Enclosure fires

In Figure 57 calculations have not been performed for “large”
openings, as it turns out that the increase in pressure is already
so low for small openings.
In most fi res the gaseous mass is not premixed. If the gaseous
mass was premixed doors and windows would be pushed
out in normal compartment fi res. But this does not actually
happen. It is very rare in house fi res for doors and windows to
crack as a result of the pressure. In Chapters 4, 6 and 7 we will
discuss in which situations we can realistically assume that the
gaseous mass is premixed. The concept defl agration is associated
with the ignition of premixed gaseous masses and is often
used when talking about fi res.
The concept smoke gas explosion is often used when talking
about fi res and means that a large smoke gas volume has been
premixed before ignition occurs. The closer the mixture is to
the stoichiometric point, the higher the increase in pressure
will be. This is because the fl ame front speed is higher. An explosion
can be defi ned as an exothermic chemical process which,
when it takes place at constant volume, results in a sudden, signifi
cant increase in pressure. Occasionally you will hear people
talking about the fi re spreading like an explosion. This is often
incorrect.
Pressure resistance in different building components
Some of the commonest hydrocarbons, e.g. methane and propane,
can in stoichiometric mixtures generate pressures of up
to 8 bar, if they ignite in a closed room. 12 No building can
withstand pressure of this size, unless they are specially designed
to do so. Usually the weakest parts in the building will
collapse, allowing the gases to escape. This will reduce the
pressure build-up. It is still possible, however, for the ventilation
areas to be too small, which means that stronger building
components can also collapse.
Table 6 (on the following page) shows the level of pressure
at which different building components will collapse. 12 The
pressure fi gures given below must be regarded as rough estimates
and vary according to the relevant material used and
how the experiment has been carried out. The resistance of
glass panes, for instance, can vary a lot.
79