Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering
Transport Technology

E.G.J. Groen Optimisation of maintenance strategies for belt conveyors in Rheinbraun's lignite mines.
Masters thesis, Report 2003.TL.6800, Transport Engineering and Logistics.

RWE Rheinbraun AG operates three opencast mining plants Garzweiler, Hambach and Inden. These mines are located in the area between the cities Köln, Düsseldorf and Aachen.

The lignite and overburden are transported mainly in and between the mines with belt conveyor systems. The lignite is further transported from the mines to the power stations with trains (Garzweiler, Hambach) or belt conveyors (Inden). The annual amount of conveyed material is approximately 100 million tons lignite and 500 million tons overburden. There are about 250 km belt conveyors (axis-to-axis distance) operational. The transport performance is 10 billion ton kilometres per year.

Bucket wheel excavators mine the lignite and overburden. The capacity of the conveyors is adapted to the capacity of the excavators.

The conveyor belts represent a large amount of invested capital. Given a total length of approximately 500 km belts installed in the conveyors and an average lifetime of seven years, the annual costs for replacement are about 35 million euros.

The belts are used under severe circumstances. Rocks up to one cubic meter fall from heights of 7 till 12 meters on the belts. These impacts cause damages to the belts and shorten their life times. Furthermore belt damages cause downtimes of the conveying system by means of failures and maintenance activities.
These damages have to be repaired periodically to prevent further growth of the damages, failure and downtime of the production process. Downtime costs are estimated at 150,000 euro per working shift (8 hours).

The total costs of maintaining the conveyor belts include manpower, use of equipment, materials and production losses. These total costs are a multiple of the 35 million euros mentioned earlier for the annual costs for replacement. The costs for belts compared to the total costs of a conveyor are in the range from 40% till 70%.

Savings in the order of tens of millions of euros can possibly be saved annually by optimizing and controlling all maintenance.

Objective of the research project is to find an optimal maintenance policy with respect to economics of the mining plant (maximum profit).
Optimization needs compromising the following objectives:
Primary target of the assignment is to develop maintenance strategies, which are focused on minimization of the costs of the plant. Secondary target is to present additional information like belt lifetimes, average costs per belt, etc.

There are several maintenance policies known from literature. These can be classified into three groups: age-based policies, condition-based policies and reliability-based policies. Reliability-based policies are the most sophisticated maintenance policies. Rheinbraun currently uses condition-based policies, but not the same for the three mines. The differences are that the policy of Inden is focused on maximum reliability, that of Garzweiler on maximizing belt life and Hambach focuses on having the repairs carried out in the workshop. These differences are based on local preferences, and are not necessarily the most economical policies. Evaluation of the most economical maintenance policy for a complicated belt conveyor system requires a computer model such a discrete-event simulation model.

A discrete event simulation model for evaluation of various strategies for maintenance of belt conveyors has been successfully developed. The model has been coded in Borland Delphi, an object oriented programming language. The program has a graphical user interface and a Windows-look. The program uses the simulation control and event handling mechanisms of TOMAS, a simulation tool developed by and available from the Delft University of Technology.
For demonstration purposes and testing of the capabilities of the model a number of simulation runs (experiments) have been carried out. A base (reference) experiment and eleven variants are carried out to study the effects of:
Using the currently available best estimates for the input parameters the following results have been obtained: The experiments are carried out using estimated distribution functions for damage occurrence frequency, size, position and growth. The quality of the input and output can be improved by using distribution functions constructed with measured data from practice. It is recommended to collect this measured data in a centralized database system.

Besides the input data the model itself can be improved with the following recommendations: Further optimization can be achieved by developing a top-down optimization model for the entire mining plant taking aboard simulation models for separate equipment such as for the belt conveyors.

Reports on Transport Engineering and Logistics (in Dutch)
Modified: 2004.03.01; , TU Delft / 3mE / TT / LT.