Installed in 2010 in the beautiful South Tyrolean Sarntal region, the Sagbach hydropower station needed an effective means of filtering the motive water at the intake.

As the mountain stream typically carries large amounts of sediment, the decision was made to use a new, patented system by specialist Wild Metal from the South Tyrolean town of Ratschings. Since then, the installed self-cleaning special screen, which exploits the so-called Coanda effect, has been working very satisfactorily. Most of the sand can be efficiently filtered out and eliminated.

The sediment and debris carried downhill by the stream poses a considerable danger to the installed turbine. However, even the most durable protective coatings will slow down the wear and corrosion of a high-pressure turbine blade only slightly, especially in constructions where the motive water carries large volumes of sediment right up to the turbines. Over time, this will typically result in unscheduled downtimes. In extreme cases, the turbine rotors need replacing every year, running up extra costs that are inhibitively high for private operators. A functional approach to solving the problem consists in efficient screening in combination with a de-sanding system. Patented and marketed by Wild Metal, the “Grizzly” – a selfcleaning special screen for filtering surface waters – carries this approach even further. Based on the positive experiences with the first screening grills of this kind at other power stations, the operator of the Sagbach station decided to have a similar construction installed at their water intake. The visible success of their decision has proved them right.

KEEPING SAND AT BAY
The “Grizzly” is a largely self-cleaning protective screen for hydropower and drinking water systems, which does not require any active propulsion. It consists basically of a durable grating made from hot-tip galvanised steel, which sits above a fine screen and is supported by protective rods whose shape follow the natural flow of the water. Thanks to a suspension frame, the whole device detaches easily from the building structure. Attached below the protective grating at the same water level is the fine screen, whose physical specifications are tailored to the local requirements. This is followed by a filter screen, which is made from acid-proof high-grade steel. It is designed to keep out at least 90 per cent of the dregs and floating debris with a grain diameter of 0.3 mm at a fine screen gap width of 0.55 mm, and 0.5 mm at a fine screen gap width of 1 mm. The Coanda effect, which causes liquid jet flows to attach themselves to nearby surfaces, combined with the shearing effect of the profile rods, causes the water to flow into the intake while keeping out debris such as leaves, needles and sand. The screen itself is largely self-cleaning, and the unwanted floating debris is carried off by the water. As a positive ecological side effect, most of the smaller stream-living animals are prevented from getting into the motive water system. Each water intake is unique, and the “Grizzly” screen must be adjusted to the local conditions. As the Sagbach installation shows, the method used by the “Grizzly” works perfectly. From time to time, the operator will have to remove the masses of sand that tend to accumulate at the bottom of the stream below the grill. Due to the large amounts of solid matter involved, the fine screen also needs replacing once every few years, but that is relatively easy to do, both financially and in terms of the workload involved. In the end, the effort is well worth it, as the turbine rotor is prevented from failing prematurely as a result of wear and corrosion.