Turbines and Mechanical Components

Extending hydropower plant life with erosion and cavitation protection

Inaki Ezpeleta of Castolin Eutectic explains how wear coatings can be tailored to provide site-specific resistance to erosion and cavitation in hydropower plants
Extending hydropower plant life with erosion and cavitation protection
(Pump refurbished with MaCaCorr)

Iñaki 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 for hydropower plants.

Erosion and cavitation can cause serious wear and reliability issues for mechanical equipment in hydropower plants. Sand particles in water is a common cause of erosion. Cavitation is a type of surface fatigue that results from sudden changes in water pressure, such as behind impeller blades. This causes bubbles of water vapor to form and collapse, creating shock waves that lead to pitting, cracking and eventual disintegration of the component.

Careful design can reduce these issues. But they cannot be eradicated completely. So maintenance teams often have to rebuild components to their original dimensions with a material that provides high surface hardness and toughness. 

Because erosion and cavitation are different wear mechanisms that can occur individually or in combination, the resulting wear is often unique to each site and component. Many factors can also apply, such as pressure and load, as well as the type and size of sand and water flow.

To address the different wear types, a broad variety of coatings is available for equipment such as Francis, Kaplan, propeller and bulb turbines, as well as draft tubes, impellers, pumps and valves, and wicket gates. Application techniques for wear-resistant alloys include HVOF (high velocity oxygen fuel), arc spraying and welding. Cold polymeric coatings can also play a role.

Resisting cavitation

Hydro-Quebec, a leading hydropower generation company in Canada, developed a special stainless-steel repair alloy to address severe erosion and cavitation on hydraulic turbine runners on 300 generating units ranging from 1 MW to 350 MW.  We produce it under the CaviTec trade name to extend the life of hydropower equipment around the world.

On test, CaviTec lasts 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.

Recent developments have focused on the application process 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 repairs for cavitation damage

Because cavitation often results in significant loss of surface material, there is 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.

Three 60 MW Francis pump turbines in Austria have benefited from coating with cavitation-resistant alloy. They can deliver a flow of 375 to 500 liters per minute against a head of 50 to 200 meters. But severe cavitation can occur on the pump turbine’s inlet edges.

Testing found applying a 5-mm-thick layer increased the repair interval from 3,000 to 9,000 hours, tripling the service life. 

Resisting erosion with thermal spraying

Thermal spraying techniques are a good solution where erosion resistance is important. They enable coatings that are both thin, for machine efficiency, and dense, to withstand the impact of hard sand particles.

Long-term performance and repairability is a vital factor, with bond strength as an important measure. That is why our field service engineers usually recommend HVOF and arc spray, as both deliver a high bond strength for new and repaired coatings. 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) that provide high protection against erosion and also resists cavitation. Using high velocity, rather than high temperature, avoids thermal effects that can impact the substrate.

HVOF coatings can be applied either in small, localized areas or as coatings across an entire component. The technology is mobile, so repairs can be performed on site.

The alternative for erosion resistance is to use amorphous materials that typically contain chromium, boron and silicon. They bond well with the underlying steel and form a dense coating. Because they are only available in the form of cored wires, they must be applied by an electric arc spraying technique. This gives faster deposition rates than HVOF, making them more economical. But they cannot match the extremely high surface hardness of tungsten carbide.

A hydropower plant in Austria was experiencing excessive erosion between the shaft seals and the runners on a 6 MW high-pressure pump turbine. This was due to heavy content of sand particles of 0.5 mm and larger in the driving water.

The result of the erosion was a service life that varied from 3,000 to 6,000 hours – and in some cases, the operator found that the wear reservoir was destroyed within a few days. 

HVOF was used to apply a 0.3 mm coating. Inspection after 2,200 hours of operation found that wear was no higher than 0.025 mm, indicating that with the coating, repair intervals can be extended considerably.

For toughness use laser cladding

Laser cladding is another method for rebuilding and re-dimensioning even very large components, up to 15 m in length, with automation for consistent high quality.

In laser cladding, metallic powder or wire on the surface of the workpiece is heated to form a melt pool that creates a strong metallurgical bond. This creates a tough coating that withstands erosion and cavitation.

The precision of the laser limits heat exposure due to a short heating time. It also offers flexibility over choice and grading of the coating as powders or wires can be mixed during application.

A good 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 aim for the operator was to prevent thermal distortion or damage, as well as eliminating the potential environmental harm from emissions produced by burning welding gases.

Our Castolin Eutectic service workshop in Stavanger, Norway, used a 6 kW laser cladding machine to build up the part using almost 250 kg of carbon steel. This enabled a rapid return to service for the high-value part.

Cast iron repair using ceramic surfacing polymers

An alternative method for enhanced resistance to cavitation and erosion is to apply a ceramic surface polymer coating. These composite materials 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.

Ceramic polymers can be applied for curative maintenance to repair and rebuild cast iron components, pipework, valve and pump bodies. They are also helpful for preventive maintenance to keep components in service for longer.

One example at a hydropower plant was applied to a water supply pipe on a cast iron storage tank for a fast return to service. It had become perforated over time, and ceramic polymer was the ideal repair material as the welding or thermal repair of cast iron is complex and challenging.

Erosion and cavitation are a significant challenge for key components in hydropower plants. The good news is that there is a variety of coating materials and application methods available that enable a fast, reliable repair while also offering the opportunity to extend service life.

This article was originally published on Power Engineering International.