What are the five conditions for a dust explosion?

Combustible dust, dispersion into a cloud, sufficient oxygen, confinement and an ignition source must all be present at once, and removing any one prevents the explosion.

What are ATEX dust zones 20, 21 and 22?

Zone 20 has a dust cloud present continuously, zone 21 occasionally in normal operation, and zone 22 only rarely and briefly, which sets the required equipment category.

Inerting replaces the air inside equipment with an inert gas such as nitrogen, keeping oxygen below the level at which an explosion can propagate.

Explosion safety in bulk handling: complete ATEX guide for powder processing

What is dust explosion risk and why is ATEX relevant to bulk handling?

Many everyday powders — flour, sugar, milk powder, metals, pharmaceuticals, plastics — are combustible. When such a dust is dispersed as a cloud in air and meets an ignition source, it can explode with destructive force. Because bulk handling constantly creates dust clouds inside conveyors, mills, mixers, filters and silos, it is one of the highest-risk activities for dust explosions. The ATEX directives (2014/34/EU for equipment and 1999/92/EC for workplaces) set the European legal framework for preventing and protecting against these explosions.

Dust explosion: the five conditions

A dust explosion needs five conditions at the same time, often shown as the explosion pentagon: combustible dust, dispersion of that dust into a cloud, sufficient oxygen, confinement, and an ignition source. Remove any one and an explosion cannot happen. This principle underlies every prevention and protection strategy: eliminate the dust cloud, control the oxygen, or remove ignition sources.

ATEX dust zones: zone 20, 21 and 22

ATEX classifies areas where combustible dust may be present into three zones. Zone 20 is where an explosive dust cloud is present continuously or for long periods — typically inside equipment such as silos, filters and mixers. Zone 21 is where a cloud is likely to occur occasionally in normal operation, for example around a filling point. Zone 22 is where a cloud is unlikely and, if it occurs, only briefly. The zone classification determines which equipment category may be installed.

Explosion-relevant dust parameters

Not all dusts are equally dangerous, and design must be based on measured parameters. Key values include Kst and Pmax (explosion violence and maximum pressure), the minimum ignition energy (MIE), the minimum ignition temperature and the lower explosion limit. These figures are determined by laboratory testing of the actual product and form the basis for correct zoning, equipment selection and protection design.

ATEX requirements for bulk handling equipment

Equipment installed in a dust zone must be certified for the corresponding ATEX category and built to avoid becoming an ignition source. That means controlling surface temperatures, preventing mechanical sparks, correct earthing to dissipate static, and appropriate ingress protection. Every component — rotary valves, filters, conveyors, level sensors — must be selected and documented for its zone.

Explosion protection: three strategies

Where prevention cannot fully exclude an explosion, protection limits its consequences. Three constructional strategies are used: explosion venting (relieving pressure safely through a rupture panel), explosion suppression (detecting the incipient explosion and injecting a suppressant within milliseconds), and explosion isolation (preventing flame and pressure from propagating through connected pipework into other equipment). These are often combined to protect a complete system.

Inerting as a primary prevention measure

The most fundamental prevention is to remove the oxygen. Inerting replaces the air inside equipment with an inert gas such as nitrogen, keeping the oxygen concentration below the limiting value at which an explosion can propagate. For high-risk metal and pharmaceutical powders, inerting turns explosion protection from damage limitation into genuine prevention. Hecht Technology designs bulk handling systems with integrated ATEX compliance — from dust testing and zoning to inerting, protection and certified equipment — so your process is safe by design.