What ATEX zone concepts, inerting solutions and pressure-resistant designs are customary for combustible dust powders?
For dust-explosive powders, protection concepts are always developed from three interlinked building blocks: zone classification, avoidance of explosive atmospheres and ignition sources, and, where necessary, constructive measures to limit an explosion. Which combination is suitable depends on the safety-relevant material data, the process, the apparatus size, the installation location and the overall plant system.
Zone classification alone does not yet describe complete explosion protection. It does, however, determine which equipment and protective systems may be used in the respective area. Inerting can prevent an explosive atmosphere from occurring. Pressure-resistant, explosion-pressure-shock-resistant or pressure-relieved apparatus, by contrast, limit the effects should an explosion nevertheless occur despite all preventive measures.
Zone concepts for dusts
Zone 20 designates an area in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, for long periods or frequently. Typical examples are the interior of mixers, silos, mills, dryers, filters, cyclones and dust-conveying pipelines. Zone 21 applies where a hazardous explosive dust atmosphere can occasionally occur during normal operation. This frequently concerns filling, discharge, filling-off and sampling points as well as the immediate area of open or not fully tight transfer points. Zone 22 comprises areas in which a hazardous dust atmosphere does not occur, or occurs only briefly, during normal operation, for example with rare leaks, faults or the dispersal of deposited dust into a cloud. The actual zone boundary must be determined on the basis of release, dust properties, tightness, extraction, cleaning, airflow and room geometry.
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 can be used. These requirements apply not only to electrical equipment but equally to non-electrical components, such as shafts, seals, bearings, mixing tools, flaps, rotary valves and conveyors. Category 1D corresponds to the very high protection level for dust atmospheres and must remain effective even in the event of rare operational faults.
Inerting solutions
Inerting is a preventive protective measure. A suitable inert gas lowers the oxygen fraction in the process space to a value at which an explosion, or, depending on the material system and gas, a fire, can no longer occur or propagate. The relevant oxygen limit concentration, internationally referred to as the limiting oxygen concentration, is product-, inert-gas- and condition-dependent. It must be determined for the specific dust-inert-gas system under the relevant temperature, pressure, moisture and particle conditions. CEN/TR 15281 explicitly treats inerting as a measure for avoiding explosions and fires.
Nitrogen is the most frequently used inert gas for many organic dusts and chemical powders. It is widely available and suitable for numerous material systems. Carbon dioxide can be an economical alternative in certain applications, but should only be used after checking product and material compatibility. With reactive metal powders, for example aluminium, magnesium, titanium or zirconium, a general recommendation for nitrogen, carbon dioxide or argon is not permissible. These metals can react with certain gases or at high temperatures. The choice of inert gas must therefore be made on the basis of the specific reactivity, temperature, particle size, moisture and possible extinguishing or decomposition reactions.
In batch processes, vacuum pressure-swing inerting is frequently used. Here a vacuum-resistant vessel is evacuated and then flooded with inert gas. The process is repeated until the oxygen content lies below the permissible limit. With apparatus that is not vacuum-resistant, or in continuous processes, flow or purge inerting can be used. A defined inert-gas stream first displaces the air and then compensates for leaks, air ingress through charging or gas exchange during discharge during operation. A slight inert-gas overpressure can reduce air ingress but must be compatible with the permissible operating pressure, tightness and safety concept.
Oxygen measurement, alarm limits and interlocks are necessary components of a safely inerted plant. The measuring points must be representative; with large or complex apparatus, several measuring points may be required. The number of sensors, the required redundancy and the functional safety are established on the basis of the risk assessment. Blanket requirements such as always two sensors, a specific SIL value or a fixed margin of a few volume percent below the oxygen limit concentration are not sufficient without specific safety evidence.
