Flashover and Backdraft are distinctly separate events which occur in different ways. Whilst there has been much scientific research associated with flashover as an event, the research efforts directed at backdrafts has been fairly sparse. However, there are various definitions that have evolved through scientific analysis of such phenomena although, in terms of content, they are all in agreement.

Fleischmann, Pagni and Williamson suggested that un-burnt pyrolysis products should be substituted for heated gases in the NFPA definition.

Whilst it is clear that flashover and backdraft are two separate events there are further situations where ignitions of the fire gases within a compartment can occur. These additional 'events' may not necessarily conform to any of the above definitions but will present a similar outcome in terms of rapid fire propagation. It is important for the firefighter to have a basic understanding of all events that may lead to such ignitions under varying conditions within a fire involved structure.

a) The formation of variable-sized flammable 'balloons' of fire gases may occur within the confines of a building. These may exist in the fire compartment itself, or in adjacent compartments, entrance hallways and corridors. They may also travel some distance from the fire source into structural voids or roof spaces. The addition of air is not a requirement for ignition of these gases which have already formed into an ideal pre-mix state, simply awaiting an ignition source. The resulting deflagration will be likened to that of a backdraft but in real terms a smoke explosion or fire-gas ignition is perhaps a better description.

At one fire in Stockholm a layer of fire gases had accumulated under a high ceiling in a warehouse and ignited with explosive force during the overhaul phase, sometime after the main fire had been suppressed. This occurred as a burning brand rose into the gas layers on the convection current. Another incident in a confined under-stairs cupboard resulted in a firefighter being blown into the hallway as he lifted debris and uncovered a smouldering fire in a smouldering pile of rags and plastic. The accumulated fire gases within the cupboard were introduced to the ignition source that had remained covered until then! Neither of these events required an inflow of air to initiate the deflagrations but rather demonstrated an ignition source being uncovered and introduced into the gases.

b) A further ignition of super-heated fire gases may occur where they mix with air as they exit the compartment. This can occur at a window or doorway and the resulting fire may burn back into the compartment through the gas layers, similar to a flashback within a bunsen burner. The author experienced this situation when gaining access to a basement apartment fire where the gases ignited outside as entry was gained. This event trapped firefighters for a few seconds at the base of the stairs leading down to the apartment as the flames rolled over their heads, cutting off the only escape route up the stairs to street level.

c) An event that creates rapid fire propagation and is often termed a flashover by on-scene firefighters can occur where a fire is suddenly stirred by a large movement of air, generally in the direction of the firefighting crew. This can occur when a crew is advancing against an attack hoseline operating in their direction; or instances where PPV is used to disadvantage; or where a window fails on the other side of the fire and a gust of wind pushes the fire at the nozzle crew. The flames will be seen to increase and searing heat will head directly at the advancing firefighters. This effect is also common in tall buildings where a negative pressure may exist behind the nozzle crew due to stack action on the access stairwell. This natural action sometimes causes windows to fail early as the air flow is window to stairwell. London firefighters experienced such an event in a high-rise fire as the crew gained access to the fire compartment. As the apartment access door was opened on the 12th floor the fire erupted out into the entrance hallway as the windows failed inwards. The heat forced firefighters to retreat down 2 floor levels before resuming their attack on the fire under the most severe of conditions! The stack action had caused flashover like conditions but this event was neither a flashover or a backdraft. Similar events have occurred at several high-rise fires, notably the Westvaco fire (NYC 1980), the Empire State Building fire (NYC 1990), and the Hotel Winecoff fire (Atlanta 1946).

d) To make matters even more confusing, there is a situation where a flashover may also be induced by increased ventilation. Chitty demonstrates this event where initially during its development, small openings within the compartment will allow a fire to reach a ventilation controlled stability point. If further ventilation is provided (a door or window is opened) the heat losses from the compartment will increase as more heat is convected out of the opening. Prior to the change in ventilation the fire will have been pyrolyzing more material than can be burnt. At this stage the amount of ventilation provided is critical - effectively, if it is sufficient then the temperature losses will be great enough to prevent a flashover. However, if the ventilation is inadequate and the temperature level is maintained then the unleashed energy of the excess pyrolyzates will create flashover conditions - a ventilation induced flashover! In some cases this could be manifest as a backdraft.

