Flashover 'Pathways' ....
'PATHWAYS' and MOMENTUM (INERTIA) FORCES ASSOCIATED WITH RAPID FIRE PROGRESS - 'FLASHOVER' - A New Theory!
Paul Grimwood July 1999 updated July 2003
'The men of the fire brigade were taught to prevent, as much as possible, the access of air to the burning materials. What the open door of the ash-pit is to the furnace of a steam-boiler the open street door is to the house on fire. In both cases the door gives vital air to the flames'.........
Chief Fire Officer James Braidwood (LONDON) in his book 'Fire Prevention & Fire Extinction' 1866
The term 'flashover' has been used generically by the fire service over many years to describe various forms of rapid fire progress that relate to phenomena such as backdraft, smoke explosion and other forms of fire gas ignitions etc. To the firefighter the various definitions can become confusing and in reality he/she is more concerned with 'actions' or 'non-actions', forming part of the firefighting operation that may directly influence the initiation of such an event.
In simple terms a 'flashover' is a sustained fire that results from a well-ventilated and developing compartment fire, leading to total room or compartment involvement. This is a heat induced situation where heat from the fire plume, at ceiling level, radiates downwards to other combustible items in the room, causing then to
pyrolize and ignite.
A 'backdraft' is generally an event caused where an under-ventilated fire receives a sudden supply of air/oxygen and an ignition source causes the mixing fire gases to ignite, sometimes with explosive force. The event may present itself in either a sustained fire where large amounts of gases have accumulated, but more likely in a brief 'flash' (non-sustained) fire.
A 'smoke-explosion' may involve the ignition of an accumulated mass of flammable fire gases existing in a room or compartment and the addition of oxygen is not a requirement as the gases already exist in a pre-mixed state.
It is important for firefighters to recognize that these events are nearly always triggered by
their own on-scene actions (or failure to act) and it is perhaps here that we need to concentrate our efforts in forming safe approach tactics and SOPs during structural firefighting operations.
An awareness of the 'ventilation profile' and the location of the 'air-track'
are of great importance here.
The first action to be considered is the creation of an entry point (or points) if this does not already exist. Initiating a point of entry (ie; by opening the street doorway) may be an extremely influential action and have major effects on the fire's development. This action may set in motion a 'gravity current' where air is 'sucked' in at low level and smoke comes 'pushing' out with high velocity at the top of the doorway. If this does not happen then the fire is either -
1. Fuel controlled and burning efficiently, or not large enough to cause rapid fire progress at this stage.
2. Confined - probably behind internal closed doors.
3. Ventilation controlled but finding or searching it's gas/smoke exit point (pathway) at another location, possibly at a higher level in the structure.
The terms 'pathway' and 'momentum' forces, in relation to rapid fire phenomena,
are new and their effects and tactical implications will be discussed later in this article.
Where a gravity current is allowed to progress un-checked the fire will be dynamic in its progression and development towards some form of dangerous 'event' associated with the above phenomena of rapid fire progress. The Incident Commander must give this some thought and consider controlling the size of entry point openings to curtail any existence of gravity currents or pathways.
Another action that can lead to an 'event' is the tactical venting by firefighters of windows or vertical openings in a structure. This strategy is often used to great effect as a means of reducing 'flashover' potential but in effect, may actually initiate a flashover or other ignition of the fire gases. Where the structure is occupied this can prove highly hazardous to occupants!
Where firefighters are to make openings in a structure they should always
coordinate this action with effective hose-line deployments in support. Where
firefighters are occupying a structure the venting actions should be carefully
applied and then, only at their request. If firefighters are occupying basements
it may be safer to bring them out to a place of safety prior to creating
openings that may create a 'pathway'. The possibility of 'thermal runaway'
(leading to flashover) or backdraft occurring following a venting action must always be a concern in an occupied structure.
If a firefighter were to uncover or reveal an ignition source within a compartment prior to venting fire gases to the outside a smoke explosion could occur. This might happen where a smoldering mattress is over-turned or a floating ember is created through cutting & overhaul or PPV smoke clearance Ops.
