Transfer Chutes (www.AppliedDEM.com)

During the 1998 Society of Mining Engineers (SME) annual meeting in Orlando, actual plant operators organized a unique “Plant Operators’ Forum” in order to highlight practical everyday issues of concern to them. One open session was titled “More Than Five Things I Never Want to See in Plant Design.” To our concern, one thing mentioned quite often was belt conveyor transfers. As one outspoken operator asked, “is it so hard to transfer material from one conveyor to another?”

Most of us can understand this sentiment all too well as we have all encountered the problems surrounding belt transfers. We have dust, spillage, abrasion and wear, and we have great potential for damage leading to unplanned downtime. Bottom line is we have the most maintenance intensive area of the whole conveyor and in many cases, the whole plant or mine. The only way to keep these areas clean and functioning properly is to allocate seemingly constant attention. And as more and more operators are required to do more with fewer people, attention is in short supply.

The problem for engineers and designers is there has not been an analytical tool available to aid in transfer design. Transfer design has been more an art than a science. The problem is indeed a difficult one as bulk materials come in endless varieties and material properties can change in hours or minutes causing changes in flow characteristics. Further, no two transfers are alike. Because the problem is complicated, our transfers are often simple and little effort is made to ‘get them right’ in the beginning.

Fortunately, with today’s rapid advances in computer and software technology, new tools are becoming available which are changing the way we approach conveyor transfers. Read further in this issue for more discussion on these advances.

Discrete Element Methods

The discrete element method (DEM) has been shown to be an excellent computational tool for simulating the material flow in transfer stations. Three-dimensional visualizations of the simulation results provide an overall feel of the flow behavior in the chute.

The DEM explicitly models the dynamic motion and mechanical interactions of each particle in the physical problem throughout a simulation and provides a detailed description of the positions, velocities and forces acting on each body or particle at discrete points in time. The particles are modeled with a system of spherical-shaped bodies. The material bodies can interact with other material bodies, with transfer chute surfaces, and with the moving rubber belt. The contact/impact phenomena between the interacting bodies are modeled with a contact force law, which has components defined in the normal and shear directions. The normal contact force component is generated with a linear elastic restoring component and a viscous damping term to simulate the energy loss in a normal collision. The linear elastic component is modeled with a spring whose coefficient is based upon the normal stiffness of the contact bodies and the normal viscous damper coefficient is defined in terms of an equivalent coefficient of restitution.

In a typical DEM algorithm the following tasks are performed:
Construct the boundaries and locate the bodies.
Solution of the dynamic equations of motion for each body in the system.
An automatic collision detection module that continuously determines which pairs of bodies are in contact.
A contact force algorithm that uses Newton’s third law to compute the forces acting on pairs of contacting bodies.
Computer graphics modules for pre- and post-processing tasks such as DEM model generation and visualization of the DEM computed results.
Belt conveyor transfer modeling is used to optimizes material flow and eliminate plugs, eliminate spillage, minimizes abrasion and damage to the belt and chute, and minimize dust and material degradation.

With the increase in size and complexity of conveyor systems, the ability to accurately predict performance is becoming increasingly important. Perhaps the greatest benefit that can be derived from the use of this tool is the sense an experienced engineer can develop by visualizing performance. A better understanding of what helps and what hinders the performance of the transfer station can be gained.

For more information go to www.AppliedDEM.com