What ATEX requirements must mixing plants for detergent powder meet in Germany?
Mixing plants for detergent powder must be designed and operated in Germany on the basis of a substance- and plant-specific explosion protection assessment. What matters is not the product designation "detergent powder", but whether the specific recipe or its fine-dust fraction can form a hazardous explosive dust atmosphere under real conditions. The assessment must also take into account organic constituents, fragrance carriers, surfactants, polymers, enzyme dusts, oxidising components and possible vapour-dust hybrid mixtures.
Legal framework and responsibility
The ATEX Product Directive 2014/34/EU governs the requirements for equipment and protective systems intended for use in potentially explosive atmospheres. It concerns the manufacturer or the party placing the product on the market. They must carry out the applicable conformity assessment procedures, prepare the technical documentation and provide the appropriate ATEX marking for the intended conditions of use. Among other things, the directive requires that equipment for dust-laden areas is designed so that dust deposits are not ignited and surface temperatures remain well below the relevant ignition temperatures.
The ATEX Workplace Directive 1999/92/EC concerns the operator. In Germany, its requirements are implemented above all through the Hazardous Substances Ordinance and the Ordinance on Industrial Safety and Health. The operator must assess the explosion hazard, define Ex zones, select suitable equipment, implement protective measures and record the results in the explosion protection document. TRGS 720 explicitly requires the risk assessment to be documented as an explosion protection document.
Technical responsibility does not end at the mixer interface. The assessment must cover the entire process chain and all transfer points: raw-material intake, dosing, mixer, extraction, filter, conveyor, silo, discharge, screening, filling, cleaning and maintenance openings, and, where applicable, solvent or fragrance systems.
Zone classification and material data
For combustible dusts, areas are classified as Zone 20, 21 or 22. Zone 20 applies where an explosive dust atmosphere is present continuously, for long periods or frequently. This frequently concerns the interior of a mixer, silo, filter or dust-conveying pipeline. Zone 21 can exist in the immediate vicinity of filling, discharge, sampling or maintenance points if dust is occasionally released during normal operation. Zone 22 applies where a hazardous dust atmosphere occurs only rarely and briefly. The actual zone classification depends on release rate, tightness, extraction, cleaning, room geometry and mode of operation.
Zone 20 generally requires Category 1D equipment. In Zone 21, equipment of Category 1D or 2D is permissible; in Zone 22, equipment of Categories 1D, 2D or 3D comes into consideration. This assignment applies not only to motors and sensors but also to non-electrical components such as mixing tools, shafts, seals, couplings, flaps, rotary valves and conveying equipment.
The design must not be based on assumed standard values. Representative safety-relevant characteristic data are required for every relevant dust fraction. These include in particular 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. With volatile liquid components and fragrances, possible vapour-dust hybrid mixtures must be assessed. TRGS 721 sets out in detail the determination and assessment of the explosion hazard and the selection of the required protective measures.
A blanket statement that detergent dusts generally belong to dust-explosion class St 1 or St 2 is not reliable. The classification results from measurement of the specific product. Oxidising components such as sodium percarbonate must also be considered separately: they are not simply to be assessed as combustible dust, but can intensify fires and release oxygen upon decomposition.
Protective measures during operation
Explosion protection follows a hierarchy. Priority is given to avoiding or limiting a hazardous explosive atmosphere and to avoiding effective ignition sources. Only where this is not sufficiently possible are constructive protective measures used to limit the effects of an explosion.
A dust-tight, as far as possible closed process operation limits dust escape. Enclosure, extraction, suitable filter technology, tightly designed transfer points and a risk-based cleaning concept reduce dust deposits in the installation room. Dust layers must not be assessed using a blanket limit such as "1 mm". Even thin, large-area deposits can contribute to a secondary explosion if dispersed into a cloud.
Avoiding ignition sources includes limiting surface temperatures, temperature monitoring of bearings and seals, detection of jamming or impermissible torque, foreign-body separators, maintenance of mechanical components, and continuous grounding with equipotential bonding. TRGS 723 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 permits a different value.
Electrostatic charging must also be considered. This concerns mixers, pipework, flexible hoses, filters, IBCs, containers, filling operations and operating personnel. TRGS 727 deals with avoiding ignition hazards from electrostatic charging. An effective concept can include conductive or dissipative components, tested grounding connections, suitable personal protective equipment and operational rules for container changes and cleaning.
If an explosion cannot be prevented with sufficient certainty, constructive protective measures must be examined. These include explosion-pressure-resistant or explosion-pressure-shock-resistant apparatus, explosion pressure relief, flameless relief, explosion suppression and the explosion-technical decoupling of connected components. Decoupling is particularly important so that flame and pressure are not transmitted via conveying or extraction lines to filters, silos or other apparatus. Suitable measures can be tested non-return flaps, quick-closing slide valves, explosion protection valves, extinguishing barriers or suitable rotary valves.
Operator obligations
The operator must prepare the explosion protection document before operation begins, keep it up to date and review it whenever the recipe or the plant changes. It contains at least the risk assessment, zone classification, safety-relevant material data, selection of equipment and protective systems, ignition-source assessment, inspection concept, cleaning rules, operating instructions and organisational measures.
