FLASHOVER - A GENERIC TERM!                                                               

Flashover is a significant killer of firefighters. In the USA NFPA statistics recorded between 1985 and 1994 demonstrated a total of 47 US firefighters lost their lives to 'flashover'

The term 'flashover' was introduced by UK scientist P.H. Thomas in the 1960s and was used to describe the theory of a fire's growth up to the point where it became fully developed. Customarily, this period of growth was said to culminate in 'flashover', although Thomas admitted his original definition was imprecise and accepted that it could be used to mean different things in different contexts. Thomas then went on to inform us in UK Fire Research Note 663 (December 1967) that there can be more than one kind of flashover and described 'flashovers' resulting from both ventilation and fuel controlled scenarios.

Thomas also recognised the limitations of any precise definition of 'flashover' being linked with total surface involvement of fuel within a compartment (room) where, particularly in large compartments, it may be physically impossible for for all the fuel to become involved at the same time. British Standards (4422) of 1969 and 1987 further attempted  to apply a more precise definition without success.

During the 1980s the generic use of the term 'flashover' evolved further still when Swedish fire engineers Krister Giselsson and Mats Rosander  encompassed a series of further events associated with the ignition of fire gases into the widening scope of the original definition.  However, this attempt at introducing 'new' terminology into previously accepted scientific language was not popular and appeared to confuse matters further although it certainly prompted a review of internationally recognised definitions to date. 

In 1989 the UK's Fire Research Station located at Borehamwood further defined the term 'flashover' as used to describe a more generic range of scientific phenomenon, but where the overall effect was one of rapid fire development - This statement was inline with other notable definitions of the time -

(1) Ignition of the flammable volatiles beneath a horizontal barrier (usually a ceiling) which have collected as a result of pyrolysis mechanisms from overheated materials.

(2) Downward radiation from flames beneath a ceiling causing rapid decomposition of 'fuel' beneath and acceleration of fire. (FRS preferred definition).

(3) 'Explosive' burning of flammable volatiles within an enclosure when the compartment is ventilated by the opening (or breakage) of a door or window.

(4) 'Explosive' burning of some special types of 'cold' smoke from the smouldering of (for example) foams.

Through the 1990s international scientists were at conflict with the recognised definitions of the term 'flashover' that consistently refused to include any reference to 'premixed' burning within its overall context. Current definitions appear as follows -

ISO Flashover - 'the rapid transition to a state of total surface involvement in a fire of combustible materials within a compartment'........

United Kingdom Flashover - 'In a compartment fire there can come a stage where the total thermal radiation from the fire plume, hot gases and hot compartment boundaries causes the generation of flammable products of pyrolysis from all exposed combustible surfaces within the compartment. Given a source of ignition, this will result in the sudden and sustained transition of a growing fire to a fully developed fire.......This is called 'flashover'......'

It is a significant feature of a 'flashover' that this transition to a state of total involvement is sustained.

BACKDRAFT - MULTIPLE EVENTS!

The USA, in the meantime, were directing their research towards the phenomenon of 'backdraft' and an NFPA definition described the event as follows - 'the explosive or rapid burning of heated gases that occurs when oxygen is introduced into a building that has not been properly ventilated and has a depleted supply of oxygen'.........

A European (UK) definition of backdraft was similar in agreement -'Limited ventilation can lead to a fire in a compartment producing fire gases containing significant proportions of partial combustion products and unburnt pyrolysis products. If these accumulate then the admission of air when an opening is made to the compartment can lead to a sudden deflagration. This deflagration moving through the compartment and out of the opening is a backdraft (backdraught).'........

In 1992 the 238 page (Fleischmann) report was produced and reported on the phenomena of backdraft -

The purpose of this project was to develop a fundamental physical understanding of backdraft phenomena. The research was divided into three phases: exploratory simulations, gravity current modeling, and quantitative backdraft experiments. The primary goal of the first phase was to safely simulate a backdraft in the laboratory. A half-residential-scale compartment was built to conduct exploratory experiments. The initial experiments concluded with a scenario describing the fundamental physics of backdrafts. The importance of the gravity current which enters the compartment after opening was identified. In the second phase, the gravity current speed and the extent of its mixed region was investigated in a series of scaled salt water experiments. In the final phase, 28 backdraft experiments were conducted in a 1.2m by 1.2m by 2.4m compartment. A methane burner was ignited inside a closed compartment and allowed to burn as long as oxygen was available. After the flame extinguished due to oxygen starvation, the burner was left on to allow the unburned fuel fraction to increase. Upon opening the hatch a gravity current enters the compartment and travels across the floor to the ignition source. After ignition a deflagration rips through the compartment and out the opening culminating in a large fireball. Histories recorded included: fuel flow rates, upper layer temperatures, lower layer temperatures, opening velocities, compartment pressures, upper layer species concentrations for O2, CO2, CO, and HC. Results indicate that unburned fuel mass fractions >15% are necessary for a backdraft to occur and that the backdraft severity strongly depends on the delay time and species concentrations

