Method for determining maximum fatigue damage point in starting and stopping process of steam turbine rotor

Abstract

The invention belongs to the technical field of turbine rotor fatigue life prediction, and particularly relates to a method for determining a maximum fatigue damage point in the start-stop process of a turbine rotor, and the method comprises the steps of geometric modeling, material parameter attribute setting, grid division, boundary condition setting, solution calculation and post-processing. According to the method, through comparison of evaluation results of fatigue damage points under different starting and stopping conditions, judgment is carried out based on results of the MLF criterion, the influence of fluctuation changes of thermophysical parameters of the steam turbine rotor on fatigue damage calculation of the steam turbine rotor is considered, and the method aims at calculating the fatigue damage of the steam turbine rotor through trend comparison of MLF results of different large thermal stress positions. Obtaining the maximum damage point in the start-stop low-cycle fatigue calculation process of the steam turbine rotor; the method is suitable for quantitative evaluation of the maximum low-cycle fatigue damage point of the steam turbine rotor, and the effectiveness and feasibility of the criterion in prediction of the maximum low-cycle fatigue damage point of the steam turbine rotor are verified through the embodiment.

Description

Technical Field

[0001] This invention belongs to the field of turbine rotor fatigue life prediction technology, and particularly relates to a method for determining the maximum fatigue damage point during the start-up and shutdown process of a turbine rotor. Background Technology

[0002] As the core equipment of modern thermal power plants, steam turbines experience complex thermomechanical loads on their rotors during frequent start-ups and shutdowns, making them prone to fatigue damage in specific areas. Steam turbine rotors also endure alternating thermal and mechanical stresses during start-ups and shutdowns, making fatigue analysis a crucial method for assessing their lifespan. Currently, research on fatigue damage of steam turbine rotors under start-up and shutdown conditions is being conducted from multiple perspectives, including finite element simulation, experimental data analysis, and life prediction models. The research covers aspects such as the thermal stress distribution patterns of high-parameter ultra-supercritical steam turbine rotors, methods for identifying critical areas, calculation of low-cycle fatigue life loss, and optimized operation recommendations, providing theoretical basis and technical reference for the design optimization, operation control, and lifespan management of steam turbine rotors.

[0003] During startup, shutdown, and load shaving, the internal medium flow and temperature changes of a steam turbine rotor over time lead to uneven temperature variations, causing significant thermal stress and increased low-cycle fatigue losses. Low-cycle fatigue is a major factor contributing to reduced turbine rotor lifespan. Therefore, safety analysis of steam turbine rotors must focus on low-cycle fatigue.

[0004] Chinese invention patent application number 201310163358.1 discloses a method for predicting the thermal stress of a throttling-regulated steam turbine rotor. The steps are as follows: First, identify the current state of the steam turbine; second, formulate the power and key steam parameter variation curves of the steam turbine; third, formulate the steam parameter variation curves of the steam turbine thermal stress monitoring points; fourth, formulate the outer surface temperature variation curve of the steam turbine rotor; fifth, formulate the volume average temperature and inner surface temperature variation curves of the steam turbine rotor; sixth, formulate the temperature difference or thermal stress variation curves of the steam turbine rotor; seventh, verify the variation curves of each key parameter and display the output. The accuracy of the thermal stress prediction can be verified using existing real-time monitoring systems for steam turbine rotor thermal stress, and relevant coefficients and constants can be dynamically learned and adjusted to improve the accuracy of thermal stress prediction.

[0005] Current calculations for low-cycle fatigue of steam turbine rotors often employ analytical or finite element methods to calculate temperature-induced thermal stress, followed by calculations of the thermal stress amplitude changes during each start-up and shutdown cycle. Finally, the linear fatigue remaining life is calculated based on the SN or EN curves. However, existing methods select the maximum fatigue damage point as the location of the maximum thermal stress calculated from a single start-up and shutdown cycle, neglecting the fact that different start-up and shutdown cycles will alter the location of the maximum thermal stress. These shortcomings introduce significant errors into the calculation of low-cycle fatigue damage of the steam turbine rotor, affecting the accuracy of the control process. Summary of the Invention

[0006] The purpose of this invention is to provide a method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor, overcoming the shortcomings of the prior art. The aim is to obtain a method for determining the maximum fatigue damage point by comparing the evaluation results of fatigue damage under different start-up and shutdown conditions, accurately determining the location of the maximum fatigue damage, and improving the accuracy of the control process.

