Enclosure fires

esult in ignition. A smoke gas explosion, on the other hand,
occurs most often in an area adjacent to the fi re room.
Generally speaking, a backdraught causes less pressure to build
up than a smoke gas explosion. A backdraught requires the ventilation
conditions to change, which, in itself, helps to ease the
pressure. In the chapter on smoke gas explosions, we also discussed
the fact that a smoke gas explosion can occur in the fi re
room, but that this is a rare occurrence. This is because there is
very often an ignition source, which consumes the combust ible
gaseous mass as soon as it is formed. As a rule, the size of the
combustible gaseous mass formed is not suffi ciently large.
We need to try and get some idea of the extent to which
ignition is dependent on whether the ventilation opening has
changed or not. If the situation has arisen due to a change in
ventilation we are dealing with a backdraught.
8.5 Differentiating between a fl ashover and
a smoke gas explosion
Both these scenarios are the simplest to differentiate between,
as a fl ashover involves diffusion fl ames, whereas premixed
fl ames are involved in a smoke gas explosion.
8.6 Summary
To sum up, we can say that it is important to understand the
processes which control how fi re-related phenomena occur so
that we can realise what has actually taken place. The terms
used, of course, also are fundamentally important, as in situations
requiring quick, appropriate action, there is no room for
any misunderstanding.
Generally speaking, it is diffi cult to differentiate between
the various phenomena which have the same type of fl ame
involved. For instance, diffusion fl ames are involved in a
fl ashover, whereas both premixed and diffusion fl ames are involved
in a backdraught. This can make it diffi cult to distinguish
between them. On the other hand, if we compare a
fl ashover with a smoke gas explosion, where premixed fl ames
are involved, it is much easier to differentiate between these.
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