How do I select the right explosion-pressure-resistance grade for a mixer in St1/St2 dust environments?
The selection is not made on the basis of dust class St1 or St2 alone. What matters is a complete explosion-protection concept built from tested material data, process conditions, apparatus geometry, pipework, installation location, and the intended measures such as inerting, pressure relief, suppression and decoupling. St1 and St2 only describe the rate of pressure rise of a dust; they do not automatically determine the required pressure-resistance class of the mixer.
The right question is therefore not “St1 or St2 – what wall thickness?”, but rather: what maximum pressure can arise in the specific mixer, including all plausible operating and fault scenarios, and which protective system demonstrably limits this pressure to what value?
Material data and process data
The design is based on representative safety-related characteristic values of the actual product. These include KSt, maximum explosion pressure pmax, minimum ignition energy, minimum ignition temperatures of the dust cloud and the dust layer, particle-size distribution, moisture, limiting oxygen concentration and electrical conductivity. Where liquids or solvents are added, possible vapour-dust hybrid mixtures must additionally be examined.
The dust explosion classes are defined via the KSt value: St1 covers 0 < KSt ≤ 200 bar·m/s, St2 covers 200 < KSt ≤ 300 bar·m/s. A higher KSt value means a faster pressure rise and can require shorter response times for relief, suppression and decoupling systems. However, it does not by itself provide a complete statement about pmax, the required vessel strength, or the suitability of a protective measure.
Literature or database values are only suitable for an initial preliminary planning stage. For a safety-relevant design, test values for the specific product or a demonstrably representative product group should be used. Formulation changes, altered moisture, finer particle sizes, organic secondary constituents or the addition of liquids can change the explosion behaviour.
Operating pressure, temperature, fill level, process atmosphere, internal fittings, rotational speed, possible dust deposits, charging, discharge, and the condition of connected pipework must additionally be taken into account. Pressure surges, high temperature, turbulence-generating internals, long connecting pipework and potential pressure piling are particularly relevant. These can significantly change the violence of the explosion and the pressure-time curve.
Choosing the right construction
In an explosion-pressure-resistant construction, the mixer is designed so that it withstands the assumed internal explosion load without dangerous failure. Whether permanent deformation is permissible must be explicitly defined in the chosen design concept. A design for the maximum explosion pressure pmax is particularly relevant where effective pressure relief or suppression is not possible, where the mixer is very tightly integrated into the plant, where product release is not permissible, or where the protection is to be based on passive vessel strength.
An explosion-pressure-shock-resistant construction can permit permanent deformation, but must not rupture or leak dangerously. It can be designed for the maximum explosion pressure or for a reduced explosion pressure. The statement “pressure-shock-resistant always means a pred design” is therefore too imprecise. The required design pressure must be clearly stated in the safety concept and verified by calculation.
A design for the reduced explosion pressure pred is possible where effective pressure relief or explosion suppression is provided. The mixer, together with its flanges, covers, sight glasses, measuring nozzles, shaft seals, discharge elements and connections, must then be designed for the actual maximum reduced explosion pressure to be expected. The design must not focus on the vessel body alone.
EN 14491 addresses the selection and design of pressure relief systems for dust explosions. It takes into account, among other things, vessel volume, dust data, opening pressure, relief area, relief ducts, reaction forces and effects outside the vessel. Long relief ducts, turbulence, pressure piling and flame jets can increase the reduced explosion pressure. The standard also points out that decoupling measures in the connecting pipework are required in most cases.
Practical approach
First, the current material data and all operating states are recorded. It is then determined whether the explosive atmosphere can be reliably prevented by inerting. If so, the inert gas supply, oxygen measurement, interlocks, leakage and failure scenarios must be designed so that the permissible oxygen value is reliably maintained. Inerting can influence the required pressure resistance, but it does not automatically replace a fallback concept for faults, start-up, opening, cleaning or loss of the inert gas supply.
