to make decisions based on knowledge and skill, not guess work or luck.

Shan Raffel EngTech MIFireE

Around 1999, firefighters were being taught to recognise the "signs and symptoms of Flashover" and the "signs and symptoms of Backdraught" This information provided a foundation for the skill of reading the fire. There was no mention of the indicators that could lead to a “fire gas ignition”, and I saw the need to have a simple method of assisting firefighters to rapidly make a risk assessment based on the key fire behaviour indicators (FBI). After a lot of research I came up with the simple mnemonic, SAHF. To be effective it is essential that fireground mnemonics are simple to remember and logical in their order.

SAHF (pronounced safe) stands for:
Smoke
Air
Heat
Flame

While there is a lot of hard and soft information that must be gathered quickly during fire size-up, the 4 key fire behaviour indicators can be used to rapidly assess the stage of fire development and the changes that are likely to occur.

Accurate fire development size up is essential to ensure that the safest and most efficient method of attack is utilised. In particular, a “Tactical Ventilation” plan cannot be safely developed or implemented until a SAHF assessment is conducted. “Reading the Fire” is an essential element in the development of an overall tactical plan as well as a critical part of the personal risk assessment that should be applied by each team member. Tasked teams should use the SAHF protocol to assess the risk in their area of operations. The critical indicators 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.

The 4 fire key behaviour indicators can be further broken up into a number of elements.

1.     SMOKE

            Height of neutral plane

            Colour and thickness

            Volume and location

            Buoyancy and energy

2.     AIR TRACK

            Velocity and direction

            Flow – turbulent or smooth

            Pulsations

            Whistling sounds

 3.     HEAT

            Blackening of windows and no flame showing

            Blistering of paintwork

            Sudden heat build up 

4.     FLAME

            Colour

            Volume

            Location

1. SMOKE

 Height of neutral plane
As the fire develops the neutral plane will lower and the thickness  of the smoke gases will increase.
 Therefore,

·         A high neutral plane could indicate that the fire is in the early stages of development.

·         A very low neutral plane could indicate very rich backdraught like conditions.

·         A sudden rise could indicate that ventilation has occurred.

·         Gradual lowering could indicate a build up in fire gases and approaching flashover.

·         Sudden lowering could indicate a sudden intensification of the fire

These photos show the lowering of the neutral plane as the fire progresses towards flashover. Photo courtesy Tim Watkins

Colour and thickness

Smoke colour can vary with the type and form of fuel package 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 smoldering combustion is occurring, the carbon in the product is released in the smoke and a very dark colour is the result. If the air supply is good, some of the carbon will be burnt in the reaction zone (flame) which will result in less smoke and a yellow flame.

Lighter coloured smoke (sometimes almost white) is produced when the fuel is heated to the pyrolysation temperature. At this temperature, the volatile components of the fuel is released and the solid carbon is left behind to form char. If the heat continues to increase and there is insufficient oxygen even for smouldering combustion, then the production of white smoke will continue. It is important to realise that as the fire develops heat will be transferred to neighbouring compartments which can cause the pyrolysis of linings and adjacent combustibles. This white smoke will accumulate and drift into uninvolved sections of the building. Even through this smoke is at a lower temperature is has a very high energy content and the introduction of flames into these spaces can result in a very sudden and powerful ignition.

When grey smoke is present it indicates that at least some smouldering combustion or flaming combustion is present. Where there is a mixing of the darker smoke from flaming or smouldering combustion with the whiter Pyrolysis smoke, the result can be a grey smoke.

Brown smoke can be released in the early stages of the pyrolysation of  timber products. This is caused when the lignin breaks down and the tar is released.

Volume and location

The volume of smoke can be a guide to the fire size and location. In some cases it can be unreliable and can actually give false indications as to the location, fire size and stage of development. Smoke can travel through concealed voids and shafts and emerge in totally unexpected locations. Most firefighters have experienced a structure releasing large volumes of smoke and later discovered that the actual fire area was quite small or in a totally unexpected location.

The basic principle is that the heated smoke will tend to rise vertically. When it reaches horizontal obstructions it will spread out and look for further openings to allow vertical travel. The longer the path of travel, the cooler the smoke will become. This will also result in partial pre-mixing of the air and smoke. As with all fire indicators it is very important not to read one indicator in isolation.

 Buoyancy and energy

Smoke seen expanding upwards and roiling indicates that the gases are at a high temperature. Even very dense fire products will be lighter than air when they are heated to high temperatures. In contrast, smoke that is released with low energy and has a tendency to slowly drift upwards or even downwards indicates that the temperature of the gases is lower. This lower temperature and buoyancy could indicate relatively low compartment temperatures or it could be caused by cooling that has occurred as the smoke has travelled some distance through uninvolved sections of the structure.  pre flashover temperatures.

