Zone 21
Definition
Zone 21 is a classification of potentially explosive atmospheres involving combustible dusts in accordance with the European ATEX Directives 2014/34/EU and 1999/92/EC. It refers to an area in which, during normal operation, a hazardous explosive atmosphere may occasionally form in the form of a cloud of combustible dust suspended in the air.
To provide a rough indication of the time frame, Zone 21 is often associated with the presence of an explosive atmosphere for between approximately 10 and 1,000 hours per year. It therefore lies between the permanently dust-laden interior of the equipment (Zone 20) and the wider surroundings (Zone 22).
Comparison with Zone 20 and Zone 22
| 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 21, Category 2D equipment with an EPL Db protection level is required. The protection must therefore remain effective even in the event of frequent faults and dust release, and not just during fault-free normal operation.
Typical Zone 21 areas at mixing plants
In process and mixing technology, dust clouds and dust deposits are inevitably generated by conveying, mixing, filling and emptying. Zone 21 primarily refers to areas in which dust is regularly stirred up or released during normal operation:
- areas adjacent to the process at filling and emptying points, such as bag feeding stations, big-bag stations and discharge spouts
- The area around sampling and inspection openings that are opened during operation or on a frequent basis
- dust-carrying transfer points where dust clouds may form during normal operation
The extent of Zone 21 in any particular case depends on the dust characteristics, the process control and the effectiveness of ventilation, extraction and cleaning.
Types of dust and explosion groups
- IIIA: flammable lint
- IIIB: non-conductive dusts
- IIIC: conductive dusts, such as metal dusts
In the case of conductive dusts in Group IIIC, the requirements for enclosure design and sealing become more stringent – for example, higher IP protection ratings are required – because deposits and dust clouds may also be associated with electrical conductivity and sparking.
Design and organisational requirements
Equipment and Plant Engineering
- Use of Category 2D devices with EPL Db
- Limiting the surface temperatures of motors, gearboxes, housings and bearings to levels well below the ignition temperature of the dust, in order to prevent smouldering and smouldering fires
- Prevention of mechanical ignition sources through appropriate clearance dimensions and design features
- electrostatic earthing and equipotential bonding of all conductive parts of the system
- where appropriate, pressure-shock-resistant or pressure-relieved design of the adjacent equipment
Sealing and dust control
- High-quality sealing systems designed to minimise dust leakage between the dust-carrying interior – often Zone 20 – and the surrounding environment
- Dust-tight or low-dust designs for filter and dust extraction systems
- Targeted extraction at filling and emptying points to reduce dust concentrations and downgrade an area from Zone 21 to Zone 22 or to a non-hazardous area
Organisation and documentation
- Zone classification as part of the risk assessment and the preparation of an explosion protection document, including areas, substances, ignition sources and protective measures
- Regular cleaning to remove layers of dust before they become airborne and form hazardous dust-air mixtures
Training of staff on how to behave in potentially explosive atmospheres, as well as on the operation, maintenance and monitoring of explosion-protected systems
The operator’s responsibility
From both a legal and technical perspective, the correct zoning is the responsibility of the plant operator. It must be carried out on the basis of the specific process conditions, material data and protection concepts, and the rationale must be clearly set out in the documentation. External specialist advice can assist with determining dust characteristics, selecting suitable equipment, defining protective measures and drawing up explosion protection documentation.