Then an extensive report from Richard Chitty of the UK Fire Research Station (5/94) went on to inform us how a 'flashover' could be caused by an increase in compartmental ventilation - but wait!  Isn't this a backdraft? No.......a tactical venting action can actually lead to both backdraft and flashover situations!  Richard Chitty's excellent report made a point of differentiating between pre-mixed flames and diffusion flames when discussing the flammability limits of smoke and fire gases, or unburnt pyrolisation products and partial combustion products. Chitty also posed the potential of a 'flashover' being induced by an increase in compartmental ventilation where the heat loss rate increases as more heat is convected through the opening. However, there is a point beyond stability where ventilation may cause more energy to be released in the compartment than can be lost and this condition of 'thermal runaway' may lead to 'flashover'. 

Chitty went on to examine the basic mechanisms which can involve a sudden change in the heat release rate of a fire in an enclosure - such changes can be divided into step events, where the heat release rate of a fire is sustained and transient events when the heat release rate returns to (approximately) its original value. There are seven ways in which a sudden change may occur - four of these are step events representing transitions between fuel and ventilation controlled states whilst three are transient events corresponding to one of the components of the fire triangle (fuel, heat, oxygen). Flashover is usually a 'step' event whilst 'backdraft' is termed as a transient event, involving short, possibly violent, releases of energy from the fire which are not sustained. It is possible for both step and transient events to occur at the same time.

In a  245 page report ( ref: 1019) (1999) from Lund University, Sweden, Lars-Goran Bengtsson presents a detailed description of the phenomena related to flashover; backdraft and also smoke explosion; which suggests  there may be additional events that might lead to rapid fire development. 

A 110 report by B.J. Sutherland of University of Canterbury in Christchurch, New Zealand (1999) further researched phenomena closely associated with smoke explosions and promotes an additional form of rapid fire development -  An explosion is defined in this study as the rapid propagation of a flame front with an accompanying pressure wave (Croft, 1980). Croft (1980) suggests that pressures as high as 5-10 kPa could be produced during a smoke explosion. Pressures this high are large enough to break windows. It is the velocity of the flame front that determines the magnitude of the pressure wave. If the pressure wave is not formed or is negligible, then the phenomenon is known as a flash-fire, and not an explosion (Wiekema, 1984). This excellent report describes how smoke/gas layers may descend onto sources of ignition; how ignition sources may ascend into the gases and how a process termed 'puffing' may precede such occurences.

Floyd Nelson (USA) introduced a further definition for a term he referred to as Forward-induced Explosions. In effect, this definition described the ignition of pockets of fire gases as they transported throughout a structure/compartment. The phenomena differed from that of backdraft in that fresh air (oxygen) is the moving force in a backdraft whilst the gases themselves are the moving force in a 'forward-induced' explosion as they move towards a supply of air. This can occur in many ways inside a fire involved structure, for example, where a collapsing ceiling forces fire gases to transport outwards from the area of collapse. On mixing with pockets of air they may come into the flammable range and can ignite with varying explosive effects.

Mr Nelson also discusses the effects of high velocity gases (in his book - Qualitative Fire Behaviour) that may gain momentum in large spaces, corridors or shafts within a structure. Where the movement and ignition of super-heated fire gases are accelerated through narrow openings or corridors or are deflected the effects can be dramatic. The deep levels of burning (referred to in the UK as a local deepening) will cause unusual patterns of burn as if an accelerant has been used to increase fire intensity. On occasions, where high-velocity gases escape to the outside without being deflected,  their flow is such that they may cross an entire street creating a flame-thrower effect from a window or doorway.

Daniel Gojkovic of Lund University in Sweden offered some most useful research to firefighters that explored some theoretical and practical aspects of tactical firefighting operations, evaluating the varied approaches to an under-ventilated compartment fire including 1. Natural venting actions; 2. 3D offensive water-fog tactics; 3.   Anti-ventilation (Close-down) and 4. PPV attack.

RAPID FIRE PROGRESS

This widespread variance in definition has resulted in a situation where the NFPA no longer keeps statistics specifically on flashover deaths. According to an NFPA analyst, it was discovered that deaths reported as flashover fatalities were really from backdrafts or other forms of rapid fire progress. So now, for the sake of accuracy, all such deaths are classed simply as due to 'rapid fire progress'.

In reality the phenomena of backdraft, flashover and smoke explosions are all closely linked and a firefighter who witnesses an event associated with rapid fire progress may not be able to differentiate between specific situations. On other occasions it may be clearly apparent as to what actually occurred. Fleischmann suggests future research should be directed at answering the following questions - (1) Can vertical ventilation prevent backdrafts? (2) What might be the effects of horizontal ventilation as an alternative? (3) What are the implications and what is the impact of firefighters closing the door on entering the compartment? Much of this was addressed in the Gojkovic research and is detailed in his report.

It is most important for firefighters to recognise the likely warning signs associated with rapid fire progress and appreciate what effect their own actions might have on inducing such events.