FireFighterNation

By Shan Raffel
Published Tuesday, August 9, 2011

In my first article on this topic, “The Art of “Reading Fire," I discussed a new mnemonic for fire behavior indicators, B-SAHF, which stands for Smoke, Air, Heat and Flame, all in the context of Building Construction. In this article, I will examine the fire behavior indicators for flashover, backdraft and fire gas ignition, and provide tactical options that can be matched to each event.

FLASHOVER PROFILES
Although there are a number of definitions for flashover, most agree on the following:

• Flashover is a transition from a developing fire to a fully developed fire.
• Once the flame spread begins, it is rapid and involves all surfaces and leads to full involvement in the compartment.(1)

One of the keys to flashover is that it requires a reasonable supply of air to allow the development of sufficient radiant heat to raise the temperature of all of the combustibles to their ignition temperature.

Let’s now re-examine the fire behavior indicators, starting with Building Construction, as all others must be considered in this context.

Building Construction
Flashover will occur in most buildings if sufficient air is available. Compartments with limited natural air flow are less likely to flashover before the available air is consumed. Heavy brick or cement rendered walls will absorb a lot of energy, which could delay flashover.

Smoke
Volume and location can be unreliable indicators that must be read in conjunction with other indicators present in the actual fire compartment. The color can vary from gray to black, with the neutral plane close to the middle of the room and gradually descending. White pyrolysis smoke will be seen coming off neighboring combustibles as the room approaches flashover.

Air
Observation of the air/smoke interface at openings in the fire compartment can provide valuable information as to the stage of development and the burning regime. Prior to flashover, the interface is generally high in the openings, and the flow is smooth or laminar. As the fire progresses toward flashover, the height of the interface will lower and become turbulent. This indicates that the fire is growing to the point where the available openings are not sufficient to allow the volume of air the fire needs to continue its growth. This shift to a ventilation controlled burning regime usually indicates that the fire is getting closer to. The size and number of openings will determine when the air/smoke interface starts to become turbulent. If the openings are relatively small, even a small fire are could move to a ventilation controlled burning regime. It is therefore important to consider all of the indicators before making assumptions.

Look for changes. If the initial observation showed a smooth interface and a short time later the interface has become turbulent, then we know that the fire is developing rapidly.    

Heat
Heat indicators are often present as flashover approaches, but this depends on the type of building construction, the type of insulation and how rapidly the fire develops. Discoloration and blistering of paintwork are usually seen higher up in the room or at openings. Surfaces may be hot to touch. Windows will darken and crack as the temperature increases. Additionally, failure of glass may allow a sudden increase in available air, which could trigger flashover or accelerate the progression toward flashover.

Flame
As the accumulated unburnt fuel approaches auto-ignition, isolated flames may travel across the ceiling. This is sometimes referred to as ghosting, and it usually proceeds rollover. This may be difficult to see if the smoke layer is thick.

Yellow flames at the base of visible burning surfaces indicate a reasonable supply of air being drawn to the base of the fire. Reddish-orange flames may indicate that less air is available or that the conditions are fuel-rich in the over-pressure region.

Orange to red turbulent flames may be present higher up on the room. When pockets of accumulated white pyrolysis gases ignite, clear to yellow flames with a long, slow wave form develop. This is a sign that the room is very close to flashover.

Possible Actions
Since the proliferation of plastics into almost every item of modern furnishings, the time from ignition to flashover can be as little as a few minutes. If the air supply to the room is limited, then the time to flashover could be increased slightly. Creating an opening in these circumstances could provide sufficient air to produce a rapid acceleration to flashover. As such, ensure that sufficient handlines are in place, and cool the smoke with water fog before fully opening up the room.

Door-Entry Techniques
Opening a doorway is a form of ventilation, and firefighters must understand the impact they can have on the fire by introducing air. Before opening doors or windows, you should conduct a quick B–SAHF assessment to determine the conditions likely to be encountered behind the door.

Further, before opening, you must take actions to stabilize the area immediately around the firefighters. Smoke that is released by creating the opening will flow over the cooled, moist surfaces and into the suspended water droplets. Some of the actions involved include:

• Apply a light film of water (termed “painting”) to the linings above and around the door to check for heat, cool the surfaces and provide a buffer.
• Suspend 3D water-fog over the doorway to cool any accumulated smoke.

One member of the team should control the door and ensure that it’s only opened wide enough to place a small amount of water into the over-pressure region. Water-fog is very efficient at cooling the gases, and if two or three pulses are used, there will be very little entrainment of air. Temperature checks can be conducted by placing short, narrow bursts of water onto the ceiling (at approximately 10 to 20 degrees). Listen for the hissing sound of water turning to steam, or watch and listen for the amount of water that comes back down. If a sizzling sound can be heard and little or no water comes back down, we know that the temperature is more than 212 degrees F (100 degrees C).

