Method and system for evaluating safety of insertion-type drum central downcomer crack

Abstract

The application discloses an insertion type boiler drum centralized descending pipe crack safety evaluation method and system, belongs to the technical field of power station equipment evaluation, solves the problems of insufficient pertinence, low evaluation efficiency and weak result reference of the prior art, and the method comprises the following steps: a three-dimensional finite element model containing a crack defect is established based on multi-source heterogeneous evaluation benchmark parameters; the residual life and crack propagation path under the current crack state are calculated based on a crack propagation prediction model combined with a stress intensity factor distribution; and the safety level of the current crack of the insertion type boiler drum centralized descending pipe is comprehensively evaluated; the application is aimed at the weld and heat affected zone crack defects of the insertion type boiler drum centralized descending pipe, a complete evaluation system covering geometric parameterized modeling, accurate stress intensity factor calculation, multi-mechanism coupled crack propagation prediction and three-dimensional weighted safety level evaluation is constructed, and full-process quantitative analysis from crack detection to maintenance decision is realized.

Description

Technical Field

[0001] This invention belongs to the field of power plant equipment evaluation technology, specifically relating to a method and system for safety assessment of cracks in the centralized downcomer of an immersion boiler drum. Background Technology

[0002] The power plant boiler is the core energy conversion equipment in a thermal power generation system. Among them, the boiler drum (steam drum) is a key pressure-bearing component for working fluid storage and steam-water separation, and its operational safety directly determines the overall reliability of the unit. The immersion-type boiler drum downcomer is the main channel for supplying circulating cooling water from the boiler drum to the water-cooled walls. It is usually made of large-diameter, thick-walled pipes welded to the boiler drum body. The interface is characterized by structural discontinuity and significant stress concentration effect. Under the combined effects of high temperature and high pressure working fluid, alternating thermal stress during start-up and shutdown, and pipe system vibration loads, the weld seams and heat-affected zone of the immersion-type boiler drum downcomer are prone to the initiation and propagation of crack defects.

[0003] Cracks in the centralized downcomer area of ​​an insert-type boiler drum mainly include fatigue cracks, stress corrosion cracks, and creep cracks. Their formation mechanisms are complex. Current safety assessments of crack defects mainly rely on traditional methods: one type is the standard-based conventional use evaluation method, which usually only considers the membrane stress under a single static pressure load, ignores the bending stress and peak stress caused by structural discontinuities, and does not fully consider the fatigue damage accumulation caused by start-stop cycles and variable load operation; the other type is to directly use general finite element software to carry out numerical simulations. However, for special structures like insert-type downcomers, there is a lack of suitable geometric parameterization modeling rules, load condition combination standards, and crack propagation quantitative judgment logic. The evaluation results have high dispersion and are difficult to directly support maintenance decisions.

[0004] Meanwhile, existing assessment processes are mostly offline and manual, relying on manual input of traditional non-destructive testing results such as ultrasonic and magnetic particle testing for crack size detection. Stress calculation and fracture mechanics assessment are disconnected, and the cycle from on-site inspection to issuing an assessment conclusion generally exceeds 7 days, failing to meet the rapid decision-making needs for unit shutdown maintenance and condition-based maintenance. Furthermore, existing methods lack a predictive model linking crack propagation rate and remaining service life, making it impossible to provide quantitative recommendations for the next inspection cycle. This often leads to the dilemma of either "over-maintenance" or "uncontrollable risks of operating with defects."

[0005] Chinese patent CN121298795A discloses a method and related apparatus for safety assessment of crack defects in power plant boiler tubes, belonging to the field of power plant equipment safety assessment technology. The method includes the following steps: crack detection of the power plant boiler tubes, and metallographic observation, crack analysis, and performance analysis of the crack defect samples to obtain metallographic observation results, crack microscopic analysis data, and material performance evaluation results; establishing a crack propagation model based on location and geometric data to predict the propagation rate; finally, assessing the remaining service life of the power plant boiler tubes to obtain the remaining safe operating life; and comprehensively conducting a safety assessment based on the above data to obtain the crack defect safety assessment results and decision recommendations, thus completing the safety assessment of crack defects in the power plant boiler tubes. However, existing methods lack crack propagation path tracking and incremental iterative simulation of the coupling effect of fatigue and stress corrosion, only providing a rough estimate of the remaining service life, and cannot output a visualized crack propagation trend and quantified safety margin, making it difficult to support accurate classification decisions and condition-based maintenance arrangements for inserted downcomers under complex start-up, shutdown, and variable load conditions.

[0006] In summary, existing technologies suffer from insufficient targeting, low evaluation efficiency, and weak referentiality of results, making them difficult to adapt to the current development needs of condition-based maintenance and intelligent operation and maintenance of thermal power equipment. To address these issues, we propose a method and system for assessing the safety of cracks in the centralized downcomer of the boiler drum. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies by providing a method and system for assessing the safety of cracks in the centralized downcomer of an insert-type boiler drum, thus solving the problems of insufficient targeting, low assessment efficiency, and weak reference value of existing technologies.

[0008] This invention is implemented as follows: a method for assessing the safety of cracks in the centralized downcomer of an insert-type boiler drum, the method comprising:

[0009] The multi-source heterogeneous evaluation benchmark parameters of the centralized downcomer of the insertion boiler drum are obtained. The multi-source heterogeneous evaluation benchmark parameters include the physical geometric parameters, material performance parameters, and historical operating condition parameters of the downcomer. The physical geometric parameters of the downcomer include the inner diameter of the boiler drum, wall thickness, outer diameter and wall thickness of the downcomer, insertion depth, weld bevel angle and transition fillet radius. The material performance parameters include hot Young's modulus, Poisson's ratio, tensile strength, yield strength and fracture toughness. The historical operating condition parameters include working pressure, start-up temperature distribution, residual stress and operating years. A three-dimensional finite element model including crack defects is established based on the multi-source heterogeneous evaluation benchmark parameters.

[0010] Load a three-dimensional finite element model, introduce preset parameters for initial crack defects into the three-dimensional finite element model, and calculate the stress intensity factor distribution at the crack tip under different operating conditions based on fracture mechanics.

[0011] Obtain the stress intensity factor distribution at the crack tip under different operating conditions, pre-build a crack propagation prediction model, and calculate the remaining life and crack propagation path under the current crack state based on the crack propagation prediction model and the stress intensity factor distribution.

[0012] Constrained by the fracture toughness of the downcomer material, the target service life, and the structural parameters of the downcomer, and combined with the remaining life and crack propagation path under the current crack state, the safety level of the current crack in the centralized downcomer of the immersion boiler drum is comprehensively evaluated.

[0013] Preferably, the method for establishing a three-dimensional finite element model containing crack defects based on multi-source heterogeneous evaluation benchmark parameters includes:

[0014] Load the multi-source heterogeneous evaluation benchmark parameters, identify the physical geometric parameters of the downcomer in the multi-source heterogeneous evaluation benchmark parameters, and establish a crack-free initial finite element model of the downcomer based on the physical geometric parameters of the downcomer. The initial finite element model of the downcomer is constructed according to the 1:1 solid scale of the boiler drum downcomer welded joint, and the small-diameter opening structure that has no effect on the stress distribution at the joint is simplified and removed.

[0015] The initial finite element model of the downcomer is meshed based on eight-node hexahedral elements, and the mesh independence is examined to determine the optimal mesh size. A mesh refinement zone is set at the connection between the inner wall of the insert boiler drum and the outer wall of the downcomer, and a transition fillet model reflecting the geometry of the weld toe is constructed. The transition fillet model is fused with the initial finite element model of the downcomer with the mesh refinement zone to output the initial finite element model of the downcomer containing the refinement sub-model.

[0016] Obtain the initial finite element model of the downcomer including the encrypted sub-model, perform stress distribution analysis on the initial finite element model of the downcomer, and identify the location of the maximum stress of the concentrated downcomer joint and the stress distribution differences at different locations of the weld toe line.

[0017] Based on the stress distribution results of the initial finite element model of the downcomer, the crack initiation location and propagation direction are determined. The crack initiation location includes the weld toe line location and the high stress zone at the inner corner. The propagation direction includes the extension direction along the weld toe line and the direction at a preset angle with the weld axis. Using material property parameters as a priori, the densified sub-model is discretized along the quarter-node singular elements to obtain a crack singularized mesh. The crack singularized mesh is applied as a boundary condition to the initial finite element model of the downcomer to obtain a three-dimensional finite element model containing crack defects.

[0018] Preferably, the method for calculating the stress intensity factor distribution at the crack tip under different operating conditions based on fracture mechanics includes:

[0019] Based on the initial defect preset parameters, crack tips in stress concentration regions are identified in the three-dimensional finite element model. The stress concentration regions include the fillet weld region at the connection between the pipe seat and the boiler drum body, the transition region between the downcomer insertion section and the inner wall of the boiler drum, and the reinforcement region at the edge of the opening in the boiler drum body. The initial defect preset parameters include crack depth, crack length, the angle between the crack plane and the principal stress direction, and the spacing between adjacent cracks.

[0020] The stress intensity factor at the crack tip under different operating conditions was calculated using the displacement extrapolation method. Polynomial fitting was performed on the stress intensity factor at different crack front positions to obtain the stress intensity factor distribution curve along the crack front. The calculation results of the displacement extrapolation method were verified by comparing with the theoretical calculation values ​​in GB / T19624-2019, and the error was controlled within the preset threshold range.

