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Putting the pedal to the metal
Handbook of Conveying and Handling of Particulate Solids A. Levy and H. Kalman (Editors) 9 2001 Elsevier Science B.V. All rights reserved.
235
Putting the pedal to the metal G. Lodewijks a, S. Kubob and A. Newman b aDelft University of Technology, Faculty of Design, Engineering and Production, Mekelweg 2, 2628 CD, Delft, The Netherlands bNesher Israel Cement Enterprises Ltd., P.O. Box 230, Ramla, Israel Upgrading an existing belt conveyor system may seem to be an easy process. However, when the upgrade includes a long overland belt conveyor with many vertical and horizontal curves and an increase of the belt speed of 30%, things change. This paper describes the upgrade of two belt conveyors located at the Ramla cement plant of Nesher Israel Cement Enterprises, Ltd. Thanks to Nesher's commitment to use state-of-the-art belt conveyor design methods and high-tech conveyor components, the upgrade turned out to be very successful. 1.
INTRODUCTION
The Ramla cement plant (see Fig. 1 for an aerial photo of the plant) has been in operation for 46 years. The original process at the Ramla cement plant to produce cement from limestone, which is the base material of cement, was a so-called wet line process. The original wet line had a capacity of 1,800 TPD (Tons Per Day). The first new production line producing cement through a so-called dry line process was commissioned in 1994. This line has a capacity of 5,000 TPD and is very successful. Building on its success, Nesher decided to build a second dry line. On the 10th of August 1997, Benjamin Netanyahu, Israel's former Prime Minister, laid the cornerstone at the Ramla plant for the second 5,000 TPD dry line. The new dry line, which is currently in the running-in stage, will join its 5,000 TPD sister dry line and the older 1,800 TPD wet line. The wet line will be phased out soon and thus the anticipated new total plant capacity will be 10,000 TPD. To save costs, Nesher decided to use the existing limestone handling and transport facilities to handle the increase in transport loads. This was made possible by relatively minor modifications to the existing infrastructure, in particular to the belt conveyor system. The raw materials needed to supply all "three" plants are now transported from a quarry 3.5 km away from the plant via the existing (upgraded) conveyor belt system. 2.
THE RAMLA CEMENT PLANT
The Ramla open-pit quarry (see Figure 2) is located 3.5 km from the Ramla plant and the deposits consist of Turonian and Senonian limestone. The limestone is extracted by conventional methods using large earth moving equipment and is transported by truck to a stationary crusher in the quarry. The limestone being excavated today has varying degrees of moisture ranging from 8% to 18%. A 700-meter long incline belt conveyor is used to lift the crushed rock 65 meters from the primary crusher to transfer station T2. A 2.3 kilometer long
236
Fig. 1. The Ramla cement plant. overland conveyor then transports the raw materials to transfer station T3. Following the addition of clay and after a second size reduction at the secondary crushing station, which is about 300 m further on, the material heading to the dry lines arrives at the first sampling station where its properties are analyzed. This station is also the split location between the dry lines 1 and 2. From here, the materials encounter similar equipment on their way to separate stacker/reclaimers, feed stations and mills. 2.1.
The primary crusher The primary crusher is located in the quarry and consists of a McLanahan 48"x72" Shale King Crusher rated at 1,000 TPH (Tons Per Hour). The driving flywheel has a diameter of 2.5 meters and is motor driven through six v-belts. The capacity of the primary crusher had to be increased to 1,250 TPH to produce enough material to serve the wet and both dry lines in the plant. To enable the crusher to operate at the higher capacity, the manufacturer recommended grooving the flywheel for two additional v-belts. To avoid the costs of disassembling, shipping and reassembling, Nesher performed the machining in-place. The operation was performed using portable tools and an auxiliary motor that turned the flywheel for machining the new grooves. 2.2.
