Conveying innovation with fill-controlled fluid couplings type TPKL World Coal, June 2005 Bernhard Schust Product group Start-up Components
Over the last decade, mine operators, belt conveyor specialists, consultants and manufacturers have had numerous opportunities for testing a wide range of actively controlled technologies for underground conveying systems. These technologies featured solutions in the form of purely electric drives, mechanical drives and hydrodynamic (turbo coupling) drive systems. All these technologies have individual characteristics. It often takes several operating years before mine operators can truly assess all the benefits of a certain drive system (procurement cost and performance compared with long-term operating and maintenance costs). Purchasing decisions for new equipment often do not take aspects such as service and maintenance (sufficiently) into account. Many of the latest electric technologies (especially AC variable-speed drives) that have been used in recent years were purchased merely for their control advantages. These technologies, whose performance can vary considerably from manufacturer to manufacturer, are, however, in a state of ongoing development, which puts long-term product support by the manufacturer into question. Additionally, some of these Fill-controlled Voith fluid coupling type TPKL. Figure 1. Progression of belt tension (in principle) with drive concentration close to the dropping head (main drive station).
technology versions have undesirable operating consequences, such as shortening the service life of the motors, harmonics in the power system and communication problems within the mine. These technologies are also extremely current-limited, which can lead to problems in the event of overloads (a condition that can lead to considerable interruptions of the conveyor system). Turbo couplings, on the other hand, are based upon a technology that has been successfully used in underground belt conveyor systems for many years. Although the fundamental hydrodynamic principles have proven themselves to be excellent for many years, especially in belt conveyor applications, Voith has continuously developed the technology further, in order to meet the growing Figure 2. Progression of belt tension (in principle) with power sharing between head drive and tripper booster drive. Figure 3. Illustration of TT principle. Figure 4. Tripper drive with active belt tensioning measurement. Figure 5. Distribution of belt tension (in principle) with different part load situations. requirements of modern belt conveyor systems. After a worldwide demand for reliable and actively controlled drive systems could be seen to emerge the development of an actively controlled coupling began in 1996. This coupling was especially designed for applications in high-performance belt conveyors in underground applications. Voith launched the new, fill-controlled coupling type TPKL in 1997 and its first concrete application was in the US. The first two such applications were of multitripper booster drives for belt conveyors at the longwall panel. Since the successful start of these belt conveyor systems in 1998, Voith has sold over 250 TPKL couplings to mines worldwide, 150 of them in the US. Why tripper booster drives? With conventional belt conveyors, where the entire drive power is close to the dropping point, the highest belt tension power is also generated at this point (Figure 1). The maximum belt tension T1 has a major effect on the belt quality and hence the belt cost. If the drive power is decentralised, the maximum belt tension powers are significantly reduced (Figure 2). Two basic possibilities for introducing tension have been launched in the marketplace: one variation is the TT (carrier belt/drive belt), also referred to as linear booster (Figure 3). Voith has gained experience with TT drives through applications at Ruhrkohle, where belt conveyors were realised with up to 20 drives at 250 kw. The second version is meant to simplify the system even further. In the loaded section, the belt winds around at least one further pair of drums, of which at least one drum is driven. Such tripper booster drives are primarily used in English speaking countries, where they are referred to as tripper drives (Figure 4). All belt conveyors with tripper booster drives need to be examined very carefully with a view to differing load conditions. Load sharing between the individual drives in dependence on the actual load is recommended. The demand for active performance control of the drives derives from this (Figure 5). Description of the coupling Hydrodynamic power transmission offers many advantages for belt conveyor drive systems. These include:
Smooth introduction of torque. Natural load sharing for belt conveyors with multi-motor drive. Long service life (often more than 10 years prior to overhaul). Additional capacity for starting overloaded systems (motor can theoretically be utilised up to pull out torque). Insensitive towards environmental conditions (humidity, temperature). Wide, open-ended performance spectrum allows utilisation of larger motors with future applications. Figure 6. Belt length 3130-4460 m, belt width 1220 mm, belt speed 4.2 m/s. The characteristics stated above apply both to constant-fill and fill-controlled couplings. However, many belt conveyor applications now require active torque, speed and cooling control. The special requirements of large-scale conveyor systems with tripper booster drives are as follows: Active belt tension control at the tripper booster drives. Active load sharing control (between primary and secondary drive drums). External heat exchangers for frequent start-ups (normally 60-120 s). Acceleration time are needed. Inspection trips with empty belt. TPKL installations in US coal mines The first TPKL drives were delivered in June 1998 and were successfully commissioned in a coal mine in Alabama. The TPKL couplings were installed in a belt conveyor with a total of four motors, of which two operate as tripper drives (Figure 6). As a result of the installation of the new TPKL drive, the mine was able to improve its output and its availability considerably. With the experience gained with this first installation, the mine decided to fit, one by one, all underground belt conveyor drives with fill-controlled couplings type TPKL. In October 2003 all underground belt conveyor drives were retrofitted with TPKL couplings (22 TPKL 562 couplings in 10 conveyors). The length of the underground belt conveyor line amounted to approximately 24 km in October 2003 and was scheduled to be extended to more than 30 km by the end of last year. Since 1998, the drive modifications in this mine led to an increase in the gross haulage capacity of 3.7 million tpa in 1997 to 7.0 million tpa in 2003. Figure 7. The conveyor contains seven TPKL 562 couplings, which are installed at three different drive stations. Figure 8. Belt length 4080 m, belt width 1525 mm, belt speed 4.0 m/s. Figure 9. Belt length 2350-2990 m, belt width 1830 mm, belt speed 4.8m/s, lifting height 40 m.
