Understanding Flashover

By Tom Kiurski

Years ago, our firefighting forefathers did not get exposed to as many flashover events as we do today. Brave as they were, the protective ensemble of the day did not allow them to become exposed to the pre-flashover temperatures we see today. With the widespread use of smoke alarms today, we are getting to fires before flashover. However, the dwindling number of personnel that arrive at our fires puts us at a great risk of entering a fire building during pre-flashover conditions, without enough personnel to engage in flashover-delaying tactics. It is important for new firefighters to have a thorough knowledge of flashover. A refresher for the veterans should be part of an ongoing training program as well.

The normal early stages of a fire see it progressing through the usual means of heat transfer, convection, conduction, and radiation. Within a confined compartment, heat continues to build at a rate higher than what the compartment can ventilate. A flashover occurs when all of the combustible materials in a room or compartment reach their ignition temperatures at the same time. Although not all fuels in a room have the same ignition temperature, the heat is rising so rapidly that the temperatures are all reached during this period of rapid heat buildup. The heat that normally accumulates at the upper level in the compartment cannot absorb heat as fast as it is being created. This causes the heat to be pushed back down onto the lower level and any fuels at any level in the compartment. The flashover ends the growth phase, where you may still find victims alive in the room or compartment. After the flashover, the fire enters the fully developed phase, and any victims in the compartment become body removals. Not all texts agree on this standard definition of a flashover, but I believe that it is accurate enough to build on for our discussion.

Today’s homes are constructed differently than the homes of years gone by. Most homes today are much better insulated than their predecessors. The insulation is better and more efficient when it comes to heating the home and saving on energy costs. Today’swindows and doors are more sturdy and energy efficient as well, holding in heat better than in prior years. This increased heat can lead to an earlier flashover. Where some windows may have broken or melted prior to or during a flashover in the past, we often see multiple panes of glass in today’s buildings that can withstand the heat buildup much better.

Statistics tell us that most of today’s homes are equipped with smoke alarms. Smoke alarms give the occupants an earlier warning of the early stages of a fire so they can take appropriate action. This includes evacuating and calling the fire department--in many cases, well before a flashover occurs. This exposes firefighters to flashovers more often than those calls that come in during or after flashover has occurred.

The contents of today’s homes include more plastics than the homes of a generation ago. Natural fibers and wooden furniture were quite common in older homes, where the furniture sat on wooden floors occasionally covered with a rug. In today’s homes, plastic furnishings, treated fiberboard, and a variety of synthetics are all sitting atop carpeting, often treated with flammable coatings to resist stains.

The outside of the structure may give no indication of the conditions going on inside the various compartments inside the home. This is where it is important for the interior crews to be aware of their situation while working on their assigned tactical objective. Each crew member has the responsibility to make other crew members aware of what they are seeing and feeling, but the interior officer has the ultimate responsibility of monitoring interior conditions while his firefighters perform tactical objectives.

Knowing your response area through company inspections can let firefighters know about homes that may be vacant. Foreclosed homes abound, and vandals often take anything of value. Malicious vandals may kick in walls and doors, allowing for a more rapid spread of fire within the building.

The indications of a potential flashover may include a growth stage fire that produces thick and dark smoke, high heat buildup, and rollover. A growth stage fire must exist, even though it may be partially or completely obscured by walls, furniture, and thick smoke. This is the only way the heat needed to flash over can be produced within a compartment. Thick and dark smoke indicates the fuels that are present are giving off vapors that can burn when exposed to high heat. The heat is intense and can build up quickly. We all thought the first fire we were involved in was a high heat condition. It is only through experience that we can realize the difference between heat conditions. Rollover is the ignition of the accumulated gases that have collected at the ceiling level. This may start off as a sporadic burst of orange flames and build up in frequency and intensity to a sea of orange flames overhead. Again, this may be partially obstructed by the smoke, but it can usually be seen by those who look for it. The intense rollover condition, characterized by the sea of orange overhead, is usually considered a late sign of an impending flashover.

With all of the advancements in today’s personal protective equipment, it is still not designed to withstand flashover conditions for longer than just a few seconds. A few seconds may save your life if you take fast and appropriate actions, but the gear will fail quickly when exposed to temperatures over 1,000°F. Keep in mind that many flashovers occur in what may look like a routine fire. Some fires build up to flashover conditions quickly, so you must be aware of your situation within the building--where is your crew, where are the doors and the windows, etc. If you suddenly become involved in a situation where there is a sea of flames with high heat, this awareness may save your life.

The only way to prevent a flashover from occurring is to extinguish the fire. With the fire out, continuous heat will be interrupted, allowing the accumulated heat to spread out and level off. This is not always possible, but there are tactics that firefighters can take to delay a flashover.

The first tactic that can delay a flashover is to ventilate the fire. This, by definition, allows heat and heated gases to escape from the compartment, replacing them with cooler air. The fire compartment now has to build up enough heat to replace the lost heat faster than it can be replaced with cooler air. We all know how effective ventilation is when done quickly and effectively.

The second tactic that can delay a flashover is to close off the compartment. By closing a door in the room that is experiencing pre-flashover conditions, air cannot enter as readily. This can decrease the rate of burning in the room, delaying the flashover. By closing a door, you are also taking the imminent flashover out of the surrounding area so that other nearby rooms can be searched in a safer manner for a longer time.

The final way that a flashover can be delayed is by cooling the atmosphere with water, high in the compartment. This would have to be done with a hoseline or water-based fire extinguisher. By aiming the stream of water into the high heat layer, the gases are cooled. This reduction in temperature slows the process of flashover within the compartment. With a hoseline, extinguishment of the burning material always eliminates a flashover, but you may not have reached the seat of the fire yet. The advantage of the high level water application is delaying a flashover, as we have said. Firefighters must also keep in mind the disadvantages of this tactic, which are the increase in steam buildup onto advancing crews and the loss of any visibility that may have been remaining. Use short bursts overhead to cool to try to keep steam in check.

Flashover is not time-dependent. Some flashovers can occur within three minutes from ignition; others may take considerably longer. Flashover times are more dependent on the size of the compartment, the fuel load within the compartment, and the construction of the compartment. Again, these variables cannot be seen from outside the structure, so the interior firefighters and officers have to be constantly aware of them.

Flashover chambers, or live burn containers, have done a lot in recent years to help train firefighters to recognize the signs that can indicate a potential flashover. If you have been in them, then you are better for the training. If you have not been in this training evolution, then make every effort to participate in this type of training. The chambers burn Class A fuel, which is strictly controlled by the arrangement of the chambers. Instead of waiting for house fires and trying to be part of an interior crew assigned to extinguishment or search, the chambers allow you to experience pre-flashover conditions firsthand. If you are unsure about the difference among low, moderate and high heat conditions, you will experience all of them inside the chamber during a live-fire evolution.

Almost every fire station in the country today has a computer with Internet access. If your does not, arrange to have one brought in on training day. Visit the video Web sites, like YouTube, and type in “Flashover. ”You can study the fire building; in some cases, you may see some of the fuel loading and the conditions that lead up to a flashover—and the flashover itself.

Flashovers kill some of our firefighters every year. It is our responsibility to enter into hostile fire conditions. By better understanding this phenomenon, we can be more aware of the conditions leading up to a flashover, how to delay a flashover, and how to prevent a flashover.

 
Tom Kiurski is training coordinator, a paramedic, and the director of fire safety education for Livonia (MI) Fire & Rescue. His book, Creating a Fire-Safe Community: A Guide for Fire Safety Educators (Fire Engineering, 1999), is a guide for bringing the safety message to all segments of the community efficiently and economically.

 

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