[0021] The geometric discontinuity factor of the inserted boiler drum centralized downcomer structure is calculated. The geometric discontinuity factor comprehensively considers the diameter ratio, wall thickness ratio, insertion depth ratio and weld shape coefficient of the boiler drum and the downcomer. Based on the geometric discontinuity factor and the preset parameters of the initial defects, the structural constraint factor is determined. The structural constraint factor characterizes the influence of the triaxial stress state at the crack tip on the fracture toughness. The structural constraint factor is determined by three-dimensional finite element analysis combined with JQ theory.

[0022] The plane strain fracture toughness is modified based on the structural constraint factor to obtain the material fracture toughness of the inserted boiler drum centralized downcomer structure. The modification process takes into account the coupling effect of residual stress and thermal stress under the hot operating conditions of the boiler drum.

[0023] The material fracture toughness of the inserted boiler drum centralized downcomer structure is obtained. A failure assessment curve is constructed based on the material fracture toughness and stress intensity factor of the inserted boiler drum centralized downcomer structure. The horizontal axis of the failure assessment curve is the load ratio, and the vertical axis is the fracture ratio.

[0024] Preferably, the crack propagation prediction model construction method includes:

[0025] A fatigue crack propagation rate model is constructed based on the failure assessment curve, and the fatigue crack propagation behavior is described by the Paris formula. The material constants in the Paris formula are calibrated based on the number of boiler drum start-up and shutdown cycles and historical data of variable load operation.

[0026] A stress corrosion crack propagation rate model was established by combining material performance parameters, and the stress corrosion crack propagation behavior was described by a power law form. The environmentally relevant constants in the power law form were determined based on the water quality parameters and temperature fluctuation data of the high-temperature and high-pressure working fluid.

[0027] A fatigue crack propagation rate model and a stress corrosion crack propagation rate model are loaded and coupled together to obtain a crack propagation prediction model. The coupling and superposition process divides the dominant intervals of each crack propagation mechanism according to the stress intensity factor level, load frequency and temperature parameters.

[0028] Preferably, the method for calculating the remaining life and crack propagation path under the current crack state based on a crack propagation prediction model combined with stress intensity factor distribution includes:

[0029] Based on the current crack geometry and location, and in conjunction with the stress intensity factor distribution, the driving force distribution for crack propagation is determined. The current crack geometry includes crack depth and crack length, and the location includes the angle relationship between the crack plane and the axial stress and circumferential stress.

[0030] The propagation direction at each point on the crack front is predicted using the maximum circumferential stress criterion or the maximum energy release rate criterion, wherein the propagation direction is perpendicular to the direction of the maximum principal stress.

[0031] The crack propagation prediction model is based on the propagation direction and uses an incremental method to gradually simulate the crack propagation process. When the stress intensity factor reaches the fracture toughness of the downcomer material or the crack depth reaches the structural wall thickness, the structure is determined to fail, and the cumulative propagation time is used as the remaining life.

[0032] The propagation direction of each point at the crack front is obtained. The position of the crack front node is updated with the propagation direction as the normal according to the preset incremental step size. The updated three-dimensional finite element model is generated by spline fitting and crack surface reconstruction. After recalculating the stress intensity factor, the next incremental iteration is entered until the failure criterion is met. The crack morphology of each iteration step is connected in series according to the number of load cycles to obtain the crack propagation path.

[0033] When there are two or more non-coplanar distributed cracks in the concentrated downcomer region of the immersion boiler drum, the stress field interaction between the cracks is calculated, and the stress intensity factor of each crack is corrected by the amplification factor of the stress intensity factor at the crack tip. The amplification factor is determined according to the crack spacing, the relative position of the cracks and the crack size ratio.

[0034] Preferably, the method for comprehensively evaluating the safety level of the current crack in the immersion boiler drum downcomer by combining the remaining life under the current crack state and the crack propagation path includes:

[0035] Load the remaining lifetime and crack propagation path under the current crack state, and extract the crack depth-time curve, crack length-time curve and crack shape evolution parameters from the crack propagation path. The crack shape evolution parameters include the rate of change of crack depth-to-length ratio.

[0036] The fracture safety margin is calculated based on the ratio of the fracture toughness of the downcomer material to the stress intensity factor under the current crack state. The fracture safety margin reflects the fracture toughness reserve that resists rapid crack propagation. The life safety margin is determined based on the ratio of the predicted remaining life to the target service life, which is determined according to the company's maintenance plan and the unit's operating years.

[0037] Obtain the crack depth-time curve and crack length-time curve in the crack propagation path, calculate the crack depth propagation rate ratio and crack length propagation rate ratio of adjacent incremental steps, and calculate the crack propagation stability index based on the crack depth propagation rate ratio and crack length propagation rate ratio. The crack propagation stability index reflects the controllability of the crack propagation trend.

[0038] Preferably, the method for comprehensively evaluating the safety level of the current crack in the immersion boiler drum downcomer by combining the remaining life under the current crack state and the crack propagation path further includes:

[0039] Based on the fracture safety margin, the lifetime safety margin, and the crack propagation stability index, a weighted comprehensive scoring method is used to determine the safety level score. The weighting coefficients in the weighted comprehensive scoring method are determined based on historical failure cases of the immersion boiler drum centralized downcomer and expert experience.

[0040] Safety levels are determined based on safety level scores. The safety levels include four levels: safe, caution, warning, and danger. The safety level corresponds to the relative position of the evaluation point and the evaluation curve in the failure assessment diagram. When the evaluation point is located in the lower left of the evaluation curve and the safety margin is sufficient, it is determined to be a safe level. When the evaluation point is close to the evaluation curve, it is determined to be a caution or warning level. When the evaluation point is located in the upper right of the evaluation curve, it is determined to be a danger level.

[0041] Based on the current safety level of the crack, a targeted maintenance recommendation report is generated. The maintenance recommendation report includes crack geometric parameters, stress intensity factor distribution, safety margin calculation results, remaining life prediction results, crack propagation path visualization diagram, failure assessment diagram, safety level assessment conclusion, and targeted maintenance recommendations. The targeted maintenance recommendations include continuing operation and setting a regular inspection cycle, shortening the inspection cycle and reviewing it during planned shutdowns, restricting operating conditions and arranging maintenance as soon as possible, and immediately shutting down for defect elimination.

[0042] On the other hand, the present invention also provides an insertion-type boiler drum centralized downcomer crack safety assessment system, the insertion-type boiler drum centralized downcomer crack safety assessment system comprising:

[0043] The finite element modeling module is used to obtain the multi-source heterogeneous evaluation benchmark parameters of the centralized downcomer of the insertion boiler drum. The multi-source heterogeneous evaluation benchmark parameters include the physical geometric parameters, material performance parameters, and historical operating condition parameters of the downcomer. A three-dimensional finite element model containing crack defects is established based on the multi-source heterogeneous evaluation benchmark parameters.

[0044] The stress intensity factor calculation module is used to load a three-dimensional finite element model, introduce preset parameters of initial crack defects into the three-dimensional finite element model, and calculate the stress intensity factor distribution of the crack tip under different operating conditions based on the fracture mechanics method.

[0045] The crack propagation prediction module is used to obtain the stress intensity factor distribution at the crack tip under different operating conditions, pre-build a crack propagation prediction model, and calculate the remaining life and crack propagation path under the current crack state based on the crack propagation prediction model and the stress intensity factor distribution.

[0046] The safety level assessment module is used to comprehensively assess the safety level of the current crack in the centralized downcomer of the insertion boiler drum, taking into account the fracture toughness of the downcomer material, the target service life, and the structural parameters of the downcomer, combined with the remaining life and crack propagation path under the current crack state.

[0047] Preferably, the finite element modeling module includes:

[0048] An initial building unit is used to load multi-source heterogeneous evaluation benchmark parameters, identify the physical geometric parameters of the downcomer in the multi-source heterogeneous evaluation benchmark parameters, and establish a crack-free initial finite element model of the downcomer based on the physical geometric parameters of the downcomer. The initial finite element model of the downcomer is constructed according to a 1:1 solid scale of the boiler drum downcomer welded joint.

[0049] The sub-model refinement unit meshes the initial finite element model of the downcomer based on eight-node hexahedral elements, and examines the independence of the mesh to determine the optimal mesh size. A mesh refinement zone is set at the connection between the inner wall of the insert boiler drum and the outer wall of the downcomer, and a transition fillet model reflecting the geometry of the weld toe is constructed. The transition fillet model is then fused with the initial finite element model of the downcomer with the mesh refinement zone set, and the output is the initial finite element model of the downcomer containing the refinement sub-model.

[0050] The stress distribution analysis unit is used to obtain the initial finite element model of the downcomer including the densified sub-model, perform stress distribution analysis on the initial finite element model of the downcomer, and identify the location of the maximum stress of the concentrated downcomer joint and the stress distribution differences at different locations of the weld toe line.

[0051] The output elements of the three-dimensional finite element model are determined based on the stress distribution results of the initial finite element model of the downcomer to determine the crack initiation location and propagation direction. With material property parameters as a priori, the singular elements along the quarter node of the densified sub-model are discretized to obtain the crack singular mesh. The crack singular mesh is applied as a boundary condition to the initial finite element model of the downcomer to obtain a three-dimensional finite element model containing crack defects.

[0052] Preferably, the stress intensity factor calculation module includes:

[0053] The crack tip identification unit identifies crack tips in stress concentration areas in a three-dimensional finite element model based on initial defect preset parameters. The stress concentration areas include the fillet weld area at the connection between the pipe seat and the boiler drum body, the transition area between the downcomer insertion section and the inner wall of the boiler drum, and the reinforcement area at the edge of the opening in the boiler drum body.

