Design Method of an Optimized Scheme for Rapid Cooling of Steam Turbine During Shutdown

Abstract

This invention provides a design method for an optimized rapid cooling scheme for steam turbine shutdown, comprising the following steps: geometrically modeling and meshing the steam turbine rotor; calculating safety criterion parameters using empirical formulas; completing a preliminary design of the rapid cooling scheme based on the calculated safety criterion parameters; setting boundary conditions for the finite element model of the steam turbine rotor, and then calculating the temperature and stress fields for the designed rapid cooling scheme; comparing and analyzing the calculated rotor temperature and stress field results under different rapid cooling schemes, and evaluating the feasibility of rapid cooling of the steam turbine unit with compressed air; and obtaining an optimized rapid cooling scheme based on the analysis results. This invention effectively shortens steam turbine shutdown time, saving valuable time for power plant maintenance and repair, and the saved shutdown time can be used for power generation to create higher economic benefits.

Description

Technical Field

[0001] This invention relates to the field of steam turbine shutdown technology, and in particular to a design method for an optimized rapid cooling scheme for steam turbine shutdown. Background Technology

[0002] Steam turbines are the core equipment in power plants for achieving thermoelectric conversion, and their effective operating time directly affects the power generation of the plant. However, under normal natural cooling conditions, the turbine shutdown and turning gear time is relatively long, which inevitably reduces the effective power generation time of the unit and lowers the economic benefits of the enterprise. How to shorten the turbine shutdown and cooling time to improve the power generation factor has become an urgent problem to be studied and solved in the power industry.

[0003] Introducing compressed air into the turbine cylinders enables rapid cooling and shutdown of the turbine. However, introducing excessively cold or excessively high-flow-rate cooling air can subject the turbine to drastic temperature gradient changes and generate thermal stress, leading to fatigue damage to the turbine rotor and affecting the unit's service life and operational safety. Therefore, it is necessary to minimize turbine downtime while ensuring unit safety. Summary of the Invention

[0004] The purpose of this invention is to provide a design method for an optimized rapid cooling scheme for steam turbine shutdown, aiming to minimize shutdown time while ensuring the safe operation of the steam turbine, thereby saving valuable time for unit maintenance and power generation.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A design method for an optimized rapid cooling scheme for steam turbine shutdown includes the following steps:

[0007] Step 1: Perform geometric modeling and meshing of the turbine rotor;

[0008] Step 2: Calculate the technical parameters of the safety criteria using empirical formulas;

[0009] Step 3: Complete the preliminary design of the rapid cooling scheme based on the safety criteria and technical parameters calculated in Step 2;

[0010] Step 4: Set the boundary conditions for the finite element model of the turbine rotor, and then calculate the temperature and stress fields for the rapid shutdown cooling scheme designed in Step 3.

[0011] Step 5: Compare and analyze the calculation results of rotor temperature field and stress field under different shutdown rapid cooling schemes designed in Step 4, and evaluate the feasibility of rapid cooling of steam turbine unit with compressed air;

[0012] Step 6: Based on the analysis results of Step 5, obtain an optimized shutdown rapid cooling solution.

[0013] Step 1 specifically includes:

[0014] Geometric modeling of the steam turbine rotor was carried out using CAD.

[0015] The established geometric model was imported into the finite element analysis software, and the corresponding coordinate system was set accurately. The unstructured mesh was initially divided into parts of the model using the software's built-in mesh generation function. Based on this, the local mesh was refined for the key parts of the rotor.

[0016] Furthermore, considering the complex geometry of the turbine rotor and the heat transfer path during the rapid air cooling shutdown process, reasonable assumptions are made about the rotor structure and energy transfer path during modeling to simplify the simulation process.

[0017] In step 2, the empirical formulas include the formula for calculating the allowable temperature difference between the cooling air and the rotor, the formula for calculating the maximum allowable temperature drop rate of the rotor, and the formula for calculating the maximum cooling air flow rate.

[0018] Furthermore, the calculation method for the allowable temperature difference between the cooling air and the rotor is as follows:

[0019] In forced compressed air cooling, the steam inlet zone is selected as the cross section for checking rotor thermal stress and life loss. The rotor surface stress is calculated using the Manson empirical formula, as follows:

[0020]

[0021] In the formula, E is the elastic modulus of the material; v is Poisson's ratio; B is the Biot number; R is the characteristic length, which is selected as the radius of the corresponding section of the rotor; λ is the thermal conductivity of the rotor; h is the heat transfer coefficient of the cooling air on the rotor surface; β is the coefficient of thermal expansion.

