Flashover!

There were multiple near miss incidents and injuries involving flashover during the month of December. These incidents point to the importance of understanding fire dynamics and reading the fire as part of initial size-up and ongoing dynamic risk assessment. Each member operating on the fireground must maintain a high level of situational awareness and communicate key fire behavior indicators and potential for extreme fire behavior phenomena.

Flashover Disrupts Firefighters’ Rescue Effort

Firefighters attempting to rescue a victim from a burning Portsmouth (VA) house on Thursday were forced to abandon the rescue attempt and exit a window when a flashover occurred.

Firefighters first entered the home through the front door, but were repelled by flames. They then made entrance through the front bedroom windows when the flashover occurred. After escaping, firefighters tried to reenter through the back of the house, but they could not.

FireEngineering.com

Ottawa Firefighter Pulled From Burning Basement

“An Ottawa firefighter had to be rescued from a burning basement after he was caught in a possible flashover yesterday afternoon. We don’t know what happened, and we haven’t had a chance yet to look into exactly what the details were, but we have a feeling that it might have been a flashover,” department spokesman Marc Messier said.

Firehouse.com

Columbus Firefighters Suffer Burns In Flashover

“Flames were coming up from the basement and out of the windows when crews arrived at the Dana Avenue house fire. There was a flashover, and fire crews quickly evacuated the duplex. Two firefighters were injured in the flashover, Battalion Chief David Whiting” said.

Nbc4i.com

Kansas City Firefighters Injured in Flashover

When they arrived, flames were coming from the first and second story of the house, firefighters said.

Kansas City, Mo., Fire Chief Smokey Dyer tells KMBC 9’s Justin Robinson what happened in a fire early Saturday that left three firefighters injured

Kansas City Fire Chief Smokey Dyer said crews went inside and started to go up the stairs, when conditions inside the house suddenly changed. He said it burned the fire hose and left the firefighters completely surrounded by flames. The firefighters sent out a mayday call for help.

“In the past 10 years, every significant firefighter injury that we have sustained in fire combat has been a result of a rapid change of conditions,” [emphasis added] Dyer said.

KMBC.com

Incidents such as these point to the need for continued emphasis on developing firefighters’ understanding of practical fire dynamics and effective strategies and tactics to control the fire environment and prevent, rather than react to occurrence of fire phenomena such as flashover.

Flashover is Just Flashover

In a recent discussion with a number of international colleagues, we were challenged to think about language, terminology, and precision when describing fire phenomena. While this is a more obvious challenge when working with firefighters, researchers, and scientists who have different first languages, it is also a day to day problem for firefighters with a common native language (e.g., English).

I have previously raised this question and proposed one approach as a starting point for classification of fire behavior phenomena based on outcome and the conditions required for the phenomena to occur (Language & Understanding: Extreme Fire Behavior and Extreme Fire Behavior: An Organizing Scheme).

Consider two recognized definitions for flashover:

• Stage of fire transition to a state of total surface involvement in a fire of combustible materials within an enclosure’ (ISO 13943, 2008, 4.156).
• A transitional phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach ignition temperature more or less simultaneously and fire spreads rapidly throughout the space resulting in full room involvement or total involvement of the compartment or enclosed area (NFPA 921-2007)

This transition is often assumed (and in many cases explicitly stated) to take place between the growth and fully developed stages. However, neither the ISO nor NFPA definition specifies this. In addition, while the NFPA definition indicates that this transition is extremely rapid (i.e., more or less simultaneously), the ISO definition does not describe the speed with which the transition to total surface involvement occurs.

In some respects, flashover is always a transition between the growth and fully developed stage (as increasing heat release rate is necessary). However, this may be a bit misleading. In the modern fire environment a compartment fire may follow an alternate path, often transitioning from growth to decay prior to flashover due to limited ventilation as illustrated in Figure 1.

Figure 1. Fire Development in a Compartment

As illustrated in Figure 1, the traditional fire development curve shows fire progressing neatly through incipient and growth stages, with occurrence of flashover resulting in transition to the fully developed stage and then decay as fuel is consumed.

The path of fire development is often quite different in the modern fire environment. The nature of common building contents provides a rapid increase in heat release rate (HRR) and corresponding oxygen consumption, resulting in the fire becoming ventilation controlled. With heat release limited by ventilation, the fire begins to decay (HRR and temperature are reduced). Uninterrupted this may cause the fire to self-extinguish. However, should an opening be created (as a result of window failure due to fire effects or opening of a door), the fire re-enters the growth stage and transitions through flashover to the fully developed stage. This is sometimes described as ventilation induced flashover (but in some respects, flashover is simply flashover).

In a spirited debate, some of my international colleagues have stated that “all flashover is ventilation induced” as ventilation is necessary to develop sufficient HRR for flashover to occur. Others have said that “flashover is temperature driven” as sufficient upper layer temperature is required. None have specifically said that flashover is a fuel dependent phenomenon, but this is true as well (given that the fuel that is burning must have sufficient energy and heat release rate for flashover to occur). In addition, flashover is dependent on compartment size and configuration, as a given fire will reach flashover in one compartment (generally a smaller one) and not in another). So, what’s the answer? It Depends!

This really boils down to being able to recognize what is important for firefighters to understand about fire development and flashover (as well as other extreme (i.e., extremely rapid changes in) fire behavior.

