The phenomenon of the flashover and backdraft. Backdrafts and flashovers. Backdrafts, rollovers and flashovers

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1 Backdrafts and flashovers By Colin Deiner, Chief Director, Disaster Management and Fire Brigade Services, Western Cape Government There are various ways to recognise the signs of a potential flashover or backdraft The phenomenon of the flashover and backdraft has over years been the second largest cause of fire deaths after structural collapse. In 1981 a flashover in a casino in Dublin, Ireland, killed 48 people. The following year a flashover in an apartment complex in Los Angeles killed 24 people. A London underground rail system fire in 1987 killed 37 people including a fire officer as fire gasses unexpectedly and violently ignited. Other major incidents occurred in St Petersburg, Russia, in 1991 when eight fire fighters were killed in a corridor flashover in a hotel and in Dusseldorf Airport in Germany in 1996 when 17 fire fighters were killed in a flashover. In 2002 five Paris fire fighters died after being trapped in two flashover related incidents. A report entitled Us Fire Service Fatalities in Structural Fires, 1999 TO 2009 (prepared by Rita F Fahy PhD) indicates that of the 78 fire fighters who died of asphyxiation in the line of duty, the main causes were structural collapse (27 deaths of which 18 were due to roof collapse) while the second most (13) was due to the progress of fire (flashover or backdraft.) The report goes on to state that: The major causes of these traumatic injuries inside structures were fire fighters becoming lost inside, structural collapse and fire progression including backdraft, flashover and explosion. South Africa is a much smaller country than the USA and respond to fewer fires with an obvious deficit in fire related injuries. The fires itself are, however, not smaller or different and the threat of flashovers and backdrafts are the same anywhere. This is why it is important for us all to understand the dangers they pose and the signs of a backdraft/flashover occurring. The most important point I can make here is that it is not up to the incident commander to recognise and manage the potential backdraft/flashover. He/she is not inside the building, so it is up to each fire fighter working the fire to recognise the warning signs and act accordingly. Definitions Some purists would criticise the use of backdraft and flashover in the same sentence as they have different causes and dynamics. This is true and we can also add fire gas ignitions to the equations. So let s attempt to define these different animals. Flashover: The sudden involvement of a room or an area in flames from floor to ceiling caused by thermal radiation feedback. Thermal radiation feedback refers to energy of the fire that radiates to the other contents of the room 30 FIRE AND RESCUE INTERNATIONAL Volume 3 No 7

2 It is not up to the incident commander to recognise and manage the potential backdraft/flashover from the surrounding surfaces (walls, floor and ceiling) which raises them to their ignition temperature. This will result in the room contents suddenly and violently igniting. Flashovers are temperature driven incidents and occur when the fire has evolved to a fully developed stage. Backdraft: Occurs when fresh air is introduced into a smouldering fire and heated gases enter their flammable range and ignite with explosive force. Other than flashovers, which are caused by temperature, backdrafts are caused by super-heated air. This makes the understanding of the two and the reading of the signs of each so important. Flashover I once attended a course presented by a very experienced fire officer from the USA who referred to fire as a non-thinking enemy. The point is that fires will follow the rules of physics. It can t change its plans or outthink you (only if you are really severely mentally challenged) and it certainly can t do what we saw in the movie. Think of this when you consider the physics of a flashover. The size of the room, its contents, ventilation and insulation will determine its flashover potential. It is therefore obvious to accept that the possibility of flashover is bigger in a smaller room than in a large structure with high ceilings due to the closer proximity of combustible materials in the more confined space. The room contents will also affect the flashover potential. More combustible materials will burn at a higher intensity, produce more heat and radiated energy and therefore increase the possibility of a flashover occurring. Due to the fact that more carbon-based interior furnishings are used nowadays than twenty years ago, we see rooms involved in fire reach peak temperatures very fast and generate larger volumes of smoke, which reaches flashover conditions much faster than in the past. A sufficient supply of air is required to ventilate the fire (although it is not air driven by definition). The introduction of improved insulation such as thermal-paned windows and ceiling insulation work together to trap heat and flammable gasses inside, causing an explosive atmosphere. Training fire fighters to recognise the signs of possible flashovers and backdrafts are imperative for their safety. As mentioned earlier, the incident commander is not in a position to recognise the signs and advise the interior crews accordingly. The first sign of a potential flashover will be the high heat generated by the fire. This can be the type and volume of materials involved and the stage of development of the fire. The longer you take to get water on to the fire, the bigger the chances of a flashover. The change of conditions such as a sudden increase in interior temperature is a sure sign of imminent flashover. In larger buildings, fire crews could experience a situation where a large fire is in contact with the ceiling, ignites the fire gasses up here and rolls along the ceiling towards them. This is generally referred to as rollover and should not be confused with a flashover. It does, of course, super heat the room much quicker and is a definite warning of a potential violent flashover. 32 FIRE AND RESCUE INTERNATIONAL Volume 3 No 7

