FLASHOVER

FLASH OVER

STRATEGY & TACTICS

Photo by Jo L Keener.

Flashover—sudden fullroom involvement in flame —is the most dangerous stage of fire development. It can trap and kill firefighters.

Flashover is caused by thermal radiation feedback. During a fire in a room, the heat is absorbed into the ceiling and upper walls and reradiated downward, gradually heating the combustible gases and contents of the burning room. When the combustibles are heated to their ignition temperatures, the room and its contents simultaneously ignite.

FIRE GROWTH

There are three stages of a room fire’s growth: the growth stage, the fully developed stage, and the decay stage.

The growth stage. During this stage, the fire increases in size from a small flame to a full fire that involves the entire room. It could take anywhere from several seconds to several hours for this to occur. Fire growth depends on the combustible content, the oxygen supply, and the size of the room. The growth stage can be identified by the upward direction of the fire curve on the time-temperature standard used by the fire service (see illustration on page 48).

The fully developed stage. In this stage, the entire room and its contents are actually burning and the fire is at its highest temperatures. The length of time a fire will remain in this stage is determined primarily by the amount of combustible material in the burning room. The fully developed stage can be identified by the horizontal portion of the fire curve.

The decay stage of a fire. The flames and temperatures begin to decrease during this final period of fire growth. The major portion of the room’s combustible material has been consumed by the fire. The decay stage can be identified by the downward direction of the fire curve.

Flashover can be identified as the steep vertical rise of the fire curve occurring near the end of the growth stage. Temperatures will reach 1,000° to 1,500°F at this time.

FLASHOVER AND FIREFIGHTING

Flashover greatly changes the fire, transforming it from smoldering flames to a fully developed blaze. Since the room is fully consumed with flame, firefighting problems are increased. It becomes more dangerous and more difficult to extinguish the fire.

Search and rescue. After a flashover, do not attempt search and rescue without the protection of a hoseline. The fire room will be too hot to enter. Position a hoseline and extinguish the fire before resuming search and rescue. Occupants cannot survive the flashover environment; anybody in the fire room during flashover will have been killed.

Fire extinguishment. If the firefighters arrive on the scene before flashover occurs, extinguishing the blaze is relatively easy. One or two firefighters can operate a portable extinguisher or small-diameter hoseline to control and extinguish the fire. However, after flashover has’ occurred, several firefighters wearing self-contained breathing equipment and protective clothing must stretch and operate a large-diameter attack hoseline to fight the blaze. They may have to break windows to vent the intense heat in the burning room. In some instances after the fire flashes over, it becomes too hot or large to be extinguished by firefighters. The fire then spreads throughout the building and destroys it.

Collapse. Before flashover occurs, the fire is fueled only by the contents of the room. After flashover, the structural supports of the building may begin to burn and fuel the fire. The fire now enters the second stage of grow th —the fully developed stage. At this point there is a danger of building collapse if the fire is not quickly extinguished.

The plaster or gypsum board ceiling and walls begin to fail, exposing the wood floor or roof beams above the fire. The side bearing walls exposed to the fire burn and weaken. If the fire is not controlled and continues to grow, the danger of collapse is more serious. Roof beams w ill cave in and bearing walls will fail.

Flashover signals the beginning of the collapse danger of a fire. Collapse potential increases during the fully developed stage of a fire’s growth and becomes even greater in the decay stage.

Photo courtesy of Houston Post.

TIME UNTIL FLASHOVER

No one can predict when flashover will occur. At some fires the room may become a sudden inferno the moment you enter it. At other fires you may search for long periods of time before flashover occurs, or it may not occur at all. Time period warnings such as “four minutes until flashover” are to inform the public of the danger. Firefighters should not use these warnings as a safety guide in search and rescue. Firefighters arriving on the scene do not know how long the fire was burning before they got there.

THERMAL RADIATION FEEDBACK THEORY

WHEN THE FIRE HEATS THE CEILING AND UPPER WALLS OF A ROOM THERE IS A RADIATION FEEDBACK EFFECT WHICH HEATS THE FIRE GASES AND CONTENTS OF THE FIRE AREA. WHEN THE COMBUSTIBLE GASES AND CONTENTS HAVE BECOME HEATED TO THEIR IGNITION TEMPERATURES, SIMULTANEOUS IGNITION OCCURS.