[0007] To achieve the above objectives, the present invention provides a method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor, comprising geometric modeling, material parameter property setting, mesh generation, boundary conditions, solution calculation, and post-processing, with the following steps:

[0008] 1) Geometric modeling: Establish a finite element model of the turbine rotor based on its geometric dimensions.

[0009] 2) Material parameter property settings, including one or more of the following: elastic modulus, Poisson's ratio, coefficient of thermal expansion, thermal conductivity, specific heat capacity, material density, and material fatigue performance parameters.

[0010] 3) Mesh generation: The turbine rotor is meshed.

[0011] 4) Boundary conditions: Using the temperature of the medium surrounding the turbine rotor under different start-up and shutdown modes as the boundary conditions, calculate the thermal stress field cloud diagram of the turbine rotor.

[0012] 5) Solve the calculation: Based on the stress cloud diagram of the turbine rotor under different boundary conditions, determine multiple thermal stress locations that reach more than 80% of the yield strength value, extract the stress time history data of the thermal stress locations with more than 80% of the yield strength value, and use the following mathematical formula to determine the location of maximum fatigue damage under various start-up and shutdown conditions.

[0013]

[0014] ψσ i,k =max{σ i,k}-min{σ i,k}

[0015] ψσi,n =max{v i,n}-min{σ i,n}

[0016] Where σ i,n Stress time history data representing the locations of the largest thermal stresses according to standard parameters, where i represents the number of locations of the largest thermal stresses, i ranging from 1 to N; σ i,k The stress time history data series represents the locations of large thermal stresses under different material parameter conditions, where k represents the number of groups under different material parameter conditions, and k belongs to 1 to M; the standard model formula for MLF is:

[0017]

[0018] 6) The stress time history data of the thermal stress location with a yield strength of more than 80% is calculated using the MLF criterion. The larger the MLF value, the greater the fluctuation of the stress time history data at the larger thermal stress location, which is the maximum fatigue damage point, thus completing the determination of the maximum fatigue damage location.

[0019] The material fatigue performance parameters in step 2) include at least the nominal yield strength σ. 0.2 Tensile strength σ b The ratio of fatigue strength to elastic modulus σ f / E, fracture strength σ f Fracture strain ε f 、Real constant b、、

[0020] The Poisson ratio is set to 0.3.

[0021] The density of the material is 8000 kg / m³. 3 .

[0022] The boundary conditions are the changes in rotor speed and the temperature changes in the airflow around the rotor during start-up and shutdown.

[0023] In the boundary conditions, the temperature rise curves of each part of the high-pressure rotor are fitted into a linear relationship, and the bearings at both ends of the high-pressure rotor are considered to be at a constant temperature.

[0024] The temperature of bearing #1 is set to 80℃, and the temperature of bearing #2 is set to 77℃.

[0025] In step 5), the transient thermal-structural coupling method is used to calculate the thermal stress of the turbine rotor.

[0026] The location of maximum stress in step 1 is set at the corner of the profile machined on the turbine rotor.

[0027] The temperature range is 20-550℃.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] 1) This invention compares the fatigue damage assessment results under different start-up and shutdown conditions, makes a judgment based on the results of the MLF criterion, and considers the influence of the fluctuation of the thermal property parameters of the turbine rotor on the fatigue damage calculation of the turbine rotor. The aim is to obtain the maximum damage point in the low-cycle fatigue calculation process of the turbine rotor during start-up and shutdown by comparing the trend of the MLF results at different locations with large thermal stress.

