Replacing the Conveyor Belt of a Long Distance Pipe Conveyor at the Skyline Mine

Design Consideration for the new Pipe Conveyor Belt

Pipe conveyors are primarily used where the key characteristics of this type of conveyor can be fully utilised:

• Due to the specific idler configuration and the tubular shape of a pipe belt, the pipe conveyor may follow the natural terrain more easily than a standard troughed conveyor. Pipe conveyors allow small curve radii and steep inclination angles.

• The suitability of a pipe conveyor for 3D-curve routings often helps to reduce the number of transfer points reducing earthwork. Also, the reduced number of mechanical components (drives) and transfer chutes minimised maintenance efforts and costs and the fewer number of transfer points increases the belt life.

• Conveyed material is fully enclosed by a pipe conveyor belt (Fig. 4). Therefore, the environment is protected from the bulk material and the conveyed material is protected from the elements like rain, snow or wind.

• Minimum spillage.

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Pipe conveyors are a specialty product for the “right” application and offer solutions conventional conveyors cannot provide. If a pipe conveyor is compared against a troughed conveyor for an “ordinary” setting certain disadvantages become evident:

• Higher design, manufacturing and installation costs.

• Higher energy consumption due to pipe forming forces, friction in the belt overlap and a larger number of idlers.

• Lower capacities because of smaller cross sectional load area in relation to the belt width.

• Significantly more effort and specific knowledge of the system are necessary for the commissioning of a pipe conveyor.

For pipe conveyors, the capacity, bulk density, maximum material lump size and the belt speed determine the pipe diameter and therefore the belt width, which determine application limits, in principal. Moreover, the safe operation of a pipe conveyor belt is determined by the required transversal stiffness of the belt, as the most important design parameter for the application. The transversal stiffness (i.e. the transversal flexibility) and the bending characteristic of the pipe belt depend on the application limits such as the required belt overlap, nominal belt strength and radii of horizontal and vertical curves in conjunction with local belt tensions. The belt’s very high transversal stiffness results in high forming forces and results in higher energy consumption due to increased running resistances. The running resistances result from intensive contact between the belt and idlers, friction in the belt overlap and the tendency of the belt to open between idler panels. If the transversal stiffness is too low, the belt buckles in curves or may collapse entirely.

Hence, an optimised pipe belt design requires sufficient transversal stiffness for maintaining the pipe form over the entire life time, as well as to provide the cross-sectional area that is necessary to ensure desired capacity. That is why it is very important that the cross-sectional area of a pipe conveyor belt, as provided for the conveyed material, is not reduced by forces acting on the belt, especially in vertical and horizontal curves. On the other hand, the stiffness of the pipe conveyor belt should be sufficiently optimised in a way that it does not induce unacceptable levels of resistance to motion as a result of increased forming forces applied to the idlers.

For pipe conveyor applications (Fig. 4), the idlers are usually fastened in a hexagonal form to the carrying panel. The configuration of idler stations is extremely important. A larger distance between idler stations leads to more bulging of the belt and the overlap opens and creates additional stresses at the following idler station as the belt edges are forced into the overlap and the pipe is closed again. Therefore, the forming force is increased.

As shown in Fig. 5 for pipe conveyor applications, it is very important to consider the influence of the conveyor routing on belt tracking. The overlap area in the perimeter of the pipe conveyor belt forms the belt section with the highest mass, as well as the highest concentration of tension member. When being moved through curves the belt therefore tries to assume a condition of lowest tension by turning the overlap towards the inner side of the curve.

When loaded, the material stabilises the pipe belt by keeping the overlap in the “12-o’clock-position”. The balance point is below the centre of the pipe and the belt is in a balanced position. When the overlap is in the 12-o’clock-position, however, the unloaded belt is unstable due to the weight distribution, with most of the belt weight positioned on top of the pipe (Fig. 6). This condition creates the tendency for the belt to rotate.

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