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 on:
- 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 kinds of the dust that are handled are combustible. These may be produced either as end-products (e.g. flour, sugar, starch, plastics, metals, coal dyestuffs and pharmaceuticals) or as by-products (e.g. 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 exposable kinds of dust 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
Light fittings and lamps should be mounted externally 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 shutdown 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 dust periodically and store metal dust which has 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 dust is 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
- 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.
- To reduce dust cloud volumes within plant units, install several small units rather than one large one.
- Where possible, handle dust in the form of slurry by mixing dust with a liquid.
- Filter ventilation air that contains 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 the 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 the 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 a plant dealing with carbonaceous materials would be 1m² of relief to every 6m³ of plant volume, whereas metal dust requires 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
People in Risk
Marcel Wood
Marcel Wood heads Etana’s risk management division. Before starting in insurance he worked for the fire department, rescuing people and (literally) putting out fires. He’s passed through Munich Re and Santam, and has designed the only risk management tool aimed specifically at the insurance industry – ARQ. Marcel is a mean guitarist, a brilliant vocalist and an accomplished soccer player with the personal motto: “Do your best all the time”.