The incline conveyor
An incline belt conveyor transports the raw material from the primary crusher at the bottom of the open pit to the surface. There the bulk material is transferred through transfer station T2 onto the overland belt conveyor. The incline conveyor, number CV 7-25-010, has a length of about 700 meters and an elevation lift of 65 meters. The belt used has a width of 1000 mm and a troughing angle is 35 degrees.
237
Fig. 2. The Ramla open pit quarry. Originally the total conveyor system supplied limestone to the wet line (1,800 TPD) and the (first) dry line (5,000 TPD). The minimum required belt conveyor system capacity was therefore 6,800 TPD. With an original capacity of 950 TPH the belt conveyors had to be in operation at full capacity for about 7.2 hours per day. With the second dry line (5,000 TPD) phased in and the wet line phased out the minimum required belt conveyor system capacity increases to 10,000 TPD. This is a capacity increase of 47%. With the same number of operational hours per day the capacity of the belt conveyors had to be increased by 47% as well to 1,400 TPH. To implement the change in conveyor capacity, Nesher enlisted the services of Huwood International, Ltd., Worchester, England (the original conveyor designers; today Continental Conveyors Ltd.) and Conveyor Dynamics, Inc. of Bellingham, WA, USA to perform a dynamic analysis of the system at the higher rated speed. Their analysis indicated the need for modifications in three areas: better speed control during conveyor starting/stopping, additional counterweight, and the installation of a capstan brake for emergency stops.
2.2.1. Belt speed The original belt speed of the incline conveyor was 3.31 m/s, which is around the DIN 22101 standard belt speed of 3.35 m/s. With a required capacity increase of 47%, the belt speed has to be increased by about 47% as well to maintain the original bulk material load and cross sectional area of the limestone body on the belt. Since all standard belt conveyor components are sized to accommodate the standard belt speeds it was sensible to follow the DIN standard to determine a new belt speed. The first step up in standard (DIN) belt speed from 3.35 m/s is 4.19 m/s, which is an increase of 25%. The next step up is 5.2 m/s, which is an increase of 55%. If the belt speed would be increased by 55% then the cross sectional area of the limestone body on the belt would decrease. In the original (950 TPH) design the edge distance was already more than required. It was therefore decided to increase the belt speed by about 25%. The increase in cross sectional area of the bulk material on the belt resulting from the 25% (instead of 47%) increase in belt speed was found acceptable. Using the
238 existing drive pulleys and a new standard gearbox the final belt speed turned out to be 4.26 m/s, which was sufficiently close to the DIN belt speed of 4.19 m/s.
2.2.2. Starting control The higher conveyor speed presented challenges in the starting/stopping of the motors, particularly since the conveyors have dual drives. The need to preserve torque capabilities at all loading conditions while maintaining equal sharing between the driving motors, led to the use of frequency-controlled drive systems (Variable Speed Drives or VSD's). The motors were provided with modem frequency converters supplied by Control Techniques, IGBT of Great Britain. The use of frequency converters enables motor startup from zero speed with full rated torque and current. This reduces mechanical stresses on the motor, gearbox and on the conveyor belt, as well as reducing transients in the power supply. "Soft-starting" of the system using a programmed S-ramp to limit the rate of change of acceleration/deceleration allows smooth speed regulation over the full operating range and improves load-sharing between the drives. An operational start is preformed by the VSD drives increasing the conveyor's velocity from rest to full speed along a 40 second S-curve velocity ramp. The incline conveyor will start after the overland conveyor has reached full speed to prevent overloading. If a start is aborted, it will be stopped with the emergency stopping control. The motors will immediately be turned off (or may already be offiine due the nature of the failure) and the capstan will be applied. As soon as the conveyor has come to rest the capstan is released. 