[0022] And calculate the local maximum stress σ of the rotor. max The formula is σ max =n·K th ·σ th In the formula, n is the safety factor; k th The stress concentration factor;

[0023] Based on the physical properties and fatigue characteristics of the rotor material, the upper limit of the maximum equivalent stress σ' of the rotor is calculated. max ,pass Calculate the maximum permissible temperature difference between the compressed air and the rotor surface.

[0024] Furthermore, the formula for calculating the maximum allowable temperature drop rate of the rotor is as follows: In the formula, η is the rate of temperature drop, and f is the shape factor.

[0025] Furthermore, the formula for calculating the maximum cooling airflow is as follows: In the formula, G a To cool the compressed air flow rate; C a Δt is used to cool the specific heat of compressed air. a To cool the temperature rise at the inlet and outlet of the compressed air; ρ a For cooling compressed air density; G mi For the mass of high and medium pressure cylinders, rotors, moving and stationary blades, etc.; C mi δt represents the specific heat of materials used in high and medium pressure cylinders, rotors, and moving and stationary blades. mi The cooling rate of the high-pressure cylinder, rotor, and moving and stationary blades.

[0026] Step 3 specifically includes: determining the minimum temperature of the cooling air introduced based on the allowable temperature difference between the cooling air and the rotor obtained in Step 2, combined with the original shutdown plan, and thus initially designing the temperature range of the cooling air introduced; and initially designing the cooling air flow rate based on the maximum cooling air flow rate and related technical implementation difficulties and economic factors.

[0027] Step 4, setting the boundary conditions for the finite element model of the turbine rotor specifically includes:

[0028] During the rapid cooling shutdown process of a steam turbine compressed air, the boundary conditions include two main categories: force boundary conditions and thermal boundary conditions.

[0029] For force boundary conditions, the stress generated by the rotor's own weight during operation, the compressive stress of compressed air on the rotor, and the centrifugal stress brought by the rotor's mass are all considered. Displacement constraints in different directions are applied to the support bearings and thrust bearings at both ends of the rotor.

[0030] For the thermal boundary conditions, there are the heat transfer coefficient between the rotor surface and the air and the temperature of the rotor surface. The bearings at both ends of the rotor are treated as the first type of boundary condition. The heat transfer between the cooling air and the rotor surface belongs to the third type of boundary condition with known heat transfer coefficient and cooling air temperature.

[0031] The calculation of temperature and stress fields for the rotor in the shutdown rapid cooling scheme designed in step 3 specifically includes: calculating the initial steady-state temperature field at the start of rapid cooling of the turbine based on the existing model; loading the initial steady-state temperature field onto the initial moment of the rapid cooling transient temperature field; and inputting the corresponding load to obtain the transient temperature field of the rapid cooling process. After calculating the rotor temperature field at each moment of the rapid cooling process, loading the calculated rapid cooling temperature field onto the transient stress field module, setting boundary conditions, and calculating the transient thermal stress field of the turbine rotor during the rapid cooling process.

[0032] In step 5, the temperature drop rate of each part of the rotor during the entire shutdown rapid cooling process is analyzed, and the temperature drop rate is compared with the maximum allowable cooling rate of the rotor calculated in step 2. If the temperature drop rate at a certain moment is greater than the maximum allowable cooling rate of the rotor, it is determined that the rapid cooling scheme does not meet the safety requirements; otherwise, it meets the requirements.

[0033] Compared with the prior art, the design method of the turbine shutdown rapid cooling optimization scheme provided by the present invention has the following beneficial effects:

[0034] The rapid cooling solution designed by this invention can be applied to the preliminary design of turbine shutdown schemes and the upgrading of existing turbine shutdown schemes after safety verification. This can effectively shorten turbine shutdown time, save valuable time for power plant maintenance, and the saved shutdown time can be used for power generation to create higher economic benefits. Attached Figure Description

[0035] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description will be briefly introduced below.

[0036] Figure 1 Flowchart of the design method for an optimized rapid cooling scheme for steam turbine shutdown;

[0037] Figure 2 This is a diagram of an axisymmetric two-dimensional geometric model of a steam turbine rotor.

[0038] Figure 3 Mesh partitioning diagram for the turbine rotor model. Detailed Implementation

[0039] The following detailed description provides further details on specific implementation methods.

[0040] like Figure 1 As shown, this invention provides a design method for an optimized rapid cooling scheme for steam turbine shutdown, comprising the following steps:

[0041] Step 1: Perform geometric modeling and meshing of the turbine rotor;

[0042] Step 2: Calculate the safety criteria technical parameters (allowable temperature difference between cooling air and rotor, maximum allowable cooling rate of rotor, and maximum cooling air flow rate) by referring to industry-recognized empirical formulas;

[0043] Step 3: Complete the preliminary design of the rapid cooling scheme based on the safety criteria and technical parameters calculated in Step 2;

[0044] Step 4: Set the boundary conditions for the finite element model of the turbine rotor, and then calculate the temperature and stress fields for the rapid shutdown cooling scheme designed in Step 3.

[0045] Step 5: Compare and analyze the calculation results of rotor temperature field and stress field under different shutdown rapid cooling schemes designed in Step 4, and evaluate the feasibility of rapid cooling of steam turbine unit with compressed air;

[0046] Step 6: Based on the analysis results of Step 5, obtain an optimized shutdown rapid cooling solution.

[0047] Step 1 specifically includes:

[0048] Based on the assembly drawings and other data of the turbine rotor, geometric modeling of the turbine rotor is carried out using CAD. Considering the complexity of the turbine rotor geometry and the heat transfer path during the rapid air cooling shutdown process, reasonable assumptions can be made about the rotor structure and energy transfer path during modeling (the rotor can be regarded as a cylindrical solid component with a stepped shaft and axisymmetric distribution during geometric modeling, so a two-dimensional model can be used for simulation calculation, simplifying the three-dimensional problem into a two-dimensional axisymmetric problem; the material properties of the rotor are only related to temperature, such as elastic modulus E and specific heat capacity C). p The simulation process is simplified by omitting factors such as thermal conductivity λ, without considering the anisotropy of materials, and neglecting small grooves and corners on the rotor surface, thus maintaining consistency with actual results as much as possible.

[0049] The established rotor geometric model is imported into finite element analysis software, and a coordinate system is set for the rotor model within the software. Specifically, the selected computational unit is a planar solid element. Considering the axisymmetry of the rotor, the planar solid element must have the y-axis as its axis of symmetry, and the entire two-dimensional model of the rotor must lie in the xy-plane to ensure the consistency of the computational unit properties. Next, the built-in meshing function of the finite element software is used to initially mesh the model with an unstructured mesh. Based on this, local mesh refinement is performed on key parts of the rotor (such as the first-stage impeller of the high-pressure cylinder and the areas before and after the extraction port). This invention verifies the mesh independence of different mesh node numbers, thereby comprehensively considering the impact of mesh quantity on computational accuracy and time factors, and finally selects a reasonable meshing scheme.

[0050] In step 2, the industry-recognized empirical formulas for calculating safety criterion parameters include:

[0051] (1) Calculation method for allowable temperature difference between cooling air and rotor:

[0052] In forced compressed air cooling, the steam inlet zone is selected as the cross section for checking rotor thermal stress and life loss. The rotor surface stress is calculated using Manson's empirical formula, as follows:

[0053]

[0054] In the formula:

[0055] E is the elastic modulus of the material, with units of MPa;

[0056] v is Poisson's ratio;

[0057] B is the number of Biots;

[0058] R is the characteristic length, in meters. In this calculation, the characteristic length is selected as the radius of the corresponding section of the rotor.

[0059] λ is the rotor thermal conductivity, with units of kJ / (m·K·h);

[0060] h is the heat transfer coefficient of the cooling air on the rotor surface, with units of W / (m²). 2 ·K);

[0061] β is the coefficient of thermal expansion, measured in °C. -1 .

[0062] Similarly, the local maximum stress σ of the rotor is calculated. max The formula is as follows:

[0063] σ max =n·K th ·σ th

[0064] In the formula:

[0065] n is the safety factor, which is usually taken as 1.5;

[0066] K th is the stress concentration factor.

[0067] Based on the rotor material properties (physical parameters and fatigue characteristics), the upper limit of the maximum equivalent stress σ of the rotor can be calculated. max If relevant information is lacking, referring to the Mitsubishi Turbine life loss criteria, it can be seen that when the maximum stress σ... max When the pressure is ≤200MPa, life loss can be disregarded; therefore, considering no life loss, the maximum allowable stress is taken as 200MPa. The maximum allowable temperature difference between the compressed air and the rotor surface is calculated using the following formula:

[0068]

[0069] (2) The formula for calculating the maximum allowable temperature drop rate of the rotor is as follows:

[0070]

[0071] In the formula:

[0072] η is the rate of temperature drop, in °C / h;

[0073] f refers to the shape factor.

[0074] (3) The formula for calculating the maximum cooling airflow is as follows:

[0075]

[0076] In the formula:

[0077] G a To cool the compressed air flow;

[0078] C a To cool the specific heat of compressed air;

[0079] Δt a To cool the temperature rise at the inlet and outlet of the compressed air;

[0080] ρ a To cool the density of compressed air;

[0081] G mi For the mass of high and medium pressure cylinders, rotors, moving and stationary blades, etc.;

[0082] C mi The specific heat of materials used in high and medium pressure cylinders, rotors, and moving and stationary blades;

[0083] δt mi The cooling rate of the high-pressure cylinder, rotor, and moving and stationary blades.

[0084] Step 3 specifically includes: determining the minimum temperature of the cooling air introduced based on the allowable temperature difference between the cooling air and the rotor obtained in Step 2, combined with the original shutdown plan, thereby initially designing the temperature range of the cooling air introduced; and initially designing the cooling air flow rate based on the maximum cooling air flow rate and related technical implementation difficulties and economic factors. Furthermore, in the design of the rapid cooling scheme described in this invention, the flow direction of compressed air within the cylinder usually needs to be considered. Co-current cooling means that in rapid cooling, the flow direction of compressed air is the same as that of steam; in counter-current cooling, the flow direction of compressed air is opposite to that of steam.

[0085] Step 4, setting the boundary conditions for the finite element model of the turbine rotor specifically includes:

[0086] During the rapid cooling shutdown of a steam turbine using compressed air, the loads on the turbine mainly include force loads and thermal loads. Therefore, the boundary conditions mainly fall into two categories: force boundary conditions and thermal boundary conditions.

[0087] For force boundary conditions, the rotor is subjected to stresses caused by its own weight during operation, compressive stresses from compressed air, and centrifugal stresses from the rotor's mass. For the support bearings and thrust bearings at both ends of the rotor, displacement constraints in different directions need to be applied.

[0088] The thermal boundary conditions include the heat transfer coefficient between the rotor surface and the air, and the temperature of the rotor surface. The rotor surface includes the shaft, both sides of the disc, the rim, and the steam seals (high-pressure rear steam seal, medium-pressure rear steam seal, and high-medium-pressure bridge steam seal). The bearings at both ends of the rotor can be treated as a Type 1 boundary condition because the return oil temperature is constant. The heat transfer between the cooling air and the rotor surface falls under a Type 3 boundary condition, where the heat transfer coefficient and cooling air temperature are known. The heat transfer coefficient of the rotor surface can be calculated using empirical formulas (as follows).

[0089] Calculation of heat transfer coefficient α1 at the optical axis:

[0090]

[0091] Nu = 0.1Re 0.68 ;

[0092]

[0093] In the formula, Re is the Reynolds number, u b ν is the circumferential velocity, ν is the kinematic viscosity of the cooling air, Nu is the Nusselt number, and λ is the circumferential velocity. s This is the thermal conductivity of the cooling air.

[0094] Calculation of the heat transfer coefficient α2 on both sides of the impeller:

[0095]

[0096] When Re ≤ 2.4 × 10 5 At that time, Nu = 0.675Re 0.5 ;

[0097] When Re>2.4×10 5 At that time, Nu = 0.0217Re 0.8 .

[0098]

[0099] In the formula, u c To calculate the circumferential velocity at the outer edge of the impeller section; R c Where is the impeller radius.

[0100] Calculation of the heat transfer coefficient α3 of the steam seal section:

[0101]

[0102] In the formula, ω is the average velocity of the cooling air, δ is the spacing of the steam seals, S is the steam seal tooth pitch, and H is the height from the rotor surface to the steam seal ring.

[0103] Heat transfer coefficient at the impeller tip:

[0104] The heat transfer coefficient at the top of the impeller can be calculated using the formula for the heat transfer coefficient of the optical axis. The qualitative temperature is taken as the average temperature of the left and right sides of the impeller.

[0105] Because the temperature of compressed air changes during flow due to heat transfer, this affects the selection of the characteristic temperature of compressed air at specific boundaries. Therefore, the calculation should be tailored to local conditions. Iterative calculation of the temperature of each part using thermal equilibrium can be considered.

[0106] Step 4 involves calculating the temperature and stress fields for the rotor under the rapid cooling scheme designed in Step 3. This includes: first, calculating the initial steady-state temperature field at the start of rapid cooling based on the existing model; then, applying this initial steady-state temperature field to the initial moment of the rapid cooling transient temperature field; and finally, inputting the corresponding load to obtain the transient temperature field during the rapid cooling process. After calculating the rotor temperature field at each moment of the rapid cooling process, the calculated rapid cooling temperature field is applied to the transient stress field module, and boundary conditions are set to calculate the transient thermal stress field of the turbine rotor during the rapid cooling process.

[0107] In step 5, when analyzing the temperature field calculation results of the rapid cooling scheme during shutdown, it is crucial to focus on the changes in the location of the highest temperature throughout the rapid cooling process. The criterion for ending the rapid cooling process is usually that the highest rotor temperature falls below the set temperature of the manufacturer or power plant. Furthermore, it is necessary to analyze the changes in the temperature drop rate of various parts of the rotor during the entire rapid cooling process and compare this rate with the maximum allowable cooling rate of the rotor calculated in step 2. If the temperature drop rate at any given moment exceeds the maximum allowable cooling rate of the rotor, the rapid cooling scheme is deemed not to meet safety requirements; otherwise, it meets the requirements.

[0108] When analyzing the stress field calculation results of the shutdown rapid cooling scheme, it is necessary to focus on the changes in the location and magnitude of the maximum stress during the entire rapid cooling process, as well as the stress changes in key rotor components (such as the first-stage impeller at high and medium pressure and the areas before and after the extraction port). The maximum stress value during the shutdown rapid cooling process should be compared with the upper limit of the maximum equivalent stress σ' of the rotor calculated in step 2. max The comparison is performed. If the value is consistently less than the latter, the safety requirement is met; otherwise, it is not.

[0109] In step 6, based on the temperature and stress field calculation results of different rapid cooling schemes in step 5, and taking into account the turbine structure, safety, economic analysis, and technical implementation difficulty, one or more preferred rapid cooling schemes for shutdown are proposed. Generally, lower air temperature, larger airflow, and co-current intake mean faster cooling speed and longer shutdown time savings. If it is desirable to minimize the equivalent stress during rapid shutdown cooling, then a higher cooling air temperature and counter-current intake are generally safer and more conservative.

[0110] For example, the specific operation steps of the present invention are as follows:

[0111] Step 1: Perform geometric modeling and meshing of the turbine rotor.

[0112] Based on the assembly drawings and other data of the turbine rotor, a geometric model of the turbine rotor is carried out using CAD. Due to the complex geometry of the rotor, reasonable assumptions need to be made. In geometric modeling, the rotor can be considered as a cylindrical solid component with a stepped shaft and an axisymmetric distribution. Therefore, a two-dimensional model can be used for simulation calculations, simplifying the three-dimensional problem into a two-dimensional axisymmetric problem. The material properties of the rotor are only temperature-dependent, such as the elastic modulus E and specific heat capacity C. p The thermal conductivity λ, etc., are not considered, and the anisotropy of the material is not taken into account. Small grooves and corners on the rotor surface are ignored. This is the two-dimensional geometric model of the turbine rotor. Figure 2 As shown.

[0113] Finite element analysis was performed using ANSYS software. The two-dimensional geometric model of the rotor was imported into ANSYS, and the selected computational element was a planar solid element. Considering the axisymmetry of the rotor, the planar solid element must have the y-axis as its axis of symmetry, and the entire two-dimensional model of the rotor must lie in the xy-plane to ensure the consistency of the computational element properties. The rotor model was automatically meshed in the Meshing platform using an unstructured meshing method, with a maximum mesh size of 30 mm. Based on this, local mesh refinement was performed on key parts of the high-pressure and intermediate-pressure rotor (high-pressure first stage, intermediate-pressure first stage, and areas before and after the extraction port, etc.), with a maximum mesh size set to 10 mm. This was achieved by reducing the basic size and adjusting the correlation.

[0114] Mesh independence was verified using different numbers of mesh nodes. As shown in Table 1, after the number of mesh nodes reached 25371, the maximum equivalent stress value remained essentially unchanged, with a relative change of only 0.1%. Since a large number of mesh nodes would significantly increase computation time, considering both time and computational accuracy, a mesh node count of 25371 was selected for subsequent calculations. The final rotor mesh generation results are shown below. Figure 3 As shown.

[0115] Table 1. Mesh independence verification results

[0116] Number of grid nodes 18620 25371 47746 78646 Equivalent stress value / MPa 53.06 52.31 51.70 52.37

[0117] Step 2: Calculate the safety criteria technical parameters (allowable temperature difference between cooling air and rotor, maximum allowable cooling rate of rotor, and maximum cooling air flow rate) by referring to industry-recognized empirical formulas.

[0118] (1) Calculation method for allowable temperature difference between cooling air and rotor:

[0119] In forced compressed air cooling, the steam inlet zone is selected as the cross section for checking rotor thermal stress and life loss. The rotor surface stress is calculated using Manson's empirical formula:

[0120]

[0121] Calculate the local maximum stress σ of the rotor max for:

[0122] σ max =n·K th ·σ th

[0123] Based on the rotor material properties (physical parameters and fatigue characteristics), the upper limit of the maximum equivalent stress σ of the rotor can be calculated. max If relevant information is lacking, please refer to the Mitsubishi Turbine life loss criteria of Japan, when the maximum stress σ max When the pressure is ≤200MPa, the life loss can be ignored. Therefore, considering the absence of life loss, the maximum allowable stress is taken as 200MPa. When the turbine life loss caused by rapid cooling is ignored and the rotor stress is considered, the safety factor n = 1.5; if calculated only according to the no-life-loss criterion, the safety factor n = 1.

[0124] The maximum permissible temperature difference between compressed air and the rotor surface is calculated using the following formula:

[0125]

[0126] Calculations show that when the turbine life loss due to rapid cooling is negligible and the rotor stress is considered, Δt... max =187.2℃. Based on the rotor's maximum temperature of 195℃ when the machine is stopped, the cooling air temperature range should be greater than 7.8℃.

[0127] (2) Formula for calculating the maximum allowable temperature drop rate of the rotor:

[0128]

[0129] Calculations show that the maximum temperature drop rate is 14.3℃ / h when the turbine life loss caused by rapid cooling is negligible and the rotor stress is considered. When calculated according to the no-life-loss criterion, the maximum temperature drop rate is 21.5℃ / h.

[0130] (3) Formula for calculating maximum cooling airflow:

[0131]

[0132] When the turbine life loss due to rapid cooling is negligible and the rotor stress is considered, the maximum cooling airflow rate can be calculated to be 83.21 m³ / s. 3 / min.

[0133] Step 3: Complete the preliminary design of the rapid cooling scheme based on the calculated safety criteria and technical parameters.

[0134] Considering the calculated maximum cooling airflow is 83.21 m³ / s. 3 / min, based on the empirical values ​​of compressed air rapid cooling air flow rates in other power plants, the cooling air flow rate here is taken as 80m³ / min. 3 / min. The initial plan is to conduct finite element analysis comparing two intake schemes: co-current and counter-current. Regarding the design of the compressed air temperature, the aforementioned calculations indicate that the cooling air temperature range should be greater than 7.8℃. To achieve better cooling, the compressed air inlet temperature should be less than 50℃. Considering the local air temperature at the power plant, the cooling air inlet temperatures are set at 10℃, 20℃, 30℃, and 40℃.

[0135] Step 4: Set the boundary conditions for the finite element model of the turbine rotor, and then perform temperature and stress field calculations for various preliminary shutdown rapid cooling schemes.

[0136] Firstly, regarding the force boundary conditions, the stresses generated by the rotor's own gravity during operation, the compressive stress exerted on the rotor by compressed air, and the centrifugal stress caused by the rotor's mass can be directly represented by the rotational speed. The stress field calculation for the rotor under only gravity and gas pressure shows a maximum stress of only 6.8 MPa. Therefore, during the subsequent rapid cooling process, the impact of stresses other than centrifugal and thermal stresses on the turbine rotor's start-up safety and lifespan loss can be considered negligible. For the support bearings and thrust bearings at both ends of the rotor, displacement constraints in different directions need to be applied. Since the rotor is an axisymmetric model, a two-dimensional model is used for simulation calculations, therefore an axial constraint needs to be applied in the horizontal direction.

[0137] The thermal boundary conditions include the heat transfer coefficient between the rotor surface and the steam, and the temperature of the rotor surface. (Refer to...) Figure 2 The rotor surface includes the optical shaft 7, the two sides of the disc 4, the rim 5, and the steam seals (high pressure rear steam seal 3, medium pressure rear steam seal 8, and high and medium pressure bridge steam seal 6). The heat exchange between the cooling air and the rotor surface belongs to the third type of boundary condition with known heat transfer coefficient and cooling air temperature. The end face 1 and center 10 of the rotor are heat-insulated. The bearings at both ends 2 and 9 are treated as the first type of boundary condition because the return oil temperature is constant.

[0138] After setting the boundary conditions, the finite element analysis of the rapid cooling process during shutdown can be carried out. The specific steps are as follows: first, the initial steady-state temperature field is loaded onto the initial moment of the rapid cooling transient temperature field, and then the corresponding load is input to obtain the transient temperature field of the rapid cooling process. After calculating the rotor temperature field at each moment of the rapid cooling process, the calculated rapid cooling temperature field is loaded onto the transient stress field module, and the boundary conditions are set in the same way to calculate the transient thermal stress field of the turbine rotor during the rapid cooling process.

[0139] Step 5: Compare and analyze the calculation results of rotor temperature field and stress field under different rapid cooling schemes in the preliminary design, and evaluate the feasibility of rapid cooling of the turbine unit with compressed air.

[0140] When analyzing the temperature field calculation results of a shutdown rapid cooling scheme, it is crucial to focus on the changes in the location of the highest temperature throughout the entire rapid cooling process. The criterion for ending the rapid cooling process is usually that the highest rotor temperature falls below the set temperature set by the manufacturer or power plant. Furthermore, it is necessary to analyze the changes in the temperature drop rate of various parts of the rotor throughout the entire shutdown rapid cooling process and compare this rate with the calculated maximum allowable cooling rate of the rotor. If the temperature drop rate at any given moment exceeds the maximum allowable cooling rate, the rapid cooling scheme is deemed to fail to meet safety requirements; otherwise, it meets the requirements.

[0141] When analyzing the stress field calculation results of the shutdown rapid cooling scheme, it is necessary to focus on the changes in the location and magnitude of the maximum stress during the entire rapid cooling process, as well as the stress value changes in key parts of the rotor (such as the first-stage impeller at high and medium pressure and the parts before and after the extraction port). The maximum stress value during the shutdown rapid cooling process should be compared with the calculated upper limit of the rotor's maximum equivalent stress. If the maximum stress value is consistently less than the latter, the safety requirements are met; otherwise, they are not.

[0142] Step 6: Provide design suggestions for an optimized shutdown rapid cooling scheme.

[0143] Since the stress fields in the calculation results all meet the requirement that the rotor stress is less than 200 MPa, the following three schemes can be proposed:

[0144] (1) A relatively conservative approach: If a conservative rapid cooling strategy is adopted, the equivalent thermal stress on the rotor should be minimized. The rapid cooling process involves introducing 40℃ and 80℃ air into the cylinder exhaust port. 3 Cooling air is supplied at a rate of / min, while simultaneously being discharged from the cylinder intake until the monitored cylinder wall temperature difference reaches the cylinder opening requirement (maximum temperature less than 100℃). During this process, the normal turning speed is maintained. In this scheme, the inlet air temperature is the highest, and a counter-flow arrangement is used. Although the cooling rate is the lowest, the equivalent thermal stress borne by the rotor is the lowest, which is relatively conservative.

[0145] (2) Relatively economical solution: Considering the economic efficiency of the rapid cooling process, it is natural to reduce the consumption of the corresponding gas production process. Since the inlet air temperature used in the calculation is within the normal air temperature range, the cooling air at ambient temperature can be directly supplied at 80m³ / h. 3 A flow rate of [flow rate] / min is introduced into the cylinder intake port and simultaneously discharged from the exhaust port until the monitored cylinder wall temperature difference reaches the cylinder opening requirement, maintaining a normal turning speed throughout the process. This method has lower consumption in cooling air pretreatment and is more economical.

[0146] (3) Common solution: Considering both the cooling effect and the control of rotor thermal stress, a co-current arrangement should be adopted to introduce 40℃ and 80m... 3 Cooling is achieved by using cooling air at a rate of / min. This method employs a larger inlet temperature but has a lower cooling rate, resulting in relatively lower thermal stress.

