Creep Life Evaluation Method for Local Low-Hardness P91 Pipe Fittings Based on Image Processing Technology

Abstract

The present invention discloses a method for evaluating the creep life of a locally low-hardness P91 pipe fitting based on image processing technology, which relates to the technical field of life evaluation of P91 material pipe fittings commonly used in the power industry. Based on image processing technologies such as the image traversal method, threshold and twenty-six neighborhood connectivity segmentation algorithm, the present invention can obtain the stress level distribution of the P91 pipe fitting, and obtain the maximum stress value of the locally low-hardness area, providing accurate stress parameters for life evaluation. The present invention can improve the accuracy of creep life evaluation, provide accurate judgment for power plant technicians, avoid unnecessary replacements, and reduce the economic losses caused by replacing pipe fittings.

Description

Technical Field

[0001] The present invention relates to the technical field of life assessment of P91 material pipe fittings commonly used in the power industry, and particularly relates to a creep life assessment method for local low-hardness P91 pipe fittings based on image processing technology. Background Art

[0002] With the rapid development of China's thermal power generation industry, super (ultra) critical thermal power units with large capacity, high parameters and high efficiency have gradually become the mainstream of the development of thermal power equipment, posing more stringent requirements on the performance of power station metal materials (high-temperature creep performance, creep rupture performance and oxidation resistance). P91 material has excellent high-temperature performance, and its creep rupture strength can reach 100 MPa under the conditions of 593 °C / 100,000 h. At present, it has been widely used in high-temperature components such as main steam pipelines, reheater steam pipelines and their by-passes, and high-temperature headers with temperatures higher than 566 °C.

[0003] In recent years, during the metal detection process, it has been found that many P91 pipe fittings have the problem of low hardness in local areas. Regarding the problem of low hardness of P91 pipe fittings, there have been many relevant studies at home and abroad. A large number of research literatures show that when the hardness value of P91 pipe fittings is lower than the standard value of 180 HB, their remaining life will be significantly shortened, and even only thousands of hours are left, posing a huge safety hazard to the operation of the unit. Therefore, it is necessary to evaluate the life of low-hardness P91 pipe fittings.

[0004] At present, the life assessment of P91 pipe fittings is generally carried out according to the isothermal extrapolation method and the L-M parameter method specified in DL / T 654-2009 "Technical Guidelines for Life Assessment of Thermal Power Units" and DL / T 940-2005 "Technical Guidelines for Life Assessment of Steam Pipelines in Thermal Power Plants". In fact, according to the existing life assessment technical guidelines, the calculated results of the remaining life of pipe fittings often deviate greatly from the actual life. One of the main reasons for the deviation is the calculation of the maximum internal pressure stress σ θmax of the pipe fitting. The above guidelines stipulate that the maximum internal pressure stress σ θmax is obtained through the stress calculation formula or finite element method. For pipe fittings with uniform hardness and microstructure, the calculated results can represent the true stress level of the whole pipe fitting. However, for local low-hardness components, the maximum stress part may not be located in the low-hardness area, but may be in the area with normal hardness. Then, it is not accurate enough to perform stress analysis through empirical formula calculation or simple finite element simulation, and there are large deviations in the analysis results, and the results are on the conservative side.

[0005] The Electric Power Research Institute (EPRI) analyzed the stress redistribution in the local soft zone (i.e., the low-hardness zone) of P91 pipes in the technical report "Effect of Soft-Zone on the Creep Performance of Grade 91" (translation: "Effect of Soft-Zone Size on the Creep Performance of Grade 91 Pipe Fittings"). It was concluded that the steady-state stress level in the soft zone is always lower than that in the surrounding normal zone, and the creep stress in the soft zone is relatively low, which is due to the redistribution of creep stress from the weak zone to the surrounding normal materials. At the same time, the literature points out that local soft zones may not be the direct cause of pipe fitting scrapping. When conducting life assessment, the size, shape, and orientation of the soft zone and the load conditions in the component should be considered. The localized soft zone may be constrained by the surrounding normal 91 materials (good materials), and the creep performance of the local soft zone can still provide a long life. The life of a pipe containing some soft zones is longer than that of a pipe with all soft zones and much shorter than that of a pipe with all normal hardness. Pipe fittings with a large number of soft zone regions will have a shorter creep life than those with a small number of soft zone regions. In addition, in all cases, the maximum principal stress is always higher than the value of the von Mises equivalent stress. Therefore, for pipes with soft zones, applying the maximum principal stress will result in a conservative estimate of the creep life.

