Cloud and layer
On the dust side there are two separate hazards, and confusing them is the most common error:
- Cloud — dust suspended in air. This is what zone classification is about.
- Layer — dust settled on surfaces. It does not itself define a zone, but it is dangerous in two ways: it smoulders on a hot surface, and a vibration, an air current or a first small explosion lifts it and feeds the secondary explosion.
In industrial dust explosions the destructive event is usually not the first explosion but the second one, fed by the layer the first one lifted.
Three zones
| Zone | Dust cloud | Typical location |
|---|---|---|
| 20 | Continuously, for long periods or frequently | Inside a silo, a filter housing, a mixer |
| 21 | Occasionally in normal operation | Discharge point, bagging station, around a hatch |
| 22 | Not expected in normal operation; brief if it occurs | The volume around zone 21, outside closed equipment |
Housekeeping is a protective measure
In dust plants, regular cleaning is not tidiness but an engineering control, and it appears in the document as one: which surface, at what frequency, by what method. Cleaning with compressed air is usually forbidden — it turns the layer straight into a cloud.
Equipment on the dust side
In dust zones the equipment categories carry the letter D, and the temperature criterion differs from the gas side: what matters is the surface temperature, assessed separately against the ignition temperature of the cloud and of the layer.