Water has been known as an extinguishing agent as long as fire has been known to man. With the exception of helium and hydrogen, water possesses the greatest specific heat capacity of all naturally occurring substances and has the greatest latent heat of vaporization of all liquids. It is estimated theoretically that a single gram of liquid water can extinguish a 50 liter flame volume by reducing its temperature below a critical value - equivalent to an 'application rate' of 0.02 liters per cubic metre. It has also been suggested that the quantitiy of water required to achieve control of a structure fire is between 10 - 18 gallons per 1000 cubic feet of fire. Again, in the UK it is further estimated4 that the majority of 'typical' compartment fires are extinguished using between 16 - 95 gallons, which is less water than one engine carries! There also are many published formulas available to firefighters, used to estimate water requirements during structural firefighting operations. These range from the Royer/Nelson view that 10 gallons per minute is required per 1000 cubic feet of fire, to the generally more acceptable National Fire Academy (USA) estimation that approximately 30 gpm would be needed for such a volume of fire.

As an extinguishing medium water has a theoretical cooling capability of 11.8 MegaWatts per gallon per second, although in the practical application of a 'direct' attack this capability is more likely to be around 2.36 MW per gallon per second. On putting such figures into perspective the firefighter is able to appreciate the true extinguishing potential of hoselines in any specific situation. As an example, the estimated Heat Release Rate (HRR) from a foam-filled chair is normally within the region of 4-500 KW. However larger fires, such as those where modern office 'work stations' comprising of furniture, stationery and a computer terminal are involved, may present a greater challenge and HRRs of 1.7 MW in five minutes (two-partition) and 6.7 MW in nine minutes (three partition) have been recorded from these items alone! Swedish style flashover 'simulators' usually approach the 3 MW level whilst at the Interstate Bank high-rise fire in Los Angeles in 1988 it was estimated that a 10 MW fire existed within two or three minutes of origin! Large amounts of water would be needed to handle such heat outputs. To the firefighter this means that the nozzle in use has a 'maximum practical' cooling capability and reliable estimates may be noted (table one). 

Water is potentially a very powerful extinguishing agent, although in order to realise this great potential, heat must be efficiently transferred from fire and its environs to the water applied during firefighting. Many scientists have closely studied the dynamics of fire suppression and extinction in general, where the dominant mode of structural fire suppression has been commonly identified as fuel cooling, although it is acknowledged that indirect cooling and inertion of the fire atmosphere also plays a part. However, few have realised the benefits and potential of gas phase cooling in terms of firefighter survival and safety, and it is the purpose of this book to introduce the techniques of three-dimensional water-fog applications as they have become increasingly popular with firefighters over a 20 year period in the inner-city capitals of Stockholm, London and Paris.

At this stage it must be clarified that such uses of water-fog are not comparable to the 'indirect' form of fire attack that became popular during the 1950s and sixties. This style of firefighting, which still has its followers today, suffered in terms of the additional hazards it created - for example, the technique relied on creating excessive amounts of super-heated steam within a reasonably 'un-ventilated' compartment (room or space). This was achieved by applying the water in spray form onto the hot surfaces, walls and ceiling within a fire involved compartment, which often necessitated firefighters working in extreme conditions and many suffered scald burns and heat exhaustion. There was also a problem caused by the 'piston' effect of the expanding steam, which would 'push' smoke, heat and occasionally fire into relatively unaffected parts of the structure, sometimes causing people to jump from exterior windows on the upper levels. In terms of application the firefighters were often trapped by their own actions as the thermal balance within the compartment was subjected to an 'envelope' effect, whereby the indirect application of water would again push fire and heat towards the far wall before moving upwards and across the ceiling and then returning down to surround the advancing firefighters! Somewhat in contrast, the main objectives of three dimensional water-fog are not aimed at dominating the mode of suppression but rather to complement the tactical approach, creating a comfortable and safe environment in which firefighters can function effectively during the overall firefighting and rescue situation. Ideally, the applications are aimed at preventing any ignition of the fire gases, but failing this, quenching, mitigating and controlling the hazards associated with flashover & backdraft. However, the application techniques are precise and rely heavily on suitable equipment, an effective operating procedure and the correct training carried out at regular intervals.

