By Shan Raffel
Published Monday, July 11, 2011
Every fire sends out signals that can assist firefighters in determining the stage of fire development and, most importantly, the changes that are likely to occur. This skill is essential to ensuring that the correct firefighting strategies 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.
Get Familiar with B-SAHF
In the late 1990s, European firefighters were being taught to recognize the “signs and symptoms” of flashover and backdraft in the context of increasing synthetic fuel loads. A focus on compartment fire behavior training was revolutionizing their approach to fire attack and tactical ventilation. The following information is based on this work and seeks to simplify the risk-recognition process into a logical and easy-to-remember format. More than a decade of research, consultation and personal experience has resulted in the simple mnemonic B-SAHF:
- Building (the context)
- Air Track
The four fire behavior indicators—the SAHF part—can be used to rapidly assess the stage of fire development and the changes that are likely to occur before, during and after fire service intervention. But what about the “B”?
The Building Construction Context
Fire Chief Ed Hartin of Central Whidbey Island (Wash.) Fire & Rescue is the owner of CFBT-US LLC, a training company specializing in compartment fire behavior training. Hartin recommended adding the “B” in front as a reminder of the need to read the indicators in the context of building construction. The style and construction method and materials can have a significant impact on fire behavior and the indicators that may be present. (For FireRescue articles by Ed Hartin, visit www.firefighternation.com/author-Ed-Hartin.)
The thermal properties of the walls and ceilings have substantial influence on how a fire will develop. Modern construction methods combine materials to make the most of the different densities, specific heat capacity and conductivity to produce building elements that are lighter, stronger and less expensive. If the linings have good insulation properties, then the heat energy is less likely to be conducted through the walls and ceiling and out of the room. Consequently, heat energy will be retained. This means that the temperature in a well-insulated room could rise more rapidly than a room with linings having high thermal mass or poor insulative properties. This can slow the growth rate, but once the linings have absorbed the heat energy, they will tend to retain it and re-radiate the energy for some time.
When double or triple window glazing is combined with good insulation, the reduced air supply will restrict fire, and these windows are much less likely to fail.
These are just a few of the main factors to consider when assessing the impact that building design and occupancy will have on fire development. Now that we’ve addressed the building construction aspect, let’s turn to the four fire behavior indicators.
SAHF: The 4 Fire Behavior Indicators
The Fire Behavior Indicators include the following four main categories and subcategories:
- Height of the neutral plane
- Color and thickness (optical density)
- Volume and location
- Buoyancy and pressure
- Velocity and direction
- Flow—turbulent or smooth
- Whistling sounds
- Blackening of windows with no flame showing
- Cracking or crazing of glass
- Discoloration of glass
- Blistering or discoloration of paintwork
- Sudden heat build-up
We’ll now address each indicator, one by one.
Height of Neutral Plane
As the fire develops, the neutral plane (the boundary between the heated smoke and the cooler air) 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 backdraft conditions.
- A sudden rise could indicate that ventilation has occurred.
- Gradual lowering often indicates a build-up in fire gases and progress to flashover.
- Sudden lowering could indicate a rapid intensification of the fire.
Color & Thickness
Dark smoke often indicates rich conditions due to restricted air supply. When flaming or smouldering combustion is occurring, soot is drawn into the plume, which results in black smoke. If the air supply is good, some of the soot will be burnt in the reaction zone (flame), which will result in less smoke and a yellow flame.
Lighter-colored smoke (sometimes nearly white) is produced when the solid fuel is heated to the pyrolization temperature. Volatile components of the fuel are released, while the soot is left behind to form char. Radiant heat in the fire compartment and heat transferred to neighboring compartments can cause the pyrolysis of linings and adjacent combustibles. This white smoke can drift into uninvolved sections of the building and accumulate. Due to the high energy content and partial pre-mixing with air, the introduction of flames to accumulated white smoke can result in an unexpected powerful ignition. Caution: Smoke can turn white after water has been applied to the fire as a result of the condensation of water vapor. I don’t agree with the theory that dark smoke can be filtered and turned white if forced through gaps.
Grey smoke generally 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 grey smoke.
Brown smoke can be released in the early stages of the pyrolysation of timber products as the tar content is released.
Caution: The color of the smoke is NOT a reliable method of determining the quantity of unburned fuel present. White smoke can have a very high fuel content, and dark smoke may have a lot of soot but smaller quantities of unburned fuel.
Volume & Location
The visible smoke volume and location is not always a reliable indicator of the location, size or stage of development. Heated smoke will rise vertically and spread out horizontally when vertical movement is restricted. Smoke can travel through concealed voids and shafts and emerge in totally unexpected locations. As with all fire indicators, it is very important not to read just one indicator in isolation.