[0147] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method of steam turbine shutdown rapid cooling optimization, characterized by, Comprising the following steps: Step 1: geometric modeling and meshing of the steam turbine rotor; Step 2: calculation of safety criterion technical parameters by referring to empirical formula; the empirical formula includes a cooling air and rotor allowable temperature difference calculation formula, a rotor allowable maximum temperature drop rate calculation formula, and a maximum cooling air flow calculation formula; the cooling air and rotor allowable temperature difference calculation method is as follows: In forced cooling of compressed air, the inlet area is selected as the rotor thermal stress and life loss checking section, and the rotor surface stress is calculated by using the Manson empirical formula, as follows: In the formula, E is the elastic modulus of the material; v is the Poisson's ratio; B is the Biot number; R is the characteristic length, which is selected as the radius of the corresponding section of the rotor; λ is the thermal conductivity of the rotor; h is the rotor surface cooling air heat transfer coefficient; β is the thermal expansion coefficient; And the rotor local maximum stress σ max The formula is σ max = n·k th ·σ th , wherein n is a safety factor; k th is a stress concentration factor; According to the physical parameters and fatigue characteristics of the rotor material, the maximum equivalent stress upper limit σ of the rotor is calculated max , by calculating the maximum allowable temperature difference between the compressed air and the rotor surface; Step 3: completing the preliminary design of the quick cooling scheme according to the safety criterion technical parameters calculated in step 2; Step 4: setting the boundary conditions of the steam turbine rotor finite element model, and then carrying out temperature field and stress field calculation of the rotor in the shutdown quick cooling scheme designed in step 3; the setting of the boundary conditions of the steam turbine rotor finite element model specifically includes: In the process of compressed air quick cooling shutdown of the steam turbine, the boundary conditions include two categories of force boundary condition setting and thermal boundary condition setting; For the force boundary condition, the stress generated by the gravity of the rotor in operation, the pressure stress of the compressed air on the rotor, and the centrifugal stress caused by the mass of the rotor, the displacement constraints in different directions are applied to the support bearings and thrust bearings at both ends of the rotor; For the thermal boundary condition, there are the heat transfer coefficient of the rotor surface and air and the temperature of the rotor surface, the bearings at both ends of the rotor are treated as the first type of boundary condition, and the heat transfer between the cooling air and the rotor surface belongs to the third type of boundary condition with known heat transfer coefficient and cooling air temperature; The temperature field and stress field calculation of the rotor in the shutdown quick cooling scheme designed in step 3 specifically includes: according to the existing model, the initial steady temperature field at the beginning of the quick cooling of the steam turbine is calculated, the initial steady temperature field is loaded to the initial time of the quick cooling transient temperature field, and the corresponding load is input to obtain the transient temperature field in the quick cooling process; after the rotor temperature field at each time in the quick cooling process is calculated, the calculated quick cooling temperature field is loaded to the transient stress field module, the boundary conditions are set similarly, and the transient thermal stress field of the steam turbine rotor in the quick cooling process is calculated; Step 5: comparing and analyzing the temperature field and stress field calculation results of the rotor under different shutdown quick cooling schemes designed in step 4, and evaluating the feasibility of the compressed air quick cooling of the steam turbine unit; Step 6: obtaining an optimized shutdown quick cooling scheme according to the analysis results of step 5.

2. The method of steam turbine shutdown rapid cooling optimization of claim 1, wherein, Step 1 specifically includes: geometric modeling of the steam turbine rotor by using CAD; importing the established geometric model into the finite element analysis software and accurately setting the corresponding coordinate system, and preliminarily dividing the model by using the mesh division function of the software, and then locally encrypting the mesh of the key parts of the rotor.

3. The method of steam turbine shutdown rapid cooling optimization of claim 2, wherein, Considering the complex heat transfer path of the steam turbine rotor geometry and the air rapid cooling shutdown process, reasonable assumptions are made for the structure and energy transfer path of the rotor during modeling, thereby simplifying the simulation process.

4. The method of steam turbine shutdown rapid cooling optimization of claim 1, wherein, The maximum temperature drop rate allowed for the rotor is calculated by the formula wherein η is the temperature drop rate and f is the form factor.

5. The method of steam turbine shutdown rapid cooling optimization of claim 1, wherein, The maximum cooling air flow calculation formula is In the formula, G a is the cooling compressed air flow; C a is the specific heat of the cooling compressed air; Δt a is the temperature rise of the cooling compressed air at the inlet and outlet; ρ a is the density of the cooling compressed air; G mi is the mass of the high and medium pressure cylinder, rotor and moving and stationary blade row; C mi is the specific heat of the high and medium pressure cylinder, rotor and moving and stationary blade row material; δt mi is the cooling rate of the high and medium pressure cylinder, rotor and moving and stationary blade row.

6. The method of steam turbine shutdown rapid cooling optimization of claim 1, wherein, Step 3 specifically includes: combining the cooling air obtained according to step 2 with the allowable temperature difference of the rotor to determine the minimum temperature of the cooling air input according to the original shutdown scheme, thereby preliminarily designing the temperature range of the cooling air input; and preliminarily designing the cooling air flow according to the maximum cooling air flow and related technical implementation difficulty and economic factors.

7. The method of steam turbine shutdown rapid cooling optimization of claim 1, wherein, In step 5, the temperature drop rate variation of each part of the rotor in the entire rapid cooling process is analyzed, and the temperature drop rate is compared and verified with the maximum temperature drop rate allowed by the rotor calculated in step 2. If the temperature drop rate at a certain time is greater than the maximum temperature drop rate allowed by the rotor, it is determined that the rapid cooling scheme does not meet the safety requirements, otherwise it meets the requirements.

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