the future development of the longwall panels. TPKL features The new TPKL coupling has a closed loop oil circuit that results in considerably reduced dimensions of the oil tank and the housing. Figure 10 shows the closed loop circuit of the TPKL coupling. Figure 10. The closed loop circuit of the TPKL coupling. five panels successfully. The third example of a tripper installation is at one of the most productive mines in the US, where over 11 million tpa is produced from two longwall systems. This mine in western Pennsylvania began in 1998 with the installation of TPKL couplings. The current coupling Figure 11. Output side performance graph of a fill controlled coupling. fleet amounts to a total of 36 TPKL 562 couplings in Prior to the drive modification, the mine had an availability of more than 85 % until 1997. This availability rate has, however, increased year after year, after more and more TPKL couplings were installed into the existing belt conveyors. The lowest availability of an individual belt conveyor was 99.19% last year, which corresponds to an absolute conveyor availability of the entire underground system of more than 97%. The second installation of new TPKL technology was carried out in another tripper-booster longwall belt conveyor, which was commissioned in a mine in Utah in 1998. For the first time, the system offered the opportunity of using TPKL technology for multi-tripper boosters. The belt conveyor shown in Figure 7 contains seven TPKL 562 couplings, which are installed at three different drive stations. With this configuration, the mine was able to complete 400 HP, 500 HP and 600 HP dual drives. The mine also decided to equip the new conveyors with 1250 HP dual drives as standard and installed several TPKL couplings type 650 in these high-performance systems. In 2001, the mine decided to install the first tripper units with TPKL couplings in its two panel conveyors to replace the previously installed frequency inverter drives. Figure 8 shows this configuration of the belt conveyor, which was commissioned in August 2002. Figure 9 shows the highest powered tripper drive in the world. This new mine in New Mexico standardised its drives to 750 HP, 4160 V, for all surface and underground belt conveyors. This tripper installation was commissioned in October 2002 and is at present approximately 2450 m long. The belt length will increase further in line with In detail Power transmission in the coupling occurs in accordance with the traditional hydrodynamic principle. The operating circuit (impeller/runner) is superimposed by an external cooling circuit. Medium flows from nozzles at the circumference of the coupling run into the pump channel, which is rotating at speed, from where it is taken off by a static head pump directed against the flow direction. As a result, the kinetic energy of the fluid is converted into flow and pressure. The hot fluid flows back through an external cooler directly into the operating circuit of the coupling (Figure 10). A supply system fills this circuit or drains fluid from it. Normally, this process is controlled via solenoid valves. With these valves, the filling level of the coupling can be steplessly adjusted from 0-100%. (From completely empty to completely full). The cooler is individually selected to comply with the existing drive configuration. If there are additional requirements for heat capacity, a separate fluid tank can be installed in the cooling circuit. The torque transmitted depends directly on the filling level of the coupling and the prevailing output speed. For the startup of a belt conveyor, the filling level has to be appropriately increased from 0-100%. In order to achieve this, an electronic control system activates the electromagnetic fill and drain valves. Operating modes Motor start-up During standstill, the coupling is virtually empty. It therefore transmits hardly any torque during the motor startup. Bearings will be lubricated by the filler pump. Breakaway of the belt conveyor By opening the fill valve, the coupling builds up more and more torque in line
with the increasing filling level, until the belt breaks away or a pre-set limit is reached. This limit can either be the maximum permissible starting torque or the current torque requirement stored during the previous switch-off of the belt conveyor, multiplied by a startup factor that can be individually chosen. Acceleration There are three individual, yet similar, procedures involved, which are described as follows: Load-independent torque limitation In this case, the transmitted torque is controlled at a constant limit by activating the fill and drain valve. The limit corresponds to the maximum belt tension to be introduced to the belt conveyor. As a result, different run-up times occur, depending on the load condition. The empty conveyor also starts up noticeably faster than the laden one. Load-adapted acceleration based on the current power requirements prior to the most recent switch-off This value (sum of all drives) is permanently updated and stored when the system is switched off. For re-starting, it is multiplied with a startup factor that can be individually chosen; it then serves as a limiting control value for the run-up. This control procedure functions exactly as described earlier, the only difference is that the limiting control value is newly set with every new startup operation. The permissible belt tension is defined as the upper margin for the variable limiting value. Figure 12. Typical belt conveyor startup. Figure 13. A typical startup profile of the belt conveyor. Constant run-up time, independent of load situation Nominal operation Once the nominal speed of the belt has been reached, the couplings are completely filled and have maximum transmission capacity. By a controlled reduction of the filling level, the coupling can be controlled down, transmitting less torque as a result of dosing the operating fluid. Switch-off Normally, the motors are (for safety reasons) always switched off to stop the conveyor and (if available) the brakes when they are applied. As a result, the drive is at a halt. The coupling runners drain most of their fill into the tank and, after a Figure 14. The appropriate temperature development measured in the coupling.