[0006] Such conservative estimation results often bring great doubts to power plant technicians. When technicians cannot accurately judge, they generally adopt conservative measures, that is, direct replacement. Direct replacement will not only cause unnecessary material waste but also cause a large amount of power loss. The resulting economic loss is in the millions, which is not worth the loss, and the significance of life assessment is greatly discounted. How to accurately analyze the stress in the low-hardness zone is an important issue facing life assessment staff. Summary of the Invention

[0007] The purpose of the present invention is to provide a more accurate life assessment method for P91 pipe fittings with local low-hardness zones. Based on image processing technology, the above deficiencies in the existing technical guidelines can be overcome. The stress level corresponding to the low-hardness zone can be obtained through image processing technology, and then the creep life of the pipe fitting can be calculated according to the relevant life assessment technical guidelines in the industry, and the stress level distribution of the low-hardness zone of P91 pipe fittings can be accurately obtained.

[0008] The technical solution adopted by the present invention to solve the above problems is: a creep life assessment method for local low-hardness P91 pipe fittings based on image processing technology, characterized by including the following steps:

[0009] Step 1: According to the geometric structure of the pipe fitting, determine the layout form of the inspection points for the surface hardness of the pipe fitting. Each inspection point should be evenly distributed and maintain an appropriate spacing to ensure that there is no inspection blind area in the whole pipe fitting. Conduct hardness inspection at each measurement point position with a portable Leeb hardness tester, record the hardness values corresponding to each measurement point, and compare them with the specified hardness standard values of P91 material to obtain the specific positions of the low-hardness value measurement points.

[0010] Step 2: Use 3D drawing software to obtain the 3D model of the pipe fitting; convert the 3D model of the pipe fitting into a 3D image, and determine the relative orientation of the hardness inspection measurement points through the image method.

[0011] Step 3: With the help of finite element software, analyze the stress of the pipe fitting under working conditions, and export the data of the pipe fitting obtained from the finite element calculation in a way corresponding to the 3D space coordinates and stress, denoted as matrix N.

[0012] Step 4: Denote the spatial pixel points of the 3D image of the pipe fitting as matrix A, and denote the relative positions of the hardness inspection measurement points and their corresponding hardness values as matrix B. Use the meshgrid function in matlab to generate azimuth matrices xq1, yq1, zq1 with the same size as A, and use the scatteredinterpolant function to perform non-linear fitting on matrix B and the three azimuth matrices xq1, yq1, zq1 to obtain a 3D matrix F, and obtain the 3D slice of matrix F. The values corresponding to matrix F are the hardness values of the pipe fitting. Perform image conversion on matrix F, and the red area of the pipe fitting is the low-hardness area.

[0013] Step 5: Use the image traversal method to count the number of hardness points in different hardness intervals in the low-hardness area of the pipe fitting, obtain the frequency distribution of different hardness interval ranges in the low-hardness area of the pipe fitting, and obtain the main concentrated falling interval of the low-hardness values of the pipe fitting.

[0014] Step 6: Use the threshold and 26-neighborhood connectivity segmentation algorithm to extract and segment the main concentrated falling interval of the low-hardness values of the pipe fitting to obtain the 3D image of the low-hardness area of the pipe fitting.