2.2.3. Stopping control and capstan brake Originally there was only one way of stopping the belt. The motors were just switched off and the belt just drifted to rest, which was an uncontrolled stop. For the upgraded conveyor two types of stopping are distinguished: the operational stop and the emergency stop. Both stops are controlled. An operation stop will be preformed by linearly decreasing the conveyor's velocity from full speed to rest in 25 seconds using the VSD's. This control method brings the conveyor to rest in a very smooth manner regardless the conveyor loading. The goal of an emergency stop is to quickly bring the conveyor system to rest in a controlled manner without the use of the motors. An emergency stop will be initiated when there is an equipment failure, power failure, PLC fault, or when an operator initiates the stop. If the motors are not used to stop the belt then the belt will drift to rest. When the conveyor is empty the belt drifts to a stop in approximately 17.5 seconds. When the conveyor is fully loaded the drift time is reduced to 5.8 seconds. Due to the extreme short stopping time of a fully loaded belt and the profile of the belt, very low belt tensions occur during a drift stop. This will lead to unacceptable high belt sag and material spillage. There are a number of options available to keep the belt tensions at an acceptable level during an emergency stop. One option is to extent the stopping time by using flywheels on the drive pulleys or on the high-speed shaft of the gearboxes. The drawback of using flywheels is that for this conveyor very large flywheels would be required. This would entail serious changes to the drive unit. It would also result in extreme long stopping times for an empty conveyor, which then have to be reduced by using a brake.
239 Another option is to install a capstan brake in the take-up system. A capstan brake has to be installed in the take-up wire rope between the counterweight and the tensioning trolley. It is designed to operate transparently during normal operation and will only actuate during an emergency stop. It was decided to use a capstan brake to maintain acceptable tension levels in the belt during an emergency stop since this was a much simpler solution to implement than using flywheels. The capstan brake assembly, see Fig. 3, consists of a multiple grooved sheave which is keyed to a shaft. A caliper disc brake manufactured by Bubenzer Bremsen of Kirchen, Germany is mounted onto the shaft. The counterweight wire rope is wrapped around the sheave to create a wrapping angle of 360 ~ The multiple grooving is necessary to avoid friction between the wire rope loops. The brake is set to slip at a pre-determined load. Actuation of the brake during an emergency stop creates an external friction force on the shaft, and, thereby, on the sheave. The tension in the take-up cable normally is 17.5 kN. The Capstan was set so that the cable tension could be increased to 30.5 kN before the take-up trolley started to move. One drawback to using a capstan is that, at least during commissioning of the system, a load cell has to be installed in the take-up wire rope to check whether or not the setting of the capstan brake is correct. In order to determine this a dynamic analysis of the conveyor is required. Commissioning of both the incline and the overland conveyor showed that the Capstans worked perfectly. No low belt tensions, high belt sag or spillage occurred.
Fig. 3. The capstan brake.
240 2.2.4. Counterweight The counterweight of the incline conveyor had to be increased from 3,900 kg to 5,350 kg to maintain acceptable tension ratios at the drive pulleys to prevent belt slip during start-up of the conveyor at its peak load.
2.3.
The overland conveyor The incline conveyor transfers the bulk material onto the overland conveyor at transfer station T2. The overland conveyor, number CV 7-25-080, has an overall length of about 2.3 km and almost no change in elevation. The overland incorporates five concave, four convex and two horizontal curves, also see Fig. 4. The radius of the horizontal curves is quite small. The first horizontal curve has a radius of 750 m, the second of 900 m. The width of the belt used on the overland conveyor is 1000 mm and the troughing angle is 45 degrees. The Conveyor Dynamics analysis indicated for the overland conveyor the need for modifications in four areas: better speed control during conveyor starting/stopping; additional counterweight; installation of a capstan for emergency stops; and a higher banking angle in the horizontal curves. 2.3.1. Belt speed & speed control The overland conveyor originally had the same belt speed as the incline conveyor. To increase the capacity of the overland conveyor to the capacity of the incline conveyor, the belt speed was increased to 4,26 m/s, also see Section 2.2.1. The speed control of the overland conveyor is equal to the speed control of the incline conveyor. The start-up time however is 90 seconds.
