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 | | Design of Belt Conveyor Transfer Stations | | | Large mass flows are generally moved by belt conveyors. Because of their technical integration into the conveying process and the way they work, transfer points are mostly inevitable. The function of a transfer point is to enable the lossless transfer of material from the feeding belt (BC1) to the removing belt (BC2).
The complex behaviour of bulk material during the transfer process makes it very difficult for an engineer to design an efficient transfer point because previously material flow could only be estimated on the basis of long-term experience and/or by using continuum mechanical models. The Discrete Element Method (DEM) is a promising planning and design tool for not only transfer point planning but also numerous other problems of conveying technology.
| | | | This article describes the processing steps, results and evaluation of the planning of a belt conveyor transfer station (transfer height 19.5m) using the DEM developed in cooperation with the IFLS (Institue for Logistics and Material Handling) at Otto von Guericke University of Magdeburg. | | | | Planning the Transfer Chute of a Conveyor System | | | | The chute to be planned is the main component of the transfer tower shown in Figure 1 for a coal-handling system. The arrows indicate the direction of the two conveyors, which have a maximum conveying capacity of 1,800t/h and 3,000t/h. | | |  | Figure 1: Transfer tower showing conveying direction, feeding point 1) and receiving point 2) | |
Verifying planning using conventional means is only possible to a limited extent. Neither estimation based on experience nor analysis by continuum mechanical models leads to a realistic reproduction of material flow. Accordingly, a design can only be validated during the implementation phase. However, correcting the design at this late stage is usually very complicated and expensive. | | | | Using the DEM to plan a Chute with a Transfer Height of 19.5m | | | The DEM enables the behaviour of bulk material in a plant to be simulated. The conveying medium is considered as a number of particles with defined characteristics, including their interaction with the design. This approach allows realistic analysis. In order to apply the DEM, suitable software is necessary. Unfortunately, software meeting the requirements of conveying technology is not yet commercially available. Consequently, the user has to have a knowledge of:
· DEM theory
· CAD
· general programming
First of all, the geometry of the transfer point has to be defined. A sufficiently accurate abstraction of the actual design is needed to minimize the work entailed and the subsequent computing time. For example, it is recommended that only those walls that could come into contact with the conveyed material and those necessary for orientation on the part of the user or anyone else viewing the design be included. Before the entire simulation can be defined, it is important to consider the criteria to be analysed. Will observing the general flow of material suffice – or should defined parameters be recorded during simulation, e.g. the load on the receiving belt? This in turn will dictate the volume of programming required.
Software solutions which already contain modules for parameterized simulation definition permit the user-friendly definition of standard transfer variants. | | |  | Figure 2: Transfer chute showing the bulk material guide, maximum transfer height
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Normally, project planning is based on experience. In cases in which the geometry of the transfer point is simple and bulk materials such as dry sand are used, satisfactory results can be obtained. However, whenever new or complicated (e.g. very cohesive) materials are to be conveyed, the application of the DEM is increasingly called for. In addition to the material flow guidance, abrasion parameters and dust parameters, the mass flows are of particular interest to the project engineer. DEM software intended for industrial application should be user-friendly and should allow different problems to be analysed.
A wide range of bulk material transfer stations is installed in FAM systems and equipment. Accordingly, diverse bulk materials are encountered in connection with complicated geometries – as shown in the following example.
| | | | Defining the Simulation Environment | | | | The rough design of a transfer point depends on how it is to be integrated into the plant. After the general design has been drawn up (see Figure 2), the abstraction has to be performed. Figure 3 shows the rationally reduced geometry. The design transferred from the CAD program is yellow. In addition to the chute to be analysed, it includes other conveyors of the transfer tower for better identification. Grey marks the parts of the operating conveyors (BC1, BC2) relevant for the simulation. | | |  | Figure 3: Reduced geometry of transfer chute with regulating flap position in areas 1) and 2) | |
| | | | DEM Simulation of the Draft Plan | | | The particle number in a simulation has a direct influence on the computing time. To obtain sufficiently accurate results, an abstraction (as performed for the geometry) is recommendable. Accordingly, the particle number that can describe the material flow as realistically as possible needs to be defined. Simulation using a few large particles is just as unsuitable as the use of very many tiny particles; the former is not realistic enough and the latter involves very protracted computing. No useful advice or values derived from experience were found in the literature. Therefore, the abstraction was based exclusively on internal experience.
Owing to the characteristics of the DEM, generating a 10sec real-time simulation involving many thousands of particles could take several hours. However, the results of a simulation need to be available as quickly as possible in order to allow design work to proceed. The number of particles should therefore be set bearing in mind the criteria to be examined. For example, will estimated values concerning speed during transfer suffice – or should detailed load parameters be recorded and analysed?
Since only the transfer process is to be analysed, reduction can be effected by suitably positioning particle generation and particle deletion It must be taken into account that the real process parameters in particular must not be influenced. For example, in the simulation the material should not be fed to close to the discharge drum to make sure the speed of the material matches the belt speed during material discharge.
In addition to attractive forces and physical parameters such as the friction and rigidity of particles and walls, the program’s own parameters such as the time step of the numerical computation routine also have a considerable influence on the simulation as a whole. Various contact models corresponding to the required material characteristics are available that enable the software to calculate the interaction between contact partners. These contact models are based on parameters related to the general theories of mechanics and process engineering. Numerous relevant publications are available [cf 3, 4, 5].
In principle, particles with a complicated shape can also be simulated over a defined grain-size range. However, since these parameters are not thought to greatly influence the criteria being examined in this example, the bulk material is represented by simple spheres with a diameter of 110-150mm.
| | |  | Figure 4: Experimental set-up for defining the angle of repose -
Figure 5: Angle of repose after the cylinder has been lifted at constant speed | |
The same plant is able to convey different types of bulk material. Furthermore, the characteristics of the same type of material may vary. For instance brown coal from Germany and Russia has very different parameters. A detailed analysis of both real and simulated bulk material is therefore not always necessary.