[0015] Step 7: Use the image traversal method to obtain the spatial coordinates of the main concentrated falling interval of the low-hardness values of the pipe fitting, denoted as matrix M; take matrix M as the spatial coordinate range, and export the corresponding stress values of matrix N. The maximum value is the maximum internal pressure stress σ for calculating the creep life of the pipe fitting. θmax ;

[0016] Step 8: According to the isothermal line extrapolation method, substitute the maximum internal pressure stress σ of the pipe fitting θmax into the creep life formula for calculation, or calculate according to the L-M parameter method with reference to the L-M parameter curve of P91 steel.

[0017] Further, the life assessment method is applicable to P91 pipe fittings with low hardness in local areas.

[0018] Further, the life assessment method is applicable to the life assessment of pipe fittings in the creep failure mode.

[0019] Compared with the prior art, the present invention has the following advantages and effects: The present invention can accurately obtain the stress level distribution in the low hardness area of P91 pipe fittings, improve the accuracy of the remaining life assessment, provide an effective control guarantee for the operation safety of the unit, provide accurate judgment for power plant technicians, avoid unnecessary replacements, and greatly reduce the economic losses caused by replacing pipe fittings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a distribution diagram of the hardness measurement points and the hardness values of each point of the pipe fitting in the embodiment of the present invention.

[0021] Figure 2 It is a relative orientation diagram of the hardness inspection measurement points of the pipe fitting in the embodiment of the present invention.

[0022] Figure 3 It is a diagram of the establishment of the finite element mesh of the pipe fitting in the embodiment of the present invention.

[0023] Figure 4 It is a three-dimensional slice diagram of the matrix F of the pipe fitting in the embodiment of the present invention.

[0024] Figure 5 It is a diagram of outlining the low hardness area of the pipe fitting in the embodiment of the present invention.

[0025] Figure 6 It is a frequency distribution diagram of the low hardness value distribution of the pipe fitting in the embodiment of the present invention.

[0026] Figure 7 It is a three-dimensional diagram of the low hardness area of the pipe fitting in the embodiment of the present invention.

[0027] Figure 8 It is a finite element analysis diagram of the stress distribution of the pipe fitting according to the conventional assessment method in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] The present invention will be further described in detail below with reference to the drawings and through embodiments. The following embodiments are explanations of the present invention and the present invention is not limited to the following embodiments.

[0029] Embodiment

[0030] In this embodiment, the creep life assessment method of local low-hardness P91 pipe fittings based on image processing technology provided by the present invention is used to evaluate the life of the P91 elbow of the high-temperature reheater steam pipeline of an in-service unit in a power plant. The specifications of the elbow are Φ609.6×12.7mm, and the steam parameters are 2.146MPa / 565.5°C. The steps are as follows:

[0031] Step 1: Since the pipe fitting to be inspected is a hot-pressed elbow, four cross-sections are taken on the elbow, and 8 points are evenly distributed circumferentially on each cross-section as the hardness inspection measuring point positions on the surface of the elbow. On-site, a Bambino2 type portable Leeb hardness tester is used to conduct hardness inspections at each measuring point position, and the hardness values corresponding to each measuring point are recorded. The test results show that there is a low-hardness area on the elbow. The hardness range of the low-value area is about 140-150HB, and the values are relatively concentrated, located in the area from the back arc surface to the neutral surface of the elbow. The hardness range of the normal area is about 190-250HB, and the values are relatively dispersed, located in the area from the neutral surface to the inner arc surface of the elbow. For the hardness measuring points and the hardness values of each point, see Figure 1 .

[0032] Step 2: Use 3D drawing software to obtain the 3D model of the pipe fitting. Convert the 3D model of the pipe fitting into a 3D image, and determine the relative orientation of the hardness inspection measuring points through the image method. See Figure 2 .

[0033] Step 3: With the help of finite element software, see Figure 3 , analyze the stress of the pipe fitting under the working conditions, and export the data of the pipe fitting obtained by finite element calculation in a way corresponding to the 3D space coordinates and stress, denoted as matrix N.

