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Automatic Filters for Hydropower Under Pressure: Automatic Filter for Swiss Hydropower Plant

| Author / Editor: Ulrich Latz* / Dominik Stephan

Minimised wear in the sliding gate valves, reduction of downtime and maintenance costs — In a hydroelectric power plant in the Swiss Alps, specially modified automatic basket strainers minimise the wear on the hydraulic control valves caused by glaciers and suspended solids. They are designed for water pressures of up to 1,160 psi (80 bar) and filter out particles with a fineness of 25 μm.

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The Swiss Kraftwerke Oberhasli hydropower plant (KWO) can generate energy on demand at any time using water dammed in the alpine Grimselsee, 1,909 meters above sea level. However, the use of natural glacial runoff and rainwater poses a problem in the form suspended matter: The glacier is continuously eroding fine stone particles from the mountains.

These solids, derived from glacial abrasion, are transported by the runoff into the reservoir and collect there with other suspended matter brought in by rainfall. These 25 to 200 micron particles pose a problem for the huge turbines power plants, as they cause heavy wear of the slide valves. Due to the water's drop height of 670 meters, very high pressures occur.

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“Until now, the valves had to be cleaned and repaired every three to four months,” explains Franz Christen, Sales and Technical Manager at BT-Hydraulik, who was commissioned by KWO to find a solution to the problem. In order to reduce or, ideally, eliminate costly and time-consuming repairs, the power plant operator looked for a new solution. “Larger settling tanks were not an option,” continues Christen.

“At water pressures of 870 psi (60 bar), 1,160 psi (80 bar) or even more than 1,450 psi (100 bar), very large and complex tanks would be needed. But that would not be feasible for economic and environmental reasons, and for lack of space. Converting to oil hydraulics was likewise not feasible for the same reasons.” Therefore, KWO decided to install filter systems upstream of the control valves and turned to BT-Hydraulik, a leading company in the field of hydraulic drive technology.

Filters Have to Withstand Up to 80 bar Pressure

The Berne-based company advised KWO to use an automatic basket strainer. “Conventional filters clog up over time and need to be serviced and replaced at regular intervals,” Christen explains. “An automatically self-cleaning filter is the more economical solution.” Yet, standard backwash filters are not designed for the high pressures in a hydroelectric power plant. Thus, BT-Hydraulik teamed up with filtration expert Eaton to work on refining the company's Eaton Model 2596.

Specially for KWO they adopted the motor driven unit to withstand pressures up to 1,160 psi (80 bar): first of all, Eaton modified the housing by increasing the wall thickness and making the cover considerably sturdier. In addition, the flushing arm drive shaft was sealed with a quadruple mechanical seal. “A challenging problem was to find a filter element that offers the highest possible filter fineness but could also withstand the high pressures and was backwashable,” says Christen.

Keeping Hydropower Plants Running

A reinforced version of the Durawedge filter proved the solution: made of V-shaped stainless steel wire profiles, even the standard version is capable of being used in demanding applications. After being further reinforced, they can withstand high pressures in the power plant, filtering out a large part of the suspended matter due to a filter fineness of 25 μm.

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Even though the test phase of the new filter system is scheduled to run for several years, initial results are already clear. “Results are very good,” says KWO. “So far we have had no outages and the system perfectly meets our needs in terms of maintaining and cleaning the baskets.” Especially compared to the turbine lines where the sliding gate valves have not yet been retrofitted, it became very clear that wear and tear can be significantly reduced — if the used filtration and separation technology can take the high pressure.

* The author is a Technical Sales Engineer at Eaton Technologies, Nettersheim/Germany

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