Pressure-resistant and pressure-shock-resistant designs
Explosion-resistant equipment is built so that it withstands an internal explosion without bursting or causing hazardous effects on its surroundings. EN 14460 describes the requirements for explosion-resistant equipment. The design is based on the expected explosion pressure and takes into account vessel geometry, volume, material, temperature, pressure, connections, seals and possible hybrid mixtures.
With an explosion-pressure-resistant design, the apparatus is designed for the relevant maximum explosion pressure. By contrast, with an explosion-pressure-shock-resistant design, permanent deformation can be permissible, provided the vessel does not fail and no hazardous consequences arise externally. In practice, these terms are often used imprecisely. The permissible pressure stage, the expected explosion load and the requirements for continued use after an event must therefore be clearly established in the project-specific safety concept.
An explosion-pressure-shock-resistant design is not automatically the same as a design for the reduced explosion pressure combined with pressure relief. Pressure relief can limit the reduced explosion pressure, but its effect depends on relief area, opening pressure, vessel volume, dust characteristics, geometry, pipe lengths and possible flame propagation. The design of pressure relief systems must follow the relevant rules and take account of the safe venting area. With indoor installation, flameless relief or other protection concepts may be required.
Explosion suppression systems detect the onset of an explosion via pressure or flame sensors and inject a suitable extinguishing agent into the apparatus. This is intended to limit the pressure rise to a manageable level. These systems are particularly relevant where pressure relief to the open air is not possible, or where the release of flames, pressure and product would not be acceptable. They require fast detection, suitable extinguishing agents, regular testing and explosion-technical decoupling of connected components.
Regardless of the design, decoupling connected plant components is frequently required. Without decoupling, pressure and flame can propagate via pipework or conveying equipment to silos, filters and other apparatus. Possible systems are explosion-protection-suitable non-return flaps, quick-closing slide valves, explosion protection valves, extinguishing barriers or certified rotary valves. The chosen device must be suitable for the actual dust-explosion class, pipe geometry, throughput, pressure conditions and installation position.
Integrated protection concept
A practical concept follows the principle of first preventing an explosion and only then limiting its consequences. The first level of protection is limiting or avoiding an explosive atmosphere, for example through closed process operation, dust avoidance or inerting. The second level of protection is avoiding effective ignition sources through suitable equipment categories, temperature limitation, grounding, equipotential bonding, foreign-body management and maintenance. The third level of protection comprises pressure relief, suppression, explosion-resistant design and decoupling.
The safety-relevant material data are the starting point for every design. These include KSt, maximum explosion pressure pmax, minimum ignition energy, minimum ignition temperature of dust cloud and dust layer, particle-size distribution, moisture, electrical conductivity, the oxygen limit concentration and, where applicable, data on hybrid mixtures. The protection strategy must take account of normal operation, start-up, shut-down, filling, discharge, cleaning, maintenance and foreseeable faults.
How amixon® supports explosion protection with dust-explosive powders
ATEX-compliant design of the product space
With dust-explosive powders, the interior of a mixer, granulator, dryer or reactor 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 classification of the entire plant area, however, is established by the operator as part of the risk assessment. It covers not only the mixing chamber but also dosing, charging, discharge, filters, conveying lines, silos, filling and the installation area.
amixon® can design its apparatus for processes with a product space classified as Zone 20. This includes, depending on series, size and project configuration, single-shaft mixers, vertical and cone mixers, container mixers, continuous mixers, and vacuum mixing dryers and mixing reactors of the VMT and AMT series. The specific ATEX design must be established separately for each application. It includes, among other things, equipment category, permissible surface temperatures, electrical and non-electrical equipment, grounding, possible process monitoring and the required documentation.
The ATEX Product Directive 2014/34/EU concerns equipment and protective systems for potentially explosive atmospheres. The ATEX Workplace Directive 1999/92/EC obliges the operator to carry out a risk assessment, classify zones and prepare the explosion protection document. For the product space in Zone 20, a very high level of protection is generally required, which regularly corresponds to a Category 1D design.
