Civil Engineering Reference
In-Depth Information
In order to prevent the formation of an explosive
atmosphere from the dispersion of dust in air or by
equipment, it is appropriate to design conveying and
removal systems to an approved standard. Such
equipment will remove the levels of dust within the
enclosure which will help to avoid primary explosions;
however, secondary explosions could still easily occur if
the lying dust is suffi ciently agitated leading to entrain-
ment in the conveying and removal systems.
The avoidance of ignition sources that may precipi-
tate an explosion for either gas or dust in the atmosphere
provides a relatively good control measure; however, in
relation to dusts, accumulations and moisture content
must be managed effectively as together they have the
potential for self-heating and therefore self-ignition, thus
the management of temperature and moisture in relation
to dust must be considered.
Mechanical inputs can also produce either glowing
sparks or hot spots and while the sparks are not
suffi ciently energetic to provide ignition repeated contact
may run the risk of igniting a dust cloud.
The removal of any foreign objects from process
streams and the use of non-sparking or spark-proof
equipment (intrinsically safe) must be considered in
either gaseous or dusty atmospheres.
Ensuring that any electrical equipment is subject to
regular maintenance (planned preventive maintenance)
must also be seen as a key area to prevent ignition
sources initiating an explosion.
Electrostatic sparks from static electricity must also
be minimised and the following should be considered:
Dust
explosion
Figure 7.31
A vented dust explosion
the vent correctly to ensure that suffi cient pressure relief
is available as it must allow suffi cient outfl ow of the
burnt fuel and air to relieve the pressure being generated
by the heat of the explosion.
There are a wide variety of differing designs of
venting dependent upon processes undertaken. These
can be simple panels that are ejected, vent covers
(attached to process vessels with clips and rubber
seals), and hinged doors that can withstand explosions
or where necessary redirect the explosion.
The vent area will depend upon the volume of the
enclosure, the enclosure's strength, the strength of the
vent cover and burning rate of the dust cloud.
There are a number of hazards caused by venting
which will need to be taken into consideration, these are:
➤
The use of conducting materials for equipment,
plant, etc. to avoid charge build-up
➤
The earthing of any equipment that may become
charged
➤
Emission of blast waves from the vent opening
➤
Ejection of fl ames from vent opening
➤
Fireballs can be ejected
➤
Emission of solid objects (parts of the vessel, vent
covers, etc.)
➤
The earthing of workers
➤
Earth non-conducting materials via an earth rod
through the storage vessel.
➤
Reaction forces on the equipment, induced by the
venting process
➤
Internal venting may also lead to secondary
explosions.
With regard to electrostatic discharges if there is any
doubt earthing should take place.
Mitigation
In relation to dust the best way to contain a primary
explosion is to ensure that the process equipment is
strong enough to withstand it. Dust explosion pres-
sures are usually within range of 5-12 bars. Designing
the plant as though it were a pressure vessel is likely
to cause it to be very expensive and beyond what is
reasonably practicable. It is therefore quite often that
designers will resort to explosion venting.
Explosion venting
is one of the most effective ways
to relieve pressure; however, it is often diffi cult to size
Consideration must be given to the location of any
explosion relief panel or venting, this must be considered
at the design stage.
If venting an explosion cannot be achieved,
explosion suppression
may be considered. Any
suppression unit must be permanently pressurised, fi tted
with a large diameter discharge orifi ce and any valve
required to operate the discharge mechanism should be
of high speed which is quite often achieved via a small
detonation charge.