TIME TEMPERATURE COURSE OF A FIRE

TEMPERATURE

The following are warning signs of flashover:

Heat buildup in a smoke-filled room. There is little danger of flashover if very little heat is mixed with smoke. However, very hot smoke that forces a firefighter to crouch down when entering a burning room signals a flashover danger. Veteran firefighters often make a mental note of the amount of heat they encounter when entering a smoke-filled fire area. The lower they are forced to crouch down, the greater the danger of flashover.

Rollover. This refers to sporadic small flashes of flame mixed with smoke seen just before flashover occurs. Flashes of flame sometimes can be seen at ceiling level of a smokeand heat-filled room and at the top of an open doorway or window mixed with billowing smoke. This can signal that flashover is about to take place inside the burning room.

These flashes of sporadic flame mixed with smoke occur when combustible gases are heated to their flammable range by the growing fire. The gases start to ignite when mixed with air at ceiling level or outside a door or window. Before entering a smoke-filled room to search, firefighters should check the smoke coming out of the door or window for signs of rollover. When crawling through a smoke-filled room to search, periodically raise an ungloved hand above your head—you can’t see rollover under such conditions —to check for a buildup of heat in the smoke. This will indicate an increased danger of flashover.

THE POINT OF NO RETURN

After a flashover occurs inside a superheated smoke-filled room, a firefighter can reach a point of no return beyond which he will not be able to escape back to safety. With today’s personal protective clothing and equipment, searching firefighters are now able to penetrate much farther into superheated, preflashover fire areas than they could in the past.

If a flashover occurs, a firefighter who is five feet from a door or less might have a chance to escape the flames by diving back out the doorway. Today, however, firefighters are crawling through several rooms and searching 20 or 30 feet into burning apartments in the preflashover stage. When these rooms suddenly burst into flames, the firefighters are forced to crash through the nearest window to escape the fire. Unfortunately there is sometimes neither a ladder nor fire escape on the other side.

Firefighters are exposed to temperatures of more than 1,000°F during flashover. Tests show that exposure to temperatures of only 280°F to 320°F will cause extreme pain and damage to all unprotected skin. A crawling firefighter wearing 60 pounds of protective equipment will travel roughly two and a half feet per second. A firefighter caught in a flashover five feet inside a room will be exposed to temperatures of 1,000° to 1,500°F for approximately two seconds. During that time his mask harness straps and face mask may melt and parts of his fire clothing may ignite before he can escape back out the doorway or window he entered—provided he can still locate it and not be blinded by pain. Therefore, the maximum distance a firefighter should craw 1 inside a superheated smoke-filled room without protection of a hoseline and still expect to get out alive is five feet.

HIGH CEILINGS

A commercial building with large unenclosed spaces and ceilings 15 to 20 feet above floor level can result in dangerously misleading flashover size-up information. When a fire occurs in a residence building compartmented by room partitions and normal eightor 10-foot ceilings, the smoke and heat from a fire will quickly accumulate inside the room and bank down—indicating the danger of flashover. The combustion products in the rooms will prevent a firefighter from moving more than several feet beyond the building’s front door.

Since most fires we respond to occur in residence buildings, they provide most of our firefighting sizeup experience and become the basis for our understanding of fire grow th and fire spread. However, residential fire experience cannot always be transferred to commercial buildings—particularly buildings with large open spaces and high ceilings.

When a firefighter enters a burning room or area and is not forced to crouch because of heat and smoke banked down below the ceiling, he subconsciously assumes the fire hazard is small. This can be a deadly judgment error. In a commercial building with a high ceiling the area betw een the firefighter’s head and the ceiling can accumulate superheated smoke, delaying the buildup of heat and smoke at the floor that normally signals flashover danger. This might allow firefighters to enter the structure, walk upright to the back of the occupancy, and become trapped by flashover occurring at ceiling level. If the area is extremely large and open there may be no buildup of heat and therefore no flashover, but the flame can spread at ceiling level above the heads of firefighters, trapping them deep inside the burning building. If a flashover or rapid flame front spreads along the underside of the ceiling, the firefighter will not be able to outrun the rapid flame.

VENTILATION

One of the most important decisions a fire officer makes during a fire is when and when not to vent. Some types of ventilation can be accomplished immediately. For example, quick roof venting of a skylight over a fire in a row’ of stores may prevent the buildup of heat and smoke necessary for a flashover or smoke explosion before firefighters enter the store to extinguish the fire. Even if a flashover or smoke explosion occurs the flame and shock waves w ill be safely diverted upward.

This is recommended as long as there is no nearby exposure problem. However, vertical ventilation often cannot be accomplished. A room on the lower floor of a 2 ‘/2-story woodframe dwelling can only be vented by opening windows and doors. Venting a growing fire can delay flashover or make it occur, depending on how far the fire has progressed in the growth stage.