3 Another positive sign of an impending flashover is the colour of the smoke. It will take on the deep black colour you see when a heap of tyres are burning. The reason for this is that the smoke contains a large volume of unburned products. When this black smoke reaches the correct air/fuel mixture, it will ignite. Due to the potential energy already existing in the smoke, there is a strong possibility of a smoke explosion higher up in the building. A note on personal protective equipment (PPE). PPE has become more specialised with time and now allows fire crews to move deeper into fires than before, enabling them to get to the seat of the fire. Flash hoods have become an essential part of the ensemble. Not so long ago the limitations of our PPE only allowed us to stay in a burning structure for a limited period. Fire fighters generally relied on this experience and instincts to determine their position and movement in an involved structure. The ability we now have will allow our crews to stay in a structure for longer and brings with it the possibility of them being exposed to serious flashover risks without being aware of it. It is therefore important that your command structure allows for the monitoring of interior crews and effective evacuation protocols. Surviving the flashover The first priority is to recognise the warning signs; especially if you are moving rapidly towards the seat of the fire. Early recognition of the warning signs could allow to adjust your fire attack such as increasing ventilation or moving crews into safe areas. Stay low as far as possible and maintain situational awareness at all times. Attack teams often tend to fixate on the fire and forget about keeping an eye on the surrounding risks such as the integrity of the structure, possible escape routes and the location of search and rescue teams or other fire suppression teams. Remember that the smaller the target, the less chance you have of being hit. Staying close to the ground and, if you are operating a combination nozzle, adjust it to full-fog above your head to disrupt the thermal balance by creating large amounts of steam. Your hand line can be very effective in slowing down the probability of a flashover. Cooling the fire area will greatly reduce the possibility of flashovers. We have all been taught not to direct water into smoke. The exception is when you detect the smoke colour getting darker and experience a rapid increase in heat. Also if you see a rollover, immediately direct your stream at the black smoke. Ventilation will always be a very effective tool for preventing flashovers. I have often mentioned that the mark of a good fire service is one that can move smoke as well as it moves water. Spending a lot of time on ventilation techniques and strategies will enable you to get to the roofs quicker or set up your horizontal ventilation in the right positions and ensure that the interior becomes more life sustaining. Care must be taken when fighting a fire in an advanced stage, not to spread the fire into unaffected areas. Any search and rescue activities in a fully involved structure should be accompanied by an attack line. The atmosphere A rollover should not be confused with a flashover inside the structure must be cooled down and ventilated as soon as possible if there is a possibility of entrapped victims. Their chances of survival will depend greatly on this. Training: The flashover simulator The flashover simulator is a training unit designed by the Swedish National Survival Board in It was adopted all over the world as the definitive training simulator for flashover awareness in subsequent years. A number of fire departments and even some training centres also built them in South Africa. If you don t have a simulator, try to find out which departments close to you does have one and arrange a training visit (remember to take doughnuts). On the other hand, it isn t an expensive prop to build yourself and will continue to provide valuable training for many years. The flashover simulator consists of two metal (14-gauge steel) shipping containers: the burn module and the observation module. They are connected and are open to each other at the connection point. Students are seated on low benches in the observation area and observe a controlled fire in the burn module. All instructors and students are suited in full turnout gear, including flash-hoods and self-contained breathing apparatus (SCBA). The goals of the flashover simulator are to teach fire fighters to recognise the warning signs of flashovers and understand the limits of their PPE. Time in the simulator is Volume 3 No 7 FIRE AND RESCUE INTERNATIONAL 33

