The primary difference between Zone 21 and Zone 22 lies in the likelihood and duration of the presence of an explosive atmosphere consisting of a cloud of combustible dust in air.
Understanding ATEX Zones for Dust
Areas where there is a risk of explosions due to flammable dust are classified into different zones according to the ATEX directive (specifically, ATEX 1999/92/EC). These zones help determine the necessary safety measures and the type of equipment (rated for specific ATEX zones) required for safe operation. The key distinction between Zone 21 and Zone 22 is the level of risk they represent:
ATEX Zone 21
According to the reference, ATEX Zone 21 is characterized by a High risk of explosive powder or dust. In this zone, Daily processes are likely to produce an environment in which hazardous dust is present. This means the explosive atmosphere is likely to occur often or for long periods during normal operations.
- Typical areas: Locations where dust is regularly handled, processed, or stored in ways that release significant amounts into the air, such as near filling heads, in dust collection systems, or inside processing equipment like mills and mixers.
ATEX Zone 22
In contrast, the reference states that ATEX Zone 22 represents a Medium risk of explosive powder or dust. For this zone, Daily processes are not likely to produce combustible dust although they may occur for a short period. This indicates that while an explosive dust atmosphere can occur, it's less frequent and typically of shorter duration than in Zone 21, usually resulting from an abnormal situation or equipment malfunction rather than routine operations.
- Typical areas: Locations where dust is generally contained but could escape under abnormal conditions, such as areas near storage vessels with infrequent access, around machinery connections that might leak, or where dust layers could be disturbed.
Key Differences Summarized
Here's a table highlighting the core distinctions based on the provided information:
| Feature | ATEX Zone 21 | ATEX Zone 22 |
|---|---|---|
| Risk Level | High risk of explosive powder or dust | Medium risk of explosive powder or dust |
| Likelihood/Duration | Likely to occur often or for long periods during normal daily processes. | Not likely to occur during normal daily processes, but may occur for a short period. |
| Frequency | Frequent / Continuous / Long-term | Infrequent / Short-term |
Understanding these zones is crucial for selecting appropriately certified equipment (e.g., vacuum cleaners, lighting, machinery) and implementing suitable safety procedures to prevent dust explosions. Equipment certified for Zone 21 is also suitable for Zone 22, as Zone 21 represents a higher level of risk.
