Back in the middle of May 2014 construction work was commenced on a new hydroelectric power station of the Austrian federal forestery agency (Österreichische Bundesforste) in the Styrian-Salzburg border area.
Forstaubach Gleiming GmbH’s plant, which is equipped with a dual machine combination, will be put into service this spring. However, before this the tricky job of laying the pressure pipes had to be completed.
Steep terrain, difficult soil, poor weather and a stream crossing: these were the greatest challenges to the laying of the penstock for the Forstaubach Gleiming power station. Given such conditions it is also not surprising that the 3.2 km long penstock could be completed only just before the end of the year after a construction time of 7 months. There’s nothing you can do against bad weather, but it was possible to counter the difficult geological conditions in one section of the line using appropriately robust material. The soil investigations conducted before the start of construction had shown that earth movements could not be ruled out in the middle part of the line. This is why ductile cast iron pipes from Tiroler Röhrenwerke TRM were used for this section whilst GRP pipes from the German manufacturer Amiantit were used for the upper and lower part of the power station line.
Everything from a single supplier
The pipes for the “hybrid line“ were provided by the Lower Austrian pipe specialists Etertec GmbH & Co KG. The distribution company based in Brunn am Gebirge convinced the operators during the tender procedure through its excellent price-performance ratio and outstanding references for comparable projects. In addition to the cast iron and GRP pipes, Etertec also provided all special parts such as couplings, reductions, discharge outlets and specially made ventilation control valves.
1 line – 3 sections
From the inlet structure built as sluice gate, the first section of the penstock was laid along a length of 738 m in the form of Flowtite GF-UP pipes with a diameter of DN1400 and pressure rating of PN 6. In the middle section of the line, cast iron pipes resistant to tensile and shearing force were used over a total length of 1,217 m due to the uncertain soil properties. A transition was created for the different pipe materials using a special moulding and the diameter of the penstock simultaneously tapered to DN1000. Leading to the power house, the final section of the pressure line of 1,203 m in length was then executed as a GRP line with a diameter of DN1200 and pressure fittings of PN 10 and 16. In view of the persistently wet weather whilst the pipeline was being laid, the soil along the pipeline could not completely dry out at any time. Combined with an extremely steep terrain in part, these were by no means favourable conditions. Nevertheless, Pitzer, the Schlaming-based company entrusted with the laying of the pipes, did a great job. Another challenge was to cross a stream with the pipeline which proved to be an involved undertaking due to the water conservation measures. Commenting on the complexity of the work, graduate engineer Gerhard Brei-tenbaumer from Austrian federal forestery agency, explains that the situation was compounded due to the fact that traffic had to be controlled in the lower area of the pipeline because it ran along a public road.
Power station soon to go into service
A combination of two machine sets is used in the machine house of the Forstaubach Gleiming power station. A vertical-axis Pelton turbine and a Francis turbine will be in operation – both from EFG. The decision in favour of a double turbine solution was made due to the special hydrological conditions in Forstaubach. With a net head of some 94 m and a flow rate of 2.100 l/s, the machine sets together will generate a total output of 1.7 MW and therefore around 6.5 million KWh per year of clean energy. The pilot operation is planned for the end of April.