FLASHOVER: THE SUDDEN, SILENT KILLER
SPEAKING OF SAFETY
(Photos courtesy of author.)
Flashovers have been happening in North America since 1590, but they seem to be having a more adverse impact on firefighters now than ever before. One of the reasons for this is lack of experience. Most firefighters rarely get to see a flashover. Either they arrive in time to keep it from happening or they arrive after it happens Unfortunately for some firefighters, the first flashover they witnessed was also their last.
Not many of us have accumulated enough firsthand experience to contribute credible advice to those who haven’t witnessed a flashover. During my 10 years with the Wisconsin state training program, however. I had the opportunity to watch more than a hundred fires progress from ignition to flashover as we intentionally burned a great variety of buildings, including residences, grain elevators, schools, factories, hotels, stores, and others.
FINDINGS
The study of flashover is as perplexing as it is fascinating. Flashover is a phenomenon that is not easily understood. It can happen so swiftly that there may not be time enough to escape; and most times it gives little or no warning.
The obvious warning signs described in current literature can be helpful if they occur, but many flashovers have occurred without those indicators; and in some instances where they do show, there may not be time enough to escape. Therefore, expecting to see these warnings can lead to a false sense of security and a sudden, dangerous surprise
DEFINING FLASHOVER
Flashover is the sudden increase in the progress of a fire in an enclosure during which flame quickly engulfs the upper portions of the room and extends through window and door openings.
Sudden onset. 1 have seen fires go from the size of a man’s hand to that of a fully involved living room in less than one minute and from a small fire in a corner of a dairy barn to full involvement of the 45,000-cubic-foot barn in a minute. In most of the flashovers I witnessed, the fires had been burning slowly for a long time. One dairy-barn fire went from ignition to flashover in three minutes. Most of the flashovers witnessed, however, followed a much longer burn period, usually about 30 minutes in a poorly ventilated room, down to 10 minutes in a room with all windows open. Some of the rooms were empty except for 50 pounds of wood and cardboard; some had furnishings appropriate to the occupancy.
Nevertheless, regardless of how long the fire had been burning before flashover, the onset of flashover was rapid in most cases. Small, slow-burning fires that could be extinguished with a gallon of w’ater escalated to fully involve the room in a minute or two. Although the time from ignition to flashover usually is less in smaller rooms, once flashover starts, it doesn’t take much longer for it to spread through larger rooms. W’e saw this in several of our tests, and there are many reports of rapid spread in ballrooms and other large-area occupancies.
CAUSES OF FLASHOVER
I must disagree with the prevalent American theory that most flashovers are caused by a “feedback” of radiant heat from the ceiling that ignites the combustible contents of the room so that they all burst into flame simultaneously. In “Anatomy of Fire,” a paper on flashover 1 wrote in 1963 and delivered at the National Staff and Command School, 1 described flashover as follows: It is the ignition of a layer of combustible gases produced by the fire and which accumulate close to the ceiling. Ignition usually takes place when flame front the fire reaches these gases As they burn, the flame rolls along under the ceiling, picking up speed, until suddenly the entire upper portion of the room is filled with the flame of the burning gases.
II that was true in 196.3, it is still true today. The laws of physics and chemistry don’t change with the times. Since then, this theory has been affirmed by the American authority W alter llaesslcr 1 and also by the Swedish government,2 which did considerable research on this subject.
I still believe this theory is correct, but I’m willing to refine it. W hat we know as flashover may occur in the following ways: (1) a lire with a continuous supply of oxygen proceeds directly to full development in a straight-line, extremely rapid progression ( Type I flashover); (2) flame ignites unburned tire gases that have accumulated near the ceiling (Type 2 flashover); (.3) gaseous products of pyrolysis and combustion, initially tix> rich a mixture to burn, suddenly obtain enough oxygen to make a combustible mixture, which ignites vigorously (Type 3 tlashover); and ( t) a large quantity of noncombustible gases, which had been slowing combustion, is suddenly transformed into copious amounts of two highly combustible gases, which ignite (Type i flashover); it may follow any of the other types of flashover. *
In terms of safety, the more we know about the causes of an event, the better we can prepare for it If w e consider flashover as a gas phenomenon, we can develop strategics and tactics for it, as the Swedes are doing and as some American fire departments have done
HARMFUL EFFECTS
W hen the gaseous nature of flashover is understood, its harmful effects can be anticipated. The gases given off by the tire contain carbon monoxide (CO), which, when mixed with air in the right proportion, will ignite at about l,100°F. This usually occurs when flame reaches the gas cloud at the top of the room. There is no explosion because the cloud also contains a large amount of carbon dioxide (CO,), which, being nonflammable, will slightly retard the burning of the CO. But, as the gases heat up above 1,2()0°F, the CO, breaks down into CO, thus changing it from a fire retardant to a fuel. At the same time, another product of the fire is changing front H,() to hydrogen and oxygen. Now. we have copious amounts of fuel, oxygen, and heat uniting rapidly and generating additional heat and fuel.
The four types of flashover.
That is just the beginning of the hazard because the rapidly expanding gases will push through available openings. If one of the openings is a doorway leading to a hall or stairway, the superheated gases will travel through that hall or up that stairway with force anti speed. Firefighters are more likely to become victims in those places than in the room of origin. There may be warning signs in the room of origin, but they would not be noticeable to firefighters elsewhere, such as those in halls, stairways, or other rooms.
