Steam turbine last-stage blade aging evaluation method based on relation between water erosion degree and hardness

Abstract

The invention discloses a steam turbine last-stage blade aging evaluation method based on the relation between the water erosion degree and the hardness. Sampling observation is conducted on different-degree water erosion parts of the steam inlet side of a steam turbine last-stage blade serving for a long time, and the hardness and the water erosion pit depth of the steam turbine last-stage blade are measured; fitting the measured data points by taking the depth of the water erosion pit as an X axis and the hardness of the water erosion sample as a Y axis to obtain a corresponding function relation curve of the water erosion degree and the hardness; the aging state of the last-stage blade of the steam turbine is evaluated by measuring the water erosion depth of the last-stage blade and then calculating the hardness of the last-stage blade. According to the steam turbine last-stage blade aging evaluation method based on the relation between the water erosion degree and the hardness, the real-time hardness of the position where the water erosion pit is located is calculated by measuring the depth of the water erosion pit online in real time, the operation service state and the aging degree of the steam turbine last-stage blade serving for a long time can be evaluated, operation is easy, cost is low, and the method is suitable for popularization and application. The evaluation accuracy is high, the stability is good, and convenience and rapidness are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of steam turbine blade service status assessment, and in particular to a method for assessing aging of steam turbine last-stage blades based on the relationship between water erosion degree and hardness. Background Art

[0002] In recent years, the proportion of intermittent renewable energy sources such as wind power and photovoltaics in the power system has increased significantly. To ensure the reliability and stability of power grid operation, thermal power units need to frequently participate in peak-shaving operations, and it has become common for steam turbines to operate under low load conditions for a long time. This operating mode differs significantly from the unit's initial design conditions, resulting in changes in the service environment of key components such as the turbine's last-stage blades, which in turn exacerbates water erosion. Water erosion causes blade material loss and profile changes, reducing the unit's thermal efficiency. In extreme cases, it can cause blade breakage and lead to serious production safety accidents.

[0003] The last-stage blades are one of the key components of a steam turbine. In service, the last-stage blades are subject to the strong effects of centrifugal force, steam pressure, and water droplet erosion in the steam zone, causing water erosion. The jagged burrs formed by water erosion on the last-stage blades of a steam turbine cause stress concentration, reduce the cross-sectional area of ​​the blade root, affect the vibration characteristics of the blades, significantly reduce the strength of the blades, deteriorate the aerodynamic performance of the cascade, and reduce stage efficiency. In severe cases, the blades may break and damage, leading to severe vibration and other serious accidents in the unit. Data shows that accidents caused by damaged turbine blades account for up to 40%. The last-stage blades of a steam turbine unit are generally more likely to fail than medium- and high-pressure blades, accounting for 58% of blade failure accidents. Most of the last-stage blade failures are caused by water erosion on the steam inlet side.

[0004] The water erosion process on the steam inlet side of a steam turbine's last-stage blades occurs when steam forms a water film on the stationary blades. The water film then tears, creating secondary water droplets that impact the steam inlet side of the moving blades. When the impact force exceeds the yield strength of the blade material, the blade surface undergoes plastic deformation, forming pits. Under the continued impact of the secondary water droplets, the depth of the pits on the blade surface gradually increases, forming water erosion pits. As the degree of water erosion accumulates, cracks are induced at the eroded areas, leading to material spalling and the formation of sawtooth-shaped sharp corners at the eroded areas. This not only degrades the blade's aerodynamic performance, but also easily causes stress concentration at the sawtooth-shaped sharp corners, forming crack sources, leading to a decrease in the blade's load-bearing capacity and even fracture, resulting in major safety accidents and economic losses. Therefore, timely assessment of the aging degree of long-serving last-stage blades can effectively prevent last-stage blade failures caused by water erosion on the steam inlet side.

