By Ed Hartin
Published Friday, August 1, 2008
| From the August 2008 Issue of FireRescue
It goes without saying: Firefighting is dangerous work. Responding, for example, to a compartment fire, firefighters are faced with dynamic, rapidly changing conditions that present a significant threat to building occupants and firefighters alike.
We often make rapid strategic and tactical decisions based on our experiences and expectations of how the fire will behave. In 1948, fire scientist Harry Gisborne made several observations about wildland firefighters’ experienced judgment that can apply to structural firefighters today: “For what is experienced judgment except opinion based on knowledge acquired by experience? If you have fought fires in every type of building with every different configuration and fuel load, under all types of conditions, and if you have remembered exactly what happened in each of these combinations, your experienced judgment is probably very good.”1
None of us, of course, has this level of experience. The ability to read the fire, anticipate fire development and take appropriate action depends on integration of experience with a sound understanding of practical fire dynamics, coupled with an appreciation of our own limitations.
Wildland firefighters use the term extreme to describe “a level of fire behavior characteristics that ordinarily precludes methods of direct control action …”2 This term “is framed within the context of our perception with ‘extreme’ defining our limited ability to control it and its potential impact on firefighter safety.”3 Extreme can also be used to describe flashover, backdraft and smoke explosion in the structural firefighting environment. This article addresses the most common of these phenomena: the flashover.
Fire Development & Flashover
There are a number of definitions or ways to describe flashover. Basically, it’s a rapid transition to a fully developed fire. But it’s not unpredictable. Understanding this phenomenon and how it occurs is critical to firefighter safety.
A fuel package such as a couch burning in open air progresses through four phases:
- In the incipient stage, the fire is small and involves only a small amount of fuel.
- As the fire moves into the growth stage, more fuel becomes involved and the speed of the combustion reaction increases.
- Eventually, the entire object becomes involved and the fire is fully developed.
- As fuel is consumed, the fire begins to decay.
Throughout this process, fire development is fuel-controlled. The speed of fire development and energy release is dependent on the characteristics and configuration of the fuel, in this example a couch.
The heat of combustion is the energy released when a specific mass of fuel is completely burned. The total energy released when an object burns is dependent on the heat of combustion and the amount (mass) of fuel burned. However, this only provides part of the picture.
When fuel burns inside a compartment, fire development becomes influenced by the characteristics of the compartment. Thermal energy released by the fire that’s retained in the compartment increases fuel temperature, which, in turn, increases the speed of combustion. The most significant difference with fire in a compartment: the compartment’s ventilation profile. The size, location and configuration of openings in the compartment influence both the oxygen available and the retention or escape of thermal energy in the hot gases and smoke produced by the fire. The speed of this energy’s release is the heat-release rate.
All of this is very interesting, you may say, but what does this have to do with flashover? As it turns out, heat-release rate has everything to do with flashover.
If the fire releases energy faster than it can escape from the compartment, temperature will increase; and if sufficient energy is released, flashover will occur and the fire will transition rapidly from the growth stage to the fully developed stage. As this occurs, the fire will spread across all combustible surfaces in the compartment and flames will exit through compartment openings.
Ventilation & Flashover
In the incipient and early growth stages of a compartment fire, the speed of fire growth is fuel-controlled, with fire development substantially influenced by the chemical and physical characteristics of the fuel. However, oxygen is required for fuel to burn and release thermal energy. As a compartment fire develops, the available air supply for combustion becomes more important. Increased combustion requires more oxygen, and as smoke fills the compartment, it restricts the flow of air into the compartment (see Figure 1). As the fire becomes ventilation controlled, heat-release rate and fire growth are limited by the available oxygen supply.
For many years firefighters have been taught that ventilation reduces the potential for flashover. Although this is often true, it’s only part of the story. Increasing ventilation to a fuel-controlled fire will allow hot gases to exit, transferring thermal energy out of the compartment and replacing the hot gases with cooler air. The combined influence of these two factors slows progression toward flashover and increases the heat-release rate required to reach flashover.
On the other hand, when a fire is ventilation controlled, heat-release rate is limited by the available oxygen. Under ventilation-controlled conditions, increased air supply (ventilation) results in increased heat-release rate and establishes a path for fire travel, which may result in flashover.