4 about fifteen minutes and, if the conditions are good, a full on flashover will happen within a controlled environment. I will deal more in more detail with flashover training in future editions. Backdraft Backdrafts occur in the smouldering phase of the fire when most of the oxygen has been consumed. The heat from the free-burning phase remains, however, and flammable products of combustion are suspended in the smoke. All of this just needs an injection of fresh air to burst into rapid, almost instantaneous combustion. This situation normally comes about due to inadequate ventilation early on. The most common cause for this injection of fresh air is someone opening a door or window of the smouldering room. In South Africa we also have to deal with the heavy tiled roof structures in a lot of residential occupancies. If the fire penetrates the ceiling and communicates with the roof void, you have a large open space with nothing but fresh air and timber trusses holding up a heavy tiled roof. Oh, and you also have a heavy geyser precariously perched up there. Not only will you be dealing with a backdraft overhead but you might also be wearing a geyser if you re not careful. So how do you recognise backdraft conditions? Well, the first would be have a common sense approach when you respond to a structure fire. The first thing will be to look at the structure you are approaching. A cold winter s morning response to a fire in a confined room in residential structure with little or no flame showing or smoke stained windows should begin to tell you that opening the door will really ruin your day. The first-in engine arriving at the fire might see smoke leaving the building in puffs or at intervals and black smoke becoming a dense greyish yellow colour. This is another sure sign of an impending backdraft. I have mentioned in previous articles the value of the firstin engine company doing a three quarters drive-by of the building and get a view of the front three sides before committing the unit to the incident. This will stop the crew from getting tunnel vision and rushing in on the most obvious opening with catastrophic results. Sector safety officers must be very attentive when they see a sudden rapid movement of air inward soon after an opening is made. This is an acute indicator of an imminent backdraft. Backdraft prevention and survival If backdraft conditions are recognised upon arrival, all efforts must be focussed on preventing it from occurring. Vertical ventilation is the best way to lessen the risk. When putting a ventilation crew on the roof, care must be taken to ensure that they will not be compromised by the sudden rush of smoke, heat and flames that will blow out of the newly created opening. Positive pressure ventilation (PPV) should never be considered when backdraft indicators are high. When using PPV in a confined structure fire, take care to ensure that an adequate ventilation outlet opening is created. The lack of an outlet for the generated air could lead to a backdraft when the fire is in its decay phase. When you are moving through a structure with multiple compartments (rooms), each compartment must be evaluated for a possible backdraft. Heat and smoke can be trapped in cells within a room and when your crew moves from a well-ventilated room into a compromised compartment, it could cause a backdraft. Once backdraft conditions develop within a compartment, it may be necessary to simply do nothing but monitor the space until the smouldering fire goes out due to lack of oxygen or fuel or to allow the gases within the space to cool below their ignition temperature. This can be accelerated if a piercing nozzle is introduced into the space. We are all taught from the start of our careers not to stand directly in front of a doorway or other openings when a backdraft is possible. Consider the V-shaped force that will blow outward near the opening. The gaseous products of the backdraft will expand as it comes through the opening because of the lesser pressure of the atmosphere outside the building. Also remember that that you may not encounter backdraft conditions on arrival but that it could develop during the course of the operation. This has often happened and incident safety officers must ensure that the tell-tale conditions are monitored continuously through the job. In closing It is a fact that fires burn hotter today than in the past. The introduction of new textiles have increased the fire load of buildings immensely over the years. The higher risk of smoke and toxic gasses has further increased the hazards you may encounter. There are various ways to recognise the signs of a potential flashover or backdraft and it is important that all fire fighters responding to a structure fire be well trained in recognising these signs. You can also only mitigate the possibility of either occurring if you understand the dynamics of the situation and why it is leading to a potential disastrous situation. Recognise that no two situations will ever be the same. Continuously evaluate and make sure you pull your crews out of a possible backdraft/flashover situation if they are already committed to fighting the fire. If you have not yet committed them, make sure you are satisfied that all potential hazards have been negated before you do so. If you are not sure, don t commit them. During a chat with the publisher recently we agreed that if we were to adhere strictly to the Occupational Health and Safety Act no one would ever do aggressive interior fire fighting. The risks are tremendous and we must use all the tools at our disposal to ensure that our fire fighters are as safe as they can possibly be when they do enter a structure involved in a well-developed fire. 34 FIRE AND RESCUE INTERNATIONAL Volume 3 No 7

5 A sequence of photographs taken by Lee Raath-Brownie at Windhoek Fire Services flashover simulator Volume 3 No 7 FIRE AND RESCUE INTERNATIONAL 35