Buoyancy & Pressure
Smoke seen rapidly expanding upward and rolling indicates that the gases are at a high temperature. In contrast, smoke that’s released with lower temperatures has a tendency to slowly drift upward or even downward. Lower buoyancy could indicate relatively low compartment temperatures, or it could be caused by cooling that has occurred as the smoke has travelled through uninvolved sections of the structure. Smoke forced out under pressure usually indicates that the exit point is close to the fire source.
2: Air Track
The air track is the flow of air toward the fire base and the movement of the heated combustion products up and out of the compartment. This helps to establish the ventilation profile by observing the movement of smoke in and out of openings.
Velocity & Direction
When an opening is created in a fire compartment, 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. High ventilation will cause a sudden in rush that is not followed by a sudden out rush.
Smoke or flame being discharged from the entire height of ventilation openings usually indicates that it’s an outlet and that the ventilation inlet is in another part of the structure. An opening that is both a ventilation inlet and outlet will show signs of smooth or turbulent flow at the air/smoke interface.
Flow—Turbulent or Smooth
When viewing an opening in the fire compartment, a slow and laminar (smooth) air/smoke interface 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 seen with a low neutral plane), this could indicate a working fire that’s in the ventilation-controlled phase. Caution: Do NOT make assumptions based on viewing only one ventilation opening; quite often there will be multiple air tracks making up the full ventilation profile.
Pulsations & Whistling Sounds
Smoke seen pulsing out of small openings can indicate variations in pressure from a limited air supply. As the fire consumes the available oxygen, the combustion process is severely hindered, which decreases the internal temperature, causing the gases to contract in volume, which creates a negative pressure that draws in air. The inrush of air allows the fire to increase slightly, which causes an increase in temperature until the air is consumed. This sets up a pulsation cycle that may continue for hours. This may be accompanied by whistling sounds, which is a classic indicator of backdraft. Extreme caution should be exercised before creating any opening in these conditions. Cool the gases and surfaces before you vent.
Blackening of Windows with no Flame Showing
Blackening indicates rich conditions (backdraft potential) and is often accompanied by oily deposits on the inside of the window. This indicator may be difficult to see on double- or triple-glazed windows.
Cracking or Crazing of Glass
Rapid heat build-up can result in cracking of glass, and crazing (finer cracks) can be seen when the heat build-up has been slower.
Blistering or Discoloration of Paintwork
This can be easily seen on lightweight internal doors, but may be absent in heavy, well- insulated exterior doors. The evaporation of a water film applied across a door surface may help to indicate the presence of the heat layer. This will occur at temperatures as low as 212 degrees F (100 degrees C), and thus can provide an indicator at a lower temperature than required for blistering and discoloration.
Sudden Heat Build-up
This indicator is frequently noted as a sign that flashover or backdraft is imminent. However, firefighters should NOT rely on this indicator, as it often occurs AFTER some form of fire gas ignition has commenced in the upper section of the compartment. This is often obscured from view, and by the time firefighters feel the heat through PPE, they will be in a very dangerous situation. The objective of reading the fire is to prevent firefighters experiencing this late and potentially fatal indicator.
Traditionally, we have been told that the color of the flame indicates the product burning. This is less applicable to a structure that is filled with multiple fuel types. In reality, the same product can burn with different colored flames depending on the efficiency of the combustion process.
The combustion of propane provides an excellent example of how the flame color is influenced by the efficiency of the process. When premixed with air, a blue flame is produced. When the fuel and air are mixed by the process of diffusion, the combustion process is less efficient and a yellow flame is produced due the presence of soot particles. Propane burning in an oxygen-deficient or fuel-rich environment can produce a red flame.
A simple example of a carbonaceous product can be seen with the combustion of particle board. When the air supply is good, the flame is yellow. When the oxygen concentration is reduced, the flame becomes a reddish-orange color.
In a compartment fire, yellow flames generally indicate a reasonable air supply. Reddish-orange flames are an indicator of less oxygen and fuel-rich conditions.
The shape or form of the flame can also give an indication of the type of combustion occurring. The reddish-orange flames are often turbulent with a short-wave form. The ignition of accumulated pyrolysis products (aka combustible gases) produces a very light-yellow flame (sometimes almost clear). Amazingly in this case, the wave form is larger and the flames seem very slow.
Predicting the behavior of fire in a compartment is difficult because of the great number of variables that need to be considered. Although there are no simple answers to such a complex problem, there are fire indicators that can assist in building up an accurate profile of the fire. And although we can never be 100% certain of all factors, these simple indicators found in B-SAHF give us a foundation for formulating the safest and most effective strategy and tactics.
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