Zone 20, 21, 22 for Dusts
Dust zone classification follows the same principles as gas zones but addresses the unique characteristics of combustible dust atmospheres. The ATEX Workplace Directive provides precise definitions that form the basis for classification decisions.
Zone 20: Continuous or Frequent Hazard
Zone 20 is defined as "a place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods, or frequently."
Zone 20 is relatively rare and typically confined to the interior of containment equipment:
- Inside hoppers and silos during filling or discharge
- Inside cyclones, filters, and dust collectors during operation
- Inside powder mixing vessels and blenders
- Inside pneumatic conveying pipes during transfer
- Inside spray dryers during operation
Zone 20 requires Category 1 equipment (marked as 1D)—the highest protection level, capable of remaining safe even with two simultaneous faults.
Zone 21: Occasional Hazard in Normal Operation
Zone 21 is defined as "a place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally."
"Normal operation" means situations when installations are used within their design parameters. Zone 21 commonly occurs:
- Areas immediately surrounding Zone 20 equipment
- Bag dump stations and manual filling points
- Areas around pneumatic conveying connection/disconnection points
- Dust accumulation areas that may be disturbed during routine operations
- Near poorly sealed access doors on dust-containing equipment
- Sacking and bagging operations
Zone 21 requires Category 2 (or better) equipment, marked 2D. This equipment maintains protection during normal operation and foreseeable malfunctions.
Zone 22: Abnormal Conditions Only
Zone 22 is defined as "a place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only."
Zone 22 represents areas where dust clouds are abnormal rather than expected:
- Areas surrounding Zone 21 where small quantities of dust may escape
- Storage areas for bagged products where bags could be damaged
- Areas where dust deposits could be disturbed by unusual air movement
- Equipment surroundings where minor leaks might occur
- Spillage collection areas
Zone 22 permits Category 3 equipment (marked 3D), which provides protection during normal operation but not necessarily during faults.
The Critical Role of Dust Layers
The directive explicitly notes that "layers, deposits and heaps of combustible dust must be considered as any other source which can form an explosive atmosphere." This is crucial because even thin dust layers can be disturbed into explosive clouds by:
- Air currents from HVAC, doors, or equipment operation
- Pressure waves from a primary explosion elsewhere
- Maintenance activities and cleaning
- Personnel movement
A rule of thumb: if dust accumulation can exceed 1mm thickness and could be disturbed, the area may need classification even if dust clouds aren't normally expected during operations.
Layer Ignition vs Cloud Ignition
Equipment temperature limits for dust zones must consider both the minimum ignition temperature of the dust cloud AND the minimum ignition temperature of the dust layer. Layer ignition temperatures are often lower because smouldering can occur over extended periods—a hot surface that won't ignite a dispersed cloud may still ignite an accumulated layer through prolonged contact.
For dust atmospheres, equipment typically shows a maximum surface temperature (e.g., T135°C) rather than a T-class. This value must be lower than both:
- ⅔ of the dust cloud ignition temperature, AND
- The dust layer ignition temperature minus 75K (or more conservative depending on expected layer thickness)
Extent of Zones
Determining zone extent is often more challenging for dusts than gases because dust dispersal patterns depend heavily on particle size, air movement, release energy, and cleaning practices. Unlike gases that disperse and dilute relatively predictably, dust may settle, accumulate, and be re-dispersed repeatedly.
The zone classification documentation in the EPD should clearly show dust zone extents, the basis for their determination, and the housekeeping requirements essential to maintaining the assessed classification.