Systematically avoiding ignition sources
Low tool speeds can reduce the mechanical energy input as well as friction, impact and abrasion processes. For the vertical mixers VM and HM, amixon® states an adjustable circumferential-speed range of approximately 0.8 to 3.5 m/s. Speed, however, is only one of several variables in the ignition-source assessment. Even at low rotational speed, possible tool-to-wall contact, jamming foreign bodies, bearings, seals, surface temperatures, electrostatic charging and electrical components must be taken into account.
Avoiding ignition sources can include temperature and torque monitoring, controlled rotational speeds, suitable material pairings, foreign-body separation, gas-tight sealing systems suited to the application, grounding and 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.
Tightness and pressure design
Product areas with few joints, ground smooth, suitable seals and closed transfer points can reduce dust escape. According to amixon®, Clever-Cut® inspection doors with OmgaSeal® seal are designed for low-dead-space sealing of the product space. The actual tightness, however, additionally depends on pressure differentials, seal material, wear, maintenance and process operation. Dust-tightness reduces possible dust deposits in the installation area but does not replace a cleaning concept or the assessment of possible secondary explosions.
The VMT and AMT series can be built gas-, vacuum- and pressure-tight. They are therefore suitable for mixing, reaction and drying processes under vacuum, overpressure or an inert-gas atmosphere. The possible vacuum level, the pressure stage and the permissible temperature ranges are executed project-specifically. Pressure or vacuum resistance is not automatically equivalent to an explosion-pressure-resistant or explosion-pressure-shock-resistant design. Such a design must be based on the safety-relevant material data, in particular KSt, pmax, minimum ignition energy, ignition temperatures, particle size, moisture and possible hybrid mixtures. EN 14460 describes requirements for explosion-resistant equipment intended to withstand an internal explosion without bursting.
An explosion-pressure-resistant or explosion-pressure-shock-resistant design is only one possible component of the protection concept. Where pressure relief, explosion suppression or other constructive protective measures are planned, all connected apparatus and pipework must be considered. Explosion-technical decoupling may be required so that flame and pressure are not transmitted to filters, silos, conveyors or downstream process stages.
Inerting and process integration
Inerting can be used as a preventive explosion protection measure. Here the oxygen content in the process space is lowered below a safe value defined for the specific dust-inert-gas system using a suitable inert gas, for example nitrogen. Vacuum mixing dryers and mixing reactors can be designed for vacuum pressure-swing processes. The space is evacuated and then flooded with inert gas; through repetition, the oxygen content can be reduced further. amixon® describes this inerting principle for vacuum mixing dryers and synthesis reactors.
Inerting presupposes a sufficiently tight apparatus, a suitable inert-gas supply, defined purge and evacuation sequences, representative oxygen measurement, and alarm and safety interlocks. The permissible oxygen value must lie with a safety margin below the oxygen limit concentration determined for the product, inert gas and operating conditions. Generally applicable oxygen setpoints are not reliable. The choice of inert gas must also be matched to chemical reactivity, temperature, corrosion behaviour, product quality and occupational safety.
Combining several steps in a single closed apparatus can reduce open transfer operations and thereby limit the risk of dust release. Whether mixing, granulating, reacting and drying can sensibly be combined in one unit, however, depends on the product, the recipe, the process-engineering requirements and the overall safety assessment. Even with integrated process operation, charging, discharge, filters, condensation, sampling, cleaning and maintenance must be included in the zone concept.
Manufacture and testing
The project-specific design follows the operator's User Requirement Specification and the safety-relevant material data. Trials with the original product can investigate process-engineering properties such as mixing quality, temperature input, liquid distribution, caking, discharge and cleanability. They do not, however, replace safety-relevant material testing, the design of explosion protection systems or the risk assessment carried out by qualified specialists.
According to its own statements, amixon® offers trials with the original product in its pilot plants. The documented results can support the choice of mixer type, tools, seals, dosing concept, heat transfer and process operation.