Civil Engineering Reference
In-Depth Information
substance in air may cause an explosion. The range is
provided with two limits which are:
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:
➤
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.
➤
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)
➤
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.
➤
The concentration of the fl ammable substance in air
within the explosive range
Flame
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.
Fuel/air
or
Fuel-oxidiser
Burned
gas
Figure 7.27
Confi ned explosion within a tank
7.5.4 Explosive conditions
Unearthed hopper
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:
Non-conductive
blocks
Fine flowing
material
➤
Coal and peat
➤
Metals such as iron, zinc, aluminium
Sparks can occur here
igniting the dust cloud
below
➤
Natural organic materials such as grain, linen,
sugar, etc.
➤
Processed materials such as plastics, organic pig-
ments (paint), pesticides, etc.
Settled material
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
Earthed vessel
Figure 7.28
Static discharge between an unearthed hop-
per and an earthed vessel having the potential to ignite dust
Figure 7.26
The smaller a substance is divided the greater the surface area