Employees require briefings on Ex zones, dust release, grounding, safe cleaning, faults, alarms and emergency measures. Hot work, grinding, welding or the use of unapproved mobile equipment in or near Ex areas require a controlled permit procedure. Explosion protection systems, grounding connections, temperature monitoring, filters, decoupling devices and pressure relief components must be regularly inspected in accordance with the risk assessment, manufacturer specifications and applicable testing obligations.
How amixon® supports the ATEX-compliant design of mixing plants for detergent powder
Mixing plants for detergent powder can only be safely designed if the actual properties of the recipe are known. What is decisive is not merely the designations of the raw materials used, but the properties of the respective dust fraction, possible organic constituents, fragrance carriers, surfactants, polymers, enzyme dusts, oxidising components and, where applicable, vapour-dust hybrid mixtures. amixon® supports the process-engineering design of the mixer; the risk assessment, zone classification and preparation of the explosion protection document are the responsibility of the operator and the explosion-protection specialists commissioned by them.
amixon® can supply mixers, granulators, vacuum mixing dryers and mixing reactors with product-space designs suitable for a Zone 20 classification. This applies, depending on series and project configuration, among others to single-shaft mixers, vertical and cone mixers, container mixers, continuous mixers, and vacuum mixing dryers and mixing reactors. A very high level of protection is generally required for Zone 20, which usually corresponds to a Category 1D design. The specific equipment category, maximum permissible surface temperature, electrical and non-electrical equipment, and the associated ATEX documentation are established project-specifically on the basis of the material data and process conditions.
The legal framework results from the ATEX Product Directive 2014/34/EU for equipment and protective systems, and the ATEX Workplace Directive 1999/92/EC for the operation of potentially explosive plants. In Germany, operator obligations are given concrete form in particular through the Hazardous Substances Ordinance, the Ordinance on Industrial Safety and Health and the Technical Rules for Hazardous Substances. The operator must assess the explosion hazard, define the Ex zones, implement suitable protective measures and document the results in the explosion protection document. TRGS 720 explicitly requires the risk assessment to be documented as an explosion protection document.
The zone classification always covers the entire process chain. The interior of a mixer, filter, silo or dust-conveying line is frequently Zone 20. In the area of filling, discharge, sampling or maintenance points, Zone 21 can exist depending on the actual dust release. Areas in which a hazardous dust atmosphere is expected only rarely and briefly can be classified as Zone 22. The final classification depends on release rate, tightness, extraction, cleaning, room geometry and mode of operation.
amixon® mixers can be designed for product flow that is as closed and dust-tight as possible. Mixing chambers with few joints, ground smooth, and suitable sealing systems can limit dust escape. According to amixon®, Clever-Cut® inspection doors with OmgaSeal® seal are designed for low-dead-space sealing of the product space. Long-term tightness, however, also depends on pressure differentials, seal material, wear, maintenance and the actual process operation. A dust-tight apparatus design therefore does not replace the necessary cleaning, extraction and control of dust deposits in the installation area.
Limiting possible ignition sources is a central part of the apparatus design. Many amixon® vertical mixers can be operated at low circumferential speeds. For the VM and HM series, amixon® states an adjustable range of approximately 0.8 to 3.5 m/s. Low tool speeds can reduce friction, impact and abrasion energy. They are not, however, general proof that no effective ignition source can occur. The assessment must include bearings, seals, shafts, mixing tools, possible tool-to-wall contact, foreign bodies, jamming, surface temperatures, electrostatic charging and electrical components, among other things.
Suitable measures can include temperature and torque monitoring, controlled tool speeds, foreign-body separation, suitable sealing and bearing concepts, grounding, equipotential bonding and preventive maintenance. For dust layers, TRGS 723 states, as a general rule, 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 must be considered for mixers, pipework, flexible hoses, filters, containers and charging operations.
Suitable dosing lances and nozzles are available for liquid additives such as surfactants, oils or fragrance formulations. The addition must be matched to liquid quantity, viscosity, spray pattern, droplet size, temperature and product movement. The aim is even distribution with limited local over-wetting and as little caking as possible. With detergent recipes containing enzyme granulates or oxidising constituents such as sodium percarbonate, moisture, temperature and mechanical stress must be controlled particularly carefully. Sodium percarbonate is an oxidising substance and can release oxygen upon thermal decomposition; inerting therefore replaces neither temperature control nor the assessment of possible decomposition reactions.
If an explosion cannot be prevented with sufficient certainty, constructive protective measures must be assessed for the overall plant. These include explosion-pressure-resistant or explosion-pressure-shock-resistant apparatus, explosion pressure relief, flameless relief, explosion suppression and explosion-technical decoupling. In particular, filters, silos, conveying equipment and pipework must be protected against the propagation of pressure and flame. Which protective measures are required results from KSt, pmax, minimum ignition energy, ignition temperatures, moisture, particle size, electrical conductivity, possible hybrid mixtures and the overall plant geometry.
amixon® offers trials with the original product in order to investigate mixing quality, temperature development, liquid distribution, caking behaviour, discharge and cleanability. These results can support the selection of mixer, tools, dosing concept, sealing systems and operating parameters. They do not, however, replace the safety-relevant material testing, the legally required risk assessment, the explosion protection document or the inspection of the overall plant by qualified specialists.