[0030] 2) The determination method proposed in this invention is applicable to the quantitative evaluation of the maximum low-cycle fatigue damage point of the turbine rotor, and finally obtains the determination method of the maximum fatigue damage point. It accurately determines the location of the maximum fatigue damage, improves the accuracy of the control process, and verifies the effectiveness and feasibility of the criterion in predicting the maximum low-cycle fatigue damage point of the turbine rotor through examples. Attached Figure Description

[0031] Figure 1 This is a turbine rotor geometric model according to an embodiment of the present invention;

[0032] Figure 2 This is a turbine rotor mesh model according to an embodiment of the present invention;

[0033] Figure 3 This refers to the change in rotational speed of the turbine rotor during startup, as described in this embodiment of the invention.

[0034] Figure 4 This refers to the change in main steam temperature during the turbine rotor startup process in an embodiment of the present invention.

[0035] Figure 5 This is the thermal stress calculation result of embodiment 35149s of the present invention;

[0036] Figure 6 This is the thermal stress calculation result of embodiment 38479s of the present invention;

[0037] Figure 7 This is a schematic diagram of the location with relatively large thermal stress according to an embodiment of the present invention;

[0038] Figure 8 This is the result of the variable parameter finite element thermal stress calculation at position 1 of the present invention;

[0039] Figure 9 This is the result of the variable parameter finite element thermal stress calculation at position 2 of the present invention;

[0040] Figure 10 This is the result of the variable parameter finite element thermal stress calculation at position 3 of the present invention;

[0041] Figure 11This is the result of the variable parameter finite element thermal stress calculation at position 4 of the present invention;

[0042] Figure 12 This is the result of the variable parameter finite element thermal stress calculation at position 5 of the present invention;

[0043] Figure 13 These are the calculated MLF values ​​at five locations in this embodiment of the invention. Detailed Implementation

[0044] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0045] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the specific embodiments used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the specific embodiments described below are some embodiments of the present invention. For those skilled in the art, other specific embodiments can be obtained based on these specific embodiments without creative effort.

[0046] The components of the embodiments of the invention described and shown in the specific embodiments herein can be arranged and designed in countless different configurations. Therefore, the following detailed description of the embodiments of the invention provided in the specific embodiments is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention.

[0047] An embodiment of the method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor according to the present invention includes geometric modeling, material parameter property setting, mesh generation, boundary conditions, solution calculation, and post-processing. The specific operation steps are as follows:

[0048] 1) Geometric modeling, see Figure 1 A finite element model of the steam turbine rotor is established based on its geometric dimensions. The steam turbine rotor is subjected to alternating thermal and mechanical stresses during start-up and shutdown, and fatigue analysis is an important method for assessing its lifespan. A precise three-dimensional geometric model of the rotor is established based on the rotor drawings. This model considers details of key components such as blade root slots, center bores, and transition fillets. Furthermore, geometric simplification is performed when necessary, retaining the main features.

[0049] 2) Material parameter property settings, including at least one of the following: elastic modulus, Poisson's ratio, coefficient of thermal expansion, thermal conductivity, specific heat capacity, material density, and material fatigue performance parameters; the setting of material properties mainly includes the following aspects:

[0050] 2-1) Elastic modulus

[0051] Table 1. Changes in elastic modulus with temperature

[0052] Temperature (℃) Elastic modulus (GPa) 25 214 100 212 200 205 300 199 400 190 500 178 600 176

[0053] 2-2) Poisson's ratio is 0.3

[0054] 2-3) Coefficient of thermal expansion

[0055] Table 2. Variation of thermal expansion coefficient with temperature

[0056] Temperature (℃) coefficient of thermal expansion 25 <![CDATA[1.199×10 -5 ]]> 100 <![CDATA[1.199×10 -5 ]]> 200 <![CDATA[1.281×10 -5 ]]> 300 <![CDATA[1.325×10 -5 ]]> 400 <![CDATA[1.366×10 -5 ]]> 500 <![CDATA[1.392×10 -5 ]]> 600 <![CDATA[1.415×10 -5 ]]>

[0057] 2-4) Thermal conductivity

[0058] Table 3. Changes in thermal conductivity with temperature

[0059] Temperature (℃) Thermal conductivity (w / (m×℃)) 25 44.8 100 44.8 200 44.8 300 42.8 400 40.8 500 37.5 600 35.3

[0060] 2-5) Specific heat capacity

[0061] Table 4. Changes in specific heat capacity with temperature

[0062] Temperature (℃) Specific heat capacity (J / (kg×℃)) 25 599 100 599 200 599 300 624 400 666 500 720 600 804

