Belt system goes into extra length

Bulgaria has always been heavily dependent on nuclear and thermal power stations. Power stations in the Maritsa lignite region form an important pillar of the national energy supply network. The region lies in the Thracian Plain, some 100km from the shore of the Black Sea. The Maritsa complex includes three power stations. Maritsa Istok 2 went into operation in 1966 and is currently, with an installed capacity of 1465 MW, the largest coal-fired power station in Bulgaria. (For comparison: the largest German lignite power station Niederaussem has a capacity of 3864 MW).

Conveyor belt stays tightly closed

The combustion of lignite from the Maritsa region produces copious quantities of ash. This has to be transported away and together with the overburden from the opencast mine dumped on the tip where it then represents a component in recultivation of the landscape.

The ash produced at the power station is washed into retention basins, where it dries out. Transport to the tip then commences with excavation of the retention basins.

The first part of the conveyor run was taken into service in 2000. This consists of a bucket chain conveyor with a capacity of up to 1100 m³/h, one mobile and one stationary conveyor system and a two-and-a-half kilometer long tubular belt conveyor (also called a "pipe conveyor").

Pipe conveyors are characterized by transporting bulk materials without loss of material as a totally enclosed system. The conveyor belt closes in the form of a tube around the material to be conveyed, the edges of the belt overlapping over the distance transported. Only at the feed-in and at the delivery point does the belt open out. A further advantage lies in its ability to be laid in curves, so that extremely flexible conveyor routes are possible.

Installation of the first pipe conveyors in Maritsa was based at that time on a FLEXOPIPE belt with aramid carcass (type: DP 1600) from Metso Minerals. The belt proved extremely reliable in service. Over the whole of its operating life, there was not a single instance of critical twisting, and even today the belt is wearing very well. In addition to this, each and every joint in the belt is still in good shape.

For planning a second and much longer run a second pipe conveyor with aramid carcass from Metso Minerals was therefore projected. Starting point for construction of the new conveyor system of 5806 meters length was the steel framework, in 2007. In parallel with the delivery of the 34 belt rollers from Norway (manufactured by Metso Minerals Kongsvinger), installation of the belt began in May 2008.

A reliable carcass for long sections

The decision of the operator in favor of a belt with aramid carcass was made on the grounds of its lightness. To reduce carriage of dead weight over a total belt length closely approaching 12 km to an absolute minimum, it was essential to minimize the belt's own weight. Normally, the high belt strength would have been achieved with a carcass of steel cable, but the high-performance chemical fiber aramid has similar tensile strength to steel, and yet it weighs only one-fifth as much.

The rubber belt covering is made of hard-wearing rubber with an extremely high resistance to UV and ozone. In the top covering, there is in addition a so-called fabric breaker integrated. The tensile forces in the belt are transmitted by four-millimeter aramid cords which are imbedded in transverse polyamide cords.

A key element of the belt is represented by a breaker of steel cables laid transversely. This, along with the fabric breaker, insures the rigidity and maintenance of the tubular shape. All the components - the aramid, the polyamide and the steel cords - make sure of the necessary operational rigidity and are also of great significance in making the joints.

Jointing - taking things to the limit

Manual preparations for the work included uncovering the rubber layers (due to the strength of the bond, an extremely time-consuming exercise, making the use of special leatherworking tools necessary, see Figures 5+6), zigzag cutting of the aramid on both sides and precise meshing on one level (see Figures 7-8). This guaranteed that the tensile forces would be transmitted exclusively via the rubber surfaces lying between the tongues. As a general rule it is important that the cutting apart and jointing is carried out on one level, so that the connection is homogeneous and permanent. If the two layers were simply bonded one on top of the other, the top layer would be continuously stretched when running round the drum, and the bottom layer continuously compressed - an untenable state of affairs for the long term. After joining the tongues together (also called fingers, or finger connections), there now follows renewal of the rubber coverings or assembling the belt above and below the aramid cords. For this purpose the factory supplies a pre-assembled "rubber package" which is composed of various qualities (hard-wearing rubber, types of rubber for bonding to fabrics, steel etc.). To make sure of a good bond between the rubber pack and the exposed aramid cords, the cut surfaces are previously sealed with a corresponding solution. Vulcanization is finally carried out with two vulcanization presses each 3200 mm long, with a power output of up to 150 kW per press (see Figure 9). Due to the length of the connection, which is almost three meters, the whole process is extremely demanding in terms of personnel, tools and materials.

From the Roll to the Draw In

For scheduling the installation, all the experience gained nine years ago during the installation of MTLT1 (the first pipe conveyor at the location) was taken into account. The major uncertainty revolved around cooperation with the company engaged for the pull-in work: here, problems of coordination were anticipated and consequently, the time scheduled for drawing in and vulcanizing was estimated at over two days for each new belt reel.

Additional working time had to be allowed for necessary re-equipment of the working area, the complicated feeding in of the belt through the end station and for the connections at head and foot. Finally, the schedule for the work was fixed at twelve weeks.

The schedule laid down by Metso for the belt installation formed the basis for further work on the system. The engineering team, consisting of representatives from Maritsa 2 and the installation engineers and other suppliers (e.g. for structural steelwork, drums, drives and electrical equipment), the contractors and Metso, met at weekly intervals for coordination discussions in order to synchronize the works.

Due to the length of time the process was scheduled to take and the pressure of work, Metso teams came from Germany, Holland, Norway and Sweden by turns, replacing each other after three weeks. In addition, there were vulcanisers from a local company on site, so that at any one time there were six to eight people plus supervisor available for both of the vulcanization plants. The work was supported and tracked throughout by Metso personnel from Sofia - a key factor for successful completion of the work.

The Metso Minerals service team in Maritsa started on jointing the belts even as the belt rolls were being delivered.  

In the middle of the installation there were two installed - one each for carrier and return run. Work was possible at both tables at the same time - thus it was impossible either to vulcanize at the same time, or to draw in both ends of the belt at the same time.

Initially the drawing in was performed by a locally-based company in the direction of the end station. Due to the high tractive force of up to twelve tons required, the work was done with a bulldozer. For this, the cable attached to the belt was routed out of the installation with a sheave attached to a second bulldozer (see Figure 10).

At the start, the estimates for both the time to form the connections and the delays which were likely to be encountered were confirmed - caused for example by the non-appearance of the crane to move the rolls, problems with power supplies or non-availability of material, equipment or personnel belonging to the other companies involved. Since the work on both the vulcanization stations was coordinated, every delay that occurred always affected each of them.

With increasing routine, corresponding levels of experience and improved coordination, the Metso Team on site was able to reduce the time required for a belt connection, including drawing it in, to 14 working hours. This was even less than had been required for the connections on the first pipe conveyor, which were over a meter shorter.

Making the connections and drawing in the total of 34 rolls was completed on schedule in twelve weeks.

Minerals (UK) Ltd
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Tel: +44 (0)1788 532100
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