It is then decided whether the mixer is designed for pmax, protected with pressure relief to pred, or limited to a verified pressure by a suppression system. For indoor installation, it must be checked in particular whether pressure relief to the outside is possible. If not, flameless venting, suppression or an explosion-resistant construction may be more suitable. The decision also depends on whether product, flames or toxic substances are permitted to be released in the event of an incident.
The selection must take the entire connected plant into account. Connecting pipework to filters, silos, conveyors, vacuum systems and filling plant can propagate an explosion or cause pressure piling. Explosion-technical decoupling is therefore often required, for example using suitable non-return valves, quick-acting slide valves, chemical barriers or qualified rotary valves.
Safety margins must not be applied as a blanket 10 or 20 percent. They result from the standard applied, the calculation method, uncertainties in the material data, design rules, pressure fluctuations, material characteristic values, and the responsible testing body concerned. Likewise, a typical design range of 0.2 to 2 bar for pred is not a general specification; such values are only permissible within the scope of validity of the method used in each case.
Particularities of St2
St2 dusts have a higher rate of pressure rise than St1 dusts. This reduces the available time window for active systems such as suppression or quick-acting slide valves. Pressure relief and decoupling must also be designed with particular care. Larger relief areas may be required, but they are not automatically sufficient: vessel geometry, internals, pipework, reaction forces and possible flame propagation must also be assessed.
An elevated operating temperature or an elevated initial pressure can change the explosion parameters. Hybrid mixtures of combustible dust and vapour can likewise lead to higher explosion pressures or altered ignition sensitivities. For St2 products, it is therefore particularly important not to rely on apparently conservative standard values, but to design for the actual conditions together with specialists in constructive explosion protection.
Documentation and inspection
The pressure design, the material data used, the protection concept, the calculations, the decoupling and the requirements for attached components must be comprehensibly recorded in the explosion-protection document and in the technical documentation. Changes to the formulation, particle size, moisture, operating pressure, temperature, throughput, pipework, relief devices or connected apparatus can make a renewed assessment necessary.
The inspection prior to initial commissioning must confirm that the mixer, protective systems and connected plant have been erected in accordance with the explosion-protection concept and function effectively. The same applies after changes subject to inspection.
How amixon® supports the design for St1 and St2 dusts
Basis: material data and protection concept
The required explosion-pressure-resistance class of a mixer does not follow from the classification of the dust into St1 or St2 alone. The dust class describes the KSt value and thus the maximum rate of pressure rise, but not automatically the required pressure resistance of the mixer. St1 covers KSt values from above 0 up to 200 bar·m/s, St2 values from above 200 up to 300 bar·m/s. For the apparatus design, pmax, minimum ignition energy, minimum ignition temperatures, moisture, particle size, operating pressure, operating temperature and possible vapour-dust hybrid mixtures are additionally relevant in particular.
amixon® can execute mixers, granulators, vacuum mixing dryers and mixing reactors for applications with a product space classified as Zone 20. However, the precise zone classification and the overall explosion-protection concept are determined by the operator on the basis of the risk assessment. In addition to the mixer, these cover dosing, filters, silos, conveying routes, discharge, filling and all connecting pipework.
The ATEX Equipment Directive 2014/34/EU governs the conformity of equipment and protective systems for potentially explosive atmospheres. The ATEX Workplace Directive 1999/92/EC obliges the operator to carry out a risk assessment, zone classification and the explosion-protection document. For the mixer interior in Zone 20, a very high level of protection is generally required, which regularly corresponds to Category 1D. Category 1D, however, is not a definition of the pressure resistance of a vessel. Pressure resistance and constructive explosion protection must additionally be defined explicitly and on a project-specific basis.
Choice of pressure design
An explosion-pressure-resistant or explosion-pressure-shock-resistant design must be based on the explosion pressure relevant to the apparatus and the protection strategy. The mixer can be designed for the maximum explosion pressure pmax or – provided effective pressure relief or explosion suppression is planned – for the demonstrably limited reduced explosion pressure pred.