[0063] 2-6) Density is 8000 kg / m³ 3

[0064] 2-7) Fatigue performance parameters of materials

[0065] Table 5 Low-cycle fatigue parameters

[0066]

[0067] 3) Mesh generation: The turbine rotor is meshed, see... Figure 2 This is a mesh model of a steam turbine rotor;

[0068] 4) Boundary conditions: The boundary conditions mainly consider the changes in rotor speed and the changes in airflow temperature around the rotor during start-up and shutdown, as follows: Figures 3-4 As shown.

[0069] To facilitate the calculation of rotor thermal stress, the temperature rise curves of various parts of the high-pressure rotor can be fitted to a linear relationship. Since the bearing temperature changes relatively little during the entire cold start process and has little impact on the overall rotor stress, bearings #1 and #2 can be considered to be at a constant temperature, i.e., bearing #1 temperature is 80℃ and bearing #2 temperature is 77℃. The fitted temperature rise curves of the high-pressure turbine rotor during cold start are shown below. Figure 4 As shown.

[0070] 5) Solving the calculation: The transient thermal-structural coupling method is used to calculate the thermal stress of the turbine rotor. Based on the stress cloud diagrams of the turbine rotor under different boundary conditions, the locations of multiple large thermal stresses are determined. The stress time history data of the locations of the large thermal stresses are extracted, and the following mathematical formula is used to determine the location of the maximum fatigue damage under various start-up and shutdown conditions;

[0071]

[0072] ψσ i,k =max{σ i,k}-min{σ i,k}

[0073] (Formula 3)

[0074] ψσ i,n =max{σ i,n}-min{σ i,n}

[0075] (Formula 4)

[0076] Where σ i,n Stress time history data representing the locations of the largest thermal stresses according to standard parameters, where i represents the number of locations of the largest thermal stresses, i ranging from 1 to N; σ i,k The stress time history data sequence represents the location of large thermal stress under different material parameter conditions, where k represents the number of groups under different material parameter conditions, and k belongs to 1 to M;

[0077] The standard model formula for MLF is:

[0078]

[0079] In post-processing, the MLF criterion is used to calculate the stress time history data of the proposed location of greater thermal stress. The larger the MLF value, the greater the fluctuation of the stress time history data at the location of greater thermal stress, which is the maximum fatigue damage point, thus completing the determination of the location of the maximum fatigue damage.

[0080] 6) Post-processing: Extract the thermal stress time history curve and select the maximum stress value for fatigue damage calculation. The thermal stress contour maps at different time points are shown below. Figure 5 and Figure 6 As shown, the location of the maximum stress calculated at different time periods is not the same, making it difficult to determine which location to use as the location of the maximum fatigue damage.

[0081] Furthermore, the above summarizes the general process for low-cycle fatigue calculation of turbine rotors. The material properties and boundary conditions considered in finite element analysis (FEM) are idealized. In reality, the material properties and boundary conditions of turbine rotors deviate from those set in the FEM calculation, and these deviations will cause changes in the location of maximum damage during fatigue calculation, making accurate calculation impossible. Therefore, this patent proposes a method to determine the location of relative maximum thermal stress. Since thermal stress and fatigue have a linear relationship, the accurate fatigue damage location can be obtained.

[0082] Because there are too many parameters affecting the fatigue damage of the turbine rotor during start-up and shutdown, four typical parameters—elastic modulus, coefficient of linear expansion, thermal conductivity, and specific heat capacity—are selected for variable-parameter finite element thermal stress calculation. Considering a fatigue safety factor of 1.2, an orthogonal test method with a variation range of 20% is used for grouping. The specific variation levels and detailed groupings are shown in Tables 6 and 7, respectively.

