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Lower Mühlebach power station in the Glarus region relying on GRP pipes

After a concession period lasting many years, building work began last summer on the construction of the lower Mühlebach power station in the Swiss municipality of Glarus Süd.

The run-of-river hydropower plant is connecting to its upstream power station directly at the tailrace channel; once operational it will generate around 1.78 GWh of electricity each year. For the manufacture of the 880-metre-long penstock, the owners of the facility, KWM Kraftwerk Mühlebach AG, are relying on the high-quality GRP material produced by the piping specialist AMIBLU. The penstock is being constructed in its entirety at a dimension of DN1000 and overcomes a height difference of around 40 metres.

There is a long tradition of using hydropower on the lower Mühlebach in the Glarus Süd municipality (known as the Engi municipality until the merging of municipalities in 2011) According to the technical report prepared by the Glarus-based planning agency Runge AG, it can be traced back to the year 1864. At the time it was still used purely for mechanical purposes: it first served to operate a matchstick factory, which was converted into a carpentry and sawmill in the years following. After the mechanical plant was destroyed following flooding in 1910, electrical energy was produced using replacement systems. To this end, water was dammed at the so-called ‘Wydensteg’ using a tainter gate and directed to a Francis turbine in the lower level using a penstock. The Sernftal tramway has made use of this section of river since 1905, thanks to the construction of a hydroelectric power plant used to electrify the regional train line. However, as rail operations were shut down at the end of the 1960s and business terminated at the carpentry in 1992, the use of hydroelectric power on the lower Mühlebach also came to a temporary end.

The existing “Mühlebach KWM” power station became operational in 2009 and makes use of the stretch of water between the Ueblital and the “Mühlebach Weseta AG” dual headquarters on Bergen road, above the Dorfbrücke bridge; alongside this plant it was suggested that efforts again be made to use the lower section of the Mühlebach for energy production. To do this, plans were made to trap the water in the tailrace channel of the upstream power station and immediately direct it to the new powerhouse via a penstock. After a concession phase lasting around 8 years, the KWM Kraftwerk Mühlebach AG, comprising Weseta Kraftwerke AG and SN Energie AG from the Glarus Süd municipality, was finally granted a building permit in Spring 2017. Just a short time afterwards at the end of May, building was able to begin with the laying of the penstock. Building and civil engineering works were carried out entirely by the local building company Marti AG. The general planning and building supervision was taken over by Runge AG, a hydroelectric project specialist based in Glarus and a sister
company of Jackcontrol AG.

Since the new power station acquires tailrace water from its upstream plant that has been cleaned of bed load and floating debris, there is no need for the otherwise obligatory system components such as weirs, watcher catch-ments and sand traps. A surge tank is installed for hydraulic decoupling. The surge chamber, which is used to transfer the tailrace water from the upstream power station, is integrated into the existing tailrace channel on the right-hand side. “When constructing the Mühlebach power station, the tailrace channel was newly constructed and provisions were put in place for the connection of the planned downstream facility. To this end, an opening was created in the tailwater channel and temporarily closed. Consequently, the surge tank for the new system can be constructed relatively easily”, explains Runge AG project manager, Cyrill Althuser. She continues: “So that the tailrace water flowing through the tailrace channel can be directed into the water surge chamber, a sluice gate is fitted at the end of the tailrace channel. This prevents the tailrace water from flowing out into the Mühlebach on the one hand, and on the other hand it prevents bed load and floating debris from getting into the tailrace channel in the event of flooding.

Directly following the surge tank and the intake shaft, the 880-metre-long penstock begins. In total, the power station’s penstock overcomes a height difference of around 40 metres. In a comparison of variants, considering economic aspects as well as building and ownership regulations, the building contractors decided to route the penstock through the largely undeveloped area along the right-hand side of the Mühlebach up to Kantonstraße. The underground crossing of Kantonstraße is shortly followed by the crossing over the Sernf, a tributary of the Linth, via the existing access bridge. The final section of the penstock travels along the left bank of the Sernf to the powerhouse. A consistent size DN1000 was chosen for the pipe diameter. If the pipe has a net cross section of 0.79 m², this results in an average flow velocity of 2.04 m/s during full-load operation, which in turn results in an economical ratio of hydraulic losses against investment costs.

For the piping material, the operators are relying on glass-reinforced plastic (GRP) produced by the company AMIBLU. The high-performance pipes, produced in centrifugal processes, offer a whole range of advantages and have been proving their quality for decades across the most varied of applications. Along with a high abrasion resistance, the material’s UV resistance is also a winning quality, and outstanding flow properties are guaranteed thanks to an extremely smooth inner surface. The minimal weight combined with the user-friendly coupling system ensures a high laying capacity. The deviation capability of the pipes of up to 3 degrees inside the coupling joint allows minor changes in direction to be uncomplicated. Additionally, for a broad curve within the penstock routing, the pipes can be optimally pre-installed by the manufacturer using bevelled cuts, meaning there is no need to construct additional moulded parts.
The AMIBLU project team was extremely pleased with the handling of the project, not least thanks to the product-specific pre-planning, and pointed to the successful collaboration with the building companies, the planners and the operators.

In order to have as minimal an effect as possible on access to the powerhouse construction site, the pipeline bridge immediately in front of it was erected just before the beginning of the building works. This section of the penstock routing leads to a steel structure on a road bridge over the Sernf and at the same time constitutes the only section of the penstock that runs above ground. On both the left and right-side banks, two “Flex 4” pipe couplings made by the Swiss firm Straub
Werke AG serve to connect the GRP pipes.

The couplings allow for distances of up to 200 mm between the ends of the pipes and, thanks to their extra strong rubber surface, ensure optimal expansion compensation within the penstock. Because the pipeline bridge marks a high point of the penstock, technical measures had to be put in place regarding ventilation and draining. The moulded parts necessary to achieve this, such as pipe bends and the T-section for ventilation, were custom-fitted by AMIBLU. Shortly before the transition of the penstock into the powerhouse, there is a low point in the penstock routing which has additional drainage.

In total the new power station will have a
volumetric flow rate of 1.6 m³/s. Due to the variable water volume of the Mühlebach and the relatively low drop height, the operators decided to use a robust cross-flow turbine, which can demonstrate its strengths above
all when operating at partial loads. At a maximum electrical output of 479 kW, the
turbine, coupled with a synchronous generator, will produce around 1.78 GWh of electricity each year.

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