The defined parameters were verified by using developed angle of repose tests. A test setup with a lifting cylinder is illustrated in Figure 4. The cylinder is elevated at a defined speed. The test is complete once the material has come to rest. The resulting angle of repose is shown in Figure 5; in this example it is 39°. This is a realistic value for the hard coal conveyed in this case.
| | | | Analysis of the Transfer Simulation | | | | Figure 6 and Figure 7 show the material flow in the transfer chute for the case of the maximum transfer height. The particles are generated in point 1) and deleted in point 2). The illustrated situation includes about 3,500 particles. The particle colour indicates the particle speed from 1m/sec (blue) to 12m/sec (red). | | |  | Figure 6: Visualization of transfer station with maximum transfer height (19.5m), view opposite the conveying direction, particle colour corresponding to speed -
Figure 7: Visualization of transfer station with maximum transfer height (19.5m), view in same direction as conveying direction, particle colour corresponding to speed
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The particle colour shows that the material is fed at a speed exceeding 12m/sec. The feeding speed and direction are important criteria concerning the optimization of the transfer process because of their fundamental influence on belt abrasion, idler loading and dust formation. If necessary, a quantitative analysis of particle speeds in defined ranges is possible, enabling different design modifications and their influence on these parameters to be compared.
Also visible in Figure 6 and Figure 7 are the main load areas of the chute, providing indications for abrasion verification. Chute abrasion results from the load caused by the bulk material. The material speed is very important in this connection.
The result is primarily the potential for optimization concerning speed minimization during transfer in order to minimize chute abrasion. Other results are the belt load and dust formation.
| | | | Optimizing the Transfer Process | | | In addition to theoretical attempts to modify the particle speed, practical experience has to be integrated into optimization because aspects such as caking cannot be taken into account under these simulation conditions.
| | |  | Figure 8: Optimization of material flow by a deflecting plate 1) on the upper segment of the chute; material flow totally deflected on the regulating flap -
Figure 9: Optimization of material flow by a deflecting plate 1) on the upper segment of the chute; compact material flow during complete transfer process
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In contrast to Figure 6 and Figure 7, compact material flow during the complete transfer process is visible, meaning reduced dust formation can be assumed.
Figure 10 and Figure 11 illustrate a simulation of 30,000 particles with diameters of 40–80mm. Concerning the simulation criteria particle speed and material flow course, this tallies well with the simulation of 3,500 particles (see Figure 8). In this case, the level of abstraction of the bulk material to 3,500 particles is sufficient.
| | |  | Figure 10: Simulation with upper deflecting plate; 30,000 particles, particle size range 40–80mm; view opposite to the conveying direction -
Figure 11: Simulation with upper deflecting plate; 30,000 particles, particle size range 40–80mm; view in same direction as conveying direction | |
| | | | Conclusion | | | The Discrete Element Method is a useful tool in different areas of conveying technology. The possibilities of the DEM have been explained in the step-by-step documentation of an example and the following analysis.
The simulation of transfer processes permits the evaluation of chute function during project planning and provides concrete indications for optimization. The simulation analysis of the design enabled parameters to be defined, allowing the strain on the chute and removing belt to be reduced. Based on practical experience, guide mechanisms were installed on the chute and tested in a new simulation.
The resulting interpretation of a DEM simulation and the ensuing designs should always take into account the respective degree of abstraction of the simulation and practical experience of bulk-handling.
The simulation results can only be compared with reality when the plant is commissioned. This in turn will increase the experience of using the DEM for the reliable planning of transfer points.
Although numerous recent publications are available on this topic, they mostly only concern research. Publications containing detailed documentation of real problems in industry are few and far between.
Experience of defining various simulation parameters and not least simulation analysis is essential for the effective application of the DEM.
The DEM is a useful tool for manufacturers of plant and equipment. Acceptance of the DEM has to be increased, which can only be achieved by its consistent application. A major contribution could be made by special software developed for problems of conveying technology which permits the user-friendly application of the DEM and also provides fast, reliable results even with large numbers of particles.
| | | References
[1] von Stein, R.: Optimierung der Übergabezone von Gurtförderanlagen, Dissertation Universität Hannover (1985)
[Author: Stein, R.: Transfer zone optimization of belt conveyor systems, Dissertation University Hannover (1985)]
[2] Herzog, M.: Untersuchungen zur Verbesserung der Schüttgutübergabe zwischen Gurtförderern, Dissertation Technische Universität Dresden (1999)
[Author: Herzog, M.: Researches about material-transfer-improving between belt conveyors, Dissertation Technical University Dresden(1999)]
[3] Gröger, T.: Partikelmechanische Untersuchungen zur senkrechten Schlauchgurtförderung, Dissertation Otto-von-Guericke-Universität Magdeburg (1998)
[Author: Gröger, T.: Particle-mechanical researches about vertical pipe-belt- conveying “Otto-von-Guericke“ University Magdeburg (1998)]
[4] Lungfiel, A.: Ermittlung von Belastungsgrößen mittels der Diskrete-Elemente-Methode für die Auslegung von Sturzmühlen, Dissertation Technische Universität Freiberg (2002)
[Author: Lungfiel, A.: Determination of force parameters by DEM for the design of ball mills, Dissertation Technical University Freiberg(2002)]
[5] Author: Gröger, T.: Tüzün U., Heydes D. M.: Modelling and measuring of cohesion in wet granular materials, Powder Technology 133 (2003)
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