What ATEX requirements apply to mixing processes with combustible dust powders (Zone 20/21), and how are they implemented in practice?
In mixing processes with combustible powders, the ATEX Workplace Directive 1999/92/EC for operators and the ATEX Product Directive 2014/34/EU for equipment and protective systems apply in particular within the European Union. Practical implementation begins with a risk assessment and an explosion protection document. In this, the dust properties, possible release points, zone classification, ignition sources and protective measures are defined for the entire plant — not only for the mixer, but also for feeders, silos, filters, conveyors, pipework, discharge and filling.
An ATEX-compliant design cannot be derived from the designation of a powder alone. That a powder is organic or very fine does not automatically mean that it is capable of a dust explosion; conversely, inorganic substances or substance mixtures can also present a corresponding hazard. The basis is representative safety-related testing of the specific product, including possible variations in particle size, moisture, solvent content and recipe.
Zones and equipment selection
For combustible dusts, areas are classified according to the frequency and duration of occurrence of a hazardous explosive dust atmosphere. Zone 20 designates areas in which an explosive dust cloud is present continuously, for long periods or frequently. In mixing processes this typically concerns the interior of closed vessels, pipework, filters or silos. Zone 21 designates areas in which a hazardous dust atmosphere can occasionally occur during normal operation, for example inside certain apparatus or in the immediate vicinity of extraction, filling and discharge points. Zone 22 applies where a hazardous dust atmosphere does not occur, or occurs only briefly, during normal operation, for example in more remote areas with infrequent dust emissions. The precise zone boundary results from the actual dust release, extraction, cleaning and room situation — not from blanket distance values.
Zone 20 generally requires Category 1D equipment. In Zone 21, equipment of Categories 1D or 2D may be used; in Zone 22, equipment of Categories 1D, 2D or 3D is permissible. The marking must match the dust atmosphere, the temperature class or permissible surface temperature, the equipment group and the zone. Not only electrical components are relevant here. Non-electrical devices such as mixer shafts, bearings, seals, flaps, conveyors, couplings, brakes and tools must also be assessed with regard to possible mechanical ignition sources.
Material data and ignition sources
The safety-relevant characteristic data of the powder actually processed form the basis of the design. These include in particular KSt, maximum explosion pressure pmax, minimum ignition energy, minimum ignition temperature of the dust cloud, minimum ignition temperature of a dust layer, particle-size distribution, moisture, electrical conductivity and, for solvent-containing recipes, the properties of possible vapour-dust hybrid mixtures. The lower explosion limit can additionally be relevant but, in operational practice, is often more difficult to use as a reliable control variable for dusts than the other characteristic data.
The most important level of protection is avoiding hazardous explosive atmospheres and effective ignition sources. Inside the mixer, hot surfaces, friction and overheating of bearings or seals, mechanical impact sparks from foreign bodies, jammed tools, electrical equipment and electrostatic discharges must be assessed in particular. In Zone 20, no friction, impact or abrasion processes that could lead to an effective ignition source may occur, even in the event of rare operational faults. TRGS 723 also states, as a general rule, a safety margin of 75°C between the minimum ignition temperature of a dust layer and the surface temperature for dust layers, unless the risk assessment specifies otherwise.
Practical measures are temperature-monitored bearings and seals, rotational-speed and torque monitoring, protection against jamming, suitable foreign-body separation, preventive maintenance, and reliable grounding with continuous equipotential bonding. Conductive or dissipative plant components, flexible connections, filter media, containers and operating personnel must be included in the electrostatic protection concept. However, a single resistance limit value such as 10^6 Ω is not universally applicable to every component and every grounding function; the requirements arise from the material, the component function and the specific explosion protection concept.
With particularly sensitive substances, or where ignition sources cannot be avoided with sufficient reliability, inerting the mixing chamber can be a suitable measure. Here the oxygen fraction is kept below a substance- and process-specifically determined oxygen limit concentration using nitrogen or another suitable inert gas. A minimum ignition energy of below 10 mJ is not a universally applicable threshold above which inerting would be mandatory. Whether it is necessary follows from the risk assessment, the material data and technical feasibility.
Constructive protection
If an explosion cannot be prevented with sufficient certainty, its effects must be limited. Possible measures are explosion-resistant or explosion-pressure-shock-resistant apparatus, explosion pressure relief, flameless pressure relief, explosion suppression and explosion-technical decoupling. The protective method must suit the explosion severity, the vessel volume, indoor or outdoor installation, the location and the connected plant technology.
Bursting discs or explosion flaps can vent pressure to a safe area. They require a correctly dimensioned relief area and a safe venting area. For indoor plants, direct pressure relief to the open air, or, where this is not possible, a suitable flameless relief system, may be required. Relief systems must be designed according to recognised rules; EN 14491 contains basic requirements for the selection and design of protective systems for explosion pressure relief in dust explosions.
Decoupling is required where an explosion could propagate via pipework to filters, silos, conveying sections or other apparatus. Suitable options are, for example, rotary valves tested for explosion protection purposes, non-return flaps, explosion protection valves, quick-closing slide valves or extinguishing barriers. A rotary valve only prevents flame or pressure propagation if it is explicitly suitable for this function, correctly sized, installed, monitored and operated. The mere use of any rotary valve is not an adequate decoupling measure.
Operation and documentation
The explosion protection document must be prepared before work begins and updated whenever changes occur. It documents, among other things, the risk assessment, zone classification, material data, the equipment and protective systems selected, ignition-source assessment, organisational measures, inspections and maintenance requirements. Recipe changes, new raw-material sources, deviating particle sizes, changed moisture, new solvents or modifications can change the explosion properties and thus the protection strategy.
