Introduction to Fire Safety Management

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.
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 combus-
tion 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 a-
gration. 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.