Fig. 4. Overland belt conveyor CV 7-25-080.
241 2.3.2. Counterweight The counterweight of the overland conveyor had to be increased from 7,500 kg to 9,480 kg to maintain acceptable tension ratios at the drive pulleys to prevent belt slip during start-up of the conveyor at its peak load.
2.3.3. Capstan An inescapable characteristic of long belt conveyors is the notable occurrence of tension waves in the belt. These waves can in particular be noticed during an emergency stop by looking at the motion of the counterweight. Tension waves travel through the belt at a particular speed determined by the belt's properties, the bulk material on the belt, and some motion resistances. Steep front tension waves, further referred to as shock waves, develop in areas of a long conveyor with considerable differences in belt load or belt resistance (low in the return strand, higher in the carrying strand and very high in the horizontal curves). These shock waves cause very low tensions and high belt sag in the return strand. They also cause belt tensions exceeding the allowable tension levels. Passage of a shock wave can result in material being thrown off the belt. During an operational stop, the shock waves are safely limited by the controlled ramp-down of the motor; however, during an emergency stop or in the event of power loss, another mechanism must be employed to attenuate these dynamic effects. To counteract the effect of the shock waves and regulate the tension in the belt during an emergency stop, it was decided to install a Capstan brake in the take-up system. The Capstan brake was set so that, during an emergency stop, the take-up tension could increase by 10 kN to 25.5 kN before the take-up trolley started to move. Commissioning showed that the application of the Capstan resulted in an acceptable belt behavior during an emergency stop.
2.3.4. Banking angles The banking angles of the idlers in the horizontal curve had to be increased as a result of the increase in belt-line tension to keep the belt centered on the rolls during fully loaded operation. Commissioning showed that the application of the VSD's, for a soft start and operational stop, and the Capstan considerably reduced the tension variations in the belt and therefore kept the side displacement of the belt in the horizontal curves within acceptable limits.
2.4. The secondary crusher The secondary crusher, which is located within the plant, consists of a McLanahan 48"x72" Heavy Duty Double Roll Crusher rated at 1,500 TPH. Because of its design, the manufacturer believed that the crusher could handle an increase in capacity without any modifications. Therefore no modifications were made.
2.5.
The transfer stations
The transfer stations T2 and T3 have drop distances exceeding 10 meters. In the past, material blockages in the chutes of the transfer stations regularly caused downtime. Due to the increase of the total plant capacity (1,800 TPD from to 10,000 TPD) Nesher had to take relatively more Senonian limestone, which is the limestone located in the quarry in the top layers. Since Senonian limestone absorbs more moisture than Turonian limestone and the new
242 limestone mix contains relatively more Senonian limestone, it was expected that the number of blockages and thus downtime would increase. Nesher, therefore, enlisted the services of Jenike & Johanson, Inc. of Westford, MA, USA, who conducted flow properties tests on the expected limestone mix to determine wall friction angles and chute angles. Based on their test results they provided recommendations for modifying the existing belt-to-belt transfer chutes. Their recommendations were to install curved chutes constructed of 3CR12 stainless steel combined with curved impact plates at the chute inlet.
2.6.
Raw material analysis Prior to its entrance into storage for stockpiling, the raw materials are analyzed for proper pre-blending mix. A self-contained Bulk Material Analyzer from Gamma-Metrics of San Diego, CA, which contained a vertical and narrow analysis chute, performed the analysis. This unit was not only liable to blockages, it would also not be able to meet the higher throughput required. A cross-belt Bulk Material Analyzer from Gamma-Metrics therefore replaced the unit. The new unit is mounted directly on the conveyor belt and measures the elemental composition of the raw material in real time. It also provides on-line analysis results of the entire belt cross-section. 3.
CONCLUDING REMARKS
Ramla Dry Line 2 is currently at the equipment running-in stage and is operating at full capacity. The changes to the raw material transport system have been completed and are on line. To date, operational results indicate that the system is functioning properly, and will meet all expectation when fully operational.
