An in-depth paper just released by Conveyor Dynamics Inc, based in Bellingham, Washington state, USA is a fascinating review the world’s longest single-flight overland belt trough conveyor, which has now been in service for a decade and was originally commissioned CDI and ELB Engineering Services in South Africa in October 2015.
The 26.8 km long conveyor is capable of delivering up to 2,400 t/h of coal from the Impumelelo mine to the SASOL coal-to-synfuels conversion plant near Secunda in South Africa.
The principal contractor for this project was ELB Group, based in Johannesburg. ELB provides Engineering Services, Construction Services and Equipment supply in the South African market, and throughout sub-Saharan Africa.
CDI specialises in the design of long distance overland conveyors (OLC). It designs trough and pipe conveyors, transfer chutes, and control systems. It also provides forensic engineering services, and develops software for the bulk material handling operations.
CDI was engaged by ELB to do the basic mechanical engineering design and a large part of the detail design for the Impumelelo overland conveyor. Additionally, it also provided the mechanical design for the principal underground incline conveyor that brings the coal from the underground storage bunker to the mine head storage bunker above ground.
CDI developed the basic mechanical and control system design with its proprietary in-house software BeltStat and BeltFlex. The conveyor has an overall horizontal length of 26,816 m, with a total change in elevation between the tail pulley and the head pulley of -50 m.
Except near the tail and head end transfers, the conveyor is at or near ground level for its entire length. The OLC follows the general contour of the local terrain and includes 54 vertical curves and four separate horizontal curves.
The original route included a transfer tower shown between station 18000 and 19000 on the map. This would have resulted in a two flight conveyor system, with lengths of about 18.5 km and 9.5 km respectively. CDI designed in-line booster drive and tripper chute station that eliminated the transfer tower. This reduced the cost of the civil and structural works, reduced the visual impact of the conveyor, and increased the conveyor reliability by removing a chute that could have plugged if the 18.5 km conveyor took longer to stop than the 9.6 km conveyor.
At peak tonnage the conveyor transports 2,400 t/h of coal at a maximum speed of 6.5 m/s. However, operators can reduce the conveyor speed to save power when they transport less tonnage. The motors are able to start the fully loaded belt at the ambient temperature recorded for this location: -5°C.
To reduce the operating expense required, CDI specified a special low-rolling resistance (LRR) bottom rubber compound and special low rolling resistance precision idlers. It also designed stringers with long span lengths between idler sets on the carry and return side. The large idler spacing, reduced CAPEX, noise, and maintenance time. Idlers from several idler manufacturers were tested to prove they met CDI’s low drag and SASOL’s low noise specification. To reduce windborne dust losses and emissions, the entire length of the cover is covered, and the idler tables are specifically designed to shield the carry side of the conveyor belt from the wind.
The super low rolling resistance (SLRR) belt was imported from Aneng, China, and produced by Goodyear Veyance (now Contitech). The belt manufacturer originally offered 500 m reels of belt for this project. CDI insisted they manufacture and transport 1,000 m reel and after review Veyance agreed. This increased shipping costs but provided substantial splicing cost and time savings. The entire belt was pulled and spliced at an average rate of 1 km/day, the installation was performed with the assistance of Conveyor Belt Technologies of Vancouver, British Columbia, Canada and could have been completed in 60 days if the crews had pulled the belt continuously.
There are a total of 4 x 1,000 kW and 2 x 450kW VFD controlled motors at three separate locations: the head, the booster station and the tail. Drives 1-4 are 1,000 kW and drives 5 and 6 are 450 kW. Belt speed and belt slip conditions are monitored using special analogue tachometers designed and manufactured by CDI.
Belt tension for motor control purposes is measured at selected locations using a specially designed array of idlers and strain gauge load cells.
The OLC accelerates to full speed in 670 seconds, including a 70 second dwell period at 5% of full speed. The dwell stretches the belt prior to applying full torque for the main acceleration ramp. Using a combination of the drives and the brakes, the belt takes 100 seconds to come to a fully controlled operational stop.
Belt stretch is removed by a 25 ton gravity take-up located at the head end of the conveyor. This take-up incorporates a capstan brake in the take-up reeving to reduce the severity of take up motion during emergency stops. Since the tape length of the belt is 54 km, a second take-up or ‘belt storage loop’ was added to the Booster Drive Station to take up the slack created by permanent elongation out of the belt as it stretches over time. This take-up is static, and is only adjusted when the dynamic take-up at the head end is observed approaching its travel limits.
The idlers were provided by Lorbrand of Pretoria, South Africa, and were tested by TUNRA in Australia to insure that they met CDI’s drag and SASOL’s noise reduction requirements. To reduce drag, CDI specified lower viscosity grease than is typically supplied by the idler bearing manufacturers. The idlers consist of a steel inner shell with a polypropylene outer covers. The plastic outer covers reduce the idler noise and make it easier to achieve a low TIR, making the idlers roll easier and with less vibration. The idler spacing was chosen to minimise the idler count while maintaining belt sag at or below 1% during normal steady state running conditions.
Due the complexity of the final pulley designs, CDI reviewed the production operations and quality control procedures of several pulley manufacturers located in Johannesburg to ensure the quality of the fabrication met the design intent. CDI says all of the pulley manufacturers surveyed in the process had excellent design, fabrication and quality control, and any of them could have produced these pulleys. ELB selected Lorbrand as the pulley manufacturer, based on CDI’s recommendations and other factors.
CDI also provided conceptual and detail design and DEM analysis of the tail feeding chute and the booster station tripper chute. DEM simulations were performed with Granular Dynamics International’s ROCKY DEM software.
This chute uses a curved hood deflector to consolidate and direct the material flow towards the centre of the receiving belt. A curved receiving spoon takes the impact of the falling material, and redirects the flow stream to be tangential to the belt motion. The drop height between the head pulley on the discharge conveyor and the belt determines the material speed as it exits the receiving spoon.
CDI summarised the project as follows: “The capital and operating expenses associated with long overland conveyors are substantial. However, significant cost savings are achievable when the designer understands that many of the rules of thumb which apply to short conveyors do not apply to long belts. Long belts differ from short conveyors in several ways including: high tensions that lead to less belt sag, longer time to complete a belt rotation leading to longer belt life, and large operating costs that justify
low rolling resistance belting and idlers.”
It adds: “The large investment in construction materials and time on long conveyors justifies devoting significant engineering hours to optimising the conveyor assemblies. Even small differences in ground module weight and erection time lead to large savings when 27 km of ground modules are needed. At Impumelelo, CDI utilised the most advanced technology available in optimising the system. We reduced the number of transfer towers, reduced the structural steel weight, reduced the number of idlers and pulleys, reduced the installation time, cut the operating costs, and improved the availability of the system. We also reduced the noise, reduced the visual impact of the system, reduced dust emissions.”
CDI concludes: “This paper highlights the complexity of optimising an overland conveyor to maximise the reliability and availability of a conveyor while reducing its cost. It is a flagship for state of the art
conveyor engineering that clearly demonstrates the advantages of using modern engineering tools and a strong understanding of the science of conveying to design overland conveyors.”