A further action that might cause an 'event' is the opening of internal doors that set-up strong airflows towards a stair or lift shaft or possibly the approach and entry to adjacent rooms or compartments some way from the fire area, where flammable gases have accumulated within.
The movement of flammable gases may be created by the collapse of a ceiling, driving gases in their pre-mixed state towards a source of ignition elsewhere in the structure. This effect may also occur where firefighters utilize wide fog patterns that 'push' gases towards the fire.
'Pathways' & 'Momentum'
However, a phenomena that has received little, if any, attention is that of creating
'pathways' where venting actions occur below and above
(or on opposite sides of) the fire area, sometimes leading to devastating effects! A routine approach to a structural fire would be to open the street entry door, normally below the fire, followed by a tactical venting action above
or opposite to the fire. Incidents come to mind where occasionally this process is reversed where the entry is made at high level before the venting action occurs at low level, creating a 'pathway'. It is the final action of creating a 'pathway' for air to enter from below the fire and smoke and energy rich gases from an under-ventilated fire to exit at high level that sometimes leads to an 'event' of rapid fire progress. The two firefighters who just made it out of the Illinois fire were lucky to have escaped with minor injuries as a 'pathway' was created between the air-inlet (street door) and fire-gas exit point (window venting action) resulting in an ignition of the energy rich gases at 2nd floor level. It was as if the action of venting the window provided the momentum or inertia for the high-energy gases to ignite in a 'flash' of combustion, unleashing tremendous amounts of heat for a brief second or two. Other fires have demonstrated similar events as this 'pathway' was created by firefighting actions - notably the Cherry Road fire in Washington DC in 1999; Seattle basement fire 1987and the Dorothy May fire in 1982. Think of occasions where this 'pathway' might have been created, leading to rapid fire progress - an air inlet (street doorway) and a fire gas/smoke outlet (window) both in existence and if fuel rich gases (black or fast moving smoke) are involved - note the inertia created, quickly followed by a subsequent ignition!
'Momentum' and inertia forces may be set up inside a structure
that will directly affect the likelihood and intensity of flashovers or
backdrafts. These could be related to air-flow dynamics caused through -
Whilst many scientists have suggested that cool air flowing, or
being drawn, into a structure might have some cooling effect on the hot gases
accumulating inside a fire compartment, we have learned through experimentation
that the opposite effect is more likely! In situations where the outside air is
cold we have observed more frequent events of rapid fire phenomena that occur
with greater intensity, than where the air is warmer. These conditions are
easily replicated in small 'dolls house' flashover demonstrators and even
in larger steel shipping containers to some extent.
This may suggest that the specific gravity, or weight, of air
features in these flashover momentum or inertia forces in some way. As the air
exchange occurs in a compartment fire, with hot gases leaving the compartment
and cold air replacing them, the density of of the outside air may have some
effect on the severity of any subsequent rapid fire phenomena. As colder air
becomes more dense, the air exchange with heated gases in a fire compartment may
be more aggressive.
The relevance of this is that rapid fire phenomena may become
more likely, and indeed more fierce, on a very cold day, especially in sub zero
temperatures and this has been noted in several real fires.
To counter this effect we need to restrict or prevent a gravity
current or pathway from occurring in the first place - this may be achieved by
reducing and controlling the width of the opening at the entry doorway. We must
also deal with any build-up of high-energy gases that may have accumulated in
the structure - tactical venting! However, before we do this we might attempt to
cool and inert the gases just prior to venting them out. This may assist in
avoiding an ignition, but where a gas ignition does occur then the water
droplets in the overhead may quench the effect. The only way these gases may be
cooled or inerted effectively prior to venting is by the use of a stream of
water droplets from a finely divided water-fog pattern, applied in a series of
brief bursts (pulses) so that they hang in the gases for several seconds.
'It is most dangerous for any persons who happen to be in other rooms of the house, particularly those above and at he back, into which, after a front window has been cut through (broken), it is probable, if not almost certain, that the fire will penetrate before the firemen can reach them.........'
Chief Fire Officer Sir Eyre Massey Shaw (LONDON) in his book 'Fire Protection' 1876
For further information on this subject also see FORCED