235
Putting the pedal to the metal G. Lodewijks a, S. Kubob and A. Newman b aDelft University of Technology, Faculty of Design, Engineering and Production, Mekelweg 2, 2628 CD, Delft, The Netherlands bNesher Israel Cement Enterprises Ltd., P.O. Box 230, Ramla, Israel Upgrading an existing belt conveyor system may seem to be an easy process. However, when the upgrade includes a long overland belt conveyor with many vertical and horizontal curves and an increase of the belt speed of 30%, things change. This paper describes the upgrade of two belt conveyors located at the Ramla cement plant of Nesher Israel Cement Enterprises, Ltd. Thanks to Nesher's commitment to use state-of-the-art belt conveyor design methods and high-tech conveyor components, the upgrade turned out to be very successful. 1.
INTRODUCTION
The Ramla cement plant (see Fig. 1 for an aerial photo of the plant) has been in operation for 46 years. The original process at the Ramla cement plant to produce cement from limestone, which is the base material of cement, was a so-called wet line process. The original wet line had a capacity of 1,800 TPD (Tons Per Day). The first new production line producing cement through a so-called dry line process was commissioned in 1994. This line has a capacity of 5,000 TPD and is very successful. Building on its success, Nesher decided to build a second dry line. On the 10th of August 1997, Benjamin Netanyahu, Israel's former Prime Minister, laid the cornerstone at the Ramla plant for the second 5,000 TPD dry line. The new dry line, which is currently in the running-in stage, will join its 5,000 TPD sister dry line and the older 1,800 TPD wet line. The wet line will be phased out soon and thus the anticipated new total plant capacity will be 10,000 TPD. To save costs, Nesher decided to use the existing limestone handling and transport facilities to handle the increase in transport loads. This was made possible by relatively minor modifications to the existing infrastructure, in particular to the belt conveyor system. The raw materials needed to supply all "three" plants are now transported from a quarry 3.5 km away from the plant via the existing (upgraded) conveyor belt system. 2.
THE RAMLA CEMENT PLANT
The Ramla open-pit quarry (see Figure 2) is located 3.5 km from the Ramla plant and the deposits consist of Turonian and Senonian limestone. The limestone is extracted by conventional methods using large earth moving equipment and is transported by truck to a stationary crusher in the quarry. The limestone being excavated today has varying degrees of moisture ranging from 8% to 18%. A 700-meter long incline belt conveyor is used to lift the crushed rock 65 meters from the primary crusher to transfer station T2. A 2.3 kilometer long
236
Fig. 1. The Ramla cement plant. overland conveyor then transports the raw materials to transfer station T3. Following the addition of clay and after a second size reduction at the secondary crushing station, which is about 300 m further on, the material heading to the dry lines arrives at the first sampling station where its properties are analyzed. This station is also the split location between the dry lines 1 and 2. From here, the materials encounter similar equipment on their way to separate stacker/reclaimers, feed stations and mills. 2.1.
The primary crusher The primary crusher is located in the quarry and consists of a McLanahan 48"x72" Shale King Crusher rated at 1,000 TPH (Tons Per Hour). The driving flywheel has a diameter of 2.5 meters and is motor driven through six v-belts. The capacity of the primary crusher had to be increased to 1,250 TPH to produce enough material to serve the wet and both dry lines in the plant. To enable the crusher to operate at the higher capacity, the manufacturer recommended grooving the flywheel for two additional v-belts. To avoid the costs of disassembling, shipping and reassembling, Nesher performed the machining in-place. The operation was performed using portable tools and an auxiliary motor that turned the flywheel for machining the new grooves. 2.2.
The incline conveyor
An incline belt conveyor transports the raw material from the primary crusher at the bottom of the open pit to the surface. There the bulk material is transferred through transfer station T2 onto the overland belt conveyor. The incline conveyor, number CV 7-25-010, has a length of about 700 meters and an elevation lift of 65 meters. The belt used has a width of 1000 mm and a troughing angle is 35 degrees.
