A method for evaluating the ultimate load-bearing capacity of elbow pipes with progressive local thinning defects

Abstract

The present invention is a method for evaluating the ultimate bearing capacity of a bend with progressive local thinning defects. The ultimate bearing capacity of a bend with a known local thinning size is determined by finite element ultimate load analysis; combined with the wall thickness fixed-point thickness measurement results, computational fluid dynamics and dynamic mesh method are used to analyze the change law of the erosion wear area and the maximum erosion thinning depth of the bend over time, and the change law of the actual local thinning morphology defect over time is obtained; the regional range and depth of the equivalent local thinning defect equivalent to the ultimate load of the actual local thinning morphology defect are determined, and the change law of the actual erosion thinning regional range and the maximum erosion thinning depth over time is obtained, thereby realizing the evaluation of the ultimate bearing capacity of the bend with progressive local thinning defects. The complex process of modeling the actual situation is avoided, and an engineering feasible method is proposed. The actual morphology is expressed by the calculation results of the CFD range, and the actual evaluation process is simpler.

Description

Technical Field

[0001] The invention relates to the field of evaluation of the ultimate bearing capacity of a locally thinned elbow due to erosion wear, and in particular to how to quickly determine the scope and size of local thinning defects during the gradual local thinning of the elbow. Background Art

[0002] Pressure pipelines are an important part of special equipment. Pressure pipelines are often used to transport various fluid media, such as gas, liquid or fluid containing particles. When in service, they are often subjected to internal pressure, bending moment and other loads, and are subjected to very harsh working conditions. When the direction of the pipeline changes, the fluid containing particles will erode the pipeline, causing local thinning of the pipeline wall. The local thinning defect reduces the bearing capacity of the pipeline. For the parts of the pipeline that are perforated due to local thinning caused by erosion and wear, the ultimate load of the pipeline is usually used to evaluate the ultimate bearing capacity of the pipeline during the evaluation process, and the progress of local thinning of the pipe wall is identified through regular thickness measurement or online thickness measurement in production. However, the evaluation of the ultimate bearing capacity of the pipeline with progressive local thinning defects requires not only the maximum erosion and thinning depth, but also the range size of the erosion and thinning defects. Obviously, fixed-point detection of wall thickness alone cannot meet the requirements.

[0003] Sufficient research has been conducted on the analysis of the ultimate load of elbows with local thinning defects: the variation patterns of the ultimate internal pressure of elbows with local thinning defects with pipe size, defect location, defect depth, defect width and defect length are given. In existing methods, the variation patterns of the ultimate load of straight pipes with internal and external double defects with pipe diameter and wall thickness, defect length and depth, and axial distance of internal and external double defects are studied, but the variation curve of the ultimate load with the depth of local thinning defects is not considered, and it is not suitable for the calculation of the ultimate load of elbows with gradual local thinning defects on the inner wall of the outer arch. However, the variation pattern of the ultimate load alone cannot make a correct judgment on the bearing capacity of pipes with gradual local thinning defects. Therefore, this application is based on the ultimate load analysis of defective pipes and combines erosion and wear analysis to provide a method for the ultimate bearing capacity of elbows with gradual local thinning defects. Summary of the invention

[0004] The purpose of the present invention is to evaluate the demand for the ultimate load-bearing capacity of a pipe bend with local thinning defects, and a method for evaluating the ultimate load-bearing capacity of a pipe bend with progressive local thinning defects is proposed based on the ultimate load analysis. The method uses computational fluid dynamics combined with measured wall thickness data to determine the range size of the local thinning defect, solving the problem that the existing technology cannot accurately determine the range size of the local thinning defect in the pipeline.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] A method for evaluating the ultimate load-bearing capacity of a pipe bend with a progressive local thinning defect, the method comprising the following contents:

[0007] S1: Taking the elbow with local thinning defect as the research object, the variation law of the ultimate load of the elbow with local thinning defect with the size of the local thinning defect is obtained;

