Belt conveyors: how to minimize vertical fall distance of material?

Minimizing the vertical fall distance of the material being transferred in turn minimizes the impact of the material on the belt and reduces air entrainment into the material. Many of the methods for minimizing drop height are also effective for controlling air.

To ensure both material containment and dust control, operators should design transfer systems that minimize drop heights, absorb rebound from the material dropping onto the receiving belt, contain the material on the belt, and contain and slow the air entering and leaving the containment.

Higher vertical drops and heavier lump sizes also create more impact on the receiving belt. This impact causes belt damage and increases the material rebound and likelihood of material spillage.

 An impact cradle system (impact bed) installed under the receiving belt will absorb material impact, softening the rebound, and provide protection to the receiving belt. These impact systems consist of an impact-absorbing material covered by a friction-reducing top cover and serve to flatten the belt’s surface at the edge.

This flattened surface provides a location where system sealing is performed in order to keep the material on the belt. In areas of low impact or no impact, light-duty, side support cradles (slider beds) can be used.

For effective design of impact cradles, refer to Conveyor Equipment Manufacturers Association (CEMA) Standard 575–2013 [CEMA Standard 575–2013]. CEMA is a trade association composed of leading manufacturers of conveyors and conveying systems that design, produce, and install all types of conveying machinery.

Impact energy should be calculated from formulas in Figure 6.5, determining both impact energies and selecting the larger of the two values. Impact energies are determined using the following procedure:

• Find the weight of the largest lump carried (W) and multiply it by the vertical fall distance (h) to calculate impact energy (IE).
• Compute the equivalent weight (We) by dividing the square of the bulk material flow rate (Q²) by the spring rate of the equipment considered (k).
• Multiply We by h to determine the alternate impact energy.
• Apply the larger of the two values to table below to determine the duty rating required for the impact cradle.

Table 1:  Impact cradle ratings to be used based on impact loadingRock boxes and

other loading devices are utilized to soften the load and reduce impact cradle code requirements. Rock boxes allow ore to fall onto ore rather than directly onto the belt. This approach shortens the drop and alters the speed and the direction of the material being transferred in order to minimize wear on the conveyor belt and the inner surfaces of the chute

Reference

NIOSH Mining Program Report of Investigations, «Dust Control Handbook for Industrial Minerals Mining and Processing», second edition.