Introduction to Fire Safety Management

Figure 7.26
The smaller a substance is divided the greater the surface area
Figure 7.27
Confi ned explosion within a tank
Burned
gas
Flame
Fuel/air
or
Fuel-oxidiser
Figure 7.28
Static discharge between an unearthed hop-
per and an earthed vessel having the potential to ignite dust
Unearthed hopper
Non-conductive
blocks
Earthed vessel
Sparks can occur here
igniting the dust cloud
below
Fine flowing
material
Settled material

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Introduction to Fire Safety Management
130
➤
The amount of explosive atmosphere suffi cient to 
cause injury or damage by ignition.
The consequences of both gas and dust explosions 
will depend upon the environment in which the mixture 
(dust or gas cloud) is contained or, in the case of the 
gas cloud, what it engulfs. A gas explosion will be clas-
sifi ed in relation to the environment in which the explo-
sion takes place, for example 
confi ned gas explosions
which occur within vessels, pipes, tunnels or channels, 
or unconfi ned gas explosions commonly referred to as 
unconfi ned vapour cloud explosions
, which occur in 
process plants and other unconfi ned areas.
Once the determination of the explosive atmosphere 
has been established it is then necessary to consider 
whether there is a presence of an ignition source which 
will precipitate an explosion; such ignition sources 
could be:
➤
Hot 
surfaces
➤
Naked 
fl ames and other hot gases
➤
Mechanically generated sparks
➤
Electrical 
equipment
➤
Static 
electricity
➤
Lightning
➤
Radio frequency (RF) – electromagnetic waves
➤
Ionising 
radiation.
Once ignited an explosion may have secondary reper-
cussions, particularly in the case of dust explosions. This