Turbine erosion and prevention measures

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Cavitation in Turbines and Preventive Measures 1 Overview In recent years, continuous innovation and development in various industries have greatly promoted the growth of related sectors, and the same is true for turbines. At present, the total capacity of hydroturbines in our country is among the highest in the world. However, in certain applications, they are affected by various factors; especially during the construction of water conservancy projects and power generation, hydroturbines suffer severe wear and tear, which not only reduces their service life but also causes economic losses. The following article provides a brief analysis of several possible causes of wear in water turbines, and proposes reasonable measures to address these causes. 2 Analysis of the causes of turbine abrasion 2.1 Sediment wear Sediment wear is a relatively severe form of damage; during hydroelectric power generation, large amounts of sediment enter the water, and this is an inevitable phenomenon when the turbine is in operation. Depending on the properties of the sediment, the degree of damage caused also varies. The sediment in river water can generate significant force during the operation of turbines, leading to wear and tear on various components within the turbine. Especially with hard sediment, the destructive impact on the turbine cannot be ignored; at high speeds, it can create cutting forces that cause the metal surfaces of the turbine to be cut and fractured. Over time, this can result in severe damage to the turbine, making it impossible for it to continue operating. 2.2 Cavitation erosion Cavitation erosion refers to the physicochemical erosion exerted on the flow components of a turbine by tiny bubbles formed due to low water pressure, which causes water to vaporize; these bubbles undergo formation, growth, and collapse during this process. The process of bubble formation and collapse occurs continuously at a high frequency, as the water flows at high speed without interruption. The shock waves generated when bubbles burst have a shock speed of 100 m/s to 400 m/s, with a shock range of 1? Zi M-25? zm, with a pressure of 100 MPa to 1000 MPa ； It is accompanied by continuous impact from micro-jets on the surface of the turbine’s flow-through components. Although the area affected by this type of impact damage is small, the high energy and density result in deep and frequent impacts, causing pitting and pinholes on the surface of the flow-through components; in severe cases, honeycomb or sponge-like cracks may even form. 2.3 Coupling effects 2.3.1 Coupling of sediment wear and cavitation wear. Sediment and cavitation are two relatively common types of wear in turbines, and they generally occur simultaneously as a result of each other. The occurrence of mud and sand phenomena creates large rough surfaces inside the turbine, thereby increasing the wear on its components. When significant resistance is generated on the surface of these turbine components, it has a severe impact on the turbine. Under the effect of external erosion, the components become loose, which makes them more susceptible to damage under stress. These two factors interact with each other, ultimately damaging the turbine and reducing its service life. 2.3.2 Coupling of abrasion and vibration. During operation, the turbine is subject to significant forces due to its own internal conditions. When these forces are interrupted by rapidly rotating bubbles, it causes severe vibrations in the remaining internal space of the turbine. The areas affected by these forces resist their impact due to inertia, which can result in the blades of the turbine separating from their mounts in an instant. Such forces pose a direct threat to the internal components of the turbine, and in severe cases, they can endanger the entire operation of the turbine, exacerbating the degree of wear and damage. 2.3.3 Coupling of electrochemical effects and abrasion. The relevant personnel have conducted numerous experiments on measures to prevent wear in turbines. By using indicator tests, they analyzed the chemical elements in water that have a significant impact on turbine components, as well as certain charged particles; it was found that these particles can precipitate in suitable conditions, and through complex physicochemical processes, they are converted into ozone, which then causes chemical corrosion on the surface of the turbines. Furthermore, when bubbles burst, the energy associated with them is released and continuously impacts the surface over which fluid flows, resulting in high temperatures in certain areas of the component – up to 300°C. When these areas of the material are heated, a temperature difference arises compared to the surrounding material, creating a thermocouple effect. Along with the continuous impact from bubble bursts, microcurrents are generated within the material, leading to electrolytic ionization and thus electrochemical corrosion. During this process, some of the minerals present in the water undergo changes, turning the water near the components into a corrosive solution; this leads to chemical corrosion on the surface of those components, further exacerbating wear and tear. 3 Protection Measures Against Turbine Abrasion 3.1 Coating Protection Coating protection is a common preventive measure in recent years, and it has been effectively utilized in the maintenance of turbines. This section provides a brief analysis and summary of the two most common coating methods; it is hoped that the information presented here can serve as a reference for those involved in this field. 3.1.1 Protection with epoxy silicon carbide coating. Epoxy emery has a compact molecular structure; this coating can effectively reduce or prevent the penetration of corrosive substances such as acids and alkalis into the substrate. It is also characterized by low cost, easy application, and good wear resistance, enabling it to provide effective protection in areas subject to sand abrasion where cavitation does not occur. In the 1970s, this technology was first applied at the Sanmenxia Hydropower Station, where this coating was used to protect the front surface of the turbine blades, the fixed guide vanes, and the intermediate ring ； Subsequent tests conducted at several hydroelectric power stations showed that this coating provides excellent protection for the non-cavitation areas of the turbine flow components. However, for areas with severe cavitation, the protective effect of this coating is not ideal. 3.1.2 Polyurethane coating protection. Polyurethane is an organic polymer compound, and this coating offers excellent performance in resisting cavitation erosion. Polyurethane coatings can absorb the impact force of sediment particles, and they possess characteristics such as wear resistance, high strength, and great elasticity. However, the coating has poor resistance to impacts from hard objects and scratches; once a local area is scratched, it can lead to large-scale peeling. Additionally, due to insufficient adhesion strength, the coating often peels off in large areas or entirely after a short period of operation of the equipment. Currently, polyurethane coatings with an adhesive strength of 30 MPa have been successfully developed in China, and they are now being used in various hydropower stations. 3.2 Optimizing turbine design: Optimizing the design of turbines during the manufacturing process can also help to reduce damage to them. Especially in the manufacturing of hydraulic turbines, the shape and size of the blades must meet the requirements of actual operation, so that the forces acting on them are evenly distributed. Furthermore, wear-resistant materials or stainless steel rotors can be used to enhance the smoothness of the material surface and reduce wear. During the design process, in order to obtain an optimal blade shape, CNC machines can be used for processing the blades. This not only helps to reduce errors but also ensures a smooth surface on the cut shapes, thereby enhancing their corrosion resistance. 3.3 Reducing sediment carried by water flow As shown through the analysis above, sediment has a highly significant impact on the proper operation of hydroturbines; therefore, it is essential to conduct a thorough analysis of measures to address this issue. Rational sediment removal methods should be developed based on the factors that arise in actual operations, in order to reduce the destructive effects of sediment on hydroturbines. Under normal circumstances, sand discharge facilities can be installed alongside the facility operations, which can effectively reduce wear on the water turbines. Not only that, but methods such as storing clear water and discharging turbid water, discharging sediment during floods, and generating electricity during normal water levels can also effectively address the relationship between sediment discharge and power generation. All of the above operation methods significantly reduce wear on the components involved in the hydro turbine’s operational processes, greatly improve work efficiency, and extend the service life of the hydro turbine. 4 Conclusion From the analysis above, it can be seen that wear is a fairly common phenomenon in the operation of hydroturbines. In actual operation, if it is possible to analyze the reasons for wear based on specific problems and to find appropriate solutions, this will have a positive effect and help extend the service life of the turbine. Not only that, but it can also help reduce costs to a certain extent, ensure the quality of turbines, and enable them to fulfill their functions properly, thereby promoting the development of related industries and bringing benefits to people.