After the pulses, close the door for 5 to 15 seconds to allow for the cooling and dilution of the gases to take effect. Any time the door is opened, the team should be observing changes in the fire behavior indicators to assist with the ongoing, dynamic risk assessment. In particular, you should observe:

Smoke

• Is the height of the neutral pressure plane low or high?
• Is the smoke color dark or light?
• How thick is the smoke in the over-pressure region?

Air

• Is the direction and velocity of the air track flat or pulsing? Fast or slow?

Heat

• Listen for the sound of the water hitting super-heated linings or the sound of water falling back to the floor.
• Look for blistering of paintwork, melted plastic fittings, etc.

Flame

• Is there any visible flame? If so, what color is it? Is it just low down or over head as well?
• Are there any visual or audible indicators that victims may be near?

This process can be repeated several times to allow for improvements in conditions. If improvements are not achieved after three to five such applications, it is unlikely that any further improvements will be achieved with this method alone. As such, any visible burning surfaces should be blacked down by pencilling before entry. Raking a jet across the ceiling will help to dislodge any loose debris, and any disruption to the thermal balance should be minimal and short lived. If possible, these actions should be coordinated with ventilation.

BACKDRAFT PROFILES
One definition for backdraft is: a fire phenomenon caused when heat and heavy smoke (unburned fuel particles) accumulate inside a compartment, depleting the available air, and then oxygen/air is re-introduced, completing the fire triangle and causing rapid combustion. Let’s consider this in the context of our fire behavior indicators.

Building
Backdraft is more likely to occur in energy-efficient buildings with good insulation, climate control and well-sealed double/triple glazing. The limited natural air flow may prevent the fire from developing to flashover, or there may be insufficient oxygen to sustain fire development after a flashover has occurred.

Insulated shafts, voids or spaces with limited air supply may also prevent a fire in these areas from developing fully, and backdraft conditions could be present for some time after the initial ignition. The fire may break out of these spaces if the heat causes part of the structure to fail, or it could occur when occupants or firefighters suddenly open the space to investigate.

Smoke
Smoke color can vary enormously and is not a reliable indicator on its own. Even smoldering combustion will produce a dark smoke. Heat transfer into non-fire-involved spaces can cause pyrolyzation of combustibles, which will release a white smoke. Timber products or structural elements will often release a brown-colored smoke.   

There are often indicators of a very low neutral plane with turbulent smoke forced out low through gaps. Smoke seen pulsing in and out of cracks in doors or windows at a low level indicates the presence of a pulsation cycle in that space.

Air
A sudden change in the direction of smoke movement in doorways or other openings is a classic backdraft indicator. Backdraft is a variable event, and ignition can occur seconds or even minutes after openings are created. Be aware that there may be multiple small air tracks feeding the fire.

Heat
Discoloration and blistering of paintwork may be present and possibly give indication of the height and temperature of the heat layer. Darkening of windows indicates rich conditions, and cracking indicates high temperature conditions. The surface may be hot to touch, but this depends on the insulation and construction materials.

Flame
Little or no flame may be visible pre-backdraft, but conditions can vary widely in different parts of the structure. Super-heated, fuel-rich smoke may auto-ignite after leaving the compartment of origin. When an opening is created, flames may occur around the smoke/air interface as air is drawn in toward the fire base. This is a very late indicator! Additionally, the presence of small red or orange flames may indicate fuel-rich conditions. Pockets of blue flames are said to be from the auto-ignition of carbon monoxide.

Possible Actions
When you recognize backdraft conditions, opening the compartment will supply the missing side of the fire triangle (air). If the unburnt fuel component is above the auto-ignition temperature (AIT), it is possible that they will ignite spontaneously when they mix with air. Sometimes the mixing process with air will cool the fuel to below the AIT. Even if there is no ignition shortly after ventilation, it is still possible that smoldering material will burst into flame as the air moves into the room.

Cool Before You Vent!
The traditional approach to dealing with backdraft has been to ventilate high. Another approach that may be faster and safer is to introduce water spray or mist into the space before opening up. This does a number of things:

• It cools the temperature of the gases and reduces the chances of heated gases auto-igniting when air is introduced.
• It cools suspended water droplets and disrupts flames.
• It cools water droplets that reach combustible surfaces and reduces pyrolyzation. Even the application of water to non-combustible compartment boundaries will reduce the overall temperature in the involved area.
• Cooling external surfaces outside of the ventilation opening reduces the chances of auto-ignition by ensuring that the exiting gases will flow over cooler, moist surfaces.   