237
Fig. 2. The Ramla open pit quarry. Originally the total conveyor system supplied limestone to the wet line (1,800 TPD) and the (first) dry line (5,000 TPD). The minimum required belt conveyor system capacity was therefore 6,800 TPD. With an original capacity of 950 TPH the belt conveyors had to be in operation at full capacity for about 7.2 hours per day. With the second dry line (5,000 TPD) phased in and the wet line phased out the minimum required belt conveyor system capacity increases to 10,000 TPD. This is a capacity increase of 47%. With the same number of operational hours per day the capacity of the belt conveyors had to be increased by 47% as well to 1,400 TPH. To implement the change in conveyor capacity, Nesher enlisted the services of Huwood International, Ltd., Worchester, England (the original conveyor designers; today Continental Conveyors Ltd.) and Conveyor Dynamics, Inc. of Bellingham, WA, USA to perform a dynamic analysis of the system at the higher rated speed. Their analysis indicated the need for modifications in three areas: better speed control during conveyor starting/stopping, additional counterweight, and the installation of a capstan brake for emergency stops.
2.2.1. Belt speed The original belt speed of the incline conveyor was 3.31 m/s, which is around the DIN 22101 standard belt speed of 3.35 m/s. With a required capacity increase of 47%, the belt speed has to be increased by about 47% as well to maintain the original bulk material load and cross sectional area of the limestone body on the belt. Since all standard belt conveyor components are sized to accommodate the standard belt speeds it was sensible to follow the DIN standard to determine a new belt speed. The first step up in standard (DIN) belt speed from 3.35 m/s is 4.19 m/s, which is an increase of 25%. The next step up is 5.2 m/s, which is an increase of 55%. If the belt speed would be increased by 55% then the cross sectional area of the limestone body on the belt would decrease. In the original (950 TPH) design the edge distance was already more than required. It was therefore decided to increase the belt speed by about 25%. The increase in cross sectional area of the bulk material on the belt resulting from the 25% (instead of 47%) increase in belt speed was found acceptable. Using the
238 existing drive pulleys and a new standard gearbox the final belt speed turned out to be 4.26 m/s, which was sufficiently close to the DIN belt speed of 4.19 m/s.
2.2.2. Starting control The higher conveyor speed presented challenges in the starting/stopping of the motors, particularly since the conveyors have dual drives. The need to preserve torque capabilities at all loading conditions while maintaining equal sharing between the driving motors, led to the use of frequency-controlled drive systems (Variable Speed Drives or VSD's). The motors were provided with modem frequency converters supplied by Control Techniques, IGBT of Great Britain. The use of frequency converters enables motor startup from zero speed with full rated torque and current. This reduces mechanical stresses on the motor, gearbox and on the conveyor belt, as well as reducing transients in the power supply. "Soft-starting" of the system using a programmed S-ramp to limit the rate of change of acceleration/deceleration allows smooth speed regulation over the full operating range and improves load-sharing between the drives. An operational start is preformed by the VSD drives increasing the conveyor's velocity from rest to full speed along a 40 second S-curve velocity ramp. The incline conveyor will start after the overland conveyor has reached full speed to prevent overloading. If a start is aborted, it will be stopped with the emergency stopping control. The motors will immediately be turned off (or may already be offiine due the nature of the failure) and the capstan will be applied. As soon as the conveyor has come to rest the capstan is released. 2.2.3. Stopping control and capstan brake Originally there was only one way of stopping the belt. The motors were just switched off and the belt just drifted to rest, which was an uncontrolled stop. For the upgraded conveyor two types of stopping are distinguished: the operational stop and the emergency stop. Both stops are controlled. An operation stop will be preformed by linearly decreasing the conveyor's velocity from full speed to rest in 25 seconds using the VSD's. This control method brings the conveyor to rest in a very smooth manner regardless the conveyor loading. The goal of an emergency stop is to quickly bring the conveyor system to rest in a controlled manner without the use of the motors. An emergency stop will be initiated when there is an equipment failure, power failure, PLC fault, or when an operator initiates the stop. If the motors are not used to stop the belt then the belt will drift to rest. When the conveyor is empty the belt drifts to a stop in approximately 17.5 seconds. When the conveyor is fully loaded the drift time is reduced to 5.8 seconds. Due to the extreme short stopping time of a fully loaded belt and the profile of the belt, very low belt tensions occur during a drift stop. This will lead to unacceptable high belt sag and material spillage. There are a number of options available to keep the belt tensions at an acceptable level during an emergency stop. One option is to extent the stopping time by using flywheels on the drive pulleys or on the high-speed shaft of the gearboxes. The drawback of using flywheels is that for this conveyor very large flywheels would be required. This would entail serious changes to the drive unit. It would also result in extreme long stopping times for an empty conveyor, which then have to be reduced by using a brake.