With a design for pmax, the vessel must withstand an internal explosion without dangerous failure. This solution can be appropriate where safe pressure relief is not possible, where the apparatus is installed inside a building, where product or flame release is not permissible, or where the safety strategy is based on a robust vessel construction. With an explosion-pressure-shock-resistant construction, permanent deformation may be permissible; however, the vessel must not rupture or leak dangerously. Which deformations are permissible and whether the vessel may continue to be used after an event must be clearly regulated in the specification and the safety concept.
With a design for pred, the pressure relief or the explosion suppression must be demonstrably effective as part of the overall system. Not only the mixer vessel itself, but also covers, flanges, shaft seals, sight glasses, measuring nozzles, inlets and outlets, filter connections and connecting pipework must be designed for the defined pressure load. EN 14491 describes the selection and design of pressure relief systems for dust explosions.
amixon® can, in particular, execute the VMT and AMT series gas-tight, vacuum-tight and pressure-tight. These properties enable process operation under vacuum, positive pressure and, where applicable, inert gas. However, they are not automatically equivalent to an explosion-pressure-resistant or explosion-pressure-shock-resistant design. If such a design is required, the permissible explosion pressure must be clearly defined in the User Requirement Specification and calculated on the basis of the material data and the chosen protection concept.
Ignition sources, tightness and inerting
amixon® vertical mixers can be operated at low circumferential speeds. For VM and HM, amixon® specifies an adjustable range of approximately 0.8 to 3.5 m/s. Low tool speeds can limit friction, impact and abrasion energy. However, they are not a general demonstration of freedom from ignition sources. Bearings, seals, shafts, possible tool-to-wall contacts, foreign bodies, blockages, surface temperatures, electrostatic charging and electrical components must be taken into account as part of the ignition hazard assessment.
Product areas designed with minimal seams and ground smooth, together with suitable seals, 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 tightness actually achieved, however, also depends on the pressure differential, seal material, wear, installation and maintenance. Dust tightness alone replaces neither extraction and cleaning nor a concept against secondary explosions.
For gas-tight and vacuum-tight apparatus, inerting can be provided as a preventive protective measure. The process space is, for example, evacuated and then flooded with a suitable inert gas. Repeated vacuum-pressure cycling can further reduce the oxygen content. Safe inerting requires a tight plant, inert gas supply, 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 dust-inert gas system.
Inerting can greatly reduce the probability of an explosion, but it does not automatically replace the design for all operating states. An assessed protection strategy must be in place in particular for start-up, shutdown, opening, cleaning, faults or loss of the inert gas supply. Inerting also does not necessarily protect against self-ignition, decomposition or exothermic reactions.
St1 and St2 in the connected plant
With St2 dusts, the rate of pressure rise is higher than with St1 dusts. This can reduce the time window available for suppression and decoupling systems. Pressure relief, suppression and decoupling must therefore be matched with particular care to vessel volume, geometry, internals, connecting pipework and dust data. A higher KSt value does not necessarily mean that an St2 mixer must always be designed for a higher vessel pressure; however, it significantly influences the requirements for the chosen protective system.
Explosion-technical decoupling is required where pressure, flames or burning particles could propagate via pipework to connected filters, silos, conveyors or other apparatus. Tested non-return valves, quick-acting slide valves, chemical barriers, explosion protection valves or qualified rotary valves can be suitable. Which technology is appropriate depends on the entire process train, the pipework geometry, throughput, flow direction and the material data.
Trials and documentation
amixon® offers trials with the original product at its pilot plant. These can examine mixing behaviour, product heating, abrasion, build-up, discharge, vacuum and inerting sequences, as well as the suitability of the process control. However, they do not replace the safety-related dust testing, nor the calculation or verification of a constructive explosion-protection system.
The pressure design, the material data used, the strength of all pressure-loaded components, the protective systems, the decoupling and the operating limits must be comprehensibly documented in the explosion-protection document and in the technical documentation. Changes to the formulation, moisture, particle size, temperature, pressure, throughput or pipework can make a renewed assessment necessary.