PROCESSING. 4R. Furthmann, AUMUND Fördertechnik GmbH, Rheinberg, Germany ZUSAMMENFASSUNG SUMMARY

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1 4R. Furthmann, AUMUND Fördertechnik GmbH, Rheinberg, Germany SUMMARY ZUSAMMENFASSUNG Criteria for minimizing wear are often of only secondary importance in the selection and design of conveying systems. Even in the plant planning phase it is possible to take measures that permanently lower the operating costs of a plant without any excessive increase in the capital costs. Capital costs versus operating costs this constantly recurring trade-off that has to be considered before every plant project will be dealt with in the article with the aid of three specific practical examples for bucket elevators and apron pan conveyors of the type used in the cement industry as well as in the entire bulk materials industry. Bei der Auswahl und Planung von fördertechnischen Anlagen haben Kriterien zur Verschleißminimierung häufig nur eine untergeordnete Bedeutung. Dabei können schon in der Phase der Anlagenprojektierung Maßnahmen ergriffen werden, die die Betriebskosten einer Anlage nachhaltig senken, ohne die Investitionskosten über Gebühr zu erhöhen. Investitionskosten gegen Betriebskosten diese immer wiederkehrende Abwägung, die vor jeder Anlagenprojektierung zu bedenken ist, wird anhand von drei konkreten Praxisbeispielen für Becherwerke und Plattenbänder wie sie in der Zementindustrie und auch in der gesamten Schüttgutindustrie eingesetzt werden, in dem Beitrag behandelt. (Translation by Robin B.C. Baker) 52 CEMENT INTERNATIONAL 4 5/ VOL. 15

2 Technical measures for reducing the wear in bucket elevators and apron pan conveyors Technische Maßnahmen zur Reduzierung des Verschleißes an Becherwerken und Plattenbändern 1 Bucket elevators for recirculating the material in cement grinding plants If bulk materials have to be transported vertically then bucket elevators continue to be the most cost-effective and efficient solution. The mechanical resistances and power losses are very low and are caused essentially by the loading and drive train itself, so the installed electrical power of the drive can be virtually fully utilized for lifting the bulk material. When operating costs are being considered the focus is therefore on the service lives of the wearing parts and the availability of the plant. There is a wide selection of bucket elevator types, in which certain systems have gained acceptance over the years on the basis of their industrial usage ( Fig. 1). For example, fast-running bucket elevators from the mixed emptying sector, with belts or plate-link chains as the traction agent, have become widely accepted in the cement industry. In most cases the plate-link chains are used as central chains. The traction agents have been subject to continuous onward development in recent years, with the result that bucket elevators have now penetrated into previously unknown performance ranges. High performance steel cord belts developed specifically for bucket elevators now permit lift heights of up to 200 m and, thanks to the use of temperature-resistant rubber mixes, are capable of long-term transport of materials with temperatures of up to 150 C. The spectrum of use of belt bucket elevators has also been extended through the development of special bucket elevators for coarse-grained materials. This means that belt bucket elevators can now handle materials with particle sizes of up to 80 mm without any difficulty. the development of forged plate-link chains with grease lubrication and labyrinth seals has achieved a significant improvement in the service lives of the bucket elevator chains. Aumund, based in Rheinberg, Germany, already has bucket elevators in all these categories in use. The highest belt bucket elevator built so far by Aumund measures 173 m and the highest chain bucket elevator measures 90 m. Conveying capacities of up to m 3 /h can be achieved with belt bucket elevators and as high as m 3 /h with chain bucket elevators. In total, more than bucket elevators produced by Aumund are in use around the world. In the past the particle size and the temperature of a ma terial were often criteria that permitted the use of only either a belt bucket elevator or a chain bucket elevator but nowadays both types of bucket elevator can be used in many instances. Making the correct choice of bucket elevator is no longer straightforward and therefore requires close coordination between plant operator and manufacturer. 1.1 Practical example A bucket elevator for recirculating the material is required for grinding cement clinker in a high-pressure roller mill. The bucket elevator is to be designed for a conveying capacity of t/h (bulk density of cement clinker is about 1.5 t/m 3 ) and should have a lift height of 40 m. The feed material has a particle size of 10 to 80 mm with a maximum temperature of 110 C. According to Fig. 2 the options are either a BWG-GK belt bucket elevator (coarse material belt bucket elevator) or a BW-D double chain bucket elevator ( Fig. 3). However, plate-link chains have also achieved a significant leap in performance in recent years through the use of improved materials and manufacturing methods. In particular, Vertical bucket elevator The BWG-GK bucket elevator would need a bucket width of mm while the BW-D would have to use 2 x 800 mm wide bucket strings, each equipped with a central chain. Traction agent Feeding Emptying Belt Chain Scooping Pouring Mixed feeding Gravity Centrifugal force Mixed emptying Fabric Steel cord Round link Plate link Figure 1: Selection matrix for different types of bucket elevators as specified in VDI 2324 CEMENT INTERNATIONAL 4 5/ VOL

