Spotlight |
Tailored coatings boost hydropower
Erosion and cavitation are wear mechanisms that can reduce the effective working life of machinery components in hydropower plants. Applying an ultra-hard wearfacing coating can add durability but varying conditions mean that every site is different. Inaki Ezpeleta, applications engineer at Castolin Eutectic, explains how wear coatings can be tailored to provide site-specific resistance to erosion and cavitation – and how this ensures high availability and reliability.
WHEN IT COMES TO excessive wear on mechanical equipment, cavitation and erosion are major issues. Cavitation is a kind of surface fatigue caused by changes in pressure as water flows through equipment. In places where the pressure drops, such as behind impeller blades, bubbles of water vapor can sometimes form spontaneously. When these bubbles collapse, the resulting shock waves results leads to pitting, cracking and eventual disintegration of the impeller. Erosion can also cause excessive wear over time
due to sand particles in the water colliding with a component.
Above: Inaki Ezpeleta
Below: Francis turbine cavitation wear before repair and after welding
Cavitation and erosion can happen to hydraulic machines of all types, sizes and ages and it’s not possible to completely eliminate them. Instead, regular repair is needed to rebuild components to their original dimensions with a material that provides high surface hardness and toughness. This will return the hydropower plant to service at its original level of efficiency, while extending its service life. A challenge is that cavitation and erosion are
different wear mechanisms and can happen separately or in combination with each other and with corrosion. The resulting wear is experienced differently at every site, depending on the factors such as the pressure and load, as well as the type and size of sand and water flow. To overcome this, a broad range of coatings is
available for repair of equipment such as Francis, Kaplan, propeller and bulb turbines, as well as draft tubes, impellers, pumps and valves, and wicket gates. Wear-resistant alloys can be applied with techniques such as HVOF (high velocity oxygen fuel), arc spraying and welding. Cold polymeric coatings can also play a role. The key to a successful repair is careful choice
of repair materials and application techniques, depending on the type and extent of wear experienced locally.
Cavitation-resistant alloy One important repair alloy was developed by Hydro-
Quebec, a leading Canadian hydropower generation company. It was experiencing cavitation and erosion on its 300 generating units ranging from 1MW to 350MW in capacity. This prompted it to develop a special stainless-steel alloy to resist damage from severe cavitation erosion on hydraulic turbine runners. We have been producing this alloy under the
10 | June 2024 |
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trade name CaviTec since 1994 to extend the life of hydropower equipment around the world. Under testing, the alloy has proven to last six times longer than other stainless-steel grades such as 308 or 309. It is an austenitic alloy containing chromium, cobalt, silicon and manganese. These act together to lend strain-hardening and shock damping properties, which are ideal to resist cavitation and erosion. Strain-hardening means that during cavitation, the
pressure waves hitting the material’s surface cause a reaction in the microstructure that increases surface hardness. This provides added durability so that the component can remain in service over much longer time periods than with other materials. Recent developments have extended the alloy’s application to improve the quality of the wear coating and its surface finish. For example, we introduced pulse welding with minimal spatter and have optimized application parameters to minimize porosity.
Welding cavitation repairs Cavitation often results in the loss of material from the
surface so material loss can become quite significant over time. This creates a need to rebuild components with welding repairs. This involves removing damaged material with arc-air or plasma gouging before grinding the gouged surface to remove oxides and slag. The final step is grinding to shape before the component can be returned to service. In cases where a part needs to be significantly
rebuilt, it’s important to lay down a base layer in a stainless steel such as grade 309L stainless steel as the overlying CaviTec layer should be no thicker than 9.5mm.
A set of three 60MW Francis pump turbines in Austria has benefited from CaviTec coating. The pumps can deliver a flow of 375 to 500 litres per minute against a head of 50 to 200 metres. The challenge is that severe cavitation occurs on the pump turbine’s inlet edges at the upper limit of this range. Changing the hydraulic shape of the component did not solve this problem, so the operations team carried out a trial of a 5 mm thick layer of cavitation- resistant alloy. Testing found that this increased the repair interval from 3000 to 9000 hours, tripling the service life.
Thermal spraying to resist erosion Where erosion resistance is important, it is possible
to use thermal spraying techniques. These enable
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