Dust explosions need five things at once. Most workshops supply four of them without noticing.
Fuel, oxygen and an ignition source give you a fire. Add dispersion into a cloud and confinement, and you have an explosion. A dust extraction system provides the dispersion and the confinement for free, which is why the filter unit is so often where these events start.
The dangerous sequence: a small primary explosion inside plant shakes settled dust off beams, ledges and ductwork throughout the building. That dust forms a much larger cloud, and the primary event ignites it. The secondary explosion is the one that takes the roof off.
Extraction, filters, cyclones, silos, hoppers, mills, conveyors and elevators. Where the cloud forms and what contains it.
Tramp metal, bearing failure, static on ductwork, hot work, and the electrics inside a dusty enclosure.
Venting, suppression, isolation and containment, and whether what is fitted is suitable for what you actually process.
Beams, ledges, cable trays, light fittings and the roof void. The places nobody cleans because nobody can see them.
What you clean with matters as much as how often. Compressed air and dry brushing make things worse.
Zone 20, 21 or 22 marked up so an electrician knows what may be installed where.
The only way to know for certain is testing, but the practical rule is that if it is organic or a metal, and it is fine enough to form a cloud, assume it is until proven otherwise. Flour, sugar, starch, wood, grain, animal feed, coal and aluminium all are.
A widely used rule of thumb is that a layer deep enough to write your name in, covering a significant part of a room, is enough to sustain a secondary explosion if it is disturbed. Secondary explosions are what turn an incident into a disaster.
Almost always inside the plant rather than in the room. Extraction ductwork, filter units, cyclones, silos, hoppers and mills contain both the cloud and the confinement. That is why a Zone 20 or 21 is usually inside the equipment and a Zone 22 outside it.
Housekeeping is one of the most effective controls there is, but not with a brush and not with an ordinary vacuum. Dry sweeping puts the dust into the air, which is exactly what you were trying to avoid.
How Zone 20, 21 and 22 are decided and drawn.
Where dust risk turns up most often.
What is legal to install once a zone exists.
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