Introduction to Fire Safety Management

Figure 7.23
Detonation wave through a fl ammable mixture
Rea
c
tion zone 
Rea
c
tion zone
Sho
c
kwave
Sho
c
kwave
Burnt material
Unburnt material

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Introduction to Fire Safety Management
128
In contrast the speed of travel of the fl ame front 
during a detonation is supersonic. In a fuel/air cloud a 
detonation wave will move at speeds of between 1500 
and 2000 
m/s and the peak pressure in front of the 
fl ame can reach 15 to 20 bar. This pressure front when 
enclosed can cause substantial damage including the 
collapse of structures.
In an explosion of a fuel/air cloud ignited by a spark, 
the fl ame will normally start out with a velocity of the 
order of 3–4 m/s. If the cloud is truly unconfi ned and 
unobstructed (i.e. no equipment or other structures are 
engulfed by the cloud) the fl ame is unlikely to accelerate 
to velocities of more than 20–25 m/s, and the overpres-
sure will be negligible if the cloud is not confi ned.
In a building or other enclosed space when the 
mixture is burning the temperature will increase and the 
fuel/air mixture is likely to expand by a factor of up to 8 
or 9, accelerating the fl ame front to several hundreds of 
metres per second.
If the ignition source is from a weak source, e.g. a 
hot surface or a spark, the explosion will initially start as 
a slow burning defl agration. Due to obstructing objects 
and confi nement, the defl agration can accelerate and 
become fast burning. When a defl agration becomes 
suffi ciently rapid, a sudden transition from defl agration 
to detonation may occur.
If this transition occurs, very high pressure loads, 
up to 50 bar, can be reached locally and severe damage 
can be expected within the compartment. If a detonation 
has been established in the compartment it may also 
propagate into any unconfi ned cloud outside, potentially 
creating a UVCE.
A defl 
agration propagating into a large truly 
unconfi ned and unobstructed cloud will slow down 
and the pressure generation will normally be negligible. 
A detonation, however, will propagate through the entire 
cloud at a high velocity and cause severe blast waves. 
The possibilities of transition to detonation will mainly 
depend on the:
➤
Type of fuel
➤
Size of cloud
➤
Enclosure conditions, such as obstructing objects 
and confi nement.