CONTROL OF INDEPENDENTLY DRIVEN PRESSURE FEEDERS ON CRUSHING MILLS

Transcription

1 CONTROL OF NDEPENDENTLY DRVEN PRESSURE FEEDERS ON CRUSHNG MLLS R.M. Farrelll, J. Johnson1 & B.P. EdwardsZ 'N.S.W. Milling Co-op, Broadwater Ma, New South Wales, Australia zsugar Research nstitute, Mackay, Qld. Factory: Engineering ABSTRACT One of the features of Australian milling tandems is the use of pressure feeders. Every mill in Australia is fitted with some sort of pressure feeder. For a long time, the speed of the pressure feeder on a mill has been a fixed ratio of the speed of the top roll because the drive to the feeder has been by chain from the mill rolls or by gears on the main gear bed. Since 1986 many hydraulic drives have been installed on pressure feeders so that it is now possible to change the ratio of feeder speed to top roll speed almost instantaneously. This offers an extra control opportunity that had been unavailable previously. This paper looks at milling control systems that not only control milling load but also the height of bagasse in the feed chute. Some experimental results on the time constants associated with control are given. Results are reported for 'two strategies applied to a bagasse mill fitted with an independent hydraulic drive on the pressure feeder. Keywords: Sugarcane, pressure feeders, crushing mills, control, crushing load, hydraulic drives, Australia. ~ODUCTON Since ~ 1%0s co~ous pressure feeders have been a standard feature of Australian milling practice. Generally these pressure feeders have been driven either by gears or by chains from the top and feed rolls. For this reason the speed of the pressure feeder Qs been a fixed ratio of the top roll speed. Commonly the pressure feeder rotational speed is 1.3 times the top roll speed. n the last 9 years, 15 1Gdraulic drives have been installed on pressure feeders, allowing the speed of the pressure feeder drive to be used in controlling a mill. Of these drives 14 are low-speed hydraulic motors (Marathon motors from Hagglunds) and one drive is a high-speed motor with planetary gearbox (Rexroth). \ This is not the first time independently driven pressure feeders have been used. Sham (1962) reported the successful operation of a system on a mill at Fairymead Mill in the early 1960s. Because of the installation of dependent mechanical drives (using gears or chains from the main drive) on pressure feeders, that experience faded from view until the new generation of hydraulically driven pressure feeders began to be installed. Paddock & Farrell (1987) reported the first use of independently driven pressure feeders in Australia in recent times and showed how successful load control on first mills could be. n Australia the feed hopper for the first mill is kept full by varying the speed of the carriers feeding the mill. Thus the first mill does not need to be involved in controlling the height of material in the feed chute. For the following mills, the control is more complicated as the feed height in the hopper as well as the milling torque is controlled by the mill itself. This paper builds on the experience of both Sham (1962) and Paddock & Farrell (1987) to look at mill control systems that aim to control not only milling torque but also-in the case of bagasse mills-the feed height of bagasse in the feed hopper. Some experimental results on the time constants associated with control are given. Results for two control strategies applied to a bagasse mill fitted with a hydraulic drive are reported. OBJECTVES OF MLLNG UNT CONTROL There are two objectives for controlling a mill in a crushing train: To control the loads on the mill to ensure a high crushing load is maintained to obtain good extraction To control the crushing rate through the mill. "Conventional" bagasse mills (i.e., those with a dependent drive on the pressure feeder) typically meet these two requirements with a load control loop, altering the feed chute flap to control turbine chest pressure (an indication of milling torque) and a crushing rate control loop altering mill speed to control the feed height in the feed chute. Mills with independent pressure feeder drives have more options.

