In many businesses, such as flour mills, wood processing facilities, grinding lines that generate metal dust, and plastic granule processing facilities, combustible dust particles suspended in the air carry an explosion risk just as serious as gas. This risk is managed with a classification system separate from gas environments, through the Zone 20, Zone 21 and Zone 22 areas. In this article we explain how dust-based zone classification works.
Definitions of Zone 20, Zone 21 and Zone 22
| Zone | Definition | Required Equipment Category |
|---|---|---|
| Zone 20 | An area in which a combustible dust cloud is present continuously, for long periods or frequently (e.g. the interior volume of a silo) | Category 1D |
| Zone 21 | An area in which a combustible dust cloud is likely to occur occasionally in normal operation | Category 1D or 2D |
| Zone 22 | An area in which a combustible dust cloud is not expected to occur in normal operation, and if it does, only for a short period | Category 1D, 2D or 3D |
The numbering system being chosen as 20/21/22, different from gas environments (0/1/2), is meant to prevent the two systems from being confused with each other; however, the logic is the same: the smaller the number, the higher the risk level.
How Dust Environments Differ From Gas
Dust-based explosive atmospheres differ from gas environments in two important ways. First, dust clouds settle on surfaces over time and form layers; these layers carry both a self-heating risk and the risk of triggering a secondary, generally much more severe, dust explosion when re-suspended into the air by an air current. Second, in dust environments, regular cleaning and control of accumulated dust are protection strategy components as important as equipment selection; for this reason, when carrying out Zone 20/21/22 classification, not only the airborne dust cloud but also possible dust accumulation areas are assessed.
Factors Considered in Determining a Zone
- Ignition properties of the dust: Minimum ignition energy, minimum ignition temperature and explosion severity (Kst value) are determined through laboratory tests.
- Particle size and concentration: Fine-particle dusts generally carry higher risk.
- Tightness of process equipment: Leak points, connection joints and maintenance openings in closed systems.
- Cleaning and maintenance regime: How frequently dust accumulation is cleaned directly affects zone boundaries.
How Does Category Compatibility Work?
Just as with gas environments, equipment with a higher level of protection can be used without issue in a lower-risk zone; Category 1D equipment can also work in Zone 22, but Category 3D equipment cannot be used in Zone 20 or Zone 21. The marking showing that equipment is certified for a dust environment uses different symbols than gas environment marking; we explain how to read these markings in our article How Is the Ex Mark Read?.
Frequently Asked Questions
According to which standard are Zones 20-21-22 determined?
The classification of dust-based explosive atmospheres is carried out in accordance with EN 60079-10-2, a separate standard from EN 60079-10-1, which classifies gas environments.
Why does dust pose a different hazard than gas?
In addition to remaining suspended in the air, dust clouds can settle on surfaces and form layers; these layers can self-heat over time or trigger a secondary dust explosion when they come into contact with an ignition source.
Is every dust explosive?
No, the explosiveness of a dust depends on properties such as particle size, moisture content and minimum ignition energy, and is determined through laboratory tests.
What is the key difference between Zone 21 and Zone 22?
In Zone 21, an explosive dust cloud is likely to occur occasionally in normal operation; in Zone 22, such an occurrence is not expected, and even if it does occur, it is only brief and rare.
Conclusion
Zone 20/21/22 classification ensures the correct grading of dust-based explosion risk and the determination of appropriate equipment and cleaning regime. Dust explosions are generally less well known than gas explosions, but their consequences can be just as devastating; for this reason, classification must be carried out with great care.
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