For mixers processing organic or other combustible powders, ATEX-compliant design is always the outcome of a material-, process- and plant-specific assessment. The product designation alone is not sufficient: what matters is the actual fine-dust fraction, moisture, particle size, possible solvent or fragrance vapours, temperature, process pressure and the operating states from start-up to cleaning. For mixers and mixing plants, VDI 2263 Sheet 10 is a central design guideline; it expressly addresses fire and explosion protection for mixers with dust, gas, vapour and hybrid mixtures.

Zone concept and material data

Zone classification belongs in the operator's explosion-protection document. Zone 20 exists where an explosive dust atmosphere is present continuously, for long periods, or frequently, inside a mixer, filter, silo or conveying line. Zone 21 covers areas where such an atmosphere can occasionally occur during normal operation, for example at filling, discharge or sampling points. Zone 22 describes areas where it occurs only rarely and briefly. The specific delineation results from dust release, tightness, extraction, cleaning and the surrounding conditions.

Zone 20 generally requires a very high level of protection; in the classic ATEX categorisation this regularly corresponds to Category 1D. For Zone 21, Category 1D or 2D is permissible, and for Zone 22, Category 1D, 2D or 3D. The assignment covers electrical and non-electrical components: motors, sensors, actuators, shafts, bearings, seals, mixing tools, flaps and conveying equipment must be considered together. The appropriate conformity assessment and the need for an EU-type examination depend on the equipment category, equipment group, equipment type and the applicable ATEX conformity assessment procedure; an EU-type examination is therefore not generally required for every Category 1D or 2D device.

Representative safety-related material data are required before detailed design work begins. These include, at a minimum, KSt, maximum explosion pressure pmax, minimum ignition energy, minimum ignition temperature of the dust cloud, minimum ignition temperature of the dust layer, particle size distribution, moisture and electrical conductivity. Where volatile liquids or solvents are added, vapour-dust hybrid mixtures must be assessed separately, since ignition and explosion behaviour can differ from that of pure dust. General minimum-ignition-energy ranges for “detergent dusts” or other product groups are not sufficiently reliable and should not be used as a basis for design.

Preventing ignition sources

Avoiding effective ignition sources is a central layer of protection. EN 1127-1 considers various types of ignition source, including hot surfaces, flames and hot gases, mechanically generated sparks, electrical equipment, stray currents, static electricity, lightning strike, electromagnetic radiation and exothermic reactions. EN ISO 80079-36 and -37 are relevant for assessing non-electrical equipment and its potential ignition sources.

Mechanical clearances, material pairings, possible contact points and foreign bodies must be designed so that no dangerous friction, impact or grinding sparks occur during normal operation or under the malfunctions to be considered. There is no generally valid maximum speed such as “below 1 m/s” for mixing tools in Zone 20. The permissible circumferential speed depends on geometry, materials, particle size, possible ingress of foreign bodies, probability of contact, temperature, the mixing task and further protective measures. It must be justified as part of the ignition-hazard assessment.

Bearings and seals are critical points. Possible measures include externally mounted bearings, effective separation of the product space from the drive space, suitable shaft seals, temperature monitoring and condition monitoring. The measures must suit the powder, the rotational speed, the pressure, the temperature and the intended cleaning procedure. 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.

Electrostatic charging can represent an ignition source, particularly with dry, fine and poorly conductive powders. A consistent earthing and equipotential-bonding concept is required, together with an assessment of conductive and insulating components, and suitable hoses, filter media, flexible connections, containers and personal protective equipment. TRGS 727 describes the requirements for avoiding electrostatic ignition hazards. A blanket earthing resistance of 10⁶ Ω cannot be applied to all plant components and applications. For Type C FIBCs, for example, TRGS 727 specifies a dissipative resistance of less than 10⁷ Ω to the earthing point; Type D FIBCs do not require earthing, but may only be used where it has been demonstrated that no hazardous charging occurs.

Electrical equipment must be suitable for the assigned zone, the dust atmosphere and the permissible surface temperature. For dust-protected electrical devices, a dust-tight degree of protection such as IP6X is often relevant; however, the required degree of protection and marking follow from the chosen protection principle and the specific equipment approval. Intrinsic safety can be sensible for certain measurement and control circuits, but it is not a universal requirement for all sensors in the mixer.

Protection concept and inerting

A protection concept follows a hierarchy. First, it is examined whether a hazardous explosive atmosphere can be avoided or limited, for example through enclosed process control, limiting dust release, or inerting. Effective ignition sources are then avoided. If both goals cannot be reliably achieved, constructive measures to limit the consequences of an explosion are added.

Inerting reduces oxygen using a suitable gas, often nitrogen, until the oxygen content lies below the limiting oxygen concentration determined specifically for the material and process. A blanket target value of less than 8 volume percent oxygen is not permissible; the permissible operating concentration must be derived from the limiting oxygen concentration, measurement uncertainty, leakage assumptions, process fluctuations and a defined safety margin. Inerting requires a sufficiently gas-tight plant, a suitable inert gas supply, oxygen measurement, defined alarm and shutdown values, and safety-related interlocks.

For recipes containing oxidising agents, such as sodium percarbonate, inerting must be assessed with particular care. Sodium percarbonate can release oxygen on decomposition and intensify fires. Reducing oxygen with nitrogen therefore replaces neither temperature control nor the assessment of thermal decomposition, possible contamination or exothermic reactions.

Constructive protective measures can include explosion-pressure-resistant or explosion-pressure-shock-resistant apparatus, pressure relief, flameless venting, explosion suppression and the decoupling of connected plant components. EN 14460 addresses explosion-resistant equipment; EN 14491 addresses explosion pressure relief, and EN 14373 addresses the design, application, effectiveness testing, operation and maintenance of explosion suppression systems. Such systems are intended for enclosed or substantially enclosed enclosures and do not apply to unstable, explosive, pyrotechnic or pyrophoric substances.

Decoupling is particularly important so that explosions cannot propagate through lines to filters, silos, dosing and filling plant. Suitable solutions can include non-return valves tested for explosion protection, quick-acting slide valves, chemical barriers, explosion protection valves, or rotary valves suited to this purpose. Every decoupling device must match the dust explosion class, the pipework, the process direction, the pressure conditions and the installation situation.

Documentation and inspection

The design must be documented comprehensibly in the manufacturer's risk assessment and in the operator's explosion-protection document. This covers material data, zones, ignition-hazard assessment, equipment categories, the protection concept, interfaces, operating limits, maintenance, cleaning, inspections, and the treatment of start-up, shut-down, recipe changes and malfunctions.

Inspections are required before commissioning and at recurring intervals, in accordance with the regulations applicable at the site. In Germany, the requirements of the Hazardous Substances Ordinance (Gefahrstoffverordnung) and the Ordinance on Industrial Safety and Health (Betriebssicherheitsverordnung) must be observed in particular. A structured HAZOP or comparable safety study can be very worthwhile for complex mixing plants, but it is not a blanket statutory requirement for every plant.