Introduction to Fire Safety Management

7.5 Explosion
As the terminology surrounding explosion is generally
less familiar than that of fi re, it is benefi cial to defi ne
certain terms to clarify future explanations.
7.5.1 Terminology
Explosion – an abrupt oxidation, or decomposition reaction,
that produces an increase in temperature, or pressure,
or in both temperature and pressure simultaneously.
Explosive atmosphere – fl ammable substances in the
form of gases, vapours, mists or dusts mixed with air
under atmospheric conditions, which, after ignition has
occurred, combustion spreads to the entire unburned
mixture.
Defl agration – a combustion wave propagating from an
explosion at subsonic velocity relative to the unburnt gas
immediately ahead of the fl ame (fl ame front).
Detonation – a combustion wave propagating from an
explosion at supersonic velocity relative to the unburnt
gas immediately ahead of the fl ame (fl ame front).
BLEVE – boiling liquid expanding vapour explosion –
an explosion due to the fl ashing of liquids when a vessel
with a high vapour pressure substance fails.
CGE – confi ned gas explosion – explosion within tanks,
process equipment, sewage systems, underground
installations, closed rooms, etc.
UVCE – unconfi ned vapour cloud explosion –
a vapour/gas explosion (defl agration or detonation) in an
unconfi ned, unobstructed cloud.
7.5.2 The mechanism of explosion
Explosions can be caused by nuclear reactions, loss of
containment in high pressure vessels, high explosives,
runaway reactions, or a combination of dust, mist or gas
in air or other oxidisers. This chapter concentrates on the
latter examples.
Reaction zone
Burnt material
Figure 7.23 Detonation wave through a fl ammable mixture
Principles of fi re and explosion
Both dust and gas explosions are very similar in
nature; when a volume of fl ammable mixture is ignited
it results in a rapid pressure increase and fi re moving
through the atmosphere or cloud.
A dust explosion occurs when a combustible
material is dispersed within the air forming a fl ammable
cloud, this allows the fl ame to propagate through it.
A gas explosion follows a very similar principle
when gas and oxygen are premixed within explosive
limits which depend upon the supply of oxygen and the
concentration of the fuel. If either of these is too high or
too low then explosion will not occur.
7.5.3 Defl agration, detonation and explosive
atmospheres
An extremely dangerous situation will occur if a large
combustible mixture of fuel and air (gas or dust) is formed
in a cloud and ignites. The time period from the release
of the fuel mixture in air through to ignition can range
from a few seconds to tens of minutes. The amount of
fuel involved can also vary. In the case of gas the amount
can vary from a few kilograms up to several tons.
The pressure generated by the explosion’s combustion
wave will depend on how fast the fl ame propagates
through the explosive mixture and how easily the
pressure can expand away from the cloud, which is
governed by confi nement.
When a cloud is ignited the fl ame can propagate in
two different modes through it. These modes are:
➤ Defl agration
➤ Detonation.
The most common mode of fl ame propagation is defl agration.
During defl agration the fl ame front travels at
subsonic speeds through the unburned gas; typical
fl ame speeds do not reach higher than 300 m/s. The
pressure developed in front of the fl ame explosion may
reach values of several bars.
Unburnt material
Shockwave
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