Civil Engineering Reference
In-Depth Information
FUEL
Flammable gases
Flammable liquids
Flammable solids
Fully developed
Fully developed
(a)
(a)
Decay
Decay
OXYGEN
Always present in the air
Additional sources from
oxidising substances
IGNITION SOURCE
Hot surfaces
Electrical equipment
Static electricity
Smoking/naked flames
Growth
Growth
Induction
Induction
Figure 7.1
Triangle of fi re
Time
Time
Figure 7.3
Stages of combustion or fi re growth
(a)
The decomposition of the material in this way is
known as pyrolysis and the smoke that can be seen
when a fi re occurs is in fact unburnt products of pyroly-
sis included in the vapours given off.
Figure 7.2 illustrates the main elements of the fi re
process.
fl ashover zone
'fl ashover point' (see Fig. 7.3). The time taken to arrive at
this fl ashover point will vary and is likely to involve more
than one area or phenomenon, including the size of the
room, the surface linings, the availability of oxygen and
a variety of complex chemical reactions.
The
fully developed
stage in a fi re is whilst the
reactions are not as rapid as in the growth stage, the
fi re continues to burn violently consuming the available
oxygen supply and fuel sources. This 'fully developed
stage' is characterised by massive fl ames and very high
temperatures (in excess of 300C). It is in fact at this
time that the fi re is controlled not by the amount of fuel
that it has to burn but by the amount of oxygen it has on
which to feed.
Finally the
decay
stage, where having consumed
all the available fuel the fi re dies down and is eventu-
ally extinguished, can be as a direct result of fi re service
intervention or can occur naturally when there is no further
oxygen or fuel to support the combustion process.
During the growth stage there is a period of time
prior to reaching the fully developed stage where there is
a serious risk of
fl ashover
. This is due to the layering of
hot gases beneath the ceiling and the oxygen concen-
tration in the air being less than normal. It is at this time,
as the concentration of gaseous fuel rises sharply in the
growth stage, that allowing air to enter causes it to mix
within the fuel layer, and with the already existent heat
and fl ame a fl ashover occurs, a process which is not
dissimilar to an explosion.
The fi re process is 'exothermic', in other words it
releases signifi cant quantities of heat. The amount of
heat produced is dependent upon the fuel involved
Fuel
Smoke
Oxygen
Fire
Heat
Ignition/heat
Figure 7.2
Diagrammatic representation of the fi re
process
7.2.2
Stages of combustion
There are four generally acknowledged stages con-
tained within the process. These start at the
induction
stage where the component parts of the triangle come
together and initiate the reactions.
The
growth
stage, where supplied with an uninter-
rupted level of oxygen or fuel the reactions become
rapid and grow in intensity, creating large volumes of
smoke (unburnt products of pyrolysis) the time taken
in the growth of a fi re may be from a few minutes to
several hours dependent upon the prevailing conditions.
The point at which the fi re involves all the combust-
ible materials within the room or area is known as the