239 Another option is to install a capstan brake in the take-up system. A capstan brake has to be installed in the take-up wire rope between the counterweight and the tensioning trolley. It is designed to operate transparently during normal operation and will only actuate during an emergency stop. It was decided to use a capstan brake to maintain acceptable tension levels in the belt during an emergency stop since this was a much simpler solution to implement than using flywheels. The capstan brake assembly, see Fig. 3, consists of a multiple grooved sheave which is keyed to a shaft. A caliper disc brake manufactured by Bubenzer Bremsen of Kirchen, Germany is mounted onto the shaft. The counterweight wire rope is wrapped around the sheave to create a wrapping angle of 360 ~ The multiple grooving is necessary to avoid friction between the wire rope loops. The brake is set to slip at a pre-determined load. Actuation of the brake during an emergency stop creates an external friction force on the shaft, and, thereby, on the sheave. The tension in the take-up cable normally is 17.5 kN. The Capstan was set so that the cable tension could be increased to 30.5 kN before the take-up trolley started to move. One drawback to using a capstan is that, at least during commissioning of the system, a load cell has to be installed in the take-up wire rope to check whether or not the setting of the capstan brake is correct. In order to determine this a dynamic analysis of the conveyor is required. Commissioning of both the incline and the overland conveyor showed that the Capstans worked perfectly. No low belt tensions, high belt sag or spillage occurred.
Fig. 3. The capstan brake.
240 2.2.4. Counterweight The counterweight of the incline conveyor had to be increased from 3,900 kg to 5,350 kg to maintain acceptable tension ratios at the drive pulleys to prevent belt slip during start-up of the conveyor at its peak load.
2.3.
The overland conveyor The incline conveyor transfers the bulk material onto the overland conveyor at transfer station T2. The overland conveyor, number CV 7-25-080, has an overall length of about 2.3 km and almost no change in elevation. The overland incorporates five concave, four convex and two horizontal curves, also see Fig. 4. The radius of the horizontal curves is quite small. The first horizontal curve has a radius of 750 m, the second of 900 m. The width of the belt used on the overland conveyor is 1000 mm and the troughing angle is 45 degrees. The Conveyor Dynamics analysis indicated for the overland conveyor the need for modifications in four areas: better speed control during conveyor starting/stopping; additional counterweight; installation of a capstan for emergency stops; and a higher banking angle in the horizontal curves. 2.3.1. Belt speed & speed control The overland conveyor originally had the same belt speed as the incline conveyor. To increase the capacity of the overland conveyor to the capacity of the incline conveyor, the belt speed was increased to 4,26 m/s, also see Section 2.2.1. The speed control of the overland conveyor is equal to the speed control of the incline conveyor. The start-up time however is 90 seconds.
Fig. 4. Overland belt conveyor CV 7-25-080.