2 R.M. Farrell, J. Johnson & B.P. Edwards CONTROL OF RATE THROUGH A MLL Crushing rate through a mill is controlled by pressure feeder speed. t is tempting to think that it is the "mill" speed that is responsible for this function; but with an independent pressure feeder drive, no matter what the mill speed, if the pressure feeder is stopped, no cane will be processed. Here reference is made to the speed of the three crushing rolls as "top roll speed," as distinct from the pressure feeder speed. CONTROL OF MLLNG LOAD Paddock & Farrell (1987) controlled milling torque on a No. 1 mill by varying the pressure feeder speed while holding the top rollispeed constant. This is not the only option for control. The pressure feeder speed could have been held constant and the top roll speed varied to control the load. n fact a bagasse mill needs to perform a combination of both of these options to control load as the pressure feeder speed varies continually to control throughput. Time constants for milling load control Experiments were conducted on the No. 1 mill at nkerman Mill to determine the time constants associated with load control. The experiments also considered the situation when the crushing mill was slowed down relative to the pressure feeder speed. There may be a dangerous rise in forces in the pressure feeder chute. For this first mill, the height of feed in the feed hopper was controlled by the cane and prepared cane carriers. Throughout each experiment the pressure feeder speed was held constant. Then in the middle of a single rake of cane, the speed of the top roll was increased by 10 %, held at that speed for a period and then decreased back to the original speed. Figure 1 shows records of chute height and top roll speed from a typical test. Top roll speed was increased at about 200 sec and then decreased at 330 sec. Figure 2 shows the response of the top roll tailbar torque (measured with strain'gauges attached to the top roll tailbar). The torque decreases after the top roll speed increase at 200 sec and then increases arer the top roll speed decrease at 330 sec. Many tests of this type were completed and analyzed using process-reaction curves for the apparent dead and rise times of the system (Fig. 3). Typically, when top roll speed was increased (and tailbar torque decreased), the top roll torque showed an apparent dead time of 12 sec, followed by a rise time of 10 sec. When top roll speed was decreased (and tailbar torque increased), the dead time was typically 20 sec and rise time 20 sec. These results show that there is no benefit to be gained from quickly changing the relative speeds of the pressure feeder and crushing mill. This is fortunate because if such an approach were taken, the control of torque would interfere with the control of the height of bagasse in the feed hopper. Repercussions in the pressure feeder chute t seemed possible that when top roll speed is reduced (and the pressure feeder speed held steady), there would be an increase of pressure in the pressure feeder chute, which could damage it. This was investigated by taking careful records of the pressure feeder torque as it was not possible to measure the forces in the pressure feeder chute itself. Use was made of work by Shann (1967), who simultaneously measured pressure feeder chute wall pressures and pressure feeder torques. Sbann found that pressure feeder torque invariably reflected the pressures being experienced within the pressure feeder chute. Based on this observation and also the theory that the driving force for pressures in the pressure feeder chute is the pressure feeder torque, pressure feeder torques were recorded during top roll speed changes to see if anything untoward appeared to be happening in the pressure feeder chute. Figure 4 shows the result of what was expected to be the most dangerous move-a sudden decrease in milling speed of about 10% in <2 sec. The pressure feeder torque (measured with the hydraulic pressure for the drive motor) did not respond; thus there seem to be no impediment to control of this type. This result was repeated many times, and no torcpe response to the change in speed was noted. PUCTCAL MPLEMENTATON A hydraulic drive was installed to drive the pressure feeder on the No. 4 mill at Broadwater Mill during the 1994 season. Two control strategies were used to control the mill: Method involved controlling: / Chute height by adjusting pressure feeder speed Mill load (at this stage turbine nozzle bowl pressure) by altering top roll speed Pressure feeder load (as indicated by hydraulic pressure to the drives) by altering the feed chute opening

3 Factory: Engineering.. - Top Roll Speed Chute Height THE (seconds) Figure 1. Chute height and top roll speed during a test THE (seconds) Figure 2. Torque on top roll tailbar during test shown in F ire 1. nput Apparent Dead Time 7 Rise Time r Apparent Final Value - Response nitial Value Figure 3. Definition of apparent dead and rise times.

