Hazardous — area classification
Hazardous area zones
20, 21 and 22.
Where a dust cloud is continuous, likely, or merely possible — three zones that decide what equipment may live there.
In brief
Hazardous areas for combustible dust are classified into three zones: zone 20 where an explosive dust cloud is present continuously (inside silos, convey lines), zone 21 where it is likely in normal operation (around filling points), and zone 22 where it is unlikely and brief (general plant areas). Equipment categories map to zones: category 1D, 2D and 3D respectively.
In detail
What it means for a powder plant.
Classification is geography: a drawing of the plant with zones marked around every source of release — fill points, transfer stations, vents. The zone then dictates the equipment: motors, sensors, valves and lights inside each boundary must carry the matching category and temperature class, with the powder's minimum ignition temperature setting the limit.
TLG produces dust hazardous-area classification as part of system design, so zones are minimised by engineering — contained transfers shrink zone 21 footprints — rather than inherited as oversized circles that make every component an Ex purchase.
- Zone 20
- Cloud continuous/long periods — vessel and line interiors
- Zone 21
- Cloud likely in normal operation — around open transfer points
- Zone 22
- Cloud unlikely, short-lived — general plant areas
- Equipment
- Category 1D / 2D / 3D respectively, per temperature class
Common questions
Frequently asked questions.
- What is the difference between zone 21 and zone 22?
- Likelihood. Zone 21 covers places where an explosive dust cloud is likely during normal operation — around bag tip stations or filling heads. Zone 22 covers places where a cloud is unlikely and would persist only briefly — a fault condition, not routine. Equipment requirements relax accordingly, from category 2D to 3D.
- Who is responsible for hazardous area classification?
- The plant operator holds legal responsibility, but classification is normally produced by competent engineers as part of system design. TLG delivers classification drawings with its hazardous-duty systems, so the operator's documentation starts complete rather than retrofitted.
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