Zone 20

Definition

Zone 20 is the highest hazard level for areas at risk of dust explosions, as defined by the European ATEX Directives 2014/34/EU and 1999/92/EC. It refers to an area in which a hazardous explosive atmosphere, in the form of a cloud of combustible dust, is present continuously, for long periods or frequently.

In powder processing, Zone 20 generally refers to the interior of dust-carrying equipment: the mixing chamber of a mixer, the interior of silos, cyclones, filter housings, rotary valves, screw conveyors and pneumatic conveying systems. As a rough guide, Zone 20 is associated with an exposure of more than approximately 1,000 hours per year.

Comparison with Zone 21 and Zone 22

The zone classification determines which equipment category and which equipment protection level (EPL) are permitted for electrical and non-electrical equipment. For Zone 20, Category 1D equipment with EPL Da is required; such equipment must remain safe even in the event of two independent faults occurring simultaneously.

Types of dust and explosion groups

Flammable dusts are classified into groups for the purposes of explosion protection; these groups influence the choice of equipment and protective measures:

• 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.

Relevance in mixed media

Due to the design of the mixing chamber, a cloud of dust is generated during filling, mixing and emptying. Above the mixture there is a free gas volume into which this cloud spreads. As soon as the processed product is flammable, the mixing chamber must therefore always be regarded as a Zone 20 – regardless of how dust-free the area surrounding the equipment is.

Typical products that result in a Zone 20 classification:

• organic powders: flour, sugar, starch, ground spices, milk powder, cocoa
• chemical and pharmaceutical powders: pigments, active ingredients, additives
• Metal powders: aluminium, magnesium, iron, graphite

• Plastics: PE, PP and PVC powders and fine granules

   

Requirements for equipment in Zone 20

Avoid sources of ignition

• Limiting the surface temperatures of all components in contact with the product and adjacent components to values well below the minimum ignition temperature of the dust
• electrostatic earthing and equipotential bonding of all conductive parts of the system, including vessels, mixing tools, pipework and doors
• Design-based prevention of mechanical ignition sources through defined clearance dimensions between rotating parts and the housing
• Monitoring of bearings, shaft bushings and drives for temperature and condition

• Preventing the formation of dead spaces where dust can accumulate and smoulder

   

Limiting the impact

As an explosive atmosphere in Zone 20 is considered to be present on a permanent basis, the risk can rarely be controlled solely by preventing ignition sources. As a rule, structural explosion protection is also required:

• Pressure-shock-resistant design of the apparatus, rated for the reduced explosion overpressure
• Explosion pressure relief via rupture discs, explosion vents or flameless relief systems
• Explosion suppression using rapid-response extinguishing systems
• Explosion isolation from upstream and downstream plant components
• Inerting, for example with nitrogen, in conjunction with oxygen monitoring

The assessment is based on the product’s safety parameters: the Kst value, maximum explosion overpressure pmax, minimum ignition energy and minimum ignition temperature. If flammable liquids or solvents are added to the powder, the hybrid mixture must also be assessed.

Waterproofing and zone demarcation

A dust-tight version of the unit confines Zone 20 to the interior. High-quality shaft and housing seals, outlet fittings with minimal dead space and tightly-sealing inspection ports prevent dust from escaping into the surrounding area. This often allows the adjacent area to be classified as Zone 22 or even as a non-hazardous area – with significantly lower requirements for peripheral equipment.

The operator’s responsibility

The division into zones is the responsibility of the operator, both legally and technically. It is carried out as part of the risk assessment, based on the specific process conditions, substance data and protection concepts, and must be clearly justified in the explosion protection document. Organisational obligations include regular cleaning, staff training and the updating of characteristic values in the event of changes to formulations or raw materials.