Each day, firefighters face the need to correctly evaluate fire behavior and develop a safe means of fire attack. Your correct evaluation of fire conditions (or events) and choice of fire attack can be the difference between life and death. There are no standard rules stating when the phenomenon of flashover will occur. Being able to recognize its warning signs however, will allow the firefighter time to alter tactics and ultimately survive this extreme event.
Flashover is a rapid, spontaneous ignition of a compartment and all of its contents. It can occur in as little as three minutes from first ignition or take considerably longer. The time that it takes for flashover to occur depends on a number of factors that include the size and configuration of the room, fuel load and heat release, location of the fire, room insulation, and ventilation. As a fire burns, thermal radiation feedback allows heat to be absorbed into the adjoining contents and, the ceiling and upper walls of an enclosed space. The radiation feedback heats furnishings and other components in the fire room to their auto-ignition temperatures that then spontaneously ignite. When flashover occurs, it is the end of the growth stage and the transition to the fully developed stage of the fire. It is the end of an effective search and rescue of any trapped occupants. This condition means sure death to any victim trapped within the now fully involved compartment and severe or even fatal injury to any trapped firefighter. Flashover is also the transition from a contents fire to a structural fire. Once the fire compartment has flashed, collapse of the structure must be considered because the fire is now attacking the structural members of the building. Choosing the correct tactics will be very critical at this point of the firefighting operations.
Heat release of fuel loads has dramatically changed over the years. This has caused flashover to occur at lower temperatures and sooner. Years ago, ordinary combustible materials used in the construction of household furnishings did not generate extremely high temperatures and thus resulted in the less likelihood of a flashover occurring. Today’s fire environment consists of furnishings made from many different synthetic materials that have an extremely high heat release and very low ignition temperatures. When ignited, the fuel load tends to release high heat in a short time. This causes flashover to occur much sooner as similar contents reach auto ignition temperatures faster. Couple this with modern, energy efficient building construction and you have the recipe for early flashover. With the advent of residential smoke detectors and modern communication systems, the fire department is arriving at building fires earlier, thus prior to or at the time of flashover. Understanding the warning signs associated with pre-flashover conditions will allow the firefighter to make sound tactical decisions and evade or escape the flashover phenomenon.
Warning Signs
Some of the first signs of impeding flashover can be observed prior to or at arrival. Obviously, if fire is showing upon arrival, flashover has already occurred. What if there is no fire showing and you arrive to a smoke condition? The smoke condition emanating from the structure can give an indication of the fire conditions within. Laminar or light, smooth, streamlined type smoke indicates that the fire compartment is still absorbing heat. This indicates that conditions within the flaming envelope have not advanced to the flashover point. Turbulent or thick dense black or brown smoke however, indicates that the flaming envelope is experiencing extreme radiant heat feedback. The compartment at this point has absorbed all the heat it can and is now looking for a release. Conditions within the compartment are on the verge of flashover. Appropriate tactics should be employed prior to entering the structure.
Heat mixed with smoke on the interior of the building can also be an indication that flashover is approaching. Modern PPE masks temperatures as much as 450oF meaning that if the wearer is forced to crouch or crawl because of the extreme heat conditions, flashover temperatures are likely just four to six feet above the floor. Again, take appropriate measures to avoid danger when this condition is discovered.
A phenomenon known as rollover may preface Flashover. Rollover is a sudden or sporadic flash of flame mixed with smoke at the upper ceiling level exiting the flaming envelope. Firefighters monitoring the overhead conditions can observe this important warning sign and reevaluate tactics.
How to Delay Flashover
Flashover can be delayed in a number of ways. First, the best way to delay flashover is to extinguish the fire before the thermal radiation feedback advances the room and contents to their ignition temperatures. The next tactic to delay flashover is ventilation. Proper ventilation coordinated with the fire attack will allow the pressurized heat and smoke within the compartment to escape ceasing the thermal radiation feedback within the space and the chance of flashover. Compartmentalization of the fire room may delay flashover from occurring by cutting off the air supply to the fire. This action may delay the inevitable for only a few moments however, and must be followed with one of the previous methods. Lastly, cooling the overhead of the flaming envelope with a stream of water will halt the buildup of superheated gases and slow the thermal radiation feedback process. This can be accomplished via hose stream or pressurized water extinguisher. Again, this tactic will only delay the flashover for a few moments, but could provide sufficient time to effect a rescue or escape the immediate area.
Surviving Flashover
The best way to survive flashover is to recognize the warning signs and avoiding getting into a bad situation altogether. A firefighter’s personal protective equipment has limitations. Advancements in PPE allow the firefighter to penetrate further into a structural fire masking the extremely high temperatures and conditions of the fire environment. Designed to reduce the flow of heat and moisture from the fire environment to the wearer, the multi-layered gear protects the wearer from minor cuts, abrasions and offers limited protection from chemical/biological contact. Often, temperatures under 400oF go undetected by firefighters because of the properties of the PPE. As mentioned above, this masking may give the firefighter a false sense of security, pushing further into the structure and further danger. The thermal protection offered by PPE is very limited and could be greatly depreciated dependent on condition and moisture. This could further contribute to early failure of the PPE at the time of flashover. Nomex garments start to degrade at approximately 750oF and approximately 1200oF for PBI, temperatures well below those experienced during flashover in the modern fire environment. If extreme heat conditions force the firefighter to crawl, then conditions may be right for flashover. Consider evasive maneuvers based on this warning information. Your personal protective equipment worn and maintained properly, and your awareness, may be the only thing that saves you. Always monitor your surroundings and constantly evaluate the conditions. Update your tactics as often as conditions dictate and if necessary, retreat to a safer environment. Lastly, attend a flashover awareness course like the one offered at the academy. If offers an informative, safe environment to recognize some of the potential warning signs associated with the flashover phenomenon and is an excellent review/refresher when actual fire incidents are low.
The flashover phenomenon does not have to be a deadly experience for the fire service. Understanding fire behavior and flashover warning signs, knowing the limitations and performance of PPE, making sound tactical decisions and understanding flashover survival techniques can make a structural fire incident safer so that everyone goes home.