[0083] Table 6 Level Factors

[0084]

[0085]

[0086] Table 7 Detailed Calculation Grouping Table

[0087] Group number coefficient of thermal expansion elastic modulus thermal conductivity Specific heat capacity 1 1 1 1 1 2 1.2 1 1 1 3 0.8 0.8 0.8 0.8 4 0.8 1 1 1 5 0.8 1.2 1.2 1.2 6 1 0.8 1 1.2 7 1 1 1.2 0.8 8 1 1.2 0.8 1 9 1.2 0.8 1.2 1 10 1.2 1 0.8 1.2 11 1.2 1.2 1 0.8

[0088] The thermal stress of the turbine rotor is calculated based on the grouping in Table 7. Extracting... Figure 7 The stress-time history curves at locations with relatively high thermal stress are shown in the figures below. Figure 8 , Figure 9 , Figure 10 , Figure 11 and Figure 12 As shown. The time-history stress calculation results based on different locations are calculated using the following formulas (1), (2), (3), and (4). The calculation results are as follows. Figure 13 As shown. Obviously, the MLF value is the largest at position 1, which is the position of the relative maximum thermal stress, i.e., the position of maximum fatigue damage.

[0089] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor, characterized in that, The process includes geometric modeling, setting material parameters, mesh generation, boundary conditions, solution calculation, and post-processing, and the steps are as follows: 1) Geometric modeling: Establish a finite element model of the turbine rotor based on its geometric dimensions; 2) Material parameter property settings, including one or more of the following: elastic modulus, Poisson's ratio, coefficient of thermal expansion, thermal conductivity, specific heat capacity, material density, and material fatigue performance parameters; 3) Mesh generation: The turbine rotor is meshed. 4) Boundary conditions: Using the temperature of the medium surrounding the turbine rotor under different start-up and shutdown modes as the boundary conditions, calculate the thermal stress field cloud diagram of the turbine rotor; 5) Solve the calculation: Based on the stress cloud diagram of the turbine rotor under different boundary conditions, determine multiple thermal stress locations that reach more than 80% of the yield strength value, extract the stress time history data of the thermal stress locations with more than 80% of the yield strength value, and use the following mathematical formula to determine the location of maximum fatigue damage under various start-up and shutdown conditions. ss i,k =max{σ i,k }-min{σ i,k } ss i,n =max{σ i,n }-min{σ i,n } Where σ i,n Stress time history data representing the locations of the largest thermal stresses according to standard parameters, where i represents the number of locations of the largest thermal stresses, i ranging from 1 to N; σ i,k The stress time history data series represents the locations of large thermal stresses under different material parameter conditions, where k represents the number of groups under different material parameter conditions, and k belongs to 1 to M; the standard model formula for MLF is: 6) The stress time history data of the thermal stress location with a yield strength of more than 80% is calculated using the MLF criterion. The larger the MLF value, the greater the fluctuation of the stress time history data at the larger thermal stress location, which is the maximum fatigue damage point, thus completing the determination of the maximum fatigue damage location.

2. The method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor according to claim 1, characterized in that, The material fatigue performance parameters in step 2) include at least the nominal yield strength σ. 0.2 Tensile strength σ b The ratio of fatigue strength to elastic modulus σ f / E, fracture strength σ f Fracture strain ε f 、Real constant b、、 3. The method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor according to claim 1, characterized in that, The Poisson ratio is set to 0.

3.

4. The method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor according to claim 2, characterized in that, The density of the material is 8000 kg / m³. 3 .

5. The method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor according to claim 1, characterized in that, The boundary conditions are the changes in rotor speed and the temperature changes in the airflow around the rotor during start-up and shutdown.

6. The method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor according to claim 5, characterized in that, In the boundary conditions, the temperature rise curves of each part of the high-pressure rotor are fitted into a linear relationship, and the bearings at both ends of the high-pressure rotor are considered to be at a constant temperature.

7. The method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor according to claim 6, characterized in that, The temperature of bearing #1 is set to 80℃, and the temperature of bearing #2 is set to 77℃.

8. The method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor according to claim 1, characterized in that, In step 5), the transient thermal-structural coupling method is used to calculate the thermal stress of the turbine rotor.

9. The method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor according to claim 1, characterized in that, The location of maximum stress in step 1 is set at the corner of the profile machined on the turbine rotor.

10. The method for determining the maximum fatigue damage point during the start-up and shutdown process of a steam turbine rotor according to claim 1, characterized in that, The temperature range is 20-550℃.

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