When does a firefighting stream become a spray? and, when does a spray become a mist, or a fog? 

In 1990 the Fire Experimental Unit in the UK completed research linked to the use of water-sprays in compartment fires. There was a distinct observation that firefighters followed a natural 'three-phase' approach when attacking the post-flashover fires.

There has been much research into the effects of 'gas -phase' cooling 

Modern firefighting nozzles produce sprays through pressure atomising effects and the result is termed a 'polydisperse' spray - that is, it comprises of a wide range of droplet sizes ranging from coarse to very fine. There are several methods of measuring droplet sizes within a spray but the results often conflict, depending on the method used. It has been suggested that there is an optimum droplet size in terms of fire suppression but this has never been achieved as the objectives are variable. In terms of 'theory' it is fairly straightforward in ascertaining the optimum size but in real situations a firefighting spray has to contend with several hindering factors when injected into a hostile mass of super-heated fire gases. The smaller the droplet the better its cooling capacity but if the droplets are too small then its likely that interaction with the buoyant fire plume may prevent droplets reaching the source of the fire. 

3

During the early 1980s following a flashover where two swedish firefighters were killed, Stockholm firefighters began to practise techniques developed by Gisselson & Rosander that were aimed at protecting firefighters from the flashover & backdraft hazards. These techniques entailed utilising a water-spray nozzle (T&A Fogfighter) to apply a fine water 'mist' into the overhead of fire gases using a series of brief 'spurts' (by resorting to a 'pulsing' technique at the nozzle). The objective was to avoid contact with hot surfaces, walls and ceilings and to place small amounts of water droplets directly into the fire gases where any cooling effect was maximised. The application avoided the massive expansions of steam and other problems associated with 'indirect' water-fog attack and created a safe and comfortable environment for firefighters to advance into prior to attacking the main source of fire. The Swedish concept (also termed 'offensive firefighting') was based on recognition of a fire's development process and great emphasis was placed upon observation of specific warning signs that might lead to an ignition of the fire gases, ie flashover & backdraft. The benefits of 3D water-fog applications are equally seen in both pre-flashover situations and post flashover fires.

To achieve effective results the 'fog-cone' and application angles are as important as the practical aspects of nozzle 'pulsing'. For example, a 60 degree fog-cone applied at a 45 degree angle to the floor into an average room (say 50 cubic metres) will contain about 16 Cu.m of water droplets. A one second spurt from a 100 lpm flow hoseline will place approximately 1.6 liters of water into the cone. For the purposes of this explanation let us suggest a single 'unit' of air heated at 538 deg.C weighs 0.45kg and occupies a volume of one cubic metre. This single 'unit' of air is capable of evaporating 0.1kg (0.1 liter) of water, which as steam (generated at this, a typical fire temperature in a compartment bordering on flashover) will occupy 0.37 Cu.m. It should be noted that a 60 degree fog-cone, when applied, will occupy the space of 16 'units' of air at 538 deg.C. This means that 1.6kg (16 x 0.1kg), or 1.6 litres of water, can be evaporated - ie; the exact amount that is discharged into the cone during a single one second burst. This amount is evaporated in the gases before it reaches the walls and ceiling, maximising the cooling effect in the overhead. It may be seen that too much water will pass through the gases to evaporate into undesirable amounts of steam as it reaches the hot surfaces within the compartment.