241 2.3.2. Counterweight The counterweight of the overland conveyor had to be increased from 7,500 kg to 9,480 kg to maintain acceptable tension ratios at the drive pulleys to prevent belt slip during start-up of the conveyor at its peak load.
2.3.3. Capstan An inescapable characteristic of long belt conveyors is the notable occurrence of tension waves in the belt. These waves can in particular be noticed during an emergency stop by looking at the motion of the counterweight. Tension waves travel through the belt at a particular speed determined by the belt's properties, the bulk material on the belt, and some motion resistances. Steep front tension waves, further referred to as shock waves, develop in areas of a long conveyor with considerable differences in belt load or belt resistance (low in the return strand, higher in the carrying strand and very high in the horizontal curves). These shock waves cause very low tensions and high belt sag in the return strand. They also cause belt tensions exceeding the allowable tension levels. Passage of a shock wave can result in material being thrown off the belt. During an operational stop, the shock waves are safely limited by the controlled ramp-down of the motor; however, during an emergency stop or in the event of power loss, another mechanism must be employed to attenuate these dynamic effects. To counteract the effect of the shock waves and regulate the tension in the belt during an emergency stop, it was decided to install a Capstan brake in the take-up system. The Capstan brake was set so that, during an emergency stop, the take-up tension could increase by 10 kN to 25.5 kN before the take-up trolley started to move. Commissioning showed that the application of the Capstan resulted in an acceptable belt behavior during an emergency stop.
2.3.4. Banking angles The banking angles of the idlers in the horizontal curve had to be increased as a result of the increase in belt-line tension to keep the belt centered on the rolls during fully loaded operation. Commissioning showed that the application of the VSD's, for a soft start and operational stop, and the Capstan considerably reduced the tension variations in the belt and therefore kept the side displacement of the belt in the horizontal curves within acceptable limits.
2.4. The secondary crusher The secondary crusher, which is located within the plant, consists of a McLanahan 48"x72" Heavy Duty Double Roll Crusher rated at 1,500 TPH. Because of its design, the manufacturer believed that the crusher could handle an increase in capacity without any modifications. Therefore no modifications were made.
2.5.
The transfer stations
The transfer stations T2 and T3 have drop distances exceeding 10 meters. In the past, material blockages in the chutes of the transfer stations regularly caused downtime. Due to the increase of the total plant capacity (1,800 TPD from to 10,000 TPD) Nesher had to take relatively more Senonian limestone, which is the limestone located in the quarry in the top layers. Since Senonian limestone absorbs more moisture than Turonian limestone and the new
242 limestone mix contains relatively more Senonian limestone, it was expected that the number of blockages and thus downtime would increase. Nesher, therefore, enlisted the services of Jenike & Johanson, Inc. of Westford, MA, USA, who conducted flow properties tests on the expected limestone mix to determine wall friction angles and chute angles. Based on their test results they provided recommendations for modifying the existing belt-to-belt transfer chutes. Their recommendations were to install curved chutes constructed of 3CR12 stainless steel combined with curved impact plates at the chute inlet.
2.6.
Raw material analysis Prior to its entrance into storage for stockpiling, the raw materials are analyzed for proper pre-blending mix. A self-contained Bulk Material Analyzer from Gamma-Metrics of San Diego, CA, which contained a vertical and narrow analysis chute, performed the analysis. This unit was not only liable to blockages, it would also not be able to meet the higher throughput required. A cross-belt Bulk Material Analyzer from Gamma-Metrics therefore replaced the unit. The new unit is mounted directly on the conveyor belt and measures the elemental composition of the raw material in real time. It also provides on-line analysis results of the entire belt cross-section. 3.
CONCLUDING REMARKS
Ramla Dry Line 2 is currently at the equipment running-in stage and is operating at full capacity. The changes to the raw material transport system have been completed and are on line. To date, operational results indicate that the system is functioning properly, and will meet all expectation when fully operational.