What explosion-protection concepts (pressure-shock-resistant design, inerting, decoupling) are customary for dust-explosion risks?
For dust explosion risks, preventive and constructive protective measures are combined. First, hazardous explosive dust atmospheres and effective ignition sources are to be avoided. Where this is not possible with sufficient certainty, constructive measures limit the effects of a possible explosion to an acceptable degree. TRGS 724 names four basic measures for this purpose: explosion-resistant construction, explosion pressure relief, explosion suppression, and explosion-technical decoupling of flames and pressure.
The choice of protection concept depends on the safety-related material data, the apparatus size, the connected pipework, the installation location, the operating concept and the possible consequences of an event. Relevant data include, in particular, KSt, maximum explosion pressure pmax, minimum ignition energy, minimum ignition temperatures, particle size, moisture, electrical conductivity and, where applicable, data on hybrid mixtures.
Explosion-resistant construction
Explosion-resistant apparatus, for example mixers, filters, silos, cyclones, dryers or pipework, are designed to withstand an expected internal explosion without rupturing. After an explosion or detonation event, the affected plant components must be checked to determine whether their explosion resistance is still intact.
A distinction is made between explosion-pressure-resistant and explosion-pressure-shock-resistant construction. Explosion-pressure-resistant components withstand the maximum or reduced explosion pressure without permanent deformation. Explosion-pressure-shock-resistant components may deform plastically but must not burst or leak dangerously. An explosion-pressure-resistant construction simultaneously meets the requirements of an explosion-pressure-shock-resistant construction.
The design can be based on the maximum explosion pressure or – where effective pressure relief or explosion suppression is provided – on the reduced explosion pressure pred. General values such as "8 to 10 bar" are not a basis for the design of a specific apparatus. They can deviate considerably depending on the dust, hybrid mixture, initial pressure, temperature and geometry. A project-specific design based on tested material data and recognised rules is required.
Inerting
In inerting, the oxygen content in the process space is reduced using a suitable inert gas, or the atmosphere is largely replaced by inert gas. This is intended to prevent an explosion or fire from occurring or developing. Inerting is therefore a preventive rather than a constructive explosion-protection measure.
Nitrogen is frequently used for organic dusts and many chemical powders. Carbon dioxide or argon can be suitable in certain cases. However, the choice requires an assessment of chemical reactivity, process temperature, possible corrosion, product quality, occupational safety requirements and material behaviour under malfunction conditions. For reactive metal powders, an unsuitable inert gas can itself cause additional risks.
The permissible oxygen value must lie below the limiting oxygen concentration determined for the specific dust-inert gas system. It is set with a safety margin and takes into account measurement uncertainty, leakage, pressure and temperature fluctuations, moisture, product changes and possible oxygen release. General target values for residual oxygen are not sufficient.
Typical methods are vacuum-pressure-swing inerting for batch processes, purge or through-flow inerting for continuous plant, and blanketing with a slight inert gas overpressure. Safe implementation requires a sufficiently tight plant, a suitable inert gas supply, representative oxygen measurement, alarm limits, interlocks and a concept for malfunctions or loss of power.
Decoupling and downstream protection
Explosion-technical decoupling prevents pressure, flames or burning particles from spreading via pipework and conveying routes to other plant components. It is particularly important at the interfaces between mixer, filter, silo, cyclone, conveyor, dryer and filling. Without suitable decoupling, an explosion in a small apparatus volume can propagate to larger connected plant areas.
Suitable decoupling measures can include non-return valves qualified for explosion protection, quick-acting slide valves, explosion protection valves, chemical barriers, or certified rotary valves. Each individual measure must match the dust explosion class, pipework geometry, flow direction, throughput, pressure conditions and installation position. A rotary valve is only effective as a decoupling device if it is expressly designed for this protective function, suitably installed and regularly inspected.
Pressure relief and explosion suppression complement the concepts described. In pressure relief, bursting discs or relief flaps open so that pressure and flames are vented to a safe area. Where venting to the open air is not possible, flameless relief systems or suppression systems may be required. Explosion suppression systems detect the onset of an event and introduce extinguishing agent to limit the pressure rise. Both methods require coordinated design, testing and maintenance.
How amixon® supports explosion protection for dust-explosive powders
ATEX-compliant design of the product space
With dust-explosive powders, the interior of a mixer, granulator, dryer or reactor is frequently classified as Zone 20. Zone 20 denotes an area in which an explosive dust atmosphere is present continuously, for long periods, or frequently. However, the final zone classification of the overall plant is determined by the operator as part of the risk assessment. Besides the mixer, it also covers dosing, filling, discharge, filters, conveying lines, silos, filling stations and the installation room.
amixon® can execute mixers, granulators, vacuum mixing dryers and mixing reactors for applications with a product space classified as Zone 20. Depending on series, size and project configuration, this applies to 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 apparatus design, equipment category, permissible surface temperature, electrical and non-electrical equipment, and required documentation are defined on a project-specific basis according to the material data and process conditions.
The ATEX Equipment Directive 2014/34/EU governs requirements for equipment and protective systems used in potentially explosive atmospheres. The ATEX Workplace Directive 1999/92/EC obliges operators to carry out a risk assessment, classify zones and maintain an explosion-protection document. A product space classified as Zone 20 generally requires a very high level of protection, which regularly corresponds to a Category 1D design.
Ignition sources and tightness
Low tool speeds can limit mechanical energy input as well as friction, impact and abrasion energy. For the vertical mixers VM and HM, amixon® specifies an adjustable range of approximately 0.8 to 3.5 m/s. With sensitive powders, this mode of operation can additionally reduce particle stress. However, it is not a general demonstration that no effective ignition source can occur in the mixer. Even at low rotational speeds, bearings, seals, shafts, tool-to-wall contacts, foreign bodies, blockages, surface temperatures, electrostatic charging and electrical components, among other things, must be assessed.
Mixing chambers and mixing tools can be designed with minimal seams and ground smooth. According to amixon®, Clever-Cut® inspection doors with OmgaSeal® sealing are designed for low-dead-space sealing of the product space. These features can reduce dust escape and deposits. However, the actual tightness achieved is also determined by pressure differentials, seal material, wear, assembly, maintenance and the specific process control. A dust-tight construction reduces the risk of dust deposits, but it replaces neither an effective cleaning concept nor the assessment of possible secondary explosions.
An ATEX-compliant overall concept may require temperature and torque monitoring, suitable bearing and seal concepts, foreign-body management, earthing, equipotential bonding and preventive maintenance. 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.
Pressure design and decoupling
The vacuum mixing dryers and mixing reactors VMT and AMT can be built gas-tight, vacuum-tight and pressure-tight. These properties are required for processes under vacuum, positive pressure or an inert gas atmosphere. However, they do not automatically mean that an apparatus is designed to be explosion-pressure-resistant or explosion-pressure-shock-resistant. A corresponding design must be expressly specified and is based on safety-related material data such as KSt, pmax, minimum ignition energy, ignition temperatures, moisture, particle size and possible hybrid mixtures.
An explosion-pressure-resistant or explosion-pressure-shock-resistant construction can be part of a constructive explosion-protection concept. Depending on the protection strategy, it is designed for the maximum explosion pressure or for an explosion pressure reduced by pressure relief or suppression. After an event, the affected components must be checked for continued integrity and fitness for use.
Explosion-technical decoupling is required where pressure, flames or burning particles could propagate to other plant components via connecting lines. This applies in particular to the interfaces between mixer, filter, silo, dryer, conveyor and filling plant. Suitable measures can include non-return valves tested for explosion protection, quick-acting slide valves, explosion protection valves, chemical barriers, or suitable rotary valves. Selection and design are carried out for the specific process train, the dust explosion class, the pipework geometry, the throughput and the pressure conditions.
Inerting as a preventive measure
Inerting can be used as a preventive explosion-protection measure. This reduces the oxygen content in the process space, using a suitable inert gas, to a safe value below the limiting oxygen concentration determined for the specific dust-inert gas system. In vacuum apparatus, for example, the atmosphere can be adjusted by repeated evacuation followed by flooding with a suitable inert gas. amixon® describes this principle for vacuum mixing dryers and synthesis reactors.
This requires a sufficiently tight plant, a suitable inert gas supply, defined evacuation and purging sequences, representative oxygen measurement, alarm limits and safety-related interlocks. The permissible oxygen setpoint must be defined for the specific recipe, the inert gas, pressure, temperature, moisture and measurement uncertainty. General oxygen setpoints are not sufficient.
Inerting reduces the probability of an explosive atmosphere but does not replace all other protective measures. It does not automatically protect against self-ignition, thermal decomposition, exothermic reactions or possible fires in deposits. For oxidising constituents and reactive powders, the suitability of the inert gas and the effectiveness of inerting must be examined with particular care.
Trials and design
The design follows the safety-related material data, the operator's User Requirement Specification, and consideration of the entire connected plant. Trials with the original product can assess process-engineering suitability, for example mixing quality, temperature development, liquid distribution, build-up, discharge, cleanability and behaviour under vacuum. They do not, however, replace safety-related material testing, the design of pressure relief, suppression or decoupling, or the risk assessment carried out by qualified explosion-protection specialists.
amixon® states that it operates more than 30 test units of different sizes at its Paderborn site; additional pilot plants are available in Japan, India, Thailand, China, South Korea and the USA. The trial results can support the selection of mixer, tools, seals, heat transfer, dosing concept and process control.