FPASA BULLETIN SF 04
DUST EXPLOSION
The Phenomenon
Almost every combustible material when in a finely divided form, is capable of producing a
dust explosion, which can be relatively mild or devastating. It is not unknown for a building
to be demolished by such a phenomenon.
The occurrence and severity of a dust explosion depends upon:
i)
ii)
iii)
iv)
the type of dust
the size of the particles
the degree of concentration of particles
the degree of ventilation in the area
It can occur only in the presence of a naked flame or spark when the dust cloud exists in
an “ideal” concentration, in exactly the same manner as an explosion involving flammable
gases or vapours will occur only if the concentration is neither too lean nor too rich.
A feature of dust explosions is the occurrence of a double explosion. The first or “pilot”
explosion may be relatively mild, but the dust clouds stirred up will result in the secondary
and more serious explosion.
In industry many of the dusts handled are combustible. These may be produced either as
end-products (eg: flour, sugar, starch, plastics, metals, coal dyestuffs and
pharmaceuticals), or as by-products (eg: wood, sawdust and textile fluff).
The dust explosions hazard should be considered at the design stage of a plant so that
precautions can be built into the plant more economically.
Explosion prevention
In the design and safe operation of industrial plant where explosible dusts are present,
sources of ignition must be excluded as far as possible. These include:
Flames
Exclude flames from within or in the vicinity of the dust handling plant by applying “no
smoking” rules. The burning of dust in furnaces must be controlled to prevent blow-back to
ducting and the ignition of extraneous dust.
Hot surfaces
Lighting fittings and lamps should be mounted external to dust handling areas, or
separated by dust-tight armoured glass windows with servicing access from outside the
dust handling enclosures. Lead lights should not be permitted in dust areas; battery
operated hand lamps are preferred.
Welding and cutting
Welding, cutting and similar hot work should be permitted only after plant shut-down and
the removal of dust from the plant.
Friction
Sparking from the entry of tramp metal and stones into grinding mills or disintegrators can
be prevented by the installation of magnetic or pneumatic separators, and the use of hand
power tools should be controlled in dust handling areas.
Automatic shut-offs of power to process machines should be provided to prevent design
loads being exceeded and feed regulators should be installed to prevent localised heating
which can occur due to clogging of dust in machinery.
Electric sparks
Electrical equipment in dust handling areas should be dust tight and all metal and
machines must be earthed and bonded to prevent static build-up.
Spontaneous heating
The immediate result of spontaneous ignition is fire, but explosions may follow if the dust is
subsequently dispersed into a cloud. To prevent a heat build-up allow hot dust to cool
before storing, test the temperature of stored dusts periodically and store metal dusts
which have been manufactured in inert atmospheres under inert gas.
Methods of dust explosion protection
Positive steps must be taken to deal with possible explosions when exposable dusts are
present which may be dispersed into a cloud in industrial plant.
Some of the methods of explosion protection, if used in conjunction with others, are
beneficial but singly do not give adequate protection.
These are:
Minimising dust cloud formation
i.
Deliver dust into hoppers through various openings in the sides of the hopper rather
than by allowing dust to fall from a high point to the surface of the deposited
material.
ii.
To reduce dust cloud volumes within plant units, install several small units rather
than one large one.
iii.
Where possible, handle dust in the form of a slurry by mixing dust with a liquid.
iv.
Filter ventilation air containing dust, as soon as practicable after it is removed from
the plant and replace and clean filters regularly.
Separation
Where possible separate exposable dust handling plants by enclosure in separate
buildings. Where such division is impractical, sections of continuous plant may be isolated
from each other by means of chokes of the rotary valve or work conveyor type to prevent
the spread of an explosion.
Venting
Explosion reliefs offer a method of protection which can considerably reduce damage to
plant and takes the form of bursting panels or explosion doors.
Explosion reliefs should be fitted on all cyclones, bag filters, silos, bins and other large,
relatively weak parts of the plant. The size of explosion relief depends on the maximum
pressure and rate of pressure risk resulting from the rapid combustion of the flammable
material, the design, construction and volume of the plant and on the construction of the
explosion relief itself.
Examples of vent ratios normally recommended for plant dealing with carbonaceous
materials would be 1 m² of relief to every 6 m³ of plant volume, whereas metal dusts
require twice this relief area.
Inerting
Explosions may be prevented in many plants and processes, where the dust is confined
within an enclosure, by the replacement of the normal atmosphere by an inert gas. This
ensures that if a dust cloud does form it will not be a flammable dust cloud.
Published by
Fire Protection Association of Southern Africa
(Incorporated Association not for Gain)
P O Box 15467
Impala Park
1472