Introduction to Fire Safety Management

Figure 7.29
Primary and secondary explosions
Figure 7.30
Diagram depicting typical consequences of the release of combustible gas into the atmosphere
Release of Gas
and/or Liquid
No Ignition
Immediate
Ignition
Fire
Ignition
(delayed)
Gas
Explosion
No damage
Damage to
Personnel
and Material
Fire
Fire
and
BLEVE
Formation of
Combustible
Fuel-Air
Cloud
(Premixed)
Dust cloud formed
Primary Explosion
Blast wave
Lying Dust
Secondary Explosion
Heat from Primary Explosion lgnites Dust Cloud

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Principles of fi re and explosion
131
can be broken down into primary and secondary explo-
sions. The concentrations required for a dust explosion 
are generally not seen outside process vessels and thus 
the most signifi cant dust explosions start from within a 
piece of equipment, e.g. mixers, hoppers and silos.
The fi rst explosion is known as primary causing 
a rupture of the vessel releasing the fl ammable gas/
air mixture into the atmosphere. With dust generally 
suspended in the air around the process equipment or 
dust lying undisturbed within the building a secondary 
explosion occurs.
A similar chain of events can be seen in relation 
to gas explosions, particularly when a gas explosion 
involves pressurised containers. The consequences of 
gas explosions range from no damage to total destruc-
tion and can lead to fi res and BLEVEs as indicated in 
Figure 7.30.
7.5.5 Principles of explosion management
The principles of explosion management can be broken 
down into two discrete areas: those of control and miti-
gation, each will be dealt with separately. The Dangerous 
Substances and Explosive Atmospheres Regulations 
(DSEAR) require the application of a hierarchy of control 
measures to manage the risk of accidental explosion.
Control
Critical to the management of explosive atmospheres is 
the avoidance or reduction of potentially explosive mater-
ials within an atmosphere.
The substitution of fl ammable substances by inert 
materials or limiting the concentrations of the fl amma-
ble substances to avoid their explosive range must be 
con sidered at the top of any explosion management 
hierarchy.
Such controls may be the replacement of a fi ne 
dusty material by a less dusty granular material or 
reducing the fl ammable gas to the absolute minimum. 
Limiting the concentration to avoid the explosive range 
with mechanical systems linked to ventilation which may 
be actuated via gas or fl ow detectors (including alarms) 
should be considered. In the case of combustible liquids 
the objective should be to reduce the concentration of 
any mist formed, below the lower explosion limit, which 
in turn will ensure it is suffi ciently below its fl ashpoint to 
prevent explosion.
An alternative mechanism may be the use of adding 
inert gases, e.g. nitrogen and carbon dioxide, utilising 
water vapour or inerting using a powdery substance 
such as calcium carbonate. With appropriate dispersal 
these materials can prevent the formation of an explosive 
atmosphere, which we term inerting.