What We Know and Why It Matters

There are a number of things that we know about compartment fire behavior that are significant when considering how and why flashover occurs:

• Fire behavior is completely predictable if you have the necessary information and the time to analyze it (but on the fireground you seldom do). Predicting fire behavior is really saying: This is what I think is likely to happen.
• Changes in the built environment have influenced fire development (but there are a number of variables that may vary from nation to nation). In the US, modern building contents have increased heat of combustion and heat release rate, resulting in more rapid fire development than in the past.
• If ventilation is adequate, the typical room (e.g., bedroom, living room, family room) has well in excess of the amount of fuel (both in heat of combustion and peak heat release rate) to allow a fire to progress to flashover.
• Smoke is fuel. This is not dependent on the size or occupancy of the building. Smoke always presents a potential flammability hazard and as the concentration of fuel and energy in the smoke increases (think temperature, even though this is not the same as energy), the hazard increases.
• When a compartment fire becomes ventilation controlled, pyrolysis continues, adding additional gas phase fuel to the smoke in the upper layer.
• Building configuration and ventilation profile has a significant impact on fire development. However, despite increased compartment size and open floor plans, fires in modern single family dwellings are likely to be ventilation controlled when the fire department arrives.
• Increasing the air supplied to a ventilation controlled fire will result in an increased heat release rate (unless you immediately put the fire out) and this can occur quickly. Where you ventilate in relation to the fire, the existing heat release rate, and energy in the upper layer will all influence how quickly these changes occur.
• Creating an opening for entry is ventilation! This change in the ventilation profile often influences development of ventilation controlled fires by increasing air supply and providing a flow path for fire travel from the current area of involvement to the entry point (watch for a bi-directional air track with air in at the bottom and smoke out at the top of the opening).
• Adding additional openings will further increase the HRR and speed fire growth (unless you put the fire out). This is true even if the openings are near the seat of the fire.
• It is unlikely that you can tactically create sufficient ventilation to return a ventilation controlled fire to a fuel controlled burning regime (meaning that as you continue to increase ventilation, HRR will continue to rise). This does not mean that ventilation is bad as you may influence fire spread and the level of the upper layer, but recognize that the fire will get larger (increased HRR).
• Wind can have a significant influence on fire behavior. Consider wind direction, velocity, and how fire behavior (e.g., HRR, flow path) may change if the ventilation profile changes.

Given what we know, how should this inform our choice of strategies and tactics? Remember that strategies and tactics are context dependent. If you arrive with a single resource and two firefighters, your capabilities are different than if you arrive with six resources and 24 firefighters. Resources change some of your tactical options and the potential for concurrent operations. However, resources and their capability do not change the chemistry and physics of fire dynamics. It is important to recognize potential fire behavior, the scope and magnitude of the problems presented by the incident and the capabilities of the resources at hand.

Recognize that there are no simple answers to the questions of how much risk is too much and what actions are appropriate in a given circumstances. That said the following are steps you can take to reduce the potential of being caught or trapped by rapid fire progress:

• Recognize the indicators of flashover potential and communicate these observations to the members of your crew. Company officers (crew/team leaders) should communicate observation of flashover indicators to their immediate supervisor (e.g., Command, Division or Group Supervisor).
• Ensure that fire attack (or any other operation that involves working inside a burning building) and tactical ventilation is coordinated. In more explicit terms this means that ventilation occurs when companies or crews assigned to fire attack can quickly put water on the fire (not when they are ready to call for water or are simply ready to enter the building).
• Ensure that you are working on a hoseline (or are protected by one) if you are working in a smoke filled environment. Without a charged hoseline you have no defense (you cannot outrun flashover or other rapid fire development phenomena).
• Take positive actions to reduce the threat. If there are hot gases overhead, cool them. If you can put water directly on the fire, do it. If you put the fire out, things will generally improve! When you can control the fire ventilate to remove the smoke and remove the hazard.
• Consider the effects of wind on potential fire behavior. Consider exterior attack and avoid advancing lines in the potential flow path when the potential for wind driven fire conditions exits. Use caution when entering from the windward side and control inlet openings (or provide adequate exhaust).

Clearly understand when you are taking a reasonable and calculated risk and when you are gambling. Think about this before you are engaged in a firefight. Make it a conscious decision and not simply a default choice. Field Marshal Erwin Rommel made this distinction between taking risks and gambling: “With a risk, if it doesn’t work, you have the means to recover from it. With a gamble, if it doesn’t work you do not. Normally, to succeed you must take risks. On occasion you have to make a gamble” (Clancy, 1997, p. 152).

What’s Next?

My next post will dig into the findings and tactical implications of the recently released research results and on-line training program from Underwriters Laboratories (UL): Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction.

This training program is of critical importance to anyone fighting fires in today’s buildings. All firefighters and fire officers should complete this training program before the end of January 2011! Take the time and get your head around the implications of this research on what we do on the fireground. This takes a bit of effort as we need to question our assumptions and standard practices, but the outcome is worth the work.

Be a student of our craft, be safe and look out for the firefighters and fire officer that work with you. Have a great New Year!

References

Clancy, T. Into the storm: A study in command. New York: G. F. Putnam & Sons

Tags: Extreme Fire Behavior, fire behavior, firefighter injury, flashover, near miss, practical fire dynamics