2. AIR TRACK 

The air track is the movement of air towards the fire base 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 backdraught event. In some cases this will be followed by an out rush, and seconds later the backdraught. High ventilation will cause a sudden in rush, but in this can it is not followed by a sudden out rush.

 Flow – Turbulent or smooth

 If 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 is lower as well) then this could indicate a working fire that is in the ventilation controlled phase. Vigorous pulsing of the air track is a strong indicator of an active ventilation controlled fire.

 Pulsations

 Smoke seen pulsing out of small openings can indicate a ventilation controlled fire. This indicates that there are variations in pressure due to limited oxygen supply. As the oxygen level decreases so does the combustion process, which in turn decreases the temperature and the gases contract. When the air is drawn in the fire starts to increase and the pressure rises again until the air is consumed and the cycle starts again. In some cases this could develop into a potential backdraught. Smoke seen pulsing out of larger openings is read very closely with the air track and is covered in the next section.

Whistling Noises 

Whistling noises may indicate that air is being pushed in and out of the compartment through small gaps or openings due to pressure variations. This indicates a ventilation controlled fire. It should be remembered that it might be difficult to notice this with all the background noise.

3. HEAT

The initial assessment should include looking for indicators of the temperature such as,

Blackening and/or crazing of windows.
The blackening indicates rich conditions (backdraught potential) and the crazing indicates high temperatures (hot rich combustion potential). Caution should be exercised in opening up under these conditions.

Blistering of paintwork
Sweeping a spray across a door or surface can also be used to test for surface heat. If the door is hot, the film of water on the upper section will rapidly evaporate. In some cases, it is possible to get an indication of the height of the neutral plane by observing the line at which the evaporation ceases.

Sudden heat buildup
 Frequently quoted as an indicator that flashover or backdraught is impending. It often indicates that some form of fire gas combustion has commenced in the ceiling area. This may be difficult for the firefighter to see. This is a very late indicator and should not be relied upon to give adequate warning time. Temperature checks can be performed by placing a small burst of water, on a very narrow pattern, into the overhead layer.

If the water returns to the ground without any hissing, it is likely that the ceiling temperature is below 100°C in that area. If on the other hand, the water does not come down, and a hissing sound is heard, it would indicate that the temperature is over 100°C. Firefighters can also carefully raise the gloved hand to feel for heat build up. If no excessive heat can be felt through the glove, the bottom of the glove can be slipped back over the palm to expose the skin, and the hand cautiously lifted overhead to feel the heat layer.

Regular checks will assist in determining temperature variations and can give the firefighter an indication of the thermal layer.   

 4. FLAME

 The colour of the flame can give an indication of what product is burning. This can however be misleading, as the same product can burn with different coloured flames depending on the combustion process. For example LPG that is premixed with air will produce a blue coloured flame (due to the presence of CO2). If the fuel and air are mixed by the process of diffusion, then the flame will be yellow due the presence of carbon particles.  

Another example is the combustion of particleboard in a compartment. When the air supply is good it will burn with a yellow flame. If the oxygen concentration is reduced the flame becomes a reddish orange colour.  

In a compartment fire, yellow flames generally indicate a reasonable air supply. Reddish orange flames are an indicator that there is less oxygen available and a rich combustion is occurring. The shape or form of the flame can also give an indication of the type of combustion occurring. The reddish orange flames that result from the rich combustion are often turbulent with a short wave form. The ignition of accumulated pyrolysis products produces a very light yellow flame, sometimes almost clear. Amazingly in this case, the wave form is larger and the flames seem very slow. The formation of blue flames around the neutral plane is said to be due the presence of pockets of carbon monoxide that have formed flammable concentrations.

 As with all of the indicators in the SAHF assessment, it is important to look at the initial flame colour and then note any changes.

 Summary

Accurate size up is essential to ensure that the safest and most efficient method of attack is utilised. In particular, a “Tactical Ventilation” plan cannot be safely developed or implemented until a SAHF assessment is carried out. Emergency risk assessment is dynamic and must be applied until the incident is completed. “Reading the Fire” is an essential element in the development of an overall tactical plan as well as part of the personal risk assessment that should be applied by each team member.

These skills should be developed by a combination of theory, small scale carbonaceous demonstrations, large scale carbonaceous demonstration and if possible carefully controlled single room burns in real structures. Further understanding can be gained by the observation of video footage from actual fires. However, it is only through operational experience and open review that these skills are fully developed.

 Bibliography

 1. Grimwood P, Hartin E, Mc Donough J, Raffel S, - 3D Fire Fighting Training, Techniques and Tactics – Fire Protection Publications, Oklahoma State University,2005

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