Introduction to Fire Safety Management

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 
Figure 7.24
Explosions at the Buncefi eld oil depot
Too much
oxygen
Too little
oxygen
LEL
FUEL
OXYGEN
UEL
100%
0%
Methane
Hydrogen
5 %
15 %
74 %
4 %
Explosive limits
Figure 7.25
Illustration of explosive limits

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Principles of fi re and explosion
129
substance in air may cause an explosion. The range is 
provided with two limits which are:
➤
Lower explosion limit (LEL)
– the minimum con-
centration of vapour in air below which the propa-
gation of fl ame will not occur in the presence of an 
ignition source. Also referred to as the lower fl am-
mable limit or the lower explosive limit.
➤
Upper explosion limit (UEL)
– the maximum con-
centration of vapour in air above which the propa-
gation of fl ame will not occur in the presence of an 
ignition source. Also referred to as the upper fl am-
mable limit or the upper explosive limit.
Managing an explosive atmosphere requires that the 
explosive range between the UEL and LEL, where explo-
sion can take place, is avoided and that the concentration 
of gases, vapours and dusts is kept outside this range. 
The mechanisms relating to how this is achieved can be 
found later in the chapter.
These limits are signifi cant in that they provide key 
information (via MSDS) that will assist the risk assessor 
in considering the risks and any risk control measures 
required.
7.5.4 Explosive 
conditions
In relation to dust any solid material that can burn in air 
will do so at a rate that increases in direct proportion to 
the increased surface area, therefore, dust, fi bres, etc., 
are more likely to cause an explosive atmosphere if they 
are extremely fi ne.
The types of materials that often cause dust 
explosions include:
➤
Coal and peat
➤
Metals such as iron, zinc, aluminium
➤
Natural organic materials such as grain, linen, 
sugar, etc.
➤
Processed materials such as plastics, organic pig-
ments (paint), pesticides, etc.
Each of the above materials has the ability to cause an 
explosion, though all combustible materials have the 
potential to react in a similar manner to a fl ammable gas 
when mixed with the correct proportion of air (or other 
supporter of combustion). The severity will be based upon 
the heat release of the substance and the size of particles.
Determining the amount and likelihood of creating an 
explosive atmosphere will depend upon the following:
➤
The presence of a fl ammable substance
➤
The degree of dispersion of the fl ammable substance 
(this will vary dependent upon its state – dusts, gases, 
vapours and mists)
➤
The concentration of the fl ammable substance in air 
within the explosive range