Zone 22
Definition
Zone 22 is the lowest hazard level for areas at risk of dust explosions under the European ATEX Directives 2014/34/EU and 1999/92/EC. It refers to an area in which an explosive dust atmosphere does not normally occur during normal operation and, if it does, only rarely and for a short period.
To give a rough idea of the timeframe, Zone 22 is associated with exposure of less than approximately 10 hours per year. In practice, Zone 22 often refers to the wider vicinity of mixers, silos and conveyor systems – in other words, the area where operating personnel are regularly present.
Classification in relation to Zone 20 and Zone 21
| Zone | Occurrence of an explosive dust atmosphere | Device category | EPL |
|---|---|---|---|
| Zone 20 | constantly, over long periods of time or frequently | 1D | Da |
| Zone 21 | occasionally during normal operation | 2D | Db |
| Zone 22 | not to be expected during normal operation; if it does occur, it is only rarely and for a short time | 3D | Dc |
For Zone 22, Category 3D equipment with an EPL Dc protection level is sufficient. The protection is primarily designed for fault-free normal operation; dust clouds are only expected in exceptional circumstances.
Typical Zone 22 areas at mixing plants
- Areas surrounding mixers, silos and conveyor systems, where dust is released only in the event of leaks, malfunctions or brief periods of dust being stirred up
- Areas with dust deposits of limited thickness, from which dust-air mixtures may form temporarily as a result of mechanical stress or air movement
- Areas surrounding dust extraction and filter systems that are not designed to be completely dust-tight
For many industrial applications, this results in a graded classification: the interior of dust-containing vessels as Zone 20, the area immediately adjacent to the process where dust is regularly released as Zone 21, and the wider surroundings as Zone 22.
The significance of dust deposits
In Zone 22, it is not the cloud of dust stirred up but the deposited layer of dust that is the key risk factor. Even thin layers can be stirred up by mechanical stress, air movement or a pressure wave, creating an explosive atmosphere. Dust layers on warm surfaces can also smoulder and act as a source of ignition – which is why the permissible surface temperatures for dust layers are more strictly limited than for dust clouds.
Consistent cleaning in Zone 22 is therefore not a regulatory measure, but rather part of the explosion protection strategy.
Requirements and measures
- Use of Category 3D devices with EPL Dc
- Limiting surface temperatures, taking into account possible dust deposits
- electrostatic earthing and equipotential bonding of all conductive parts of the system
- dust-tight or low-dust designs for equipment, filter and dust extraction systems, to prevent the zone from spreading unnecessarily
- Targeted extraction at dust sources and defined cleaning intervals
- Documentation of areas, substances, ignition sources and protective measures in the explosion protection document
Ensuring that the mixer and its connections are well sealed is the most effective measure: if the dust remains inside the unit, the surrounding area can often be downgraded from Zone 21 to Zone 22 or removed from the zoning classification altogether. This significantly reduces the requirements for the entire peripheral equipment.
The operator’s responsibility
The correct zoning is, from both a legal and technical perspective, the responsibility of the plant operator. It must be carried out on the basis of the specific process conditions, substance data and protection concepts, and the rationale must be clearly set out in the explosion protection document.