[0005] Currently, there are few methods for assessing the aging condition of long-serving steam turbine last-stage blades. Most existing technologies use ultrasonic testing for internal damage or numerical simulation to simulate the stress field of the blades. However, ultrasonic testing can only produce ultrasonic images of internal damage, typically requiring experienced personnel to make rough estimates based on experience. Numerical simulation also requires consideration of complex actual operating conditions, resulting in poor accuracy and stability in the resulting simulation results. Summary of the Invention

[0006] The purpose of the present invention is to provide an aging assessment method for the last-stage blades of a steam turbine based on the relationship between the degree of water erosion and hardness, so as to solve the above-mentioned defects.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention proposes a method for evaluating the aging of a last-stage blade of a steam turbine based on the relationship between the degree of water erosion and hardness, comprising the following steps:

[0009] S1. Select a long-serving steam turbine last-stage blade in a working environment similar to that of the actual last-stage blade to be evaluated. Take samples from the steam inlet side of the blade from the root to the tip at locations with varying degrees of water erosion and measure their hardness.

[0010] S2. Observe the morphology of water erosion pits on samples with different degrees of water erosion and measure the depth of water erosion pits to determine the degree of water erosion;

[0011] S3. With the water erosion pit depth as the X-axis and the hardness of the water erosion sample as the Y-axis, the measured data points are fitted to obtain the corresponding functional relationship curve between the water erosion pit depth and the hardness;

[0012] S4. By using the functional relationship curve between water erosion pit depth and hardness, the actual hardness of the last-stage blades actually in service can be calculated by measuring the water erosion pit depth, thereby evaluating the aging status of the last-stage blades actually in service.

[0013] Preferably, in step S1, according to GB / T 8732-2014, the hardness of the last-stage blade of the steam turbine should be no less than 293 HBW.

[0014] Preferably, in step S1, the hardness of the water erosion sample is tested, and the specific method is: use a small load Brinell hardness tester with a force of 5 kg to measure, and each sample is tested three times or more, and then the average value is taken.

[0015] Preferably, in step S2, the water erosion degree is determined by qualitatively observing the morphology of the water erosion pits and quantitatively measuring the depth of the water erosion pits, wherein the depth of the water erosion pits is tested three or more times and then an average value is taken.

[0016] Preferably, in step S4, the depth of the water erosion pit is measured by replicating the erosion area of ​​the blade using a silicone rubber replica material, measuring the erosion depth of the replica using an optical microscope or an optical scanner, or measuring the remaining wall thickness using a portable ultrasonic thickness gauge using the reflection time of ultrasonic waves in the material, and calculating the erosion depth by comparing it with the original thickness.

[0017] The beneficial effects of the present invention are:

[0018] The aging assessment method for the last-stage blades of a steam turbine based on the relationship between the degree of water erosion and hardness of the present invention can be used to assess the operating status and aging degree of the last-stage blades of a long-term steam turbine by measuring the depth of the water erosion pits online in real time. The method has simple operation, low cost, high assessment accuracy, good stability, and is convenient and fast. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 : A photograph of a low-pressure last-stage blade of a steam turbine according to an embodiment of the present invention;

[0020] Figure 2 : Water erosion condition and sampling diagram of the low-pressure last stage blade of the steam turbine in the embodiment of the present invention;

[0021] Figure 3 : Electron microscope photos of water erosion pits of different samples in the embodiment of the present invention;

[0022] Figure 4 : Fitting curve of water erosion pit depth and hardness of the last-stage blade in the embodiment of the present invention after 10 years of service. DETAILED DESCRIPTION

[0023] The present invention is further described below with reference to the embodiments. It should be noted that these are merely examples and illustrations of the concept of the present invention. Those skilled in the art may make various modifications or additions to the described specific embodiments or replace them in a similar manner. As long as they do not deviate from the concept of the invention or exceed the scope defined by the claims, they should be deemed to fall within the scope of protection of the present invention.

[0024] Example 1:

[0025] like Figure 1-4 As shown, the present invention proposes a method for evaluating aging of the last-stage blades of a steam turbine based on the relationship between the degree of water erosion and hardness, comprising the following steps:

[0026] S1. Select a long-serving steam turbine last-stage blade in a working environment similar to that of the actual last-stage blade to be evaluated. Take samples from the root to the tip of the blade at different degrees of water erosion on the steam inlet side and measure their hardness.

[0027] Take the low-pressure last stage blades of a steam turbine that has been in service for 10 years in a power plant as an example. Figure 1 A photo of a low-pressure last-stage blade from a steam turbine that has been in service for 10 years at a power plant. The blade is made of 0Cr17Ni4Cu4Nb (17-4PH) steel. According to GB / T 8732-2014, the hardness of a steam turbine last-stage blade should be no less than 293 HBW.

[0028] Different water erosion locations of the last-stage blade of the steam turbine were sampled and observed along the blade direction. The degree of water erosion of the blade was graded according to the average depth of the water erosion pits and the water erosion morphology, and the hardness of the water erosion sampling location of the last-stage blade was measured. Figure 2 The following is the water erosion situation and sampling diagram of the low-pressure last stage blade of the steam turbine. After sampling and observing the water erosion area, SEM photos of four samples with different water erosion degrees were obtained, as shown in the following figure: Figure 3 As shown, the hardness of the front edge of the water erosion pit is measured.

[0029] The specific method for the hardness test of water erosion specimens is as follows: a Brinell hardness tester with a small load of 5 kg is used for measurement, and each specimen is tested three times or more, and then the average value is taken.

[0030] S2. Observe the morphology of water erosion pits on samples with different degrees of water erosion and measure the depth of water erosion pits to determine the degree of water erosion.

[0031] The degree of water erosion is determined by qualitative observation of the morphology of water erosion pits combined with quantitative measurement of the depth of water erosion pits. The depth of water erosion pits is measured three or more times and the average value is taken. Figure 3 As shown, the morphology of the water erosion pits was observed and the depth of the water erosion pits was measured.

[0032] S3. With the water erosion pit depth as the X-axis and the hardness of the water erosion sample as the Y-axis, the measured data points are fitted to obtain the corresponding functional relationship curve between the water erosion pit depth and the hardness.

[0033] Long-term water erosion impacts the last-stage blades of steam turbines, resulting in numerous pits and stress concentration. This long-term stress concentration triggers the initiation and propagation of microcracks within the material, reducing the structural density and load-bearing capacity at the leading edge of the erosion pits, leading to a decrease in hardness. Furthermore, during operation, centrifugal force increases stress concentration at the leading edge of the erosion pits, inducing dislocation annihilation or polygonization in this area, reducing dislocation density and causing softening, which further reduces hardness. This indicates that the degree of water erosion in the last-stage blades of long-serving steam turbine units is negatively correlated with the hardness of the erosion leading edge.

[0034] Therefore, with the water erosion pit depth as the X-axis and the hardness of the water erosion sample as the Y-axis, the measured data points are fitted to obtain the corresponding functional relationship curve between the water erosion pit depth and hardness, which can reflect the relationship between all water erosion pit depths and hardness.

[0035] like Figure 4 As shown in the figure, the measured water erosion pit depth of the blade is taken as the X-axis, the hardness of the front part of the water erosion pit is taken as the Y-axis, and the coordinate points are linearly fitted to obtain a linear relationship curve between water erosion pit depth and hardness. The dotted line in the figure is the lower limit of the blade material hardness specified in GB / T 8732-2014.

[0036] S4. By using the functional relationship curve between water erosion pit depth and hardness, the actual hardness of the last-stage blades actually in service can be calculated by measuring the water erosion pit depth, thereby evaluating the aging status of the last-stage blades actually in service.

[0037] To measure the water erosion depth, a replica material such as silicone rubber is used to replicate the eroded area of ​​the blade. The erosion depth of the replica is measured using an optical microscope or optical scanner, or the remaining wall thickness is measured using a portable ultrasonic thickness gauge using the reflection time of ultrasound in the material. The erosion depth is then calculated by comparing it with the original thickness. This method is simple to measure and can be used for on-site measurements during actual evaluations.

[0038] Therefore, based on the relationship curve between the water erosion pit depth and Brinell hardness of the last-stage blade that has been in service for 10 years, the water erosion pit depth of a last-stage blade in a certain service state can be measured by using silicone rubber replica materials to replicate the blade erosion area, or using a portable ultrasonic thickness gauge. The degree of hardness reduction of the water erosion area can be obtained, and its hardness can be judged to be qualified, thereby evaluating the aging status of the last-stage blade.

[0039] The aging assessment method for the last-stage blades of a steam turbine based on the relationship between the degree of water erosion and hardness of the present invention can be used to assess the operating status and aging degree of the last-stage blades of a long-term steam turbine by measuring the depth of the water erosion pits online in real time. The method has simple operation, low cost, high assessment accuracy, good stability, and is convenient and fast.

[0040] The above is an exemplary description of the invention. Obviously, the specific implementation of the present invention is not limited to the above-mentioned method. As long as such non-substantial improvements are made by adopting the method concept and technical solution of the present invention, or the concept and technical solution of the invention are directly applied to other occasions without improvement, they are all within the scope of protection of the present invention.

Claims

1. A method for evaluating the aging of the last stage blades of a steam turbine based on the relationship between the degree of water erosion and hardness, characterized in that: The following steps are involved: S1. Select a long-serving steam turbine last-stage blade in a working environment similar to that of the actual last-stage blade to be evaluated. Take samples from the steam inlet side of the blade from the root to the tip at locations with varying degrees of water erosion and measure their hardness. S2. Observe the morphology of water erosion pits on samples with different degrees of water erosion and measure the depth of water erosion pits to determine the degree of water erosion; S3. With the water erosion pit depth as the X-axis and the hardness of the water erosion sample as the Y-axis, the measured data points are fitted to obtain the corresponding functional relationship curve between the water erosion pit depth and the hardness; S4. By using the functional relationship curve between water erosion pit depth and hardness, the actual hardness of the last-stage blades actually in service can be calculated by measuring the water erosion pit depth, thereby evaluating the aging status of the last-stage blades actually in service.

2. The method for evaluating aging of the last stage blades of a steam turbine based on the relationship between water erosion degree and hardness according to claim 1, characterized in that: In step S1, according to GB / T8732-2014, the hardness of the last-stage blade of the steam turbine should be no less than 293 HBW.

3. The method for evaluating aging of the last stage blades of a steam turbine based on the relationship between water erosion degree and hardness according to claim 1, characterized in that: In step S1, the hardness of the water erosion sample is tested. The specific method is: use a small load Brinell hardness tester with a force of 5 kg to measure, and each sample is tested three times or more, and then the average value is taken.

4. The method for evaluating aging of the last stage blades of a steam turbine based on the relationship between water erosion degree and hardness according to claim 1, characterized in that: In step S2, the degree of water erosion is determined by qualitatively observing the morphology of water erosion pits and quantitatively measuring the depth of water erosion pits, wherein the depth of water erosion pits is measured three or more times and then an average value is taken.

5. The method for evaluating aging of the last stage blades of a steam turbine based on the relationship between water erosion degree and hardness according to claim 1, characterized in that: In step S4, the water erosion depth is measured by using a silicone rubber replica material to replicate the blade erosion area, and then measuring the erosion depth of the replica using an optical microscope or an optical scanner, or using a portable ultrasonic thickness gauge to measure the remaining wall thickness using the reflection time of ultrasonic waves in the material, and then calculating the erosion depth by comparing it with the original thickness.

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