Dust extraction in Ex zones – how to design a safe system and avoid secondary explosions?

Dust extraction in industry is not just a matter of cleanliness or compliance with health and safety regulations. In potentially explosive atmospheres (Ex), the design and operation of dust extraction systems must take into account much more serious risks, including the risk of dust explosion and its secondary propagation. One oversight is enough: a poorly selected filter, lack of explosion isolation or an uncleaned duct can make the risk a reality. In this post, you will find a specific guide: from the basics of dust explosiveness, through zone classification, to the components that really matter. Everything is based on figures, standards and implementation practice.

Dust extraction in Ex zones – why is dust removal a matter of safety as well as occupational health and safety?

Dust extraction systems operating in Ex zones are not just about removing solid particles from the air. It is about preventing the formation of an explosive atmosphere or, worse still, its ignition and escalation. This requires a completely different approach to system design. Unlike standard systems, it is impossible to completely eliminate fuel here, because dust, by definition, is part of the process.

The dust explosion pentagon clearly indicates that an explosion requires combustible dust, oxygen, an ignition source, the right mixture, and a confined space. And while you can limit some of these elements, there are always two factors that cannot be eliminated: dust and oxygen.

Therefore, in dust extraction for Ex zones, it is crucial that:

• eliminate ignition sources (spark detectors, ESD, separators),
• implement systems to prevent secondary explosions (flaps, HRD, diverters),
• control dust distribution – both in ducts and on surfaces.
  

Important: a secondary explosion can have up to 10–15 times higher pressure than the primary one. It is responsible for the greatest losses in production plants. If dust settling is possible in your installation, this risk must be taken into account at the design stage.

The key factor is that a secondary explosion usually occurs not directly at the source of dust, but after the settled dust has dispersed and come into contact with an ignition source – often completely inconspicuous. A spark, a glowing particle or an overheated component of the installation can trigger a chain reaction. If you do not know what the real sources of ignition are in potentially explosive atmospheres, it is easy to overlook something crucial.

Industrial dust extraction and classification of zones 20, 21 and 22 – how does the Ex zone change the architecture of the entire system?

The Ex zone classification directly affects the design of each element of the dust extraction system – from the filter, through the ducts, to the sensors and valves. It is not only about selecting ATEX-certified equipment, but above all about the level of protection, which must be adequate to the frequency and intensity of the dust explosion hazard.

As a reminder:

• Zone 20 – explosive dust atmosphere is present continuously or for long periods (>1000 hours/year),
• Zone 21 – explosive atmosphere is present intermittently (10–1000 hours/year),
• Zone 22 – explosive atmosphere is present rarely (<10 hours/year).
  

What does this mean in practice?

• In zone 20, you must design a system with the highest level of protection – e.g. HRD or pressure relief + explosion isolation + ignition detection.
  
• In zone 21, non-return valves, diverters, decompression panels and good sensors are often sufficient.
  
• In Zone 22, you can rely on spark detection systems and proper filtering, but you still need to consider maintenance, cleaning, and monitoring.
  

Failure to match safety devices to the zone is one of the most common errors in Ex installations. And unfortunately, it is also the easiest to overlook.

Industrial dust extraction systems – filters, materials and regeneration that cannot become a source of ignition

The choice of filter in an industrial dust extraction system has a direct impact on explosion safety. It is not only about its effectiveness in retaining particles, but above all about whether the filter itself becomes a hazard – through electrostatic charging, sparking or improper regeneration.

The most important design principles in this area:

• Use only antistatic or conductive (ESD) fabrics.
  
• Select the filter class according to the explosion hazard:
  
  • Class L – for low-risk ST1 dusts,
    
  • Class M – for ST1–ST2 dust (flour, sugar, wood),
    
  • Class H – for ST2–ST3 dusts (aluminium, magnesium, pharmaceuticals).
    
• Do not use pure cotton, bare polyester or materials that tend to generate static electricity.
  
• Ensure an effective filter regeneration system (e.g. pulse jet or reverse flow), with parameters tailored to the dust.
  

An inappropriate filter or lack of cleaning not only reduces efficiency, but also poses a real risk of dust ignition from an overheated filter bag. We know of cases where the spark did not come from the device, but from the filter itself.

Explosivity indices, such as Kst and Pmax, directly define the level of hazard for a given substance – from relatively calm ST1 dusts to extremely dangerous ST3 dusts. Their value not only determines the choice of safety system, but also influences the classification of Ex zones. In practice, however, many people are still unable to interpret these parameters correctly, which means that the risk is sometimes underestimated – and not everything that produces dust is safe.

Industrial dust extraction systems – what explosion protection measures should be included in your design?

A dust extraction system in an Ex zone without explosion protection is a ticking time bomb. And that is no exaggeration. When you ignore the need to implement isolation, relief or suppression, you leave the installation without control over the propagation of pressure waves and flames. And all it takes is one spark to set off a chain of events.

Therefore, when designing an industrial dust extraction system, you need to ask yourself a few key questions.

• Is primary dust ignition possible in my process?
  
• Are the channels and filters prepared for its propagation?
  
• Has insulation been installed (non-return flaps, quick-closing valves, VENTEX)?
  
• Is an HRD system or decompression chamber provided, depending on the zone?
  
• Are the safety devices compliant with the PN-EN 14373 standard?
  

In zone 20 and for ST2–ST3 dusts, an HRD system or a combination of relief and insulation is essential. Only such layered protection can reduce the risk of explosion to an acceptable level. It is also good practice to install spark detection at the filter inlet. This allows for immediate neutralisation of ignition before it can cause any undesirable phenomena.

The choice between an HRD system, decompression panels or flameless solutions cannot be random. Each of these technologies protects the installation in a different way, and their effectiveness depends on the correct match to the zone, dust type and process parameters. That is why it is worth approaching this strategically and implementing multi-layered dust explosion protection instead of limiting yourself to a single solution.

Air dust extraction and elimination of ignition sources – how to prevent an explosion before it even starts?

Too many dust extraction systems focus solely on the effects. However, prevention is key – specifically, eliminating ignition sources. In industrial installations, the main “culprits” are sparks, hot particles and electrostatic discharges. The latter occur particularly easily in dry environments and in the presence of electrostatic materials – i.e. where inappropriate filters, seals or connections have been used.

Spark detection and extinguishing systems (e.g. with IR sensors and water nozzles) are now standard in zones 21 and 22. Importantly, modern solutions:

• respond in less than 200 ms,
  
• operate across the entire channel,
  
• do not require replacement parts after activation – they only use water,
  
• have full diagnostics (activation history, error signals, contamination).
  

A well-chosen detection system can reduce the risk of ignition by over 95%. And although it cannot replace insulation or venting, it is the first line of defence. This is especially true where processes are dynamic and employees are physically unable to monitor the situation in real time.

How can we assist you?

You don’t have to design a dust extraction system in an Ex zone on your own. At Atex Consulting, we help you at every stage: from risk analysis and zone classification, through the selection of technical solutions, to implementation and team training.

• Are you designing a new plant?
  
• Are you modernising an outdated system?
  
• Do you need an expert opinion for your insurer?
  

Write to us or arrange a consultation. We will prepare a consistent safety concept, compliant with ATEX, PN-EN standards and the actual needs of your production.

Frequently asked questions about dust extraction in Ex zones