| Spotlight


Right: Hydropower repair by laser cladding in Castolin Eutectic Norwegian workshop


coatings that are both thin and dense. Low thickness is important for a machine’s efficiency, while density helps to withstand the impact of hard sand particles over time.


However, it’s also important to consider the long-


term performance and repairability. An important measure of this is bond strength. With this in mind, our technicians typically prefer HVOF and arc spray. Both have a high bond strength for new and repaired coatings, and this ensures a good service life. The choice of technique depends on the type of coating. HVOF applies coatings of powdered materials such as tungsten carbide cobalt chromium (WC/Co/Cr) at high velocity. This provides very high protection against erosion and also resists cavitation. Because the technique uses high velocity, rather than high temperature, it avoids thermal effects affecting the underlying component. This type of coating can be applied either in small spots or as coatings across an entire component. The technology is mobile, making it possible to carry out repairs in situ.


Another option for erosion resistance is so-called amorphous materials. These materials typically contain chromium, boron and silicon and bond well with the underlying steel and create a dense coating. They are only available in the form of cored wires, so need to be applied using arc spray, which uses an electric arc to deposit material with faster deposition rates than HVOF. This makes them more economical than HVOF but do not match the extremely high surface hardness of tungsten carbide. One hydropower plant in Austria was experiencing


excessive erosion on a 6MW high pressure pump turbine due to heavy content of sand in its driving water. Sand particles of 0.5mm and larger were leading to erosion between the shaft seals and the runners.


Depending on the water quality, this resulted in a service life that varied from 3000 to 6000 hours – and in some cases, the operator found that the wear reservoir was destroyed within a few days. As a result, the turbine was experiencing excessive leakage, requiring immediate repair. As a preventive measure, a repair technician used


the HVOF technique to apply a 0.3mm coating. Inspection after 2200 hours of operation found that wear was no higher than 0.025mm. This indicates that with HVOF, the repair intervals can be considerably longer at the site.


Laser cladding for toughness Laser cladding is another technology for rebuilding


and redimensioning components. It is possible to use it on very large components, up to 15m in length, with automated welding for consistent high quality. During laser cladding, metallic powder or wire on the surface of the workpiece is heated with a laser to form a melt pool that creates a strong metallurgical bond with the surface. This strong bond creates a tough coating for withstanding erosion and cavitation. It has the additional benefit of short heating times due to the precision of the laser, which limits heat


exposure. Furthermore, it offers flexibility over choice and grading of the coating as powders or wires can be mixed during application. One example of a laser-clad repair in the


hydropower industry was a six-ton component in Norway. It had experienced heavy wear and required significant rebuilding. The operator wanted to avoid risking thermal distortion or damage to the part, along with potential environmental harm from emissions produced by burning welding gases. Therefore, our subsidiary Castolin Trio Norway used a 6kW laser cladding machine to build up the part with almost 250 kg of carbon steel. The approach enabled the return of the high- value part into service quickly while minimizing environmental impact by using zero-emission Nordic electricity to power the laser and eliminating emissions that would be required for a traditional welding repair by burning welding gases.


Ceramic surfacing polymer for cast iron repair


Ceramic surface polymers are another type of material that can provide resistance to cavitation and erosion. These are composite materials that combine the strength and heat resistance of ceramics with the flexibility and ease of processing of polymers. Available under the MeCaTec name, they are supplied as a compound in two parts that are combined on site and then applied with a trowel, brush or as a spray. They can be used for curative maintenance to repair


and rebuild cast iron components, pipework, valve and pump bodies. Ceramic polymers are also helpful for preventive maintenance to keep components in service for longer. One example at a hydropower plant was on a water supply pipe on a cast iron storage tank. It had become perforated over time. Ceramic polymer was the ideal repair material as welding or thermal repair of cast iron is complex and challenging. The repair compound was used to repair and fill the perforations to return the component to service quickly. When carrying out repairs of hydropower components, it’s essential to evaluate the application to choose the most appropriate materials and techniques.


www.waterpowermagazine.com | June 2024 | 11


Above: On-site intervention teams for both emergencies & planned maintenance


Below: MeCaTeC coated samples


Bottom: Ceramic surface polymers to fight cavitation & erosion


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