Faced with the prospect of costly refurbishment, Kraftwerke Birsfelden undertook the ambitious task of rebuilding part of its run-of-river plant in Sitzerland. Werner Maier* explains the reasons behind the company's decision.
The liberalisation of the electricity market, which is starting to take effect in Switzerland, is forcing power producers to face up to the pressures of a competitive market place. Birsfelden, a Swiss run-of-river plant located on the upper reaches of the river Rhine, is undergoing an extensive refurbishment and modernisation programme in order to retain its competitive edge.
Since commissioning Birsfelden in 1954 Kraftwerke Birsfelden (KWB), the plant operator, has run the station as a joint stock company constituted under civil law. The electricity produced is sold to three power distribution companies in northwest Switzerland, which together meet 16% of the electricity needs of the Greater Basle region.
Birsfelden houses four 7.2m-diameter Kaplan turbines. Two of these were supplied originally by the then Swiss company Charmilles (now Voest-Alpine MCE of Austria) and two by the German company Sulzer Ravensburg (the Swiss Sulzer Group). Refurbishment has featured heavily in Birsfelden’s history as cavitation damage to the turbine rotor vanes and rotor casings has been the cause of high repair costs. Every two years each machine had to be taken out of service for eight to ten weeks to undergo refurbishment, resulting in lost production hours. Furthermore, only 50% of the reactive power of the generators could be utilised as heating of the stator winding, which was caused by occasional short-circuits in the coil, had to be strictly limited.
At the mid-point of the plant’s 80-year concession period (1994) all machines were due to undergo a general overhaul. Such refurbishment would have been costly, but energy output would have remained unchanged. Electricity production costs would therefore have increased. It was at this time that KWB’s board of directors took the far-sighted decision to make the Birsfelden power station more cost-effective, and they instigated an ambitious refurbishment and modernisation programme.
Turbine design has advanced considerably since the original turbines were built at Birsfelden. On the basis of technical studies and economic calculations it was shown that, compared with the usual overhaul programme at Birsfelden, total renewal of the rotor vanes and rotor casing would lead to considerably greater capital costs. However, in the long term — as a result of a marked improvement in efficiency and 4.4% extra production, equal to about 1000M kWh to the end of the concession in 2034 — this constituted the most cost-effective alternative.
Technical studies showed that optimising power station operation would also contribute to absorbing the modernisation costs. As a result, installing a control system that allows fully-automatic operation and makes possible a reduction in staff, assumed a central role in the Birsfelden modernisation project.
The additional energy production resulting from such improved efficiency will also comply with the Swiss government’s environmental initiatives. Energy 2000 is one such scheme, launched at the beginning of the 1990s. This, among with other objectives, sets a target of increasing ‘green’ power production in Switzerland by 5%. Those responsible at KWB want to support this intention and plan to dredge out the river downstream of the power station by an average of 80cm over a length of 2087m, increasing the fall of water at the turbine output by up to 33cm and producing an extra 4.7% of energy — a somewhat greater amount than that brought about by improving the efficiency of the machines. The additional energy to be expected from both measures at Birsfelden amounts to an overall 9.1%.
KWB’s modernisation programme started in 1995 and is scheduled for completion by 2001 at a cost of Sfr100M. The plant’s annual energy output will be increased to 600M kWh, due to the lowered tailwater level. At less than 4 centime/kWh, the energy production costs, even after this major capital investment, can be regarded as extremely favourable and competitive with regard to the liberalisation of the European energy market.
The main part of the modernisation work is being carried out by the two Swiss companies ABB and Sulzer Hydro. The automation work has been awarded to SAT in Vienna. After modernisation, the previous maximum output of 82MW will be increased to 92MW and, after deepening the Rhine, to about 100MW. The work is divided into projects as follows:
Sub-project 1, costing Sfr50M:
•Replacement of rotor vanes (new design).
•Renewal of the rotor casings (boring out and coating with 5mm CrNi steel).
•Renewal of the generators (stator iron-parts, stator winding, rotor coils).
Sub-project 2, costing Sfr30M:
•Deepening of the Rhine downstream of the power station.
Sub-project 3, costing Sfr20M:
•Installation of power station control technology (to allow the power station operation to be fully automated).
•Closed cooling-water system ( to permit maintenance-free operation and more efficient output of waste heat to the regional district heating system).
•Replacement of the most maintenance-intensive auxiliary and subsidiary works.
•Optimisation of in-house requirements in respect of efficient use of electrical energy.
The present situation
Three of the four turbine generators are already back in service after successful modernisation. Since 1995 each of the four machines have been modernised consecutively — the first machine took 40 weeks to complete. In each case the work was fitted into the seasons of lower water flow, in order to minimise surplus water flow over the weir.
