Introduction to Fire Safety Management

Introduction to Fire Safety Management
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 overpressure
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
Figure 7.24 Explosions at the Buncefi eld oil depot
128
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.
Explosive limits and the fl ammable/explosion
range
The explosive limits or fl ammable/explosion range refers
to the range in which the concentration of a fl ammable
100%
Too much
oxygen
0%
Methane
Hydrogen
LEL
Explosive limits
FUEL
OXYGEN
UEL
Too little
oxygen
5 % 15 %
4 %
74 %
Figure 7.25 Illustration of explosive limits