Which safety concepts (e.g. inerting, pressure relief) are state of the art for dust-explosion protection?
The state of the art is not a fixed package of always the same measures. It consists of a risk-based overall concept that takes into account the actual material data, apparatus, pipework, installation conditions and operating states. The protection strategy follows a clear sequence: avoid or limit explosive atmospheres, avoid effective ignition sources, and – where this cannot be achieved with sufficient reliability – limit the effects of a possible explosion through constructive measures.
The most important characteristic values include, among others, KSt, maximum explosion pressure pmax, minimum ignition energy, minimum ignition temperatures of the dust cloud and dust layer, particle size, moisture, electrical conductivity and, where applicable, data on vapour-dust hybrid mixtures. Without this data, no reliable selection of inerting, pressure relief, suppression or decoupling is possible.
Preventive measures
Inerting is an important preventive measure for enclosed apparatus such as reactors, mixers, dryers, mills or conveying plant. Nitrogen, carbon dioxide or other suitable gases lower the oxygen content below a limiting oxygen concentration determined for the specific dust-inert gas system. This includes a sufficiently tight apparatus, a suitable inert gas supply, representative oxygen measurement, safety margins, alarm limits and interlocks. The gas must suit the product, the temperature, the chemistry and occupational safety. For reactive metal powders or oxidising products, suitability must be examined with particular care.
Limiting the concentration below the lower explosion limit is, for powders, only of limited suitability as a sole measure. TRGS 722 points out that a mean dust concentration within a plant can be of limited significance: because of the inhomogeneous distribution of dust, explosive local zones can nevertheless arise. Limiting the dust concentration can therefore only be used as a protective measure if it is reliably monitored and maintained under all relevant operating states.
Avoiding dust deposits involves enclosed transfer points, tight apparatus, effective extraction, filtration technology, smooth and cleaning-friendly surfaces, and documented cleaning and inspection schedules. Dust deposits can, if raised into suspension, promote secondary explosions. A blanket limit for permissible layer thicknesses is not sufficient, however; the assessment must take into account dust properties, area, possible resuspension and ignition sources.
Controlling ignition sources
Where an explosive atmosphere cannot be reliably excluded, possible ignition sources must be systematically avoided. These include hot surfaces, friction, mechanically generated sparks, blocking drives, foreign bodies, overheating of bearings and seals, electrical equipment, electrostatic discharges, and smouldering nests.
The state of the art comprises zone-appropriate electrical and non-electrical equipment, limitation of permissible surface temperatures, temperature monitoring of critical bearing and seal points, suitable material pairings, foreign-body separators, earthing, equipotential bonding and preventive maintenance. Spark-detection and spark-extinguishing systems can be worthwhile or required in conveying and extraction ducts where hot particles or sparks have been identified as a relevant ignition source. However, they are not a general requirement for every dust-carrying plant.
Constructive protection
Where an explosion cannot be prevented with sufficient reliability, constructive protective measures limit its consequences. TRGS 724 names explosion-resistant construction, explosion pressure relief, explosion suppression, and explosion-technical decoupling as the central measures.
Pressure relief uses, for example, bursting discs or explosion relief flaps. These open at a defined response pressure and limit the internal pressure to a reduced explosion pressure permissible for the vessel. The design takes into account KSt, pmax, vessel volume, geometry, opening pressure, relief area, possible relief ducts, and the effects of pressure and flame outside the apparatus. EN 14491 addresses the selection and design of explosion pressure relief systems for dust explosions; EN 14797 addresses the requirements for the relief devices themselves.
Pressure relief must vent into a safe area. For indoor installation, flameless relief systems can be an option, provided they are suitable for the process. They hold back flames but still require an assessment of the pressure wave, heat, noise generation, possible product discharge and the safety of the surrounding area. Flameless venting is therefore not a standard solution generally suitable for every indoor installation.
Explosion suppression is an active protective system. Rapid pressure or flame detectors identify the onset of an explosion; an extinguishing agent is introduced in such a way that the pressure rise is limited to a permissible value. The required response time, quantity and type of extinguishing agent, and the design of the control system, must be matched to the material, the vessel and the process. The method is particularly worthwhile where venting to the outside is not possible or where release of flames and product is not acceptable.
Explosion-pressure-resistant and explosion-pressure-shock-resistant apparatus are designed to withstand an internal explosion without dangerous failure. Whether a design for the maximum explosion pressure or a reduced explosion pressure is required follows from the protection concept. Pressure relief and suppression can limit the load, but they do not replace the required strength of the protected vessel.
Explosion-technical decoupling prevents the propagation of flames, pressure and burning particles via pipework and conveying routes. Suitable solutions include non-return valves, quick-acting slide valves, chemical barriers, explosion protection valves, or rotary valves qualified for this purpose. Decoupling must take into account the interfaces between mixer, filter, silo, dryer, conveyor and filling. Its suitability depends, among other things, on pipework geometry, dust class, throughput, flow direction, pressure and installation situation.
Organisation and evidence
The state of the art includes an up-to-date explosion-protection document, zone plans, material data, operating instructions, cleaning and maintenance schedules, regular inspections of the protective systems, training, and regulated release procedures for hot work. The explosion-protection document must present the overall technical and organisational strategy in a comprehensible way.