standstill period of 60 seconds, are available again for a follow-up start. Re-cooling at standstill After the switch-off, the oil system is cooled until its temperature falls below 60 C. For this purpose the filler pump is activated and supplies oil back into the tank via the cooler. The cooling output of the closed loop circuit of the TPKL coupling is highly effective. Heat (resulting from extended startup times or long-time operation at reduced speed) is generated directly in the transmitting medium (oil) and is immediately dissipated by the external cooler. Figures 13 and 14 illustrate how quickly the TPKL oil circuit cools down after (or even during) the startup of a laden 2 x 500 HP belt conveyor. The closed TPKL circuit is at its highest temperature at approximately 60% of the torque ramp of the conveyor startup and returns to its starting temperature shortly afterwards. This belt conveyor can easily cope with 20 load starts/hour. Figure 13 also shows a typical startup profile of the belt conveyor. The fact that the belt speed remains relatively constant should be taken into account, although the torque generated by the TPKL coupling (see motor 1 and 2 A) continuously increases after the breakaway. As a result of constantly rising belt tension, the belt is slowly prestressed (this belt conveyor was approximately 3660 m long and had a nominal speed of 3.5 m/s). During the pre-stressing stage in the startup phase (approximately 30 seconds at 15% speed) the belt is extended by 15-20 m, which is Figure 15. TPKL control philosophy for tripper booster drives. Figure 16. The torque response characteristics of the TPKL during a tripper simulation test carried out at Voith Turbo in Crailsheim. Figure 17. Active tripper booster control and load sharing. absorbed by the tensioning system. Voith has devised a calculation program that simulates startup procedures iteratively. The run-up was calculated with the known conveyor data (Figure 13 and 14). The results are shown in Figure 15. The calculation results coincide with the measured values with sufficient accuracy. The coupling behaviour can therefore be simulated in advance with a high amount of validity. Actively controlled drive systems for belt conveyors can reach their maximum potential only if their electronic control systems are optimised for this purpose. In this context, it is not only the startup procedure that needs to be taken into account, but also the distribution of load between the individual drives (depending on the load situation). Many actively controlled drive technologies naturally react almost promptly to any control signals. While this might initially look like an advantage, belt conveyors normally react negatively to abrupt torque changes. The TPKL turbo coupling, however, shows a rather smooth reaction to control signals, as a result of which longitudinal oscillations in the belt are avoided. Apart from the stepless adjustability of the torque, the measurements show a transient delay of the response (torque change = change of oil volume). Results Practical measurements of the startup procedure, with signals from the head drive and the belt tension control at the tripper, both in laden condition, were taken. These trends were recorded during the commissioning
of a tripper booster longwall panel belt conveyor (two head drives and two booster drives). Such an observation of trends is highly recommended for the commissioning of a system. Figure 17 shows the active tripper booster control and active load sharing. The trend also shows a phenomenon that frequently occurs with underground belt conveyor systems: if a new belt section is added (extension of the distance between axes), the new belt section is sometimes inserted upside down. Different thicknesses of top carrying cover and bottom cover result in a change of the effective pulley diameter at the drive drum, as soon as the upside-down belt section enters the drive. Figure 18 shows a startup at nominal load, followed by 20 minutes of operation, during which the belt is unloaded. The measurements show the motor currents of the tripper drives together with the output speeds at the head drive. With decreasing load in the tripper section, the output of the drives in this section is controlled down, while the head drives still need to provide full power. During this measurement, the control parameters were optimised, as performance fluctuations of up to two minutes were observed. Figure 18. Startup at nominal load, followed by 20 minutes of unloaded operation. Conclusion Over the last five years, TPKL couplings have proven their suitability for complex belt conveyor systems, as well as their extreme reliability. The TPKL couplings underwent a general overhaul after four years of operation. The disassembly revealed no recognisable wear. The mine therefore decided to extend the operating time of the TPKL until the first official overhaul after eight years. Commercially efficient mines expect high availability, high output and many years of trouble free service from their belt conveyor systems, requirements the TPKL aims to fulfil.