3 In this case the belt bucket elevator proved to be the better option than the chain bucket elevator and it was assumed that the belt would have the longer service life with respect to wear, so the choice was initially made to use the coarse material belt bucket elevator. However, a different picture emerged after consultation with the plant operator over the precise conditions of use Consideration of the particle size According to the available sieve analysis the specified particle size of up to 80 mm could only be guaranteed with a probability of about 93 %. Individual pieces of material of up to 120 mm can occur on the collecting belt conveyor after the metering hoppers, especially with the additives such s as limestone and gypsum. According i to the plant configuration the coarse fractions f should in fact be removed by b screening before the input to the high-pressure h roller mill but time and again a they reach the bucket elevator as individual i pieces. Fig. 4 shows the particle p size distributions recommended by b Aumund for the use of different types of o bucket elevator Material temperature and its influence f on the service life of the belt The T operating conditions of a high-pressure s roller mill are a particular challenge for bucket elevators as not only steadystate s operation but also abnormal occurrences c have to be taken into account. Several S times in the course of a year it may m be necessary to separate the rollers from the mill so that the working surfaces Figure 2: Possible applications of Aumund bucket elevators (Graphics: Aumund) of the rollers can be hard-faced. This can overfill the foot of the bucket elevator and, depending on the quantity of material, it can lead to shutdown of the bucket elevator. Depending on how frequently this type of overfilling occurs and how long the cement clinker with temperatures of up to 110 C remains in the bucket elevator foot such occurrences can result in a substantial shortening of the service life of the belt. Above temperatures of about 70 C the rubber gradually ages and hardens under the influence of temperature. Prediction of the service life of the bucket elevator belt usually assumes that the material is transported in the buckets and is not in direct local contact with the belt over fairly long periods ( Fig. 5). BWG-GK BW-D Figure 3: BWG-GK belt bucket elevator for coarse material (left) compared with BW-D chain bucket elevator (right) (Graphics/photos: Aumund) 54 CEMENT INTERNATIONAL 4 5/ VOL. 15

