Status:
completed 12/2011
Aims:
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Development of new approaches on the explosion protection of mixers
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Creation of model risk assessments for a variety of mixers in various industries
The model risk assessments should provide the opportunity for manufacturers and operators of mixers to conduct risk assessments according to the state of the art.
Activities/Methods:
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Concentration measurements and explosion tests and in addition to this study of mixers in practical experiments
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Leading of the results of the investigations to practical explosion protection concepts for different constructions
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Reconstruction and extrapolation of the results by computer aided modelling.
Results:
The results of this research project are:
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A classification of explosion protection concepts for different industries is not effective, since a cross-sector range of mixer configurations and operational modes and safety-related characteristics of the products used in practice is present.
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Depending on branch of industry it is possible to suppose special boundary conditions, which do not relate primarily to the explosion protection, but which can have an influence on this. Examples for these special boundary conditions are the avoidance of foreign materials in pharmaceutical and food industries as well as large amounts of inert bulk materials in the building materials industry.
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Protection concepts for mixers, based on design measures for explosion protection, are implemented in praxis only in exceptional cases.
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Result from studies to the dust concentrations is that the filling degree of a mixer is not a reliable criterion for assessing the probability of occurrence of explosive dust / air mixtures.
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The probability of occurrence of explosive dust / air mixtures within mixers is dependent on material, operational and structural conditions.
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Without additional measures, such inerting, to avoid the occurrence of an explosive atmosphere, the interior of mixers is basically classified in zone 20.
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Except for vertical mixers with circumferential velocities < 1 m x s-1, which are used for the homogenization of bulk materials of the dustiness group 1 according to VDI 2263 Part 9. For these configurations an explosive dust/air mixture occurs only during the filling and emptying, so the inside of these vertical mixers can be classified in zone 21.
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The modelling of the mixing process by numerical simulations showed that the dustiness of a bulk material depends on a variety of material properties which can hardly be described by a mathematical model.
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The grain size distribution and bulk density are as the only parameters inadequate to characterize the dustiness of a bulk material by a mathematical model.
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For the modelling of the mixing process taking into account the large number of particles with the exact discrete element method, there are insufficient hardware resources by today's standards, so that a simulation of the dust concentrations in mixers cannot be practically implemented under economic aspects.
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The research results are summarized in a guideline for evaluating the operational fire and explosion risk and pattern risk assessments for typical explosion protection concepts.
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Based on this summary, it will be easier in particular for small and medium companies to recognize hazards and to derive and to implement appropriate protective measures.
Last Update:
21 Sept 2021