Before venting, a firefighter should ask, What do I want to accomplish and w hat type of fire condition am I going to create after I vent? You may want to delay flashover in order to give yourself time to make a quick search. Or you may want to delay venting to give the hose team time to position the initial attack line. At other times flashover may not be a factor in your decision—you simply may need to vent the room to release the heat, flame, and gases opposite from where the hose attack team is advancing.

There are times when venting is the best way to delay flashover and there are times when not venting is the best option.

Venting to delay flashover. Say a small fire burns in a confined bedroom. Heat buildup has not occurred, but smoke is dense and the first firefighter on the scene wants to delay flashover to gain several seconds to search for the fire’s origin or a trapped victim. He might quickly vent several windows to release the smoke and improve visibility. By venting the small amount of heat in the room, the firefighter would have temporarily delayed the buildup of heat necessary for thermal radiation feedback and flashover.

Not venting to delay flashover. Assume the same bedroom fire burned in a confined area for a longer period of time prior to the arrival of firefighters, and the heat buildup in the room was severe; heat and smoke were banked down to within one or two feet of the floor and firefighters could not enter the room. To delay flashover several seconds in order to position the hoseline for an attack, you might close the door to the burning room and not vent the windows. You can do this when all occupants have been accounted for and you want to coordinate venting with the hoseline advance. Not venting would delay flashover by starving the fire of oxygen and delaying the buildup of heat.

A flashover fire in a Fort Worth, Texas appliance store results in building collapse.

(Photos by Butch Hall.)

Telltale black smoke and flames shoot out from a house near Rosenberg, Texas.

SMOKE DETECTORS

The installation of smoke detectors has led to the early detection of fires in residential buildings. Fires are being reported more rapidly to local fire departments, and firefighters are arriving on the scene sooner than in years past.

Arriving before the fire flashes over has its good and bad points. It is easier to extinguish the blaze before it reaches the flashover stage. This saves lives and reduces property damage. However, it is more dangerous for firefighters to arrive before flashover, especially those involved in search and rescue. Firefighters often are caught in a flashover when arriving at a fire during the growth stage. Unforeseen complications—defective hydrants, burst hoselines, failure to quickly locate the exact origin of the fire, and broken centrifugal pumps— can make it even more dangerous for a firefighter searching in a superheated smoke-filled room.

COMBUSTIBLE FURNISHINGS

Today’s synthetic furnishings — drapes, rugs, chairs, tables, and beds— are more dangerous when they burn than were the home furnishings of 30 years ago. Furniture used to be made of organic materials such as wood, paper, and natural cloth fibers. Now drapes, rugs, stuffed chairs, and mattresses are partially or wholly made of plastic. When plastic furniture burns it generates smoke more rapidly then burning wood or cotton and produces thick, black smoke. The smoke area becomes rapidly obscured.

A pound of plastic generates more heat than a pound of wood or paper. For example, one pound of wood or paper, when completely burned, will give off 7,000 to 8,000 Btu’s of heat. On the other hand, one pound of plastic furnishings may generate up to 18,000 Btu’s of heat during a fire. Polystyrene, a hard, rigid plastic, can give off 18,000 Btu’s for every pound. Polyurethane, often used in upholstered furniture and bedding, can give off 12,000 Btu’s per pound.

In addition to rapidly generating dense smoke and a hotter fire, some plastic furniture gives off flammable gases. Pentane gas, used in some foamed plastics, creates an added flashover danger—a flammable gas mixed with smoke and heat. Pentane gas has properties similar to gasoline. Its flashpoint is — 40°F, its ignition temperature is 588°F, and its flammable range when mixed with air is one to seven percent.

Because burning plastic furniture creates rapid, dense smoke, many firefighters become disoriented and cannot find their way back out of the room before flashover occurs. The added heat from the burning plastic also decreases the time to flashover, thus decreasing search time.

ROOM CONSTRUCTION

Size. If two fires of equal intensity and development were burning in two separate rooms, one very large and one very small, and both fed oxygen only by the open door, flashover would occur in the smaller room first. For flashover to occur under normal fire conditions, smoke from the fire must be contained and fill up the room to at least 50 percent of its height, and enough heat must be present to begin reradiation. Without smoke and heat buildup rollover may occur but flashover will not. So a building with smaller rooms, such as a residential building, is more likely to flash over.