Now, by resorting to Charles Law calculations we are able to observe how the gases have been effectively cooled, causing them to contract. Each 'unit' of air within the cone has now been cooled to about 100 deg.C and occupies a volume of only 0.45 Cu.m. This causes a reduction of total air volume (within the confines of the cone's space) from 16 Cu.m to 7.2 Cu.m. However, to this we must add the 5.92 Cu.m of water vapor (16 x 0.37) as generated at 538deg.C within the gases. The dramatic effect has created a negative pressure within the compartment by reducing overall volume from 50 Cu.m to 47.1 Cu.m with a single burst of fog! Any air inflow that may have taken place at the nozzle will be minimal (around 0.9Cu.m) and the negative pressure is maintained. (Metrication is used to simplify the calculation).

Of course, in reality the entire area is a seething mass of heat where the 'air temperature' and 'compartmental pressures' will immediately rise again unless the applications are effectively progressed. With practise, the actual nozzle 'pulsations' may only last 0.1 to 0.5 of a second, allowing hoselines with greater flow capabilities to be used with equal effect. These are my calculations based upon three-dimensional water-fog theory and do not, to my knowledge, appear elsewhere. Whilst a far more elaborate calculation would be required to satisfy any scientific 'nit-picking', I am advised by scientists at the uk national Fire Research Station that, taking into account the variables associated with droplet sizing, the end result would remain similar to my own.

The application of of 3D water-fog into 'real' fires requires nozzle operators who possess a clear understanding of the objectives and capabilities of such techniques. These firefighters must also be extremely well practised in nozzle handling and 'pulsing' actions. Such skills can only be aquired through regular training in purpose built fire simulators, or converted steel shipping containers. Further attention should be directed to the provision and maintenance of suitable equipment and nozzles and an effective firefighting strategy should be adopted to complement the techniques.

The 'pulsing' action at the nozzle is created through rapid 'on-off' motions of the flow control lever or trigger. This is achieved with some practise and some nozzles are more suited to the action than others. Ideally, individual 'pulses' should last between 0.1 - 0.5 of a second and will place a fine range of water droplets into the overhead for a few brief seconds. As pulses of water spray evaporate the area becomes 'fogged' with water vapour but this occurs under strict control of the nozzle operator who, with experience, will learn to apply the pulses to optimum effect. Any 'sweeping' motion of the nozzle is most likely to upset the thermal balance within the compartment and force heat down to the lower parts of the room occupied by the firefighting crew, and continuous bursts of more than a second may cause a 'piston' effect to 'push' fire into uninvolved areas, roof spaces etc. The technique of 3 dimensional water-fog application has often been termed 'hole-punching', whereby the nozzle operator will attempt to 'puncture' the fire gas 'pillow' that hangs in the overhead with brief injections of water droplets. This effect will cause the gases to cool and contract and create an 'inerting' effect within the pillow itself.

This following account was presented by the author to an Irish Fire Chiefs convention in 1998. It represents a typical structural fire simulation bordering on flashover conditions and demonstrates how 3D water-fog may be utilised to complement tactical ventilation or PPV operations.

It can be seen that the use of gas-phase cooling techniques can effectively and safely complement the operational aspects associated with tactical fire venting or the use of Positive Pressure Ventilation (PPV). As with any strategy, it is important to ensure fire scene communication levels are established and maintained. The interior crews are the ones who are in a position to decide when and if ventilation operations should commence and their requests should be passed to the incident commander who has the overall responsibilty to initiate such actions.

The tactical implications concerning the use of 3D water-fog are initiated prior to gaining entry to a fire involved structure. All crews should observe, where possible, 29 CFR 1910.134 - the 'two-in / two-out' rule. Ideally, where manpower allows, a second support (back-up) line should be laid to operate behind the first-in line. In terms of application, european firefighters have demonstrated extremely low flow-rates whilst using 3D water-fog with hosereel/booster lines discharging as low as 25 gpm. However in line with safe practise and NFA guidelines concerning flow capabilities, an estimation of one 200 gpm hoseline with fog nozzle for every 6000 Cu.ft of fire involvement within a structure is normally sufficient. This figure is of particular relevance where open floor space in high-rise buildings becomes involved.

