Which safety functions are mandatory when handling dust-explosion-hazardous powders?
When handling combustible dusts, there is no uniform catalogue of individual functions that is mandatory for every plant. What is mandatory, rather, is a documented, risk-based explosion-protection concept. It must take into account the specific recipe, material data, zone classification, process control, ignition sources, installation location and the entire connected plant. The protective measures result from the risk assessment and the explosion-protection document.
In Germany, TRGS 720 to 724 set out the approach in detail: avoid or limit hazardous explosive atmospheres, avoid effective ignition sources, and – where necessary – limit the effects of an explosion through constructive measures.
Avoiding the hazard
Enclosed and, as far as possible, dust-tight process control, suitable extraction, filtration and risk-based cleaning are basic measures. They are intended to prevent the formation of hazardous dust clouds or large-area dust deposits. A general limit such as "a dust layer of 1 mm or more is critical" is not technically reliable. Even thin deposits can accumulate over large areas and, if raised into suspension, enable a secondary explosion; permissible deposits and cleaning intervals must therefore be derived from the specific risk assessment.
Inerting is required where it has been defined as an effective measure within the protection concept. It lowers the oxygen content, using a suitable inert gas, below a product- and process-specific limiting oxygen concentration. This requires a tight plant, a defined inerting sequence, the necessary oxygen measurement, alarm and shutdown values, and safety interlocks. Not every dust-explosion-hazardous plant has to be inerted; whether this is required depends on the material data, the process and alternative protective measures.
Keeping the concentration of combustible dust permanently below a lower explosion limit is, in practice, often not a reliable primary protection concept for powder processes. Dust concentrations are difficult to measure continuously and representatively; in addition, local resuspension and deposits can occur. A combination of enclosed process control, dust minimisation, ignition-source avoidance and, where applicable, constructive protection is therefore usually chosen.
Avoiding ignition sources
Where an explosive atmosphere cannot be reliably excluded, all potential ignition sources must be assessed and controlled. This includes electrical and non-electrical equipment, hot surfaces, friction, mechanical sparks, blocked drives, overheating of bearings and seals, foreign bodies, electrostatic discharges, open flames and hot work.
The functions that are strictly required follow from the specific design. Typical measures are temperature-monitored bearings and seals, rotational-speed or torque monitoring, limitation of surface temperatures, suitable material pairings, foreign-body separators, controlled maintenance, and suitable ATEX equipment for the defined zone. Earthing and equipotential bonding of all relevant conductive parts are central components of the protection concept. The requirements also apply to pipes, filters, flexible hoses, drums, IBCs, FIBCs, filling stations and, where applicable, operating personnel.
A blanket dissipative resistance of at most 10⁶ Ω is not correct for every component and every application. Requirements for dissipative materials depend on the component, the material and the application. 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 the requirements for the intended application are met.
Spark-detection and spark-extinguishing systems are not generally mandated. They can be required in pneumatic conveying lines, extraction ducts, woodworking plant or other applications where hot particles or sparks have been identified as a relevant hazard. Their necessity, installation position and safety function must follow from the risk assessment.
Limiting the consequences
Where explosive atmospheres and ignition sources cannot be avoided with sufficient reliability, constructive protective measures must be used. TRGS 724 names explosion-resistant construction, explosion pressure relief, explosion suppression, and explosion-technical decoupling as measures intended to limit the effects of an explosion to an acceptable degree.
Pressure relief can be achieved using suitable bursting discs or relief flaps. Explosion suppression requires very rapid detection and a suitable extinguishing agent. Explosion-technical decoupling prevents pressure and flames from propagating via pipework to filters, silos, conveyors, dryers or other apparatus. Suitable options can include non-return valves, quick-acting slide valves, chemical barriers, explosion protection valves, or rotary valves tested for this purpose.
The correct measure depends on material data such as KSt, pmax, minimum ignition energy and particle size, as well as on the apparatus volume, pipework geometry, throughput, indoor or outdoor installation, and possible hazard to personnel. A constructive protective system does not automatically protect every person in every environment; for pressure relief in particular, the venting direction, flame emission, pressure wave, released product and safe areas must be taken into account.
Organisation and inspection
A risk assessment and an explosion-protection document are mandatory wherever a hazardous explosive atmosphere can occur. The document must be prepared before work begins, kept up to date, and adjusted for relevant changes. It contains, among other things, material data, zone classification, protective measures, equipment selection, inspections, operating limits and organisational rules.