4 R.M. Farrell, J. Johnson & B.P. Edwards Although this system was successfbl at Harwood Mill during most of the 1993 season, some basic flaws soon became evident at Broadwater Mill. This was because the nozz e bowl pressure setpoint was close \ to the high pressure steam main pressure, which was not steady. This meant that the mill was prone to stalling; and when this occurred, the pressure feeder would just keep on feeding, making stalling even more likely. Various antistall control strategies were incorporated including taking account of speed in the mill from nozzle bowl pressure, Figure 4. Speed and pressure feeder torque change during step reduction in top roll speed. setting absolute limits on the pressure feeder to mill ratio, and limiting the pressure feeder to mill ratio as the nozzle bowl pressure increased. Although these modifications did improve operations, none was fully satisfactory as the mill simply did not bve enough reserve power to speed up and reduce its own load. This control strategy was used for 17 wk when it was abandoned. On a positive note, the system did control chute height very well. At about the same time as system A was abandoned, load sensing washers were placed under the king bolts on the plain side of this mill (being a small mill, it has only two bolts on each side), which has a fixed top roll. After a trial period, the load sensing system was deemed reliable enough to use as a load sensor, which gives a direct indication of top roll load without any coaiision caused by milling speed or settings. m. This load sensor system was then used with the secona strategy as shown in Figure 5. n Strategy B: Chute height was controlled by the top roll speed, with the pressure feeder speed slaved to the top roll speed by a ratio. Top roll load was controlled by altering the pressure feeder-to-mill ratio. This ratio was the combination of an ~peratbr~supplied ratio and adjustments by an automatic controller that monitored the top roll load. The pressure feeder torque was controlled by altering the setting at the base of the feed hopper to prevent overloading and stalling of the pressure feeder drive. This control would also prevent damage to the pressure feeder chute (although experience has shown that the chute on this mill appears to be indestructible). Given the Harwood Mill experience, this strategy was originally considered less desirable than strategy A; but because of the stalling problem, it had to be implemented. Strategy A was considered more desirable because the control Chute height Mill Speed - loop for chute height involved the pressure feeder roll speed only. Given that pressure Operator Ratio feeder speed should be responsive, control should be good. n practice, Strategy B has proven stable and works well although it has not Mill Load --= been tested exhaustively. Figure 6 shows some operating data obtained with this control system. This particular mill is a fixed mill, with a modest-sized feed hop- P.F. Oil Pressure PD Feed chute base setting - ger, and the load measurement is made by a load cell measuring the load on the top Figure 5. Control strategy B.

5 Factory: Engineering r( i' TOP loli speed d,,.,.--, *~?p-,,?-,/ ; - $. -,>.",>:,,,.,- -,?,,,--,.*-,.?; -,-,., -<;.,, ;, X - "' Y- i,: \# *<, - r -9; u Preasura Feeder to Mill Ratio E. 8 E a:*- ^e 3 - P !.- u 0 8 o -.A 1 s u 0 % zoo 3 s r( Tim (minutes) Figure 6. Operating data with Strategy B. 3 a r( roller. This is probably the most demanding control situation. A mill with the extra load control offered by hydraulic cylinders loading the top roll is significantly easier to control. Nevertheless, there can still be problems. f the top and feed rolls are not feeding properly, mill load will be low and the pressure feeder speed will increase. The torque on the pressure feeder drive will increase and so, presumably, will pressure feeder chute compactions and loads. The pressure feeder torque control then restricts the torque by closing in the chute, but the feeder can still respond by increasing speed again. f the feed roll is poor and the mill has a fixed-top roll, it will not reach load setpoint. t would be better to limit the ratio increase on high-pressure feeder torque (as measured by the hydraulic pressure) once no more gain can be achieved. The occurrence of this sort of problem should be flagged to the operator as it is an indication that the mill needs attention. CONCLUSONS Both control strategies would generally be suitable for mill control; howe"er, Strategy A was not appropriate for the particular problems with steam pressure and the small power reserves of the turbine driving the crushing section of the Broadwater milling unit. f these problems were not present or better feedback of the top roll speed to the pressure feeder were implemented, then Strategy A could be used at Broadwater as well. Strategy B delivered successful control and will continue to be used at Broadwater Mill. ACKNOWLEDGMENTS The authors would like to thank the management of the New South Wales Sugar Milling Cooperative for permission to present this paper. The assistance of the staff at nkerman Mill and Broadwater Mill in preparing the information presented in this paper is also acknowledged. Australian Sugar Mills funded the research component of this work. REFERENCES Paddock, D.R. & Farrell, R.M. (1987). Control of a fixed mill top roll load using a load cell and variable speed pressure feed roller drives. Proc. Aust. Soc. Sugar Cane Technol. 9: Shann, D.S. (1962). Some recent developments in mill feeding devices. Proc. Qld. Soc. Sugar Cane Technol. 29: Shann, D.S. (1967). Mill pressure chute monitoring. Proc. Qld. Soc. Sugar Cane Technol. 34: