R. Fauerbach, M. Otrebski and J. Kunicki, South Africa

Courtesy : Trans Tech Publications - Bulk Solids Handling Journal

1. Introduction

1.1 The Project

On 1 July 1993 the coal handling system for the Sasol Wonderwater Mine was successfully commissioned.

The plant, located between Vanderbijl-park and Sasolburg, serves to haul the full production load of the new Sasol Wonderwater strip mine to the Sigma underground mine, from which the Sasol One plant is served with its entire coal requirements, and has a design capacity of 3 million t/a of ROM coal.

The plant consists of:

• a truck unloading station with 200 m hopper

• an apron feeder

• a double roll crusher

• a conveyor

• a 3.2 km long curved conveyor

• an underground shaft bunker with a capacity of 500 m

• an underground hopper with feeder conveyor

• auxiliary equipment such as: sampler, coal analyzer, overband magnet, belt scales, dust suppression, etc.

The plant furthermore complies with the high demand for improved pollution control against dust, noise and dirt, the conveyor being close to residential areas and also crossing a river (Fig. 1).

Fig. 1: The plant at the Sasol Wonderwater Mine, South Africa

The order for the plant was placed with Krupp South Africa in September 1992 and consisted of the turnkey contract, including the civil work. In only 9 months the whole system was designed, fabricated, erected and commissioned. Almost 100% of the plant was designed and built in South Africa, with the exception of the basic design which was obtained from Krupp Germany, and the double roll crusher which was purchased from MMD England.

1.2 The Task

Not only is the task of the system to receive, crush and haul the coal, but also to function as a flexible buffer between the discontinuous truck loading from the mine and the interrupted transport underground. The choice of this route of transport from Wonderwater Mine to Sasol One was made by weighing up far lower investment costs versus additional overland transport. This advantage however, was further influenced by the fact that production is dictated by spare haulage capacity of the underground conveyor system, i.e., its availability, capacity, outage, etc. This buffer capability was achieved by a sophisticated control philosophy.

2. Plant Description

2.1 Truck Unloading Station

The ROM coal from the nearby Wonderwater open pit mine is hauled by trucks to the loading station and dumped into a 250 m concrete bin lined with steel liners. The coal is drawn out of the bin by a speed controlled apron feeder which drops it directly into a double roll crusher (Fig. 2).

2.2 The Crusher

A double roll crusher, in perpendicular orientation to the mass flow direction, sizes the coal for belt conveying. A scroll operation is possible to remove foreign and oversized material. The side wall can be opened for this purpose, by means of a pneumatic cylinder. From there it is taken by the so-called sacrificial conveyor out of the bunker area above ground to the tail end of the overland conveyor.

2.3 Sacrificial Conveyor

The conveyor is a simple head end driven conveyor with gravity take-up, which includes:

• two belt scales for weighing and totalizing the product captivity

• an overband magnet for removal of tramp iron parts

• a coal analyzer to analyze the ash content

• coal samples for taking consecutive samples of the product.

2.4 Overland Conveyor

The 3.2 km long overland conveyor then curves under the provincial road to haul the coal over the Leeuwspruit River and to the underground bunker. The conveyor has a single drive on the head pulley and a take-up trolley which tensions the belt by means of a gravity take-up. The head pulley drive (only 340 kW are required because of the downhill shape of the conveyor) is put directly on the head pulley, without a brake. The modules, fitted onto the concrete sleepers to accommodate the force of wind onto the shed are of a simple bolted, rolled section design. This allows for easy adjustment of the idler brackets in order to train the belt by inclined brackets as well as by deflected idlers.

2.5 Underground Bunker

The underground bunker, a 40m deep, 4m wide former air shaft, containing approx. 500 m of coal, was lined and water sealed with concrete. This bin leads into a hopper with a spile bar gate under which a belt feeder draws the coal and feeds the underground conveyor system.

3. Design

Facing a very competitive market called for increased investigative/design work during the early stages of tendering and it was obvious that the client would be presented with a wide ranging choice of offers giving both technical and financial benefits. Thus the level of expertise provided at the earliest possible stage of tendering could very well have been the most decisive factor in the award of contract.

The work concentrated on the following aspects:

1. The establishment of possible levels of conveyor resistance and consequently power demand and tension levels;
2. The optimisation of the drive system, bearing in mind the results of the work done in a) above;
3. The selection of the most suitable tensioning device;
4. The influence of all of the above on the curved section of the conveyor.

Performance has lead to the acceptance of f = 0.018 as being the highest expected level of conveyor resistance, while it has been recognised that the value of 'f' will be lower with the levels depending on various factors which are difficult to control in the long run.

Serious consideration was given for a long time to the idea of a multiple drive system. Together with a motorised winch tensioning device such a solution looked very promising. At a later stage however, this concept was revised due to two important factors: Firstly, the process control was modified to a variable speed type of installation. A frequency converter was selected to achieve this goal and tipped the balance towards a single drive type of arrangement. Secondly, a gravity take-up system was preferred.

The use of the frequency converter lead to the development of a more complex control strategy for start-up, stopping and speed change. This in turn allowed for the application of a decreased safety factor for the belt (6:1 as opposed to the more commonly used 6.7:1) and consequently lead to the preservation of a good balance between technical and economical demands.

Dynamic simulation performed later supported all aspects of the original design.

3.1 Dynamic Simulation of the Overland Conveyor

At the request of the client, and in order to verify proposed concepts, a series of dynamic simulations of the overland conveyor were performed. The simulation programme used was an upgraded version of the software utilised in the Syferfontein Project [1].

Apart from modelling development, the programme was expanded with the addition of a sub-programme allowing for the dynamic simulation of belt movement within the horizontal curve. The main aim of this process was to evaluate conveyor behaviour during the transient stages of operation (see Fig. 3). Considerable concern still existed in respect of the stabilfty of the belt at the end of a downhill section of the conveyor. Simulation furthermore allowed for verification of the performance of the proposed control strategies.

At this stage it was possible to demonstrate the very stable behaviour of the belt, even in cases of total power failure during start-up. This fact somehow influenced the decision not to use fly-wheels to control the stop. Instead, it was decided to make full use of the existing electric/control system which would be capable of taking care of most operational cases.

3.2 Simulation of Belt Behaviour Within the Horizontal Curve

It was mentioned earlier that the simulation of belt behaviour within the nominated curve formed part of the dynamic simulation of the conveyor. The combining of these two allowed for the observation of sideways movement of the belt under the influence of the dynamics of the system, the changes of conveyor resistance, the flow of material, etc.

Idler frames within the curve were banked in accordance with the calculated values. At the same time, the additional forward tilt of wing rolls was allowed in order to aid belt training.

The use of a gravity take-up system proved to be beneficial for the behaviour of the return strand, thereby making the belt movements more consistent throughout the various stages of operation and under the load conditions. As a result it was easier to deal with even if the maximum movements were quite significant.

The upper strand, on the other hand, required that a compromise be found between extreme movements due to various load and tension cases. In this respect the application of fully controlled start-up and stopping was generally of assistance but, as mentioned before, the prospects of total control and/or power failure had to be considered (see Figs. 4, 5 and 6).

It must be pointed out at this stage that such an old and well-known device as a fly-wheel stands up very well next to the products of modern technology and is most useful when complex techniques fail, as does happen from time to time.

4. Process Control

4.1 System Capacity Control

As mentioned above, the plant production is dictated by the spare haulage capacity of the underground Sigma mine. This task is achieved via the following procedure (Fig. 7):

• The underground conveyor No 06 which hauls Sigma mine coal, has a weighbridge which specifies the spare capacity up to 2,000 t/h. The scale is located the same distance from the feeding point (a), as the distance between the belt feeder and this feeding point (b). The scale signals can therefore be used to control the speed of the belt feeder in order to achieve a constant load of 2,000 t/h at C6, no matter what the load on this belt.

• The underground bin will maintain as much coal as possible in order to reduce the drop depth because of coal degradation. Thus a variation of the bin level of only 7 m can be used as a buffer.

• An ultrasonic bin level detector differs between three levels, i.e., the low and high levels, and the high high level. These signals are used as:

• low level: to run the apron feeder at full capacity, thereby accelerating the overland conveyor to full speed and full capacity;

• low low level: to stop the belt feeder;

• high level: to decelerate the apron feeder down to 10% of its capacity, thereby decelerating the overland conveyor down to 10% of its maximum speed;

• high high level: to stop the apron feeder, the sacrificial conveyor and the overland conveyor consecutively.

4.2 Feeding Capacity Control

In order to provide the feed to the crusher, and thus to the system, as constant as possible, the apron feeder is equipped with a frequency controlled drive. The speed control receives its signal from an ultrasonic sensor which measures the bed depth of the apron feeder. The feed control is furthermore superseded by the capacity signal (conveyor speed) of the overland conveyor, i.e., at the lowering of the overland conveyor speed the apron feeder speed is also lowered accordingly. However, this control proved to be insufficient to provide a constant feed to the overland conveyor since the coal avalanches into the crusher mouth, partly due to the polygon effect of the drive. This caused a load variation of 300 t/h within say, 12 seconds, meaning: a variation of two thirds of the overland conveyor capacity. This would have meant that the conveyor should be restricted by an average capacity of only 600 t (see Fig. 8).

5. Concepts of Control

It was decided from the beginning that an installation of this size and complexity requires a specific control system. All aspects of the conveyor's operation such as start-up, stopping and change of speed have to be precisely controlled for is correct performance. Change of speed facility, not very often found in installations of this type and required by the process control constraints (see Section 4), has excluded the usage of a fluid coupling and left 'frequency converter' as the best possible option. Thus all mentioned operations are implemented by the careful PLC control of the frequency converter applied.

The following paragraphs present the two strategies considered, show their respective bases, indicate the methods used for their parameter evaluation and finally state the reasons for selection of one of them, as well as for all the following simplifications deemed necessary for the economic efficiency of the whole project.

5.1 Proposed Concepts of Control

From the beginning of the project, two Control Concepts were considered. One was based on mechanical torque supplied to the system, and the other on belt velocity as a controlled quantity. Theoretically, both of these concepts are equivalent to, and thus exchangeable for, one another, but in practice there are some differences especially in the way such control concepts can be implemented.

5.1.1 Torque Control

Initially Torque Control was selected for implementation. This concept follows precisely an idea developed by the University of Hannover and Dr. FUNKE for previous installations of similar size constructed by Krupp SA in South Africa. It stipulates that the torque must be delivered to such a system in a linearly increasing fashion up to the maximum demand for specific speed and load condition. The torque should then be kept constant for some time while the conveyor attains its designated speed. After that the torque is decreased to the value specific for steady state operation. Fig. 9 shows the resulting torque curve in principle.

The details of such strategy are found as a result of the exhaustive dynamic simulation of the conveyor when all of these parameters are carefully optimised. Similar procedures were clearly applied to find the optimum parameters for all possible 'operations' of the conveyor, such as: start-up (to full and reduced speed, as well as with full or reduced load), stopping (with the same variations as for start-up) and change of speed. Previous experiences made by Krupp lead to such careful design process and detailed dynamic simulation of the conveyor, thereby proving its financial efficiency over other possible approaches which attempt to achieve similar end results via elaborate experimentation on site.

5.1.2 Belt Velocity Control

Belt Velocity Control was considered as an alternative option for the Torque Control as it was understood that the latter would be easier to implement, more precise in operation and clearly following the underlying physical and engineering principles. This strategy uses a set of well known formulas (Eqs. (1) and (2)), based on [2] which result in a family of belt speed curves, each for different 'operation' of the conveyor.

V(t) = V(2t/T)           (1)

0 < t < T/2        

V(t) = V(-1 + 4t/T - 2t/T)            (2)

T/2 < t < T        

Parameters for these curves/formulae can be derived from the results of simulations performed for the Torque Control strategy. In this way both concepts are more or less equivalent (Fig. 10).

5.2 Hardware

Beyond the mechanical components of the conveyor, the following elements of the system played an essential role in the selection of a particular control strategy and its further simplifications. These were:

Electric Motor
Frequency Converter
Programmable Logic Controller
and Field Instrumentation related to Control of the Conveyor.

5.2.1 Electric Motor Data

Squirrel Cage type Induction AC Motor Rated Output Power = 315 kW.

5.2.2 Frequency Converter

Siliconics
Rated Output Power = 400 kW
Frequency Synthesis Method = PWM
Torque Control = Fixed (with start-up correction)
Frequency Control = Current Loop 0 - 20 mA.

5.2.3 Programmable Logic Controller

Allan Bradley
ID Control = available (not used).

5.2.4 Field Instrumentation (related to control system)

Belt Velocity Sensor (not used in the final installation).

5.3 Solution Selected for the Control System

Due to careful considerations regarding efficiency, the Belt Velocity control concept was used and implemented in the installation. The basic schematic of such type of motor control is as in Fig. 11.

The control system presented in Fig 11 requires that a rotor speed sensor is applied to create an external belt speed feedback control loop and to ensure that the motor operates at a prescribed speed. The importance of such approach lies in the fact that the conveyor must accelerate along the precisely specified velocity curve and such control arrangement would ensure that this happens.

It was decided, however, that every change of speed (obviously including start-up, stopping, etc.!) is instituted via control of the converter output signal frequency. Such approach is, for all practical reasons, equivalent to an internal forward speed control and should force long acceleration periods due to lack of control over the motor slip. Long-lasting and excessive motor slip may in turn cause overheating, and thus require either extra cooling or replacement by a higher rated one.

Furthermore, due to specific inertias of the whole installation, it became possible to use a single average velocity curve for all various conveyor operations. This provided great simplification in the control schedule chart and also in the PLC memory requirements.

This 'average' belt velocity curve is shown in Fig. 12.

Note that the acceleration time as shown in Fig. 12 is substantially longer than that specified initially by Eqs. (1) and (2).

5.4 Analysis of Performance

It is important to state at this stage that the system constructed did indeed perform as was expected. It was operational shortly after commencement of commissioning and did not exhibit any unstable or unexpected behaviour.

The following modifications have since been implemented:

5.4.1 Acceleration Time

Acceleration time was extended substantially from 140 sec (this was due to the applied feed-forward speed control strategy).

5.4.2 Electric Motor

The electric motor was replaced with a larger one (rated power of the new motor = 450 kW). This replacement became necessary when the original motor started overheating.

5.4.3 Elimination of Vibrations

Some mechanical vibrations of the motor itself were eliminated by the skipping of a few related frequencies at the converter.

6. Conclusion

The satisfactory performance of the conveyor confirmed Krupp's capabilities in designing and delivering installations of this size and complexity.

The sophisticated control system as initially envisaged would further enhance the operational parameters of the conveyor. However, the simplifications introduced did not impair the client's and/or engineering specifications of the conveyor significantly, allowing for its increased economic efficiency.

References

1. FAUERBACH, R. and OTREBSKI, M.: The Syferfontein Overland Conveyor System at the Sasol Secunda Plant in South Africa; bulk solids handling, Vol. 13 (1993) No. 2, pp. 289-295.

2. NORDELL, L.K.: Belt Dynamics - An Alternative View; lnternational Materials Handling Conference, Beltcon 4, Johannesburg 1987.