Employees require operating instructions and training on Ex zones, dust release, earthing, cleaning, fault clearance, alarms and emergency measures. Release procedures must apply to hot work, grinding, welding or the use of unapproved equipment in Ex areas.
Explosion safety must be inspected before initial commissioning and after changes subject to inspection. The inspection covers the explosion-protection document, the technical protective measures and organisational provisions, and their interaction. This includes, among other things, ventilation, gas warning, inerting, equipment, protective systems, and safety, monitoring and control devices.
Safety-related functions must have a risk-based design. Whether SIL in accordance with IEC 61511, Performance Level in accordance with EN ISO 13849, or another method applies depends on the type of control system, the safety function and the standard applied. A specific SIL or Performance Level is not universally mandatory for all O₂ measurements, suppression systems or spark-extinguishing systems.
How amixon® supports safety with dust-explosion-hazardous powders
ATEX-compliant design of the product space
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 and project configuration, this includes single-shaft mixers, vertical and cone mixers, container mixers, continuous mixers, and the vacuum mixing dryers and mixing reactors VMT and AMT. Zone 20 generally requires a very high level of protection, which regularly corresponds to a Category 1D design. The specific configuration of the apparatus space, drive, sensors, seals, temperature limits and documentation is defined on a project-specific basis.
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 the preparation and maintenance of the explosion-protection document, as well as inspection of the plant. amixon® can provide the documents for the delivered apparatus; assessment and release of the overall plant remain the operator's tasks.
Limiting ignition sources and dust escape
Low circumferential speeds can limit friction, impact and abrasion energy in the mixed product. For VM and HM vertical mixers, amixon® specifies an adjustable circumferential-speed range of approximately 0.8 to 3.5 m/s. With sensitive bulk materials, this can simultaneously reduce particle stress. However, a low rotational speed is not, on its own, proof that no effective ignition source can occur. Bearings, seals, shafts, possible tool-to-wall contacts, foreign bodies, blockages, hot surfaces, electrostatic charging and electrical equipment must additionally be assessed.
Suitable safety functions can include temperature monitoring at bearings and seals, rotational-speed and torque monitoring, foreign-body management, controlled maintenance, earthing and equipotential bonding. Which functions are actually required and how they must be designed follows from the risk assessment. 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 a different value is justified.
Product areas designed with minimal seams and ground smooth, together with suitable sealing systems, can limit 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 and maintenance. A tight apparatus design therefore replaces neither extraction, cleaning and maintenance, nor the assessment of possible dust deposits and secondary explosions.
Inerting and pressure processes
The vacuum mixing dryers and mixing reactors VMT and AMT can be built gas-tight, vacuum-tight and pressure-tight. They are therefore suitable for processes in which mixing, reacting, conditioning or drying is to take place under vacuum, positive pressure or an inert gas atmosphere. Pressure and vacuum resistance, however, are not equivalent to an explosion-pressure-resistant or explosion-pressure-shock-resistant design. This must be expressly agreed on a project-specific basis and designed on the basis of the safety-related material data.
Inerting can be a preventive explosion-protection measure. In vacuum apparatus, for example, the mixing chamber is evacuated and then flooded with a suitable inert gas. Repeated pressure cycling can lower the oxygen concentration further. Inerting requires a tight plant, a suitable inert gas supply, defined purging sequences, 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 product-inert gas system.
Inerting does not automatically prevent self-ignition, thermal decomposition, fire or exothermic reactions. It is therefore only one building block of a complete explosion-protection concept. The suitable protection strategy may additionally require explosion-pressure-resistant or explosion-pressure-shock-resistant apparatus, pressure relief, explosion suppression, and the decoupling of connected plant components.
Pilot plant and design
The project-specific design begins with the safety-related material data, for example KSt, pmax, minimum ignition energy, minimum ignition temperatures, moisture, particle size, electrical conductivity and possible hybrid mixtures. This data determines the zone concept, permissible surface temperatures, equipment category, tightness requirements, earthing, inerting and possible constructive protective measures.
amixon® states that it offers process trials with the original product at its pilot plant. These can examine mixing quality, heat input, liquid distribution, build-up, discharge, cleanability and behaviour under vacuum or inert gas. Such trials can support the process-engineering design but do not replace the safety-related material testing or the risk assessment of the overall plant.