For the second machine, the main suppliers were granted a share in the profit arising from any early resumption of production, with the result that the installation times turned out to be 10% shorter (reduced from 40 to 36 weeks). Such early completion of the last three machines will mean they will be in service for a total of 12 extra weeks, allowing the station to generate more power during this time. The fourth and last machine will be able to resume production in May 1999.
During the modernisation programme the turbine rotor casing was not replaced but bored out by 9mm (radially) and re-coated to a depth of 3mm (radially), giving a slight increase in the intake capacity of the turbine. Rotor vanes and rotor casings were renovated at the same time, so making it possible to increase the overall efficiency, and above all to achieve better control over the cavitation problem in the future.
The automation work is for the most part completed. Before the control technology was chosen, a power station of practically identical construction which had just been modernised (Jochenstein on the Danube) was sought out. The designs for the plant were then taken over for Birsfelden, with practically no additional requirements.
Finally, negotiations with objectors who oppose the Rhine-deepening project on environmental protection grounds are currently in progress. If these proceed positively, it should be possible to start the estimated 15 months of dredging work (removing approximately 350,000m3) in October 1999.
Factors which have contributed to the success of the Birsfelden project include the fixed overall prices established by contract for supplies, as well as the fixed cost-ceilings for installation and commissioning work. Simultaneous work on the greatest possible number of sub-projects with short deadlines has also had a positive effect on costs.
Personnel featured heavily throughout the Birsfelden project. The main suppliers were required to recruit 5-10% of their tendered installation labour from the power station staff, against an offset. This transfer of experience is highly advantageous for the future care of the plant. Indeed, to ensure that after one machine is completed the best craftsmen are not taken away by promotion to another worksite or to an office job, an appropriate provision should be made in the contract when placing the order.
Thanks to the extra production achieved and lower maintenance required, the money invested in this project can be written off well before the 80-year concession period expires. The real gain for KWB lies in the fact that the costs for a ‘normal’ general overhaul (which would in any case have arisen at the mid-point of the concession) plus the replacement of the control system and various auxiliary plant, are already included in this ambitious, comprehensive modernisation package — and thus in effect no longer arise. In the future the intervals between refurbishment work at Birsfelden can be increased to five years, compared with two years as before.
|Development and preparation – Sulzer Hydro’s story**|
| In the summer of 1993, Sulzer Hydro was given the assignment of performing development tests on a model machine. The objective was to develop two new types of vertical, four-bladed Kaplan turbines with high efficiency and good cavitation behaviour to replace the four old Escher Wyss and Charmilles runners. The bladings, designed on the strength of calculations and tests for the two slightly different Escher Wyss and Charmilles machines, exceeded the set targets. With the four new runners the annual turbine energy production can be raised by 23GWh. Maximum turbine output with a head of 9.45 m is about 23.7MW. Furthermore, up to 2% higher efficiency is obtained with the new blading.
Machining the blades and assembling the runner weighing 100t was performed at Sulzer Hydro in Ravensburg, Germany. The work on the runner hub and adjusting mechanism was done at Kriens in Switzerland. It was also here that the dismantled components were reconditioned and the change to greaseless bearings took place. KWB prides itself on being an environmentally aware company and so it wanted to avoid the risk of water contamination through grease residues.
At Birsfelden, the discharge ring and the bottom distributor ring of the particular turbine were machined in situ. The original discharge ring was of cast steel and grey cast iron, which in the past were repaired by welding with different electrode material. The divergent material hardnesses resulting from this procedure presented difficulties in turning operations.
Compared with installing a new discharge ring, machining in situ is always preferable for the operator, since it makes substantial savings possible. Sulzer Hydro can claim a position as the only turbine maker with its own mobile and modular turning machine for large diameters. When the discharge ring is machined in situ, very good concentricity is achieved between the stationary and rotating parts, and this exact centering allows minimal radial clearance between runner and ring. In turn, closer runner clearance means smaller energy losses. Radial clearance at Birsfelden now measures 3-3.5mm. Before the rotor was dismantled, a variety of radial clearances up to 8mm were measured between runner and ring.
The same machine used for turning the discharge ring is also used at Birsfelden for coating. After preliminary machining of the ring, a 4-5mm layer of CrNi steel (Metcoloy 2) was sprayed on to the base material. Final machining still leaves a coating about 3.5mm thick. The layer deposited in this way is homogeneous and oxide-free. The selected coating material has been used with success for some time now in Switzerland and other countries. At Birsfelden a first inspection after a year of operation has not revealed any signs of abrasion or cavitation on the discharge ring.
Effective collaboration between the client and the contractor has led to the success of this project. In June 1996, the first rebuilt machine was reconnected to the grid, the second in June 1997, the third followed in the summer of 1998, and the last will resume operation in 1999.