4 For the example shown here a belt service life of about 2.5 years could therefore be assumed, bearing in mind possible abnormal occurrences, however, considering operation under normal conditions, much longer lifetimes for the belt would be possible. By comparison, significantly longer service lives are achieved with forged, grease lubricated, Aumund chains with labyrinth seals when handling cement clinker in comparable plants. A service life of about five years was predicted for this application. 2.1 Detailed considerations Clinker temperature and temperature-resistant belts The process control during the burning and cooling of cement clinker is becoming increasingly difficult due to a number of new influencing factors. In particular, the plant operator is often faced by certain challenges due to the increased use of alternative fuels. The fact is that throughout the world there is an increase in upset conditions that can permanently affect the clinker outlet temperature Result of the considerations The customer eventually decided in favour of the approximately 10 % more expensive BW-D bucket elevator as the additional costs are paid off relatively rapidly through the longer service life of the chain. The use of a chain as the traction agent also represents a more robust solution for this application with significantly minimized risk of stoppages. 2 Silo discharge belt conveyor for cement clinker Cement clinker is an intermediate product in the production of cement. It is sintered in a rotary kiln at temperatures of about C, cooled in a clinker cooler to about 150 to 200 C and then transferred by suitable transport equipment to a clinker silo ( Fig. 7). Content of particles [%] Typical grain size distributions 1 < Particle size [mm] Figure 4: Recommended particle size distributions for the use of different types of bucket elevator (Graphics/photo: Aumund) Apron pan conveyors are used almost exclusively for the transport from the clinker cooler as temperature peaks of up to 800 C can occur repeatedly due to the nature of the process. On the other hand, belt conveyors are also used for the onward transport after the clinker silo. The distance between centres of the apron pan conveyors is usually less than 100 m for an operating time of about h/a. Belt conveyors are better than apron pan conveyors from the point of view of the initial capital investment but certain aspects must be borne in mind in the design and selection for this specific application. The following assumptions, that are not always universally correct, are often made as selection criteria for the use of belt conveyors: Figure 5: Effect of the material temperature on the belt in normal operation and during abnormal occurrences (local temperature exposure after overfilling) (Graphics/photos: Aumund) Expected chain life time after the clinker silo the clinker temperature is always non-critical and the grades of belt have also improved with respect to the usage temperatures, and belt conveyors permit higher transport conveying speeds than apron pan conveyors and can therefore be made smaller and more compact, so that in turn there is a saving in the construction Center distance [m] costs for steel or concrete construction, Figure 6: Estimated service life of a grease-lubricated bucket elevator chain such as for tunnels or belt bridges. (Graphics/photo: Aumund) CEMENT INTERNATIONAL 4 5/ VOL

5 Apron pan conveyor Apron pan conveyor or belt conveyor Figure 7: Typical layout of a kiln plant for producing cement clinker (Graphics/photos: Aumund) Clinker silos usually have large storage capacities of up to t, so depending on the silo design and residence time, so-called hotspots with temperatures of 300 to 500 C can be retained for weeks in the silo ( Fig. 8). Cement clinker is also very abrasive and frequently contains a large fraction of fines so a suitable compromise between temperature resistance. Therefore abrasion resistance must be found when choosing a belt. SBR rubber mixes are in fact characterized by greater abrasion resistance than EPMD mixes but it must be understood that the abrasion resistance decreases with increasing ageing of the belt. The best decision for this situation is therefore ST belts made of SBR rubber mixes with a proportion of EPDM because the belt does not age so rapidly due to the greater temperature resistance Belt conveyors permit greater transport speeds Basically, belt conveyors can be operated at significantly higher speeds than apron pan conveyors. However, the recommended transport speeds for discharge from a clinker silo are limited to 0.5 to 0.7 m/s. Because of its good flow properties the cement clinker is usually discharged by gravity through silo discharge outlets ( Fig. 9). These do not require any additional dedusting, which is an advantage as the space conditions under a silo do not permit the inclusion of a filter plant and also the use of a filter would lead to higher overall operating costs. The optimum conditions for use of silo discharge outlets occur with belt speeds in the range from 0.3 to 0.5 m/s and a belt troughing angle of 45 degrees. Figure 8: Typical cement clinker temperatures between clinker cooler and silo (Graphics: Aumund) Higher speeds lead to increased wear on the belt and to large quantities of dust, which has a negative effect on the service lives, especially of the roller supports Belt conveyors save construction costs There are hardly any differences in the dimensions of the tunnels or belt bridges for installing an apron pan conveyor or a belt conveyor, as is shown by the following comparison of two completed projects ( Fig. 10). The fact that, because of their higher transport speeds, belt conveyors can necessarily always be made narrower than apron pan conveyors is not universally correct as substantially higher filling cross-sections can be achieved with apron pan conveyors. Figure 9: Silo discharge outlets for free-flowing bulk materials (Graphics: Aumund) Apron pan conveyors for clinker can also handle inclinations of up to 30 degrees without any problem but belt conveyors should only be used for slopes of up to a maximum of 15 degrees. This means that greater transport heights can be achieved with smaller distances between centres with apron pan conveyors than with belt conveyors. 56 CEMENT INTERNATIONAL 4 5/ VOL. 15

