With the development of steam turbines in high-power power plants, the research on water erosion damage protection methods of steam turbine last stage blades continues to be widely valued by personnel engaged in power industry at home and abroad. The serious water erosion of the blade will not only cause the fracture and damage of the blade, resulting in vicious accidents such as strong vibration of the unit, but also reduce the efficiency of the stage. According to statistics, 13 of the 35 low-pressure cylinder blade damage accidents are mainly fracture accidents caused by water erosion

         The development of the last stage long blade is one of the key technologies to expand the capacity of steam turbine. When developing the last stage long blade, there are high requirements for the aerodynamic performance, vibration characteristics, material performance and machining process performance of the blade. At the same time, due to the growth of blade length, the protection of blade water erosion occupies a very important position. The low-pressure blades of steam turbine work in wet steam, and the blades produce water erosion under the action of water droplets. In particular, the low-pressure last stage blade is vulnerable to water erosion due to high steam humidity and high circumferential speed

Mechanism of water erosion

         The water film from the stationary blade surface to the water outlet edge is broken by steam flow and forms water droplets with a diameter of tens or hundreds of microns. When the water drop impacts the surface of the moving blade, it changes from spherical to membrane, so a high pressure is generated in the contact part between the water drop and the moving blade. The pressure exceeds the yield limit of the material, resulting in local plastic deformation and strain hardening of the blade material. Under the repeated action of this pressure, fatigue cracks begin to appear when the blade reaches the fatigue limit of the material. When the water drop strikes the inside of the crack, the pressure inside the water drop will make the crack develop deeper, causing the blade material to separate from the blade surface and form water erosion

Development of water erosion

         The water erosion rate of moving blades (water erosion per unit area per unit time) develops rapidly in the initial stage, and then will slow down significantly. Therefore, the water erosion rate is not equal in each period. (see Figure 1)

Figure 1 divides the process of water erosion into four areas, namely:

Latent zone: the weight of blade material decreases continuously, and only local plastic deformation and strain hardening occur

Acceleration zone: due to the continuous accumulation of fatigue in the material during the latent period, damage began to occur, the blade material separated, and the amount of water erosion increased sharply

Attenuation zone: water erosion rate decreases rapidly and water erosion slows down

Stable area: the water erosion rate almost maintains a fixed value and remains in a stable state

         As mentioned earlier, when the water erosion progresses to a certain extent, a layer of water film will remain on the roughened surface, which plays a buffering role because the water film absorbs the impact pressure of water droplets and greatly weakens the water erosion. In addition, even in the part without water film retention, after the blade surface becomes rough, there is a tendency for water droplets to impact its inclined plane, and the vertical component of water droplet velocity acting on the inclined plane is significantly reduced, resulting in a significant reduction in water erosion rate

         Due to the continuous occurrence of water erosion and fracture accidents of steam turbine blades, it poses a certain threat to the safety of steam turbine. Since the 1970s, many countries have carried out large-scale experimental research to find solutions, including the formation and concentration mechanism of corrosive medium on the metal surface, improving water chemistry conditions, ensuring steam quality, improving the operation level of power plants and strengthening monitoring during operation, looking for stable materials to prevent blade water corrosion, etc. However, the water erosion damage of blade is a very complex fracture phenomenon, which involves chemistry, metallography, physics and mechanics. There are more than 50 low concentration harmful compounds in steam as corrosion medium, but the effect of different impurities on blade water erosion is still unknown. The understanding of the formation and propagation process of blade water erosion fracture is basically empirical. In this regard, there are still a lot of experimental research work to be further carried out