[0034] Step 4: Denote the spatial pixel points of the elbow as matrix A, and denote the relative positions of the hardness inspection measuring points and their corresponding hardness values as matrix B. Use the meshgrid function in matlab to generate azimuth matrices xq1, yq1, zq1 with the same size as A, and use the scatteredinterpolant function to perform non-linear fitting on matrix B and the three azimuth matrices xq1, yq1, zq1 to obtain a 3D matrix F, and obtain the 3D slice of matrix F. See Figure 4 , the value corresponding to matrix F is the hardness value of the elbow. It can be known that the minimum hardness of the elbow is 140HB and the maximum hardness is 250HB; perform image conversion on matrix F, and the red area of the elbow is the low-hardness area. See Figure 5 .

[0035] Step 5: Use the image traversal method to count the number of hardness points in different hardness intervals in the low-hardness area of the elbow, and obtain the frequency distribution of different hardness interval ranges in the low-hardness area of the elbow. See Figure 6, it can be observed that the hardness values in the low-hardness area of the elbow mainly concentrate in the range of 145 - 155 HB.

[0036] Step 6: Use the threshold and the twenty-six neighborhood connectivity segmentation algorithm to extract and segment the low-hardness area of the elbow with a hardness range of 145 - 155 HB to obtain the three-dimensional image of the low-hardness area of the pipe fitting. See Figure 7 .

[0037] Step 7: Use the method of image traversal to obtain the spatial coordinates of the low-hardness area (145 - 155 HB) of the elbow, denoted as matrix M. Taking matrix M as the spatial coordinate range, export the stress values corresponding to matrix N, and take the maximum value as σ required for the creep life of the elbow θmax , after calculation, the maximum stress σ θmax of the low-hardness area (145 - 155 HB) of this elbow is 36 MPa.

[0038] Step 8: Substitute the maximum stress σ θmax = 36 MPa of the low-hardness area of this elbow into the creep life formula for calculation, and it can be obtained that the remaining life of this elbow is 187,000 hours.

[0039] Note: Regarding the values of the 10,000-hour and 100,000-hour creep rupture strengths in the formula , when conducting a Class II life assessment according to the standard DL / T 940, for their creep rupture properties, the lowest values can be taken by referring to relevant materials. Here, by referring to the literature "Study on the Microstructure and Properties of Low-Hardness P91 Steel", a pipe sample with a lowest hardness value of 140 HB was obtained through simulating the heat treatment system of an out-of-control process. Its mechanical properties, microstructure, and heat treatment system are consistent with those of the elbow in this power plant. Therefore, the creep rupture strength value of this elbow can take the median value in the literature, that is:

[0040] To verify the accuracy of this method, it is compared with the conventional assessment method. There are usually two calculation methods for conventional stress calculation: one is to use the circumferential stress formula recommended by DL / T 940; the other is to use finite element modeling based on the actual dimensions of the pipeline. The larger stress value of the two is adopted to ensure that the life assessment data is on the safe side.

[0041] The calculation formula for the maximum circumferential stress σ θmax at the elbow part:

[0042]

[0043] In the formula:

[0044] e - roundness of the elbow;

[0045] P - calculated pressure, MPa;

[0046] Do, Di——The outer diameter and inner diameter of the elbow, respectively, in mm;

[0047] S——The minimum wall thickness of the pipeline;

[0048] v——Poisson's ratio, 0.3;

[0049] E——The elastic modulus of the material, 1.700×10 5 MPa.

[0050] Substitute the relevant data of the elbow into the above formula respectively, and the reduced stress σ of the internal pressure of the elbow can be obtained θ = 37.62 MPa.

[0051] According to the actual size of the elbow, finite element modeling and finite element analysis are carried out, and the stress distribution of the elbow is obtained. See Figure 8 . It can be seen that the maximum stress of the elbow appears on the inner arc pipe wall, and the circumferential stress σ θ = 43.47 MPa. For safety considerations, the working stress is selected as the larger value. Substitute it into the creep life formula for calculation, and the final remaining life of this elbow is obtained as 27,000 hours. If the remaining life of this elbow is evaluated according to the above conventional stress analysis results, this elbow has reached the end of its life and needs to be replaced. However, according to the hardness distribution of this elbow, the low-hardness area of this elbow is mainly located near the back arc side, and the hardness value on the inner arc side meets the standard requirements. Obviously, it is inaccurate to evaluate the creep life of the low-hardness area with the stress analysis value on the inner arc side.

