Introduction to Fire Safety Management

Principles of fi re and explosion
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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 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 300
C). 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