Observing the fire's behaviour - The nozzle operator must observe conditions within close proximity and assess the likelihood of any potential for a fire gas ignition. The overhead should be assessed for signs of flaming in the gas layers for this is a sure sign of a flashover approaching. Lower down the existence of fire 'balloons' (pockets of fire gases) igniting briefly about 2-3 feet from the floor is another warning of an imminent flashover. Signs of a rapid air movement below the interface is a sure signal to back out behind a pulsing spray as a backdraft may be seconds away. The firefighter should also look for 'rolling' smoke, particularly black smoke, which can sometimes be noted on entry as this is another 'backdraft' warning. A further example of hazardous conditions is the presence of 'blue' colored flaming - which may also serve as an indicator of 'backdraft' where pre-mixed flaming may exist. Where visibility is severely restricted through thick smoke the firefighter must rely on his senses - a sudden increase in compartmental temperature, forcing the firefighter to crouch extremely low, is a sure sign of an impending flashover.

The notion that three-dimensional water-fog applications can be used to suppress or quench flammable atmospheres is well founded. However, the scientific research to date has concentrated on WMFSS and suggests that extremely fine sprays are needed to mitigate or prevent the effects of a propagating flame in a mixture of gas and air. Various trials and tests have been carried out involving explosion suppression of all types of flammable gases and liquid vapors where extremely fine mists have successfully arrested propagating flames and inerted atmospheres to a stage where combustion would not take place. An FRDG4 report!refers to several of these studies and informs that droplet sizes below 100 microns (0.1mm) were used with great effect to achieve suppression. In terms of firefighting sprays, the existence of such fine droplets across the entire spatial angle of a cone does not normally exist during the 'average' application, but it is suggested that nozzles producing droplets within the 0.3 mm range will still provide an effective level of quenching within the flammable gas layers. If an ignition of the gas layers did occur then it is further suggested that the droplet break-up of the parent spray will serve to mitigate the explosive effects.

Whilst further research is required in this area in terms of the effectiveness of firefighting sprays it is generally accepted that a constant 'pulsing' application of water droplets, suspended in the overhead of a super-heated fire compartment, will prevent the likelihood of gaseous combustion and greatly increase the survival parameters of firefighters occupying the space.

Modern open-plan office floors are a common feature of high-rise buildings and present certain difficulties to firefighters. The large open area provides an abundance of air to feed any fire and modern office furnishings present a fuel source associated with extremely high Heat Realease Rates (HRR). These facts, coupled with a time delayed response to the fire floor, ensure that firefighters are often faced with a hot and smokey fire situation, particularly where sprinklers aren't installed. The fire may be bordering on flashover and the design of partitioned work stations may present the firefighters with a view of the flames at high level but will prevent a direct hit at the source unless it is close by. This situation will allow a highly flammable layer of fire gases to accumulate at ceiling level, or in the plenum, across the entire expanse of the fire floor! Where such floor areas are likely to exceed 200,000 Cu.ft the extent of the problem can be clearly seen. One hindering factor in mounting a successful fire attack under such circumstances is the availability of water on the upper floors of a high-rise. It can be seen (in table two) that the NFA flow requirements guideline of 33 gpm per 1000 Cu.ft is rarely, if ever achieved during high-rise firefighting operations. In fact, firefighters have commonly had to contend with flows as low as ten percent of 'normal' requirements at such conflagrations and still put the fire out!