[0052] In summary, this life evaluation method based on image processing technology is applicable to the situation where there is local low hardness in P91 pipe fittings, can improve the effectiveness of pipe fitting life evaluation, and help technicians make accurate judgments.

[0053] The content not described in detail in this specification belongs to the prior art well-known to those skilled in the art.

[0054] Although the present invention has been disclosed above with embodiments, it is not intended to limit the protection scope of the present invention. Any changes and modifications made by those skilled in the art without departing from the concept and scope of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for evaluating the creep life of local low-hardness P91 pipe fittings based on image processing technology, characterized in that, It includes the following steps: Step 1: According to the geometric structure of the pipe fitting, determine the layout form of the inspection points for the surface hardness of the pipe fitting. Each inspection point is evenly distributed and maintains an appropriate spacing to ensure that there is no inspection blind area in the whole pipe fitting; Conduct hardness inspection at each measuring point position with a portable Leeb hardness tester, record the hardness values corresponding to each measuring point, and compare them with the specified hardness standard values of P91 material to obtain the specific positions of the measuring points with low hardness values; Step 2: Use 3D drawing software to obtain the 3D model of the pipe fitting; convert the 3D model of the pipe fitting into a 3D image, and determine the relative orientation of the hardness inspection measuring points through the image method; Step 3: With the help of finite element software, analyze the stress of the pipe fitting under the working conditions, and export the data of the pipe fitting obtained by finite element calculation in a way corresponding to the 3D space coordinates and stress, denoted as matrix N; Step 4: Denote the spatial pixel points of the 3D image of the pipe fitting as matrix A, and denote the relative positions of the hardness inspection measuring points and their corresponding hardness values as matrix B. Use the meshgrid function in matlab to generate the azimuth matrices xq1, yq1, zq1 with the same size as A. Use the scatteredinterpolant function to perform non-linear fitting on matrix B and the three azimuth matrices xq1, yq1, zq1 to obtain the 3D matrix F, and obtain the 3D slice of matrix F. The value corresponding to matrix F is the hardness value of the pipe fitting. Perform image conversion on matrix F, and the red area of the pipe fitting is the low hardness area; Step 5: Use the image traversal method to count the number of hardness points in different hardness intervals in the low hardness area of the pipe fitting, obtain the frequency distribution of different hardness interval ranges in the low hardness area of the pipe fitting, and obtain the main concentrated falling interval of the low hardness value of the pipe fitting; Step 6: Use the threshold and twenty-six neighborhood connectivity segmentation algorithm to extract and segment the main concentrated falling interval of the low hardness value of the pipe fitting to obtain the 3D image of the low hardness area of the pipe fitting; Step 7: Use the image traversal method to obtain the spatial coordinates of the main concentrated falling interval of the low hardness value of the pipe fitting, denoted as matrix M; Taking the matrix M as the spatial coordinate range, the stress value corresponding to the matrix N is derived, and its maximum value is the maximum internal pressure stress σ for calculating the creep life of the pipe fitting θmax ; Step 8: According to the isothermal extrapolation method, substitute the maximum internal pressure stress σ of the pipe fitting θmax into the creep life formula for calculation, or calculate according to the L-M parameter method with reference to the L-M parameter curve of P91 steel.

2. The creep life assessment method for local low-hardness P91 pipe fittings based on image processing technology according to claim 1, characterized in that, The said life evaluation method is applicable to P91 pipe fittings with low hardness phenomena in local areas.

3. The creep life evaluation method for local low-hardness P91 pipe fittings based on image processing technology according to claim 1, characterized in that, The said life evaluation method is applicable to the life evaluation of pipe fittings with creep failure mode.

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