6 Figure 10: Comparison of the construction space for apron pan conveyors and belt conveyors (Graphics: Aumund) Summary of the considerations Apron pan conveyors are more suitable when it is a matter of longevity and high availability during the transport of hot abrasive bulk materials. The higher capital investment costs are usually paid off after five to seven years. If it is possible to exclude temperature peaks of > 400 to 500 C and if the clinker temperature is as a rule not more than 120 to 180 C then belt conveyors can definitely be a favourable alternative. However, the conditions of use should be precisely agreed with the plant supplier during the selection and planning so as not to be surprised by eventual high operating costs ( Fig. 11). During the discharge of hot cement clinker from a silo it is recommended that transport speeds of 0.5 m/s should not be exceeded as otherwise dust-free discharge from the silo cannot be achieved without additional filter plants and the service life, e.g. of the carrying rollers, is significantly reduced by the very fine and abrasive cement dust. A steel cord belt with high temperature resistance as well as good abrasion resistance should be used for preference. The joint between the cover and the carcass represents a weak spot if textile-reinforced belts are used under these conditions. 3 Modification of a coal feeder Two new power plants were commissioned at the RWE Neurath power station in August Each of these two power plants has eight service hoppers for storing raw lignite. The lignite is placed in intermediate storage in these service hoppers and transferred to the eight coal mills by apron pan conveyors ( Fig. 12) that are positioned under each of the service hoppers. After an operating period of only two years RWE decided on a complete modification of all 16 of the apron pan conveyors from another supplier and in 2014 awarded Aumund the contract to upgrade the coal feeders. The high operating costs (all CAPEX+OPEX ( ) [%] Standard EP belt Steel chord belt EPDM Compound mix Steel apron pan Converyor Belt costs with production losses afters ÿre 500 t/h, CC: 126 m Year of operation Figure 11: Example of the comparison of the capital investment costs and operating costs of different conveying systems for discharging hot cement clinker from a silo (Graphics/photos: Aumund) CEMENT INTERNATIONAL 4 5/ VOL

7 Round link chain Bushed conveyor chain Point contact at the hinge point Linear contact at the hinge point Figure 12: Apron pan conveyor as a coal feeder under a service hopper after successful modification (Photo: Aumund) Figure 13: Comparison of the contact areas at the hinge point of a round link chain and a bushed conveyor chain (Graphics: Aumund) Figure 14: Body of the conveyor, consisting of carrier plates, chains and carrier rollers (left: before modification, with 125 mm pitch round link chain, right: after modification, with Aumund bushed conveyor chain with 250 mm pitch) (Photos: Aumund) the apron pan conveyors had to be overhauled after only one year) were the reason for replacing the conveying equipment. therefore to unplanned failure of the apron pan conveyor because the chain jumps over the chain wheels. The main problems were the severe wear on the plant and the ingress of false air into the coal mills because the entire housing was not sufficiently air-tight. The carrier plates, carrier rollers and chains as well as the chain wheel sprockets were particularly affected by the wear. There was also the problem that the apron pan conveyors had been fitted with round link chains as the traction agents. Because of their smaller chain pitch, round link chains have more hinge points per metre than bushed conveyor chains, which leads to higher surface pressures at the hinge points ( Fig. 13). This explains the greater chain wear and the associated greater chain elongation. With limited tensioning travel at the tensioning station the great chain elongation also leads to slack in the chain and The entire body of the conveyor, consisting of carrier plates, chains and carrier rollers ( Fig. 14), was replaced by selfsealing carrier plates with bushed conveyor chains. At the same time the conversion to bushed conveyor chains made it possible to use sprockets with wear-reducing pitch design, which doubled the service life of the chain wheel segments. Since the conversion just under two years ago there has been no failure of the apron pan conveyors. 4 Final remark The possible savings potential in initial capital investment is frequently overvalued when measured against the possible operating or consequential costs that can arise through incorrect choice and design of a conveying system. Low-cost capital investment can often become more expensive in the end! 58 CEMENT INTERNATIONAL 4 5/ VOL. 15