A highly dangerous variant of this situation occurs when the fire originates in a small room and flashes over into a larger adjoining room through an open doorway. VCc ignited a couch in a 220-square-foot living room. Flashover occurred in 18 minutes. later. we placed a chair that matched the couch into a 32-square-foot closet that opened onto the living room. Six minutes after ignition of the chair, the tire flashed over in the closet and extended into the living room. One minute later, the living room was fully involved.
A similar event took place in a New York school building. The fire started in a cloak closet at the rear of a classroom. Members of the first-due truck companyhit the fire with an extinguisher, but it did not go out completely. As they waited in the classroom for a hoseline to arrive, flashover happened so quickly that some of them jumped from the third-story windows to escape the flames.
FORMULATING STRATEGY
The following facts will help you formulate a strategy for dealing with flashover:
Fact 1: Flashover rarely occurs under a cool ceiling. When I joined the Florida State Fire College, I w’as disappointed to find that we couldn’t get a flashover in our burn building, even wTten we burned 10,000 lbs. of wood pallets. The nine-inchthick concrete ceiling could absorb the heat of an 1,800°F fire without getting hot enough to flash over. Ill is heat-absorbing quality of a heavy noncombustible ceiling is one reason there are so few flashovers in foreign countries and they can get by with fewer fire stations and longer responsetimes.
This strategic consideration can be translated into tactics that will keep a ceiling cool by wetting it. thus preventing flashover. For a hundred years or more, this tactic has been used by Manhattan (New York City) companies in the “heavyduty” tenant factory (loft) areas. The companies sweep a solid stream along as much of the ceiling as can be reached from the doorway before they advance into the fire area.
More recently, the Swedes have developed a technique that obtains the same results in a slightly different fashion. Unlike most of Europe, Sweden has many homes with particle board as the interior wall surface. This material, made of wood chips and resin, is conducive to flashover. Swedish researchers now advocate cooling the gases at the ceiling with a fine spray of water, which will extinguish the gases that already have flashed over as well as prevent ignition of those that haven’t.
The Swedes emphasize that the objective is to cool the gases, not the ceiling; they claim that water striking the ceiling will be converted to steam, something they want to avoid. They have developed a training module for practicing this tactic.
Fact 2: Heat in a room varies directly with the distance from the floor. Sometimes floor and ceiling temperatures vary by several hundred degrees, and a person close to the floor has a better chance of survival than one who is standing. Although today’s protective envelope for firefighters enables them to enter the danger area in an upright posture, it is much safer to stay close to the floor.
Fact 3: Our present protective clothing is designed to allow a firefighter 14 seconds to escape flashover. If beyond a point from which a safe retreat can be made within that time, firefighters could he in real danger. Remember, protective clothing is meant to let a firefighter escape flashover, not walk into it.
Fact 4: This is most important: There is only one reliable indicator that flashover is about to happen—a sudden increase in temperature for no apparent reason. When this happens, the time is short; but with modern protective clothing, the firefighter may not be able to detect the change. The clothing is designed to keep firefighters from feeling heat. The Swedes have attempted to cope with this problem by using gloves that are less protective in the knuckle area so that this critical heat increase can be felt. A company in Maryland is developing a rate-of-rise temperature probe that firefighters can carry’ on their body; it will sound an audible alarm and also send a signal to a computer at the command vehicle when a sudden rise in temperature occurs.
Some other signs that warn firefighters of an impending flashover only some of the time, however, include the following:
- Rollover. This is flame rolling along under the ceiling, which appears a minute or two before flashover. If you see it, believe it; but don’t expect to see it every time.
- The upper half of the room is filled with hot, sooty smoke, and fingers of flame are reaching down from it. This is so obvious that no one in his right mind would venture into the room unless it is to rescue someone close to the entrance who could be pulled to safety in a few seconds. (As hazardous as this situation may be, the danger can be reduced to a safe level with a quick shot of water of adequate volume.)
- Any change, especially one that would admit air, such as the breaking of a window by the heat or a firefighter—or any other change that theoretically can help the fire—may hasten flashover.
- OFFENSE OR DEFENSE
When should you stay and fight, and when should you run? The answer is very simple. Stay and fight when you have a charged hoseline capable of giving the Royer-Nelson rate of flow, which translates to one gpnt for every 100 cubic feet of the potential involvement zone—not just the portion already involved. If you don’t have that capability, be wary; above all. don’t advance a dry line into the danger zone. It is better to wait at a safe distance until water is at the nozzle.
Firefighters searching or ventilating without water protection should keep ready retreat in mind; “How can I get out, and how long will it take to do so?” These questions should be answered before entering.
The information given here is not to engender a fear of flashover but to foster a healthy respect for it. When properly applied, the above information will reduce the hazards of flashover and help firefighters do their job swiftly, surely, and safely.*
Endnotes
- Haessler, W.M 1989. Fire. Fundamentals and Control. (New York, N Y Marcel Dekker, Inc).
- Taylor, J 1991. Flashover Training (a report on Swedish fire research), London, England.
- Clark, W.E. 1991 Firefighting Principles and Practices, second edition. (Saddle Brook, N.J. Fire Engineering Books and Videos).