A practical concept includes closed, dust-tight transfer points, effective extraction, ATEX-compliantly designed filters, regular removal of dust deposits and documented inspections. Dust layers must not be assessed using a blanket limit such as "1 mm"; even thin deposits can favour a secondary explosion, depending on area, dust properties and possible dispersion into a cloud. Cleaning intervals and permissible deposit quantities must therefore be set on a risk basis.
In addition, briefings, release procedures for hot work, rules for mobile equipment, checking of grounding connections and controlled change management are required. Pressure relief devices, suppression and decoupling systems, and temperature, pressure and rotational-speed monitoring must be inspected and kept functional at the prescribed intervals by competent persons or specialist bodies.
How amixon® supports explosion protection with dust-explosive powders
ATEX-compliant apparatus design
In mixing processes with combustible dusts, the interior of a mixer frequently has to be classified as Zone 20. Zone 20 designates an area in which an explosive dust atmosphere is present continuously, for long periods or frequently. The final zone classification of the overall plant, however, is part of the risk assessment and is the operator's responsibility. It covers not only the mixer but also dosing, filling, discharge, filters, conveying lines, silos and the installation area.
amixon® can design its mixers, mixing dryers and reactors for applications with a product space classified as Zone 20. Depending on the series and configuration chosen, this concerns, for example, single-shaft mixers, vertical and cone mixers, container mixers, continuous mixers, vacuum mixing dryers and mixing reactors. The specific ATEX category, equipment group, temperature class or maximum permissible surface temperature, electrical and non-electrical equipment, and the required documentation must be established for each project on the basis of the material data and process conditions.
The ATEX Product 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, classify zones, prepare an explosion protection document, and select and test suitable equipment. For the mixer interior in Zone 20, a design with the very high protection level of Category 1D is generally required.
Minimising ignition sources
Mixing intensity and circumferential speed are relevant influencing variables for the assessment of mechanical ignition sources. Many amixon® vertical mixers can be operated at low circumferential speeds; for VM and HM mixers, amixon® states an adjustable range of approximately 0.8 to 3.5 m/s. A low tool speed can reduce friction, impact and abrasion energy while also being gentle on the particle structure. It does not, however, constitute complete proof that no effective ignition source can occur.
The ignition-source assessment must include bearings, seals, tools, shafts, possible tool-to-wall contact, foreign bodies, jamming, surface temperatures, electrostatic charging, electrical components and maintenance condition, among other things. Suitable protective measures can include temperature and torque monitoring, controlled rotational speeds, foreign-body separation, suitable sealing concepts, equipotential bonding, grounding and preventive maintenance. Particularly in Zone 20, no effective ignition sources may occur even in the event of rare faults.
Tightness and constructive protection
Mixing chambers with few joints, ground smooth, and suitable inspection and sealing systems can support dust-tightness and cleanability. According to amixon®, Clever-Cut® doors with OmgaSeal® seal are designed for low-dead-space sealing of the product space. Whether a plant remains permanently dust-tight, however, also depends on pressure differentials, seal material, wear, installation, maintenance and the actual process operation.
Dust-tight process operation can reduce dust escape and deposits in the installation area. It does not, however, replace a cleaning and maintenance concept: even small leaks or product losses can lead to depositable dust. Preventing secondary explosions therefore requires closed transfer points, effective extraction, suitable filter technology, risk-based cleaning and regular inspection of critical areas.
For mixing dryers and reactors of the VMT and AMT series, gas-, vacuum- and pressure-tight designs are possible. The apparatus can be constructed for temperature, pressure and vacuum processes. The possible vacuum level, the permissible pressure stage and whether a vessel must be designed explosion-pressure-resistant or explosion-pressure-shock-resistant are established project-specifically. The design requires in particular pmax, KSt, minimum ignition energy, temperature data, product moisture, possible solvent vapours and the assessment of hybrid mixtures. Pressure or vacuum resistance alone is not equivalent to an explosion-resistant design in accordance with DIN EN 14460.
Inerting and the process chain
Inerting can be used as a preventive explosion protection measure. Here the oxygen content of the process space is lowered below a safe value defined for the specific material system using suitable inert gases, for example nitrogen. With vacuum mixing dryers or reactors, this can be achieved through repeated evacuation and flooding with inert gas. amixon® describes this principle for its mixing and vacuum drying apparatus.
Inerting is only effective if the apparatus space is sufficiently tight, the inerting sequence has been defined, oxygen measurement and safety interlocks are in place, and the permissible oxygen value lies with a safety margin below the substance- and process-specific oxygen limit concentration. Inerting does not automatically replace further explosion protection measures for connected filters, condensers, conveyors or discharge points.
Closed process chains can reduce the number of open transfer points and thus possible dust releases. Nevertheless, dosing, charging, sampling, discharge, filtering, cleaning and maintenance must be separately included in the zone concept. Whether a process step such as mixing, granulating and drying is sensibly combined in the same apparatus depends on the product, recipe, process objective and safety assessment.
Design and testing
The project-specific design begins with the safety-relevant material data and the operator's User Requirement Specification. These include at least KSt, pmax, minimum ignition energy, minimum ignition temperature of dust cloud and dust layer, electrical conductivity, moisture, particle size, and, for volatile substances, the data for possible vapour-dust hybrid mixtures. On this basis, the zone concept, equipment category, temperature limits, grounding concept, tightness, inerting, pressure relief, suppression or decoupling are established.
Trials with the original product can subsequently assess mixing behaviour, temperature input, possible caking, product abrasion, discharge and cleanability. They do not, however, replace standard-compliant safety testing or the risk assessment carried out by qualified specialists. The results can support the process-engineering design and secure the selection of suitable mixer or dryer series.