[0054] The distribution curve generation unit uses the displacement extrapolation method to calculate the stress intensity factor at the crack tip under different operating conditions, and performs polynomial fitting on the stress intensity factor at different crack front positions to obtain the stress intensity factor distribution curve along the crack front. The calculation results of the displacement extrapolation method are verified by comparing with the theoretical calculation values ​​in GB / T19624-2019.

[0055] The structural constraint factor determination unit is used to calculate the geometric discontinuity factor of the inserted boiler drum centralized downcomer structure. The geometric discontinuity factor comprehensively considers the diameter ratio, wall thickness ratio, insertion depth ratio and weld shape coefficient of the boiler drum and the downcomer. Based on the geometric discontinuity factor and the preset parameters of the initial defects, the structural constraint factor is determined. The structural constraint factor characterizes the influence of the triaxial stress state at the crack tip on the fracture toughness. The structural constraint factor is determined by three-dimensional finite element analysis combined with JQ theory.

[0056] The fracture toughness output unit corrects the plane strain fracture toughness based on the structural constraint factor to obtain the material fracture toughness of the inserted boiler drum centralized downcomer structure. The correction process takes into account the coupling effect of residual stress and thermal stress under the hot operating conditions of the boiler drum.

[0057] The failure assessment curve generation unit is used to obtain the material fracture toughness of the inserted boiler drum centralized downcomer structure, and to construct a failure assessment curve based on the material fracture toughness and stress intensity factor of the inserted boiler drum centralized downcomer structure. The horizontal axis of the failure assessment curve is the load ratio, and the vertical axis is the fracture ratio.

[0058] Compared with the prior art, the embodiments of this application have the following main advantages:

[0059] This invention addresses the crack defects in the welds and heat-affected zones of the centralized downcomer pipes in immersion boiler drums. It constructs a complete evaluation system encompassing geometric parametric modeling, precise calculation of stress intensity factors, multi-mechanism coupled crack propagation prediction, and three-dimensional weighted safety level assessment. This system achieves quantitative analysis throughout the entire process from crack detection to maintenance decision-making. By introducing structural constraint factors to correct fracture toughness, employing an incremental method to progressively simulate the crack front propagation path, and integrating fracture safety margin, life safety margin, and crack propagation stability index for weighted scoring, this invention effectively solves the problems of insufficient specificity for the special structure of immersion downcomer pipes, high dispersion of evaluation results, and difficulty in directly supporting maintenance decisions in existing technologies. It provides rapid, accurate, and quantifiable technical support for boiler serviceability evaluation and condition-based maintenance.

[0060] This invention constructs a multi-mechanism coupled crack propagation prediction model that integrates the Paris formula fatigue crack propagation model and the power-law stress corrosion crack propagation model. Based on the stress intensity factor level, load frequency, and temperature parameters, it dynamically divides the dominant intervals of each propagation mechanism, achieving an accurate characterization of the crack propagation behavior of the centralized downcomer tube of the inserted boiler drum under complex service environments. This model overcomes the limitations of single-mechanism models, and can simultaneously capture the synergistic effect of fatigue damage generated by start-up and shutdown cycles and stress corrosion damage caused by high-temperature and high-pressure water environments. It significantly improves the accuracy of remaining service prediction and crack propagation path tracking, providing a reliable quantitative basis for boiler condition-based maintenance and differentiated maintenance strategy formulation.

[0061] This invention constructs a crack propagation path tracking system that integrates crack propagation driving force analysis, multi-criteria propagation direction prediction, incremental step-by-step simulation, adaptive crack surface reconstruction, and correction of non-coplanar multi-crack interaction effects. This system accurately depicts the entire evolution process of cracks in the centralized downcomer of an insert boiler drum from initiation to failure. It scientifically predicts crack deflection behavior using the maximum circumferential stress criterion or the maximum energy release rate criterion, dynamically updates the crack front geometry using incremental iteration and automatically reconstructs the finite element model, and effectively handles the interaction of multi-crack stress fields using an amplification factor correction mechanism. This significantly improves the accuracy of remaining service prediction and the visualization capability of crack propagation paths, providing reliable quantitative technical support for boiler service evaluation and differentiated maintenance decisions. Attached Figure Description

[0062] Figure 1 A schematic diagram of a crack in the centralized downcomer of an insert boiler drum is shown in an embodiment of the present invention.

[0063] Figure 2 A schematic diagram of the insertion downcomer welding joint structure in an embodiment of the present invention is shown.

[0064] Figure 3 This is a schematic diagram illustrating the implementation process of the method for assessing the safety of cracks in the centralized downcomer of an insert-type boiler drum provided by the present invention.

[0065] Figure 4 A schematic diagram of the implementation process of establishing a three-dimensional finite element model containing crack defects based on multi-source heterogeneous evaluation benchmark parameters is shown.

[0066] Figure 5 A schematic diagram of the initial finite element model of the inserted boiler drum centralized downcomer in an embodiment of the present invention is shown.

[0067] Figure 6 This illustration shows a schematic diagram of an initial finite element model of a descender with a mesh refinement zone set when meshing the initial finite element model of a descender based on eight-node hexahedral elements in an embodiment of the present invention.

[0068] Figure 7 A schematic diagram showing the relationship between mesh size and stress magnitude during mesh generation is provided.

[0069] Figure 8 The diagram shows the stress distribution cloud map of the downcomer when performing stress distribution analysis on the initial finite element model of the downcomer in an embodiment of the present invention.

[0070] Figure 9 The geometric model of a semi-elliptical surface crack provided at the weld toe line on the inner wall of the downcomer in an embodiment of the present invention is shown.

[0071] Figure 10 A schematic diagram of the singularized mesh in the crack tip region of the downcomer in an embodiment of the present invention is shown.

[0072] Figure 11 A schematic diagram of a crack singularity mesh containing crack defects is shown.

[0073] Figure 12 The stress distribution diagram is shown when the stress field at the crack front of the downcomer is decomposed.

[0074] Figure 13 The diagram shows a comparison of stress intensity factors when simulating a surface crack of a semi-elliptical weld with a crack length c of 5 mm in an embodiment of the present invention.

[0075] Figure 14 The diagram shows a comparison of stress intensity factors when simulating a surface crack of a semi-elliptical weld with a crack length c of 10 mm in an embodiment of the present invention.

[0076] Figure 15 A schematic diagram of the stress intensity factor of crack JH1 in an embodiment of the present invention is shown.

[0077] Figure 16 A schematic diagram of the stress intensity factor of crack JH2 in an embodiment of the present invention is shown.

[0078] Figure 17A schematic diagram of the crack safety assessment results based on the failure assessment curve is shown in an embodiment of the present invention.

[0079] Figure 18 A schematic diagram of the structure of an insertion-type boiler drum centralized downcomer crack safety assessment system is shown. Detailed Implementation

[0080] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein in the specification of the application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application; the terms "comprising" and "having," and any variations thereof, in the specification, claims, and foregoing drawings of this application are intended to cover non-exclusive inclusion. The terms "first," "second," etc., in the specification, claims, or foregoing drawings of this application are used to distinguish different objects, not to describe a particular order.

[0081] Existing technologies suffer from insufficient targeting, low evaluation efficiency, and weak reference value, making it difficult to adapt to the current development needs of condition-based maintenance and intelligent operation and maintenance of thermal power equipment. In short, the method described above first establishes a three-dimensional finite element model containing crack defects based on multi-source heterogeneous evaluation benchmark parameters. Then, based on the crack propagation prediction model and stress intensity factor distribution, it calculates the remaining life and crack propagation path under the current crack state. Combining the remaining life and crack propagation path under the current crack state, it comprehensively evaluates the safety level of the current crack in the centralized downcomer of the insertion boiler drum. This invention addresses the crack defects in the welds and heat-affected zones of the centralized downcomer pipes in immersion boiler drums. It constructs a complete evaluation system encompassing geometric parametric modeling, precise calculation of stress intensity factors, multi-mechanism coupled crack propagation prediction, and three-dimensional weighted safety level assessment. This system achieves quantitative analysis throughout the entire process from crack detection to maintenance decision-making. By introducing structural constraint factors to correct fracture toughness, employing an incremental method to progressively simulate the crack front propagation path, and integrating fracture safety margin, life safety margin, and crack propagation stability index for weighted scoring, this invention effectively solves the problems of insufficient specificity for the special structure of immersion downcomer pipes, high dispersion of evaluation results, and difficulty in directly supporting maintenance decisions in existing technologies. It provides rapid, accurate, and quantifiable technical support for boiler serviceability evaluation and condition-based maintenance.

[0082] This invention uses a high-pressure boiler in a biomass power plant as the evaluation object. During internal inspection after shutdown, magnetic particle testing revealed cracks on the inner walls of two central downcomer pipes in the boiler drum. One more severe crack (numbered JH1) extends along the weld toe line, measuring approximately 220 mm in length, with an ultrasonic depth of approximately 21 mm. The other crack (numbered JH2) extends at approximately 90° to the weld axis, measuring approximately 52 mm in length and approximately 15 mm in depth. The two cracks are approximately 140 mm apart and are non-coplanar. The crack morphology is as follows: Figure 1 As shown, the structure of the insertion-type downcomer welded joint is as follows: Figure 2 As shown. Considering the limited shutdown and maintenance time, and the fact that the boiler has been operating safely for over 10 years, it is desirable to determine whether the defect can be retained for continued operation through a serviceability evaluation. According to the provisions of TSG 11-2020 "Boiler Safety Technical Regulations", defects discovered during periodic inspections should be handled according to the principle of serviceability. If the serviceability evaluation determines that the crack will not propagate, the defect can be retained for continued service. In addition, the wall thickness at this location is relatively thick. If the defect is repaired by welding, heat treatment is required. The large restraint stress generated during local heat treatment may adversely affect the entire pipe joint of the downcomer and even the boiler drum wall base material. Therefore, conducting a serviceability evaluation for this crack has significant engineering value.

[0083] This invention provides a method for assessing the safety of cracks in the centralized downcomer of an insert-type boiler drum. Figure 3 A schematic diagram of the implementation process of the safety assessment method for cracks in the centralized downcomer of an insert-type boiler drum is shown. The specific steps of the safety assessment method for cracks in the centralized downcomer of an insert-type boiler drum include:

[0084] S10, obtain the multi-source heterogeneous evaluation benchmark parameters of the centralized downcomer of the insertion boiler drum. The multi-source heterogeneous evaluation benchmark parameters include the physical geometric parameters of the downcomer, material performance parameters, and historical operating condition parameters. The physical geometric parameters of the downcomer include the inner diameter of the boiler drum, wall thickness, outer diameter and wall thickness of the downcomer, insertion depth, weld bevel angle, and transition fillet radius. The material performance parameters include hot Young's modulus, Poisson's ratio, tensile strength, yield strength, and fracture toughness. The historical operating condition parameters include working pressure, start-up temperature distribution, residual stress, and operating years. Based on the multi-source heterogeneous evaluation benchmark parameters, establish a three-dimensional finite element model including crack defects.

[0085] In this embodiment of the invention, the physical geometric parameters of the downcomer include: inner diameter of the boiler drum 1600mm, wall thickness 70mm, outer diameter and wall thickness of the downcomer, insertion depth, weld bevel angle and transition fillet radius; the material performance parameters include: material grade 13MnNiMoR, Young's modulus 193GPa, Poisson's ratio 0.31, tensile strength 570MPa, yield strength 315MPa under hot conditions; the historical operating condition parameters include: working pressure 10.43MPa, starting temperature distribution, residual stress 83.1MPa, and operating years of more than 10 years.

[0086] S20, load the three-dimensional finite element model, introduce the preset parameters of the initial defect of the crack into the three-dimensional finite element model, and calculate the stress intensity factor distribution of the crack tip under different operating conditions based on the fracture mechanics method.

[0087] S30: Obtain the stress intensity factor distribution at the crack tip under different operating conditions, pre-build a crack propagation prediction model, and calculate the remaining life and crack propagation path under the current crack state based on the crack propagation prediction model and the stress intensity factor distribution.

[0088] S40 comprehensively evaluates the safety level of the current crack in the centralized downcomer of the immersion boiler drum, taking into account the fracture toughness of the downcomer material, the target service life, and the structural parameters of the downcomer, combined with the remaining life and crack propagation path under the current crack state.

[0089] This invention addresses the crack defects in the welds and heat-affected zones of the centralized downcomer pipes in immersion boiler drums. It constructs a complete evaluation system encompassing geometric parametric modeling, precise calculation of stress intensity factors, multi-mechanism coupled crack propagation prediction, and three-dimensional weighted safety level assessment. This system achieves quantitative analysis throughout the entire process from crack detection to maintenance decision-making. By introducing structural constraint factors to correct fracture toughness, employing an incremental method to progressively simulate the crack front propagation path, and integrating fracture safety margin, life safety margin, and crack propagation stability index for weighted scoring, this invention effectively solves the problems of insufficient specificity for the special structure of immersion downcomer pipes, high dispersion of evaluation results, and difficulty in directly supporting maintenance decisions in existing technologies. It provides rapid, accurate, and quantifiable technical support for boiler serviceability evaluation and condition-based maintenance.

[0090] This invention provides a method for establishing a three-dimensional finite element model containing crack defects based on multi-source heterogeneous evaluation benchmark parameters. Figure 4 A schematic diagram illustrating the implementation process of a method for establishing a three-dimensional finite element model containing crack defects based on multi-source heterogeneous evaluation benchmark parameters is shown. The method specifically includes:

[0091] S101, Load the multi-source heterogeneous evaluation benchmark parameters, identify the physical geometric parameters of the downcomer in the multi-source heterogeneous evaluation benchmark parameters, and establish a crack-free initial finite element model of the downcomer based on the physical geometric parameters of the downcomer. The initial finite element model of the downcomer is constructed according to a 1:1 solid scale of the boiler drum downcomer welded joint, and the small-diameter opening structure that has no effect on the stress distribution at the joint is simplified and removed. Figure 5 A schematic diagram of the initial finite element model of the inserted boiler drum centralized downcomer in an embodiment of the present invention is shown;

[0092] S102, the initial finite element model of the downcomer is meshed based on eight-node hexahedral elements, and the mesh independence is examined to determine the optimal mesh size. A mesh refinement zone is set at the connection between the inner wall of the insert-type boiler drum and the outer wall of the downcomer, and a transition fillet model reflecting the geometry of the weld toe is constructed. The transition fillet model is fused with the initial finite element model of the downcomer with the mesh refinement zone, and the output is the initial finite element model of the downcomer containing the refinement sub-model. In this embodiment of the invention, considering that the mesh size and number directly affect the accuracy and time of the calculation, the mesh independence is examined. In order to save calculation time and ensure calculation accuracy, the final mesh size is 20mm, and the total number of elements is 266422 for simulation. Figure 6 This illustration shows a schematic diagram of an initial finite element model of a downcomer with a mesh refinement zone set during mesh generation based on eight-node hexahedral elements in an embodiment of the present invention. Figure 7 A schematic diagram showing the relationship between mesh size and stress magnitude during mesh generation is provided.

[0093] S103: Obtain the initial finite element model of the downcomer, including the encrypted sub-model; perform stress distribution analysis on the initial finite element model of the downcomer; identify the location of the maximum stress at the concentrated downcomer joint and the stress distribution differences at different locations along the weld toe line; among which... Figure 8 The diagram illustrates the stress distribution cloud map of the downcomer during stress distribution analysis of the initial finite element model in this embodiment of the invention. The stress cloud map shows that the maximum stress at the concentrated downcomer joint is located at the inner corner, which matches the actual stress distribution. It is also noted that the stress distribution on the inner surface of the weld is uneven, and the stress distribution varies at different locations along the weld toe line. The maximum stress at the weld toe line is not located on the central axis, but rather near an angle of approximately 45° to the axis.

[0094] S104. Based on the stress distribution results of the initial finite element model of the downcomer, the crack initiation location and propagation direction are determined. The crack initiation location includes the weld toe line location and the high-stress zone at the inner corner. The propagation direction includes the extension direction along the weld toe line and the direction at a preset angle with the weld axis. Using material property parameters as a priori, the sub-model is discretized along the quarter-point singular element to obtain the crack singular mesh. The quarter-point singular element is a method that moves the middle node on the element edge to 1 / 4 of the distance from the crack tip. The crack singular mesh is applied as a boundary condition to the initial finite element model of the downcomer to obtain a three-dimensional finite element model containing crack defects.

[0095] In this embodiment of the invention, when constructing the crack singularity mesh, material property parameters are used as priors. The refined sub-model is discretized along the quarter-node singular elements to obtain the crack singularity mesh. The SOLID187 element in ANSYS software is used to discretize the refined sub-model. The SOLID187 element is a 10-node tetrahedral element. In the crack tip region, the edge-mid-nodes of the SOLID187 element are adjusted to the quarter-node position to form equivalent singular element characteristics. Figure 9 The geometric model of a semi-elliptical surface crack at the weld toe line on the inner wall of the downcomer in an embodiment of the present invention is shown, as follows: Figure 9 As shown, the crack plane is perpendicular to the principal stress direction, and the crack front is a semi-elliptical curve. Figure 10 This diagram illustrates a singularized mesh in the crack tip region of the downcomer in an embodiment of the present invention. Figure 10 As can be seen, the edge nodes of the adjacent elements at the crack tip (marked by dots in the figure) move toward the crack tip and are located at 1 / 4 of the edge length from the tip. The singularized mesh is a multi-layered mesh that is radially distributed with the crack tip as the center. The inner layer is dense and the outer layer is gradually sparse. The wedge-shaped or tetrahedral elements around the crack tip can accurately reproduce the stress singularity.

[0096] In the embodiment of the present invention, the crack singularization mesh is applied as a boundary condition to the initial finite element model of the downcomer, so as to obtain a three-dimensional finite element model containing crack defects. Considering the influence relationship between the crack plane and the principal stress direction, the cracks are regularized into two semi-elliptical cracks. The JH1 crack plane is perpendicular to the axial stress, while the JH2 crack plane is perpendicular to the circumferential stress. The crack defects are regularized and characterized according to GB / T 19624-2019 "Safety Assessment for In-service Pressure Vessels Containing Defects", where the crack depth h=21mm, and the projected length in the circumferential direction L=200mm (the crack opening plane after regularization is perpendicular to the axial stress). The wall thickness B=70mm, and it is concluded that h≤0.7B and h<L / 2. Therefore, the defect is regularized and characterized as a semi-elliptical inner surface crack with c=L / 2=100mm and a=h=21mm. The JH2 crack is characterized in the same way, in which, Figure 11 shows a schematic diagram of the crack singularization mesh containing crack defects, in Figure 11 , the solid187 tetrahedral mesh is used to construct the singular element mesh of JH1 and JH2 cracks, and the total number of mesh elements is 270553. Table 1 shows the basic parameters of JH1 and JH2 cracks.

[0097] Table 1

[0098] Crack number Length 2c (mm) × Height a (mm) Surface distance (mm) Axial force angle Circumferential force angle JH1 200×21 21 90° 0° JH2 52×15 15 0° 90°

[0099] In the embodiment of the present invention, the method for calculating the stress intensity factor distribution at the crack tip under different operating conditions based on the fracture mechanics method includes:

[0100] S201: Identify the crack tip in the stress concentration region in the three-dimensional finite element model based on preset initial defect parameters. The stress concentration region includes the fillet weld region at the connection between the pipe seat and the drum cylinder, the transition region between the downcomer insertion section and the inner wall of the drum, and the reinforcement region at the edge of the opening of the drum cylinder. The preset initial defect parameters include the crack depth, crack length, included angle between the crack plane and the principal stress direction, and the spacing between adjacent cracks;

[0101] S202: Calculate the stress intensity factors at the crack tip under different operating conditions by using the displacement extrapolation method, perform polynomial fitting on the stress intensity factors at different positions of the crack front, and obtain the stress intensity factor distribution curve along the crack front. The calculation results of the displacement extrapolation method are verified by comparison with the theoretical calculation values of GB / T 19624-2019, and the error is controlled within a preset threshold range;

[0102] In this embodiment of the invention, before calculating the stress intensity factor, stress decomposition is performed along the thickness direction at the crack front location to obtain the distribution patterns of membrane stress, bending stress, and peak stress. The membrane stress is the average stress uniformly distributed along the wall thickness, the bending stress is the stress gradient linearly distributed along the wall thickness, and the peak stress is the additional stress generated by local stress concentration. Based on the stress decomposition results, a stress concentration factor is calculated, which is selected with reference to relevant pressure vessel standards, to obtain the stress condition at the crack tip. Specifically, a detailed decomposition of the stress field at the crack front location is required to obtain accurate stress input parameters. Stress decomposition is performed along the thickness direction of the downcomer of the insert-type boiler drum, resulting in the stress value distribution at the crack front location of the downcomer as shown below. Figure 12 As shown. Figure 12 The stress distribution diagram includes the membrane stress distribution curve, bending stress distribution curve, peak stress distribution curve, and total stress distribution curve. Among them, the membrane stress distribution curve shows that the membrane stress at the crack location is uniformly distributed in the thickness direction, reflecting the average tensile stress generated by the in-plane load such as internal pressure on the structure. The bending stress distribution curve gradually decreases from 53.3 MPa at the inner surface, reaching 0 MPa at a depth of about 39.4 mm (near the midpoint of the wall thickness), and then continues to increase until it reaches a maximum value of 43.8 MPa at the outer surface, exhibiting a linear distribution characteristic, reflecting the stress gradient generated by the bending deformation of the structure. The peak stress distribution curve has a large variation range, including stress concentration effects and additional stress caused by local structural discontinuities. The total stress distribution curve shows a large fluctuation in stress value with changes in thickness, and is the result of the superposition of membrane stress, bending stress, and peak stress. The stress concentration factor at the crack front position is calculated to obtain the stress situation at the crack tip. The stress concentration factor can be selected by referring to relevant standards for pressure vessels (such as GB / T 150, JB / T 4732, etc.) and determined by referring to tables or calculation based on structural geometric characteristics (such as nozzle openings, weld transitions, etc.).

[0103] In this embodiment of the invention, to ensure the representativeness of the stress intensity factor calculated at the crack tip under different operating conditions using the displacement extrapolation method and to facilitate theoretical calculation, simulations were performed using semi-elliptical weld surface cracks with crack lengths c of 5 mm and 10 mm. The crack depth a varied, and these cracks were combined with the crack length c to form a total of 14 crack groups. The stress intensity factor values ​​of these 14 crack groups were theoretically calculated and compared with finite element calculations.

[0104] The theoretical calculations adopt the formulas from GB / T19624-2019 "Safety Assessment of Defective Pressure Vessels in Use":

[0105]

[0106] in, Let σ be the dimension of the crack in the depth direction. m σ B The stresses are membrane stress and bending stress, respectively, obtained using the finite element stress decomposition method. The maximum value of the resultant stress in the stress path is selected based on a relatively conservative principle. The shape factor can be found in Appendix D of GB / T19624-2019 "Safety Assessment of Defective Pressure Vessels in Use". For the elliptic integral of the second kind, please refer to Appendix D.

[0107] Table 2 shows the relative crack sizes when simulating surface cracks in semi-elliptical welds.

[0108] Table 2

[0109] crack <![CDATA[a1 / mm]]> <![CDATA[c1 / mm]]> <![CDATA[a2 / mm]]> <![CDATA[c2 / mm]]> 1 1 5 2 10 2 1.5 5 3 10 3 2 5 4 10 4 2.5 5 5 10 5 3 5 6 10 6 3.5 5 7 10 7 4 5 8 10

[0110] in, Figure 13 The diagram shows a comparison of stress intensity factors during simulation of a semi-elliptical weld surface crack with a crack length c of 5 mm in an embodiment of the present invention. Figure 14 The figure shows a comparison of stress intensity factors when simulating a surface crack of a semi-elliptical weld with a crack length c of 10 mm in an embodiment of the present invention. Figure 13 As shown, the stress intensity factor at the crack tip increases with increasing crack depth *a*, and the trends of theoretical calculations and finite element simulations are consistent. Figure 14 As shown, the stress intensity factor also exhibits a trend of increasing with crack depth. The theoretical calculation and finite element simulation results agree well. Therefore, when c=5mm, the percentage difference between the theoretical calculation and the finite element simulation is 4.46%; when c=10mm, the percentage difference is 5.45%. This demonstrates that in engineering applications, using the finite element method to simulate the fracture mechanics analysis of weld cracks at insert pipe joints is relatively accurate, indicating that finite element calculation has high precision and feasibility. Therefore, this method can be used to simulate the interaction and fracture mechanics parameters of two non-coplanar cracks, compensating for the deficiency in GB / T19624-2019 "Safety Assessment of In-Service Pressure Vessels with Defects" regarding the lack of calculation of the mutual interference coefficient for non-coplanar cracks near the weld toe line.

[0111] S203, calculate the geometric discontinuity factor of the inserted boiler drum centralized downcomer structure. The geometric discontinuity factor comprehensively considers the diameter ratio, wall thickness ratio, insertion depth ratio and weld shape coefficient of the boiler drum body and the downcomer. Based on the geometric discontinuity factor and the initial defect preset parameters, determine the structural constraint factor. The structural constraint factor characterizes the influence of the triaxial stress state at the crack tip on the fracture toughness. The structural constraint factor is determined by three-dimensional finite element analysis combined with JQ theory.

[0112] S204, Based on the structural constraint factor, the plane strain fracture toughness is modified to obtain the material fracture toughness of the inserted boiler drum centralized downcomer structure. The modification process takes into account the coupling effect of residual stress and thermal stress under the hot operating conditions of the boiler drum.

[0113] In this embodiment of the invention, the boiler drum is considered to be in a hot state during operation, and its wall thickness is relatively thick. Therefore, it is necessary to consider the thermal stress borne by the boiler drum. The thermal stress generated during boiler startup is the greatest and directly affects the stress distribution of the structure. The temperature distribution of the boiler drum during startup is imported into the temperature input module, and the measured residual stress of 83.1 MPa is loaded into the model to obtain the stress value after coupling thermal stress with structural and weld residual stress. The stress intensity factor of the crack is then directly solved. Figure 15 A schematic diagram of the stress intensity factor of crack JH1 in an embodiment of the present invention is shown, while Figure 16 A schematic diagram of the stress intensity factor of crack JH2 in an embodiment of the present invention is shown.

[0114] S205, obtain the material fracture toughness of the inserted boiler drum centralized downcomer structure, and construct a failure assessment curve based on the material fracture toughness and stress intensity factor of the inserted boiler drum centralized downcomer structure. The horizontal axis of the failure assessment curve is the load ratio, and the vertical axis is the fracture ratio.

[0115] In this embodiment of the invention, the material fracture toughness of the inserted boiler drum centralized downcomer structure is obtained. When constructing the failure assessment curve based on the material fracture toughness and stress intensity factor of the inserted boiler drum centralized downcomer structure, the stress intensity factor is substituted into formula (GB / T19624-2019 "Safety Assessment of In-Service Pressure Vessels with Defects" Formula 18) to calculate the fracture ratio Kr of cracks JH1 and JH2. The calculated Kr(JH1) = 0.5202 and Kr(JH2) = 0.6045 are as follows: The formula for calculating the fracture ratio Kr is as follows:

[0116]

[0117] in, , The crack depth. The distance between the inner and outer corners of the pipe is measured in mm. and For the direction perpendicular to the crack plane at B min Stress distributed across the thickness, MPa Let JH1 be the yield strength of the material, in MPa. The calculated coordinates of JH1 (0.8950, 0.5202) and JH2 (0.9563, 0.6045) are plotted on a conventional FAD failure assessment chart to obtain the crack safety assessment results based on the failure assessment curve. Figure 17 The diagram shows the crack safety assessment results based on the failure assessment curve in an embodiment of the present invention. Table 3 shows the crack serviceability evaluation results. It can be seen that crack JH1 is located in the lower left of the curve, and crack JH2 is close to the evaluation curve and is in a critical state. Considering that the pressure and temperature of the boiler drum fluctuate during actual operation, crack JH1 has a small safety margin, and crack JH2 is already in a critical state. Therefore, it is recommended to perform defect elimination treatment on the cracks. If necessary, the LM method can also be used to analyze the remaining life of the base material.

[0118] Table 3

[0119] Crack number <![CDATA[ (MPa﹒mm^ 0.5 )]]> Kr Lr in conclusion JH1 1091.5 0.5202 0.8950 pass JH2 1268.3 0.6045 0.9563 Critical state

[0120] Among them, the conventional assessment FAD (Failure Assessment Diagram) is an engineering tool used for safety assessment of pressure vessels and pipeline structures with defects. It constructs a two-dimensional assessment coordinate system with the load ratio Lr as the abscissa and the fracture ratio Kr as the ordinate. Figure 17 A failure assessment curve, determined by both the material's fracture toughness and yield strength, is plotted. This curve divides the coordinate plane into two regions: the lower left of the curve represents the safe zone, indicating that the structure will not fracture or experience plastic instability under a given load; the upper right of the curve represents the failure zone, indicating that the structure no longer meets the conditions for safe operation. During the assessment, the load ratio Lr (the ratio of the actual reference stress borne by the structure to the material's yield strength) and the fracture ratio Kr (the ratio of the stress intensity factor at the crack tip to the material's fracture toughness) of the crack to be assessed are plotted as coordinate points on the graph. The safety status of the structure is determined based on the relative position of the assessment point to the assessment curve. If the assessment point is located in the lower left of the curve and is relatively far away, it indicates that the structure has a sufficient safety margin; if the assessment point is close to the assessment curve, it indicates that the structure is in a critical state and requires close monitoring and consideration of limiting operating conditions; if the assessment point is located in the upper right of the curve, it indicates that the structure has failed and must be shut down immediately. This evaluation method comprehensively considers both linear elastic fracture mechanics and elastoplastic failure mechanisms, and can intuitively and quantitatively reflect the safety margin of defective structures. It is the core criterion for evaluating suitability for use.

[0121] This invention provides a method for constructing a crack propagation prediction model, which specifically includes:

[0122] S301, a fatigue crack propagation rate model is constructed based on the failure assessment curve, and the fatigue crack propagation behavior is described by the Paris formula. The material constants in the Paris formula are calibrated based on the number of boiler drum start-up and shutdown cycles and historical data of variable load operation.

[0123] S302, a stress corrosion crack propagation rate model is established by combining material performance parameters, and the stress corrosion crack propagation behavior is described by a power law form. The environmentally relevant constants in the power law form are determined based on the water quality parameters of the high-temperature and high-pressure working fluid and temperature fluctuation data.

[0124] S303, Load the fatigue crack propagation rate model and the stress corrosion crack propagation rate model, and couple and superimpose the fatigue crack propagation rate model and the stress corrosion crack propagation rate model to obtain the crack propagation prediction model. The coupling and superposition process divides the dominant interval of each crack propagation mechanism according to the stress intensity factor level, load frequency and temperature parameters.

[0125] This invention constructs a multi-mechanism coupled crack propagation prediction model that integrates the Paris formula fatigue crack propagation model and the power-law stress corrosion crack propagation model. Based on the stress intensity factor level, load frequency, and temperature parameters, it dynamically divides the dominant intervals of each propagation mechanism, achieving an accurate characterization of the crack propagation behavior of the centralized downcomer tube of the inserted boiler drum under complex service environments. This model overcomes the limitations of single-mechanism models, and can simultaneously capture the synergistic effect of fatigue damage generated by start-up and shutdown cycles and stress corrosion damage caused by high-temperature and high-pressure water environments. It significantly improves the accuracy of remaining service prediction and crack propagation path tracking, providing a reliable quantitative basis for boiler condition-based maintenance and differentiated maintenance strategy formulation.

[0126] In this embodiment of the invention, the method for calculating the remaining life and crack propagation path under the current crack state based on a crack propagation prediction model combined with stress intensity factor distribution includes:

[0127] S401, based on the current crack geometry and location, and in conjunction with the stress intensity factor distribution, determine the driving force distribution for crack propagation. The current crack geometry includes crack depth and crack length, and the location includes the angle relationship between the crack plane and the axial stress and circumferential stress.

[0128] In this embodiment of the invention, based on the calculated stress intensity factor distribution, the equivalent stress intensity factor at each point at the crack front is extracted as the driving force for crack propagation. The driving force distribution exhibits non-uniform characteristics: the driving force is greatest at the deepest point of the crack and gradually decreases towards both ends of the crack; crack JH1, due to its greater depth and location in a high-stress zone, has a significantly higher driving force than crack JH2.

[0129] S402, using the maximum circumferential stress criterion or the maximum energy release rate criterion, predicts the propagation direction at each point on the crack front, wherein the propagation direction is perpendicular to the direction of the maximum principal stress;

[0130] The maximum circumferential stress criterion is implemented as follows: a local polar coordinate system is established at each point at the crack front, the change of circumferential stress with angle is calculated, and the direction corresponding to the maximum circumferential stress is taken as the crack propagation direction.

[0131] Maximum energy release rate criterion implementation: Calculate the energy release rate G when the crack virtually propagates in different directions, and take the direction corresponding to the maximum value of G as the crack propagation direction.

[0132] In this embodiment, the following applies: Crack JH1 has a plane perpendicular to the axial stress, the principal stress direction is clear, and the propagation direction mainly penetrates along the wall thickness direction; Crack JH2 has a plane perpendicular to the circumferential stress, and is affected by multiaxial stress, so the propagation direction has a tendency to deflect, and it is necessary to combine the two criteria for comprehensive judgment.

[0133] S403, the crack propagation prediction model is based on the propagation direction and uses an incremental method to gradually simulate the crack propagation process. When the stress intensity factor reaches the fracture toughness of the downcomer material or the crack depth reaches the structural wall thickness, the structure is determined to fail, and the cumulative propagation time is used as the remaining life.

[0134] S404, obtain the propagation direction of each point at the crack front edge, update the position of the crack front edge node with the propagation direction as the normal according to the preset incremental step size, generate the updated three-dimensional finite element model through spline fitting and crack surface reconstruction, and enter the next incremental iteration after recalculating the stress intensity factor until the failure criterion is met. Connect the crack morphology of each iteration step according to the number of load cycles to obtain the crack propagation path.

[0135] S405, when there are two or more non-coplanar distributed cracks in the concentrated downcomer region of the insertion boiler drum, calculate the stress field interaction between the cracks, and correct the stress intensity factor of each crack by the amplification factor of the stress intensity factor at the crack tip. The amplification factor is determined according to the crack spacing, the relative position of the cracks and the crack size ratio.

[0136] In this embodiment of the invention, cracks JH1 and JH2 are approximately 140 mm apart and are not coplanar. For such adjacent non-coplanar cracks at weld joints, GB / T19624-2019 "Safety Assessment of Defective Pressure Vessels in Use" does not provide the mutual interference effect coefficient between the two cracks, making it impossible to directly solve for the fracture ratio Kr using the formula. This method directly calculates the multi-crack interaction effect using the finite element numerical method, filling the gap in this standard regarding the interference coefficient of non-coplanar cracks.

[0137] This invention constructs a crack propagation path tracking system that integrates crack propagation driving force analysis, multi-criteria propagation direction prediction, incremental step-by-step simulation, adaptive crack surface reconstruction, and correction of non-coplanar multi-crack interaction effects. This system accurately depicts the entire evolution process of cracks in the centralized downcomer of an insert boiler drum from initiation to failure. It scientifically predicts crack deflection behavior using the maximum circumferential stress criterion or the maximum energy release rate criterion, dynamically updates the crack front geometry using incremental iteration and automatically reconstructs the finite element model, and effectively handles the interaction of multi-crack stress fields using an amplification factor correction mechanism. This significantly improves the accuracy of remaining service prediction and the visualization capability of crack propagation paths, providing reliable quantitative technical support for boiler service evaluation and differentiated maintenance decisions.

[0138] This invention provides a method for comprehensively evaluating the safety level of a current crack in an insert-type boiler drum downcomer by combining the remaining life under the current crack state and the crack propagation path. Specifically, this method includes:

[0139] S501, Load the remaining life and crack propagation path under the current crack state, and extract the crack depth-time curve, crack length-time curve and crack shape evolution parameters from the crack propagation path. The crack shape evolution parameters include the rate of change of crack depth-to-length ratio.

[0140] S502, calculate the fracture safety margin based on the ratio of the fracture toughness of the downcomer material to the stress intensity factor under the current crack state. The fracture safety margin reflects the fracture toughness reserve that the crack can resist rapid propagation. The life safety margin is determined based on the ratio of the predicted remaining life to the target service life. The target service life is determined according to the enterprise's maintenance plan and the unit's operating years.

[0141] S503, obtain the crack depth-time curve and crack length-time curve in the crack propagation path, calculate the crack depth propagation rate ratio and crack length propagation rate ratio of adjacent incremental steps, and calculate the crack propagation stability index based on the crack depth propagation rate ratio and crack length propagation rate ratio. The crack propagation stability index reflects the controllability of the crack propagation trend.

[0142] S504, based on the fracture safety margin, the life safety margin and the crack propagation stability index, a weighted comprehensive scoring method is used to determine the safety level score. The weight coefficients in the weighted comprehensive scoring method are determined based on historical failure cases of the insertion boiler drum centralized downcomer and expert experience.

[0143] S505, safety levels are divided based on safety level scores. The safety levels include four levels: safe, caution, warning, and danger. The safety level corresponds to the relative position of the evaluation point and the evaluation curve in the failure assessment diagram. When the evaluation point is located in the lower left of the evaluation curve and the safety margin is sufficient, it is determined to be a safe level. When the evaluation point is close to the evaluation curve, it is determined to be a caution or warning level. When the evaluation point is located in the upper right of the evaluation curve, it is determined to be a danger level.

[0144] S506, Generate a targeted maintenance recommendation report based on the current safety level of the crack. The maintenance recommendation report includes crack geometric parameters, stress intensity factor distribution, safety margin calculation results, remaining life prediction results, crack propagation path visualization diagram, failure assessment diagram, safety level assessment conclusion, and targeted maintenance recommendations. The targeted maintenance recommendations include continuing operation and setting a regular inspection cycle, shortening the inspection cycle and reviewing it during planned shutdowns, restricting operating conditions and arranging maintenance as soon as possible, and immediately shutting down for defect elimination.

[0145] In this embodiment of the invention, considering that the pressure and temperature of the boiler drum fluctuate during actual operation, the safety margin for crack JH1 is small, and crack JH2 is already in a critical state, it is recommended to perform defect elimination treatment on the cracks. The targeted maintenance recommendations include:

[0146] Continue running and set a regular inspection cycle: applicable to safety levels where cracks are stable and there is no risk of propagation;

[0147] Shorten the inspection cycle and recheck during planned downtime: Applicable to the level of caution, where cracks pose a certain risk and require enhanced monitoring;

[0148] Limit operating conditions and arrange maintenance as soon as possible: This applies to warning levels, such as situations where the safety margin for JH1 cracks is small;

[0149] Immediately stop the machine and perform troubleshooting: This applies to hazardous levels, such as when JH2 is already in a critical state.

[0150] On the other hand, embodiments of the present invention also provide a safety assessment system for cracks in the centralized downcomer of an insert-type boiler drum. Figure 18 A schematic diagram of an insertion-type boiler drum centralized downcomer crack safety assessment system is shown. The system specifically includes:

[0151] The finite element modeling module 100 is used to obtain the multi-source heterogeneous evaluation benchmark parameters of the centralized downcomer of the insertion boiler drum. The multi-source heterogeneous evaluation benchmark parameters include the physical geometric parameters, material performance parameters, and historical operating condition parameters of the downcomer. A three-dimensional finite element model containing crack defects is established based on the multi-source heterogeneous evaluation benchmark parameters.

[0152] The finite element modeling module 100 includes:

[0153] The initial building unit 110 is used to load the multi-source heterogeneous evaluation benchmark parameters, identify the physical geometric parameters of the downcomer in the multi-source heterogeneous evaluation benchmark parameters, and establish a crack-free initial finite element model of the downcomer based on the physical geometric parameters of the downcomer. The initial finite element model of the downcomer is constructed according to the 1:1 solid scale of the boiler drum downcomer welded joint.

[0154] Sub-model refinement unit 120 meshes the initial finite element model of the downcomer based on eight-node hexahedral elements, and examines the independence of the mesh to determine the optimal mesh size. A mesh refinement zone is set at the connection between the inner wall of the insert boiler drum and the outer wall of the downcomer, and a transition fillet model reflecting the geometry of the weld toe is constructed. The transition fillet model is fused with the initial finite element model of the downcomer with the mesh refinement zone set, and the output is the initial finite element model of the downcomer containing the refinement sub-model.

[0155] The stress distribution analysis unit 130 is used to obtain the initial finite element model of the downcomer including the densified sub-model, perform stress distribution analysis on the initial finite element model of the downcomer, and identify the location of the maximum stress of the concentrated downcomer joint and the stress distribution differences at different locations of the weld toe line.

[0156] The three-dimensional finite element model outputs 140 elements. Based on the stress distribution results of the initial finite element model of the downcomer, the crack initiation location and propagation direction are determined. With material property parameters as a priori, the sub-model is discretized along the quarter-node singular elements to obtain the crack singular mesh. The crack singular mesh is applied as a boundary condition to the initial finite element model of the downcomer to obtain a three-dimensional finite element model containing crack defects.

[0157] The stress intensity factor calculation module 200 is used to load a three-dimensional finite element model, introduce preset parameters of initial defects of cracks into the three-dimensional finite element model, and calculate the distribution of stress intensity factor at the crack tip under different operating conditions based on the fracture mechanics method.

[0158] The stress intensity factor calculation module 200 includes:

[0159] The crack tip identification unit 210 identifies crack tips in stress concentration areas in a three-dimensional finite element model based on initial defect preset parameters. The stress concentration areas include the fillet weld area at the connection between the pipe seat and the boiler drum body, the transition area between the downcomer insertion section and the inner wall of the boiler drum, and the reinforcement area at the edge of the opening in the boiler drum body.

[0160] The distribution curve generation unit 220 uses the displacement extrapolation method to calculate the stress intensity factor under different operating conditions at the crack tip, and performs polynomial fitting on the stress intensity factor at different crack front positions to obtain the stress intensity factor distribution curve along the crack front. The calculation results of the displacement extrapolation method are verified by comparing with the theoretical calculation values ​​in GB / T19624-2019.

[0161] The structural constraint factor determination unit 230 is used to calculate the geometric discontinuity factor of the inserted boiler drum centralized downcomer structure. The geometric discontinuity factor comprehensively considers the diameter ratio, wall thickness ratio, insertion depth ratio and weld shape coefficient of the boiler drum body and the downcomer. Based on the geometric discontinuity factor and the preset parameters of the initial defects, the structural constraint factor is determined. The structural constraint factor characterizes the influence of the triaxial stress state at the crack tip on the fracture toughness. The structural constraint factor is determined by three-dimensional finite element analysis combined with JQ theory.

[0162] Fracture toughness output unit 240 corrects the plane strain fracture toughness based on structural constraint factor to obtain the material fracture toughness of the inserted boiler drum centralized downcomer structure. The correction process takes into account the coupling effect of residual stress and thermal stress under the hot operating conditions of the boiler drum.

[0163] The failure assessment curve generation unit 250 is used to obtain the material fracture toughness of the inserted boiler drum centralized downcomer structure, and to construct a failure assessment curve based on the material fracture toughness and stress intensity factor of the inserted boiler drum centralized downcomer structure. The horizontal axis of the failure assessment curve is the load ratio, and the vertical axis is the fracture ratio.

[0164] The crack propagation prediction module 300 is used to obtain the stress intensity factor distribution at the crack tip under different operating conditions, pre-build a crack propagation prediction model, and calculate the remaining life and crack propagation path under the current crack state based on the crack propagation prediction model and the stress intensity factor distribution.

[0165] The safety level assessment module 400 is used to comprehensively assess the safety level of the current crack in the centralized downcomer of the insertion boiler drum, taking into account the fracture toughness of the downcomer material, the target service life, and the structural parameters of the downcomer, combined with the remaining life and crack propagation path under the current crack state.

[0166] In summary, this invention provides a method and system for assessing the safety of cracks in the centralized downcomer of an insert-type boiler drum. Targeting crack defects in the weld and heat-affected zone of the centralized downcomer in insert-type boiler drums, this invention constructs a complete assessment system encompassing geometric parametric modeling, accurate calculation of stress intensity factors, multi-mechanism coupled crack propagation prediction, and three-dimensional weighted safety level evaluation. This achieves quantitative analysis throughout the entire process from crack detection to maintenance decision-making. By introducing structural constraint factors to correct fracture toughness, employing an incremental method to progressively simulate the crack front propagation path, and integrating fracture safety margin, life safety margin, and crack propagation stability index for weighted scoring, this invention effectively solves the problems of insufficient specificity for the special structure of insert-type downcomers, high dispersion of assessment results, and difficulty in directly supporting maintenance decisions in existing technologies. It provides rapid, accurate, and quantifiable technical support for boiler serviceability evaluation and condition-based maintenance.

[0167] It should be noted that, for the sake of simplicity, the foregoing embodiments are all described as a series of actions. However, those skilled in the art should understand that the present invention is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to the present invention. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to the present invention.

[0168] It should be understood that the disclosed apparatus can be implemented in other ways, given the several embodiments provided in this application. For example, the apparatus embodiments described above are merely illustrative; the division of units described above is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or communication connections shown or discussed may be through some interfaces; the indirect coupling or communication connections between devices or units may be telecommunications or other forms.

Claims

1. A method for assessing the safety of cracks in the centralized downcomer of an insertion-type boiler drum, characterized in that, The method includes: Obtain the multi-source heterogeneous evaluation benchmark parameters of the centralized downcomer of the insertion boiler drum, and establish a three-dimensional finite element model containing crack defects based on the multi-source heterogeneous evaluation benchmark parameters. Load a three-dimensional finite element model, introduce preset parameters for initial crack defects into the three-dimensional finite element model, and calculate the stress intensity factor distribution at the crack tip under different operating conditions based on fracture mechanics method. Obtain the stress intensity factor distribution at the crack tip under different operating conditions, pre-build a crack propagation prediction model, and calculate the remaining life and crack propagation path under the current crack state based on the crack propagation prediction model and the stress intensity factor distribution. Constrained by the fracture toughness of the downcomer material, the target service life, and the structural parameters of the downcomer, and combined with the remaining life and crack propagation path under the current crack state, the safety level of the current crack in the centralized downcomer of the immersion boiler drum is comprehensively evaluated.

2. The method for assessing the safety of cracks in the centralized downcomer of an insert-type boiler drum as described in claim 1, characterized in that: Methods for establishing three-dimensional finite element models containing crack defects based on multi-source heterogeneous evaluation benchmark parameters include: Load the multi-source heterogeneous evaluation benchmark parameters, identify the physical geometric parameters of the downcomer in the multi-source heterogeneous evaluation benchmark parameters, and establish a crack-free initial finite element model of the downcomer based on the physical geometric parameters of the downcomer; The initial finite element model of the downcomer is meshed based on eight-node hexahedral elements. A mesh refinement zone is set at the connection between the inner wall of the insert boiler drum and the outer wall of the downcomer. A transition fillet model reflecting the geometry of the weld toe is constructed. The transition fillet model is fused with the initial finite element model of the downcomer with the mesh refinement zone to output the initial finite element model of the downcomer containing the refinement sub-model. Obtain the initial finite element model of the downcomer including the encrypted sub-model, perform stress distribution analysis on the initial finite element model of the downcomer, and identify the location of the maximum stress of the concentrated downcomer joint and the stress distribution differences at different locations of the weld toe line. Based on the stress distribution results of the initial finite element model of the downcomer, the crack initiation location and propagation direction are determined. With material property parameters as a priori, the sub-model is discretized along the quarter-node singular elements to obtain the crack singularized mesh. The crack singularized mesh is applied as a boundary condition to the initial finite element model of the downcomer to obtain a three-dimensional finite element model containing crack defects.

3. The method for assessing the safety of cracks in the centralized downcomer of an insert-type boiler drum as described in claim 2, characterized in that: The method for calculating the stress intensity factor distribution at the crack tip under different operating conditions based on fracture mechanics includes: Based on the initial defect preset parameters, identify the crack tip in the stress concentration region in the three-dimensional finite element model; The stress intensity factor at the crack tip under different operating conditions was calculated using the displacement extrapolation method, and polynomial fitting was performed on the stress intensity factor at different crack front positions to obtain the stress intensity factor distribution curve along the crack front. Calculate the geometric discontinuity factor of the centralized downcomer structure of the inserted boiler drum, and determine the structural constraint factor based on the geometric discontinuity factor and the initial defect preset parameters; Based on the structural constraint factor, the plane strain fracture toughness is modified to obtain the material fracture toughness of the inserted boiler drum centralized downcomer structure. The material fracture toughness of the inserted boiler drum centralized downcomer structure is obtained, and a failure assessment curve is constructed based on the material fracture toughness and stress intensity factor of the inserted boiler drum centralized downcomer structure.

4. The method for assessing the safety of cracks in the centralized downcomer of an insert-type boiler drum as described in claim 1, characterized in that: The method for constructing the crack propagation prediction model includes: A fatigue crack propagation rate model was constructed based on the failure assessment curve, and the Paris formula was used to describe the fatigue crack propagation behavior. A stress corrosion crack propagation rate model was established based on material performance parameters, and the stress corrosion crack propagation behavior was described using a power-law form. By loading the fatigue crack propagation rate model and the stress corrosion crack propagation rate model, and coupling and superimposing the fatigue crack propagation rate model and the stress corrosion crack propagation rate model, a crack propagation prediction model is obtained.

5. The method for assessing the safety of cracks in the centralized downcomer of an insert-type boiler drum as described in claim 4, characterized in that: A method for calculating the remaining life and crack propagation path under the current crack state based on a crack propagation prediction model combined with stress intensity factor distribution includes: Based on the current crack geometry and location, and in conjunction with the stress intensity factor distribution, the driving force distribution for crack propagation is determined; The propagation direction at each point on the crack front is predicted using the maximum circumferential stress criterion or the maximum energy release rate criterion, wherein the propagation direction is perpendicular to the direction of the maximum principal stress. The crack propagation prediction model is based on the propagation direction and uses an incremental method to gradually simulate the crack propagation process. When the stress intensity factor reaches the fracture toughness of the downcomer material or the crack depth reaches the structural wall thickness, the structure is determined to fail, and the cumulative propagation time is used as the remaining life. The propagation direction of each point at the crack front is obtained. The position of the crack front node is updated with the propagation direction as the normal according to the preset incremental step size. The updated three-dimensional finite element model is generated by spline fitting and crack surface reconstruction. After recalculating the stress intensity factor, the next incremental iteration is entered until the failure criterion is met. The crack morphology of each iteration step is connected in series according to the number of load cycles to obtain the crack propagation path.

6. The method for assessing the safety of cracks in the centralized downcomer of an insert-type boiler drum as described in claim 1, characterized in that: The method for comprehensively evaluating the safety level of the current crack in the centralized downcomer of the insertion-type boiler drum by combining the remaining life under the current crack state and the crack propagation path includes: Load the remaining lifetime and crack propagation path under the current crack state, and extract the crack depth-time curve, crack length-time curve and crack shape evolution parameters from the crack propagation path. The crack shape evolution parameters include the rate of change of crack depth-to-length ratio. The fracture safety margin is calculated based on the ratio of the fracture toughness of the downcomer material to the stress intensity factor under the current crack state, and the life safety margin is determined based on the ratio of the predicted remaining life to the target service life. Obtain the crack depth-time curve and crack length-time curve in the crack propagation path, calculate the crack depth propagation rate ratio and crack length propagation rate ratio of adjacent incremental steps, and calculate the crack propagation stability index based on the crack depth propagation rate ratio and crack length propagation rate ratio weighted.

7. The method for assessing the safety of cracks in the centralized downcomer of an insert-type boiler drum as described in claim 6, characterized in that: The method for comprehensively evaluating the safety level of the current crack in the centralized downcomer of the insertion-type boiler drum by combining the remaining life under the current crack state and the crack propagation path further includes: Based on the fracture safety margin, the lifetime safety margin, and the crack propagation stability index, a weighted comprehensive scoring method is used to determine the safety level score. Security levels are determined based on security level scores; A targeted maintenance recommendation report is generated based on the current safety level of the crack.

8. An insertion-type boiler drum centralized downcomer crack safety assessment system, used to implement the insertion-type boiler drum centralized downcomer crack safety assessment method as described in any one of claims 1-7, characterized in that: The insertion-type boiler drum centralized downcomer crack safety assessment system includes: The finite element modeling module is used to obtain the multi-source heterogeneous evaluation benchmark parameters of the centralized downcomer of the insertion boiler drum, and to establish a three-dimensional finite element model containing crack defects based on the multi-source heterogeneous evaluation benchmark parameters. The stress intensity factor calculation module is used to load a three-dimensional finite element model, introduce preset parameters of initial crack defects into the three-dimensional finite element model, and calculate the stress intensity factor distribution of the crack tip under different operating conditions based on the fracture mechanics method. The crack propagation prediction module is used to obtain the stress intensity factor distribution at the crack tip under different operating conditions, pre-build a crack propagation prediction model, and calculate the remaining life and crack propagation path under the current crack state based on the crack propagation prediction model and the stress intensity factor distribution. The safety level assessment module is used to comprehensively assess the safety level of the current crack in the centralized downcomer of the insertion boiler drum, taking into account the fracture toughness of the downcomer material, the target service life, and the structural parameters of the downcomer, combined with the remaining life and crack propagation path under the current crack state.

9. The safety assessment system for cracks in the centralized downcomer of the boiler drum as described in claim 8, characterized in that: The finite element modeling module includes: The initial building block is used to load the multi-source heterogeneous evaluation benchmark parameters, identify the physical geometry parameters of the downcomer in the multi-source heterogeneous evaluation benchmark parameters, and establish a crack-free initial finite element model of the downcomer based on the physical geometry parameters of the downcomer. The sub-model encryption unit meshes the initial finite element model of the downcomer based on eight-node hexahedral elements. A mesh encryption zone is set at the connection between the inner wall of the insert boiler drum and the outer wall of the downcomer. A transition fillet model reflecting the geometry of the weld toe is constructed. The transition fillet model is fused with the initial finite element model of the downcomer with the mesh encryption zone set, and the output is the initial finite element model of the downcomer containing the encryption sub-model. The stress distribution analysis unit is used to obtain the initial finite element model of the downcomer including the densified sub-model, perform stress distribution analysis on the initial finite element model of the downcomer, and identify the location of the maximum stress of the concentrated downcomer joint and the stress distribution differences at different locations of the weld toe line. The output elements of the three-dimensional finite element model are determined based on the stress distribution results of the initial finite element model of the downcomer to determine the crack initiation location and propagation direction. With material property parameters as a priori, the singular elements along the quarter node of the densified sub-model are discretized to obtain the crack singular mesh. The crack singular mesh is applied as a boundary condition to the initial finite element model of the downcomer to obtain a three-dimensional finite element model containing crack defects.

10. The safety assessment system for cracks in the centralized downcomer of the boiler drum as described in claim 9, characterized in that: The stress intensity factor calculation module includes: The crack tip identification unit identifies crack tips in stress concentration regions in a three-dimensional finite element model based on initial defect preset parameters. The distribution curve generation unit uses the displacement extrapolation method to calculate the stress intensity factor at the crack tip under different operating conditions, and performs polynomial fitting on the stress intensity factor at different crack front positions to obtain the stress intensity factor distribution curve along the crack front. The structural constraint factor determination unit is used to calculate the geometric discontinuity factor of the inserted boiler drum centralized downcomer structure, and to determine the structural constraint factor based on the geometric discontinuity factor and the initial defect preset parameters. The fracture toughness output unit corrects the plane strain fracture toughness based on the structural constraint factor to obtain the material fracture toughness of the inserted boiler drum centralized downcomer structure. The failure assessment curve generation unit is used to obtain the material fracture toughness of the inserted boiler drum centralized downcomer structure, and to construct the failure assessment curve based on the material fracture toughness and stress intensity factor of the inserted boiler drum centralized downcomer structure.

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