Explosion safety must be inspected before initial commissioning and after changes subject to inspection. This assesses not only individual components but also the interaction of apparatus, protective systems, control system, inerting, decoupling and organisational measures.
How amixon® supports up-to-date dust explosion protection
Zone 20-compliant apparatus design
With dust-explosion-hazardous powders, the interior of a mixer, granulator, dryer or reactor is frequently to be classified as Zone 20. Zone 20 denotes an area in which an explosive dust atmosphere is present continuously, for long periods, or frequently. The zone classification of the entire plant – including dosing, charging, discharge, filters, conveying routes, silos and the installation area – is part of the risk assessment and is the operator's responsibility.
amixon® can execute mixers, granulators, vacuum mixing dryers and mixing reactors for applications with a product space classified as Zone 20. Depending on series, size and project configuration, this applies to single-shaft mixers, vertical and cone mixers, container mixers, continuous mixers, and vacuum mixing dryers and mixing reactors of the VMT and AMT series. Zone 20 generally requires a very high level of protection, which in the classic ATEX categorisation regularly corresponds to Category 1D. The specific design, including maximum surface temperatures, electrical and non-electrical components, seals, sensors and documentation, is derived from the material data and process conditions.
The ATEX Equipment Directive 2014/34/EU concerns equipment and protective systems for use in potentially explosive atmospheres. The ATEX Workplace Directive 1999/92/EC obliges the operator to carry out a risk assessment, zone classification and explosion-protection document. amixon® can provide the documentation for the delivered apparatus configuration; assessment of the overall plant and selection of the overarching protection concept remain the operator's tasks.
Ignition sources and dust escape
Limiting ignition sources is a central part of explosion protection. Many amixon® vertical mixers can be operated at low circumferential speeds. For the VM and HM series, amixon® specifies a range of approximately 0.8 to 3.5 m/s. Low tool speeds can limit friction, impact and abrasion energy and reduce particle stress. However, they are not a sole demonstration that no effective ignition source can occur. Bearings, seals, shafts, tool-to-wall contacts, foreign bodies, blockages, surface temperatures, electrostatic charging and electrical components must additionally be assessed.
Suitable safety functions can include temperature and torque monitoring, controlled rotational speed, suitable material pairings, foreign-body separation, earthing, equipotential bonding and preventive maintenance. For dust layers, TRGS 723 generally specifies a safety margin of 75 °C between the minimum ignition temperature of the dust layer and the surface temperature, unless the risk assessment permits a different value.
Product areas designed with minimal seams and ground smooth, together with suitable seals, can reduce dust escape. According to amixon®, Clever-Cut® inspection doors with OmgaSeal® sealing are designed for low-dead-space sealing of the product space. The actual tightness achieved, however, depends on seal material, pressure differentials, wear, assembly, maintenance and process control. A tight apparatus design can limit dust deposits, but it replaces neither effective extraction nor a risk-based cleaning and maintenance concept.
Inerting and constructive protection
Inerting can be used as a preventive explosion-protection measure. Vacuum mixing dryers and mixing reactors of the VMT and AMT series can be built gas-tight, vacuum-tight and pressure-tight. With a suitable design, the process space can be evacuated and then flooded with a suitable inert gas. Repeated pressure cycling can lower the oxygen content further.
Safe inerting requires a sufficiently tight plant, a suitable inert gas supply, a purging sequence matched to the product, representative oxygen measurement, alarm and shutdown values, and safety interlocks. The permissible oxygen value must lie, with a safety margin, below the limiting oxygen concentration determined for the specific dust-inert gas system. General oxygen setpoints are not reliable. Inerting also does not automatically protect against self-ignition, thermal decomposition, exothermic reactions or fires in dust deposits.
Pressure and vacuum resistance alone are not equivalent to an explosion-pressure-resistant or explosion-pressure-shock-resistant design. If an apparatus is to withstand a specific pressure load in the event of an explosion, this must be expressly specified and designed on the basis of KSt, pmax, minimum ignition energy, ignition temperatures, moisture, particle size and possible hybrid mixtures.
Where an explosion cannot be prevented with sufficient reliability, constructive measures such as pressure relief, flameless venting, explosion suppression, explosion-pressure-resistant or explosion-pressure-shock-resistant construction, and explosion-technical decoupling are used. Decoupling is particularly required at interfaces to filters, silos, conveyors and other apparatus where flames or pressure could propagate via pipework. The suitable measure must be designed for the specific connected plant, the dust characteristic data, pipework geometry, throughput and installation location.
Design through trials
The design begins with the safety-related material data and the operator's User Requirement Specification. Trials with the original product can additionally examine the process-engineering properties, for example mixing quality, product heating, particle abrasion, liquid distribution, build-up, discharge and cleanability. These trials support the choice of apparatus design and operating parameters, but they do not replace standard-compliant dust testing or the explosion-protection assessment of the overall plant.
amixon® states that it operates more than 30 test units at its Paderborn site and additionally maintains further pilot plants in Japan, India, Thailand, China, South Korea and the USA. The results can be used to design the mixer type, tools, seals, dosing concept, heat transfer and process control.