Insulation. During the oil shortage of the 1970s, the attics and wall spaces in many homes were insulated to prevent heat loss and conserve energy. A room with ceiling and wall insulation will flash over faster than a room that is not insulated. The insulated ceilings and walls conduct less heat; therefore, more heat is reradiated hack to the burning room, causing flashover to occur sooner.

Combustible wall surface. Wood paneling and ceiling tiles sometimes are coated with flammable plastics, or flammable glue is used to attach a decorative covering to the wall or ceiling. The surface of the ceiling or upper walls will ignite easily during a fire, having the same effect on flashover as an insulated room. There will be less heat conducted out of the room through the burning walls or ceiling. This causes flashover to occur more rapidly, assuming the development of the fire, the room size, and the vent openings are the same.

SURVIVAL TECHNIQUES

Recognize the warning signs of flashover: heat and rollover.

  1. Know the flashover point of no return. The maximum distance a firefighter can crawl inside a superheated, smoke-filled room without protection of a hose stream and still get back out alive after flashover occurs is only five feet.
  2. Use defensive search procedures to protect against flashover. When at the doorway leading to a room filled with smoke and heat, first check the smoke flowing out of the top of the doorway for signs of rollover. If there are no signs of rollover, attempt to enter the room. If you have to crouch dow n to the lower half of the room to escape heat from the fire, beware: A flashover may occur. In this instance do not go beyond the point of no return. Check behind the door for an unconscious person, crawl no more than five feet inside the superheated smoke-filled room, sweep the floor area around the doorway for a victim using a tool or your hand, then back out, closing the door. When the hoseline arrives and is ready to advance, open the door and move in behind the advancing hose stream to continue your search.

When venting a window where there is a report of a trapped victim inside the room, do not enter the window if flames and smoke are exploding out of the upper portion of the window opening or if the smoke is hot enough to indicate a flashover danger. Instead, crouch down low and attempt to sweep the area directly beneath the window sill inside the room with a tool or your gloved hand. Sometimes you will find people alive who have collapsed directly below the sill.

If you are unable to enter after sweeping the area, try venting, entering, and searching the adjoining rooms. Then after the fire is extinguished you can enter the room and continue searching.

  1. Avoid disorientation. The most effective defense against disorientation is an organized search procedure. When searching a small room, proceed clockwise or counterclockwise and stay in contact with the walls. When moving down a hallway, brush against the wall with one shoulder as you enter and with the other shoulder as you exit.

When searching an area 75 square feet or larger, you must use a search rope. Secure one end at the entrance and use the rope as your guide back out to safety when smoke reduces your visibility.

  1. Use fireground preplanning. Encourage firefighters to study floor plans and layouts. This can reduce the dangers of being disoriented and then trapped in a flashover. Condominiums, large housing developments, and townhouses usually have similar interior room layouts. During training sessions firefighters should analyze drawings of various floor plans to _ learn the most common locations of exits, windows, dead-end corridors, fire escapes, and stairways to second floor bedrooms. This information could give firefighters confidence during search operations and reduce their chances of being caught in a flashover.
  2. Return through the same door you enter. Fear of crime has led to the elimination of many second exits in residence buildings. Twenty or 30 years ago, firefighters would size up a burning building before entering to determine if a front porch, outside balcony, or fire escape was available for a quick escape in case of flashover. If an escape route was available, a firefighter would consider taking a risk of passing fire to help a trapped victim.

Firefighters today no longer can depend on this escape. Sliding scissor gates or steel bars are being placed on the window leading to a porch or fire escape. These bars cannot be opened from inside or outside, often trapping occupants.

Sometimes this window is made of unbreakable plastic. A firefighter’s ax or halligan tool will bounce off the plastic —in order to open the window the entire metal frame must be removed.

When a firefighter is trapped in a room about to flash over, a quick escape by smashing a window pane is no longer an option. If a firefighter takes a risk and passes the fire to rescue a trapped victim, he should return through the entrance door with the victim.

  1. Carry a search light at all times. In some instances firefighters have fallen through floors and become trapped, unable to return to safety before flashover occurred. A flashlight could have possibly helped them see unstable areas in the floors and avoid them.
  2. Wear protective clothing and equipment. No firefighter responding to an alarm expects to get caught in a flashover. Yet it happens every year. A firefighter’s survival could depend on the amount of protective clothing he wears. A helmet, gloves, boots that are pulled up, a turnout coat that is fully closed, and operable SCBA are the last line of defense for a firefighter trapped in a flashover. Protective clothing and equipment are cumbersome and hot and slow down a searching firefighter. But the design, fit, and extent of protective equipment a firefighter wears will determine whether he survives a flashover and how serious his injuries will be if he survives.

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