In 1999, two firefighters in Washington, D.C., died and two others were severely injured as a result of being trapped by flashover.4 The fire occurred in the basement of a two-story townhouse-style apartment (two stories in the front, three stories in the rear). The first-arriving crew entered the first floor from the front of the building to search for the seat of the fire. Another crew approached from the rear and made entry to the basement through a patio door. Due to confusion about the configuration of the building and command’s belief that the crews were operating on the same level, the crew at the rear was directed not to attack the fire. The additional air supplied by these changes in ventilation resulted in a rapid increase in heat-release rate and subsequent flashover.
The National Institute for Standards and Technology (NIST) performed a computer simulation of fire dynamics in this incident.5 Figure 2 illustrates the timing of changes to the ventilation profile and resulting influence on heat-release rate.
As this case illustrates, the location and configuration of exhaust and inlet openings determines air track (movement of smoke and air) and the path of fire spread. When a compartment fire reaches flashover, flames extend out of compartment openings, often into adjacent compartments, following the air track to the exhaust opening.
At this incident, the patio door to the basement acted as an inlet, providing additional air to the fire. The front door and windows on the first floor opened for ventilation served as exhaust openings and provided a path for fire travel when flashover occurred.
Reading the Fire
Fire behavior indicators include a wide range of factors that firefighters may see, hear or feel. Some factors are relatively static (i.e., building construction) and others are quite dynamic, changing as the fire develops (i.e., smoke conditions and flames).
Figure 3 lists indicators of potential for flashover.6 The left column focuses on flashover potential during the growth stage, while the right examines indicators of potential for ventilation-induced flashover. (Note: Many of these indicators are common to both vent-induced flashover and backdraft.)
Training is often focused on what to do when things go wrong. But if firefighters must react to the occurrence of extreme fire behavior such as flashover, it’s already too late. Turnout gear and SCBA, as the last line of defense, provide limited protection. Most firefighters who have been severely injured and killed by flashover were wearing appropriate personal protective equipment (PPE) but were incapacitated by extreme thermal insult and unable to escape the fire building. Firefighters must place much more emphasis on staying out of trouble than on what to do when faced with the last few minutes of their lives!
Firefighters must read the fire on an ongoing basis throughout the incident and maintain an awareness of changing conditions. Recognizing the potential stage of fire development and if the fire is in a fuel or ventilation-controlled burning regime provides a starting point for managing the risk of flashover. However, firefighters should be proactive and control the fire environment to prevent flashover whenever possible.
Managing the risk of flashover should include the following:
- Wear your PPE properly. Although this is the last line of defense against being injured or killed by flashover, it needs to be one of the first steps that you take.
- Read the fire and watch for changing conditions and indicators of flashover potential. This is everyone’s job. If you see something, say something!
- Identify and maintain an awareness of potential escape routes (alternate exits) and areas of refuge (uninvolved compartments). Although the best approach is to control the environment and prevent flashover, it’s important to be able to escape if conditions change.
- Identify the current ventilation profile and how changes to ventilation may influence fire behavior. Control the air track to limit potential for flashover. Remember that your access point is a ventilation opening (inlet, outlet or both).
- Do not perform tactical ventilation (increasing air supply to the fire) until a charged hoseline is in place.
- Reduce the temperature overhead by applying brief pulses from a fog stream into the hot gas layer while moving to a point where direct attack on the fire is possible. This should not wait until flashover is imminent; rather, it must be an ongoing process when hot gases are present.
Remember, protecting yourself from flashover requires proactive—rather than reactive—firefighting based on a solid understanding of practical fire dynamics integrated with experience. If you or your crewmembers see indicators of flashover, speak up! A scene as dynamic as the fireground requires constant vigilance and anything less can quickly prove deadly.
- Gisborne, H. (1948) Fundamentals of fire behavior. Fire Control Notes 9(1), 13–24.
- National Wildfire Coordinating Group (NWCG). (2006). Glossary of Wildland Fire Terminology.
- Close, K. (2005) Fire behavior vs. human behavior: Why the lessons from Cramer matter. Paper presented at the Eighth International Wildland Fire Safety Summit, Missoula, Mont.
- National Institute for Occupational Safety and Health (NIOSH) (1999) Death in the Line of Duty, Report 99-21.
- Madrzykowski, D. & Vettori, R. (2000). Simulation of the Dynamics of the Fire at 3146 Cherry Road NE Washington D.C., May 30, 1999, NISTR 6510.
- Grimwood, P., Hartin, E., McDonough, J., & Raffel, S. (2005). 3D Firefighting: Techniques, Tips, and Tactics. Stillwater, Okla.: Fire Protection Publications.
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