A fairly recent example of such a fire occurred in 1992 when the seventh floor of a 200 ft (12 storey) office tower in Los Angeles became involved. The fire that began in a work-station spread to involve most of the 400,000 Cu.ft 7th floor level. On arrival, just after 1005 hours LAFD firefighters noted flames 'blowtorching' from two windows at level seven. The building was itself sited just a few blocks from the Interstate Bank Tower - the scene of a major conflagration in 1988. On the fire floor, Engine 3 Captain Don Austin said his crew encountered heavy smoke banking down to floor level with moderate heat conditions. The LAFD firefighters advanced their 2 inch attack hoseline, equipped with an automatic nozzle, about twenty feet into the fire floor when they observed an orange glow ahead. Even though they attempted to hit the fire the 2 inch line appeared to have no effect on the flames. Within sixty seconds of opening the nozzle the fire flashed across the ceiling and the crew were trapped with flames above and behind them. Austin and his crew , with helmets melting in the heat, managed to crawl the twenty feet back to the 'safety' of the lobby on their stomachs. It was about this time that the entire north side of the structure 'lit-up' as flames pushed out of all twenty 7th storey windows on that side of the buiding. The fire was eventually brought under control by the 263 firefighters on scene within one hour and nineteen minutes from the outset.

A recent report!by the United States Fire Administration reviewed firefighting tactics in high-rise buildings and addressed some of the problems encountered by firefighters, particularly in terms of pressure and water availabilty at upper levels. 

Developments in the use of water additives and Compressed Air Foam Systems (CAFS) have demonstrated that the use of water as a fire suppressant can be improved upon even further by the use of such solutions. What is even more enlightening is the research into the use of such applications in spray form, where it has been shown that the cooling effect of fine droplets in suspension is increased by the presence of such additives. The ideals of gas-phase cooling and 3D water-fog applications are optimized even further and a marked improvement in performance over plain water sprays is seen in all respects, particularly in terms of explosion suppression.

The Swedish flashover 'simulator' is a training unit designed by the Swedish National Survival board in 1986, following some earlier trials and experimentation by firefighters in Stockholm. There are now several versions of the system being manufactured but most are based on the original style of steel shipping containers connected together to form both burn and observation modules. The burn module is lined with panels of half-inch particleboard and a small wood crib fire is ignited to heat the boards, allowing the accumulation of copious amounts of flammable fire gases before they ignite in repeated simulations. This enables firefighters to observe a fire's growth and development stages; the formation of flammable gas layers; fire 'snakes' in the overhead and ignitions of the gases themselves. The effects are fairly dramatic with Heat Release Rates approaching 3MW but stringent safety controls ensure the danger to firefighters is minimised.

It is an effective way to take firefighters through such conditions with an element of 'control' and they learn to 'read' a fire and witness the effects of fire gas ignitions. Whilst the 'flashovers' are not true flashovers in the broadest definition, they most certainly present the severest of training conditions and both firefighter and protective clothing are tested to the limit!

Whilst in the container, firefighters are taught not only how to recognise the dangers of fire gas ignitions, but also how to tackle both 'pre' and 'post' flashover situations. The procedures related to gas-phase cooling and 3D water-fog applications are practised over and over until operators become efficient in the use of effective pattern diameters; application angles and nozzle 'pulsing' techniques. However, it is important to observe a strict safety code with special attention paid to the following:

The suspension of small amounts of water droplets directly into the accumulating fire gases in the overhead is the most effective action a firefighter can take during his/her approach to the fire's source. This application, to be effective, demands great precision and controlled use of the nozzle. It requires regular training and the provision of suitable equipment to achieve optimum results. The firefighters of the new millenium will soon realise there is only one way to effectively deal with the hazards associated with flashovers, backdrafts and fire gas ignitions - and that is to prevent them in the first place!

Excerpts.................................

Copyright Fire Tactics.Com 1999 - All rights reserved

Paul Grimwood

Fogattack@USA.com 

Paul Grimwood is a retired London firefighter having served 26 years as a professional firefighter. His book FOG ATTACK, published in 1992, greatly influenced major changes in the strategic approaches of fire departments around the world and brought the 'new-wave' Swedish techniques of applying water-fog in short rapid bursts to control the hazards of flashovers to the forefront.