Transporting vast tonnages of coal overland requires the highest levels of expertise in sophisticated conveyor and control systems
When coal processing plants are an appreciable distance from the mine, how best to transport the coal from one to the other is a key consideration. It is not unusual for the processing plant to be 10 of miles away, which might at first glance suggest delivery by truck.
But overland conveyors can often provide a far more economical solution, providing cost reductions in the order of 20-40 per cent or more.
Of course with a conveyor that may run for miles, there are many design considerations to take into account; all of which will impact on long-term reliability, availability and maintenance requirements. Transporting vast tonnages of coal overland, particularly where there are inclines involved, requires the highest levels of expertise in sophisticated conveyor and control systems.
One of the most important design considerations is the braking technology to be employed.
Disc brakes meet the requirements of overland coal conveyors in several areas. As well as being the suitable technology for normal stopping and parking operations, they also provide vital emergency stop operation.
In normal operation, depending on the installation the disc brakes may only be used during the final stages of braking, when they are applied to lock the conveyor in its parked position. In other applications, they may be needed to provide general slowing and stopping duties. Most importantly, in the event of a power failure, the brakes will be required to stop and hold a fully loaded belt.
Twiflex specialises in 'Power Off' braking systems. To operate the brakes, powerful springs apply force to compress the brake pads against the disc. To release the brakes the springs are compressed by a retraction device, which in overland conveyor applications will typically be hydraulically actuated. Any failure in the supply to the retraction device would cause the springs to be released, so providing 'Power Off' braking.
Overland coal conveyors
Twiflex braking systems are used on overland coal conveyors across the globe, but some of the most demanding of these are installed in the USA. One example is a conveyor transporting 165 tons of coal per hour on a 5° incline.
Here, two spring applied, hydraulically released VKSD type disc brakes are used, acting on 1in thick discs with 42in diameter to provide 77,000 lbs ft of braking torque. The VKSD is part of Twiflex's modular series and is a standard two-module design, with the brake provided as standard with two pre-fitted brake pads. The brakes were supplied complete with an electro-hydraulic power pack with provision for variable deceleration control.
As an example of a longer conveyor, Bucim mine, Macedonia, has an installation that runs to 8845ft long, transporting 5181 tons per hour. Here two VKSD disc brakes act on 1.5in thick discs of 48in diameter, providing 44,500 lbs ft of braking torque. The brakes were supplied complete with an electro-hydraulic power pack with the provision for soft braking.
In a more demanding installation, VKSD disc brakes are used to provide 217,000 lbs ft of braking torque on an overland coal conveyor for Bowie Resources, Colorado, USA. The conveyor carries a payload of 1500 tons per hour along a 2750ft conveyor with a 13.25° decline. The two VKSD brakes act on 1.5in thick discs of 56in diameter.