In backdraft scenarios, it is vital to ensure that charged hoselines are in place before ventilation is commenced.

Scandinavian firefighters have pioneered to the use of “fognails” (photo #?), which can produce a fine fog at normal pressure. There are a number of extinguishing tools that produce an ultra-fine, high-pressure fog that can be combined with an abrasive cutting compound. This allows firefighters to rapidly pierce through a wide range of building materials. Once the small hole has been created, the abrasive is turned off and the water fog fills the compartment without the introduction of air.

FIRE GAS IGNITION PROFILES
Although flashover and backdraft are reasonably understood, it is essential for firefighters to realize that not all sudden fire events occur in or near the fire compartment. In some circumstances, large volumes of unburnt fuel can move through the structure and premix with air. This can be difficult to detect, and while the smoke cools and thins out, it may still contain sufficient fuel to form flammable mixture that could be ignited with explosive force.(2)

Building
Voids, ducts, shafts, balloon frame, large open plan, high/suspended or false ceilings, etc., may allow smoke to move through a structure and accumulate in areas adjacent to the compartment of origin, or some distance from it. Building modifications or additions can create unexpected openings or voids, particularly when combined with poor or damaged smoke/fire stopping.

Smoke
Smoke can accumulate and emerge some distance from the source, giving a false indicator of the location of the fire compartment. Smoke that has travelled some distance from the fire compartment may appear lighter in color due to partial mixing with cooler air as it moves through the structure. The neutral plane may be less defined as the smoke mixes and cools. Don’t think that this cooler and thinner smoke isn’t dangerous; it is!

Air
The velocity will slow as the smoke moves farther from the source and spreads out (mushrooming). The further the smoke moves from the compartment of origin, the more likely it is that the air interface will be smooth.

Heat
The farther the smoke has travelled, the greater the cooling effect. This could lead to a lack of heat indicators. Note: There is a danger that cooler, light smoke may not be perceived as a risk!

Flame
No flame may be present in the space immediately prior to ignition. A common trigger for these types of events: fire suddenly breaking out of the compartment of origin and propagating through the accumulated smoke layer.

Possible Actions
Look for indicators of modifications or building features that could allow smoke to travel through voids, ducts, shafts or other openings. Be vigilant for signs of smoke in areas remote from the known fire area. Additionally, use aerial appliances/ladders to check the roof for signs of accumulating smoke. Use thermal imaging cameras to check for possible extension into void spaces. If in doubt, cool and open up suspect spaces. Treat all accumulated smoke as potentially dangerous. Cool and/or ventilate it!

CAUTION AHEAD
With all this in mind, consider the following cautions related to flashover, backdraft and fire gas ignition:

• Never rely on one indicator.
• Remember that there may be more than one air track.
• Look for changes in the indicators as the fire develops.
• The indicators are most reliable when looking at the compartment involved, rather than the smoke that has travelled some distance from the fire room.
• Risk assessments can only be as complete as the information gathered. There are a great number of variables and it is almost impossible to know all the factors.
• Building construction features, such as thick walls and/or energy-efficient insulation, may make it difficult to see heat indicators until the fire is well advanced.

Final Thoughts
Firefighters regularly work in a dangerous and rapidly changing environment. They have to make decisions in seconds, with very limited information, and take actions that may save (or endanger) lives. The best we can do is to base those decisions on our knowledge of building construction and fire behavior indicators at each stage of the incident. There are no “easy answers,” but as scientific research enlightens our knowledge of fire behavior, we must be prepared to challenge our traditional strategies and open our minds to new possibilities.

References
1 Kennedy PM, Kennedy KC. Flashover and Fire Analysis: A Discussion of the Practical Use of Flashover Analysis in Fire. (2003). Retrieved July 10, 2011, from www.kennedy-fire.com/PDFs/Flashover.pdf.

2 Grimwood P, Hartin E, McDonough J, Raffel S. 3D Fire Fighting Training, Techniques and Tactics. Fire Protection Publications. 2005 (s1).

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  Shan Raffel

  Shan Raffel is currently serving as an operational station officer in Brisbane, Australia. He is dedicated to exploring ways of improving efficiency and safety. During his 28-year career in the fire service, he has conducted numerous international study tours--including a 10-week Churchill Fellowship to research planning, preparation and response to emergencies in tunnels--in the USA (FDNY), Canada, Germany, Austria, Scandinavia and Switzerland. Read Full Bio

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