[0008] S2: Use computational fluid dynamics (CFD) and the dynamic grid method to simulate and analyze the erosion wear of the elbow to obtain the time-varying curve of the local thinning defect size, and use the fixed-point wall thickness measurement results to correct the erosion wear model parameters in the CFD according to the actual working conditions until the erosion wear prediction results match the actual ones, and determine that the time-varying curve of the local thinning defect size at this time is the time-varying law of the actual local thinning morphology defect; the local thinning defect size includes the axial length, circumferential length and depth of the local thinning defect;

[0009] S3: at different times, the actual local thinning morphology defects are subjected to equivalent processing. If the average value of the relative error between the limit loads of the elbow containing the actual local thinning morphology defects and the elbow containing the equivalent local thinning defects is less than 5%, the axial and circumferential range values ​​of the erosion wear area of ​​the elbow containing the equivalent local thinning defects at this time are determined to be the equivalent local thinning defect range of the elbow containing the actual local thinning morphology defects, and the equivalent local thinning defect range is obtained;

[0010] S4: obtaining the time-varying curves of the circumferential range and axial range of the equivalent local thinning defect in the time-varying law of the actual local thinning morphology defect in step S2 within the equivalent local thinning defect range of step S3, and obtaining the time-varying curve of the maximum erosion thinning depth, and then obtaining the time-varying curve of the size of the progressive local thinning defect;

[0011] S5: Substitute the values ​​at different times corresponding to the curve of the change of the size of the progressive local thinning defect with time obtained in step S4 into the law of the change of the ultimate load of the elbow with local thinning defects with the size of the local thinning defect obtained in step S1, and evaluate the ultimate load-bearing capacity of the elbow with progressive local thinning defects during the erosion process.

[0012] In step S2, the method for determining the variation law of the actual local thinning morphology defect over time is:

[0013] Computational fluid dynamics is used to establish an erosion wear model. Based on the dynamic grid erosion wear calculation, the flow field physical quantity information and wall erosion wear results are calculated to obtain the wear depth and wear area range under different erosion times.

[0014] According to the actual situation and the pipeline fixed-point wall thickness monitoring data, the average mass flow rate of particles, particle size function, impact angle function, particle relative flow velocity, and exponential function of particle relative flow velocity in the erosion wear model are adjusted through trial calculation until the impact wear prediction results are consistent with the actual results. The time-varying laws of erosion depth and erosion thinning area range are given at this time, thereby obtaining the time-varying laws of actual local thinning morphological defects; the pipeline fixed-point wall thickness monitoring data are measured data.

[0015] The process of equivalent quantification in step S3 is: through the limit load equivalence, the size and depth of the equivalent local thinning defect range are determined, and the time variation law of the size and depth of the equivalent local thinning defect range is obtained; the maximum erosion thinning depth is selected as the equivalent local thinning defect depth; when the erosion thinning area is equivalent, the area range is determined according to the maximum erosion thinning depth, and the values ​​of the axial and circumferential range sizes of the local thinning defects are determined according to the maximum and minimum values ​​of the area boundary coordinates corresponding to the edge depth of the equivalent erosion thinning range at the time when the limit internal pressure is equal to the maximum erosion thinning depth.

[0016] The process of determining the regional range is: taking a fixed percentage of the maximum erosion thinning depth as the edge depth of the equivalent erosion thinning range; the values ​​of the axial and circumferential range dimensions of the local thinning defects are determined according to the maximum range of the erosion area length and width corresponding to the edge depth of the equivalent erosion thinning range, and the equivalent regional range is determined at this time.

[0017] In step S5, the limit load is calculated based on the load state point p l Whether it is close to 1.0 is used as the criterion for risk assessment of pipe bending, and the load state point p is obtained. l Curve of variation with defect size; p l =P T / P LS , where P T is the design internal pressure of the piping system, P LS is the ultimate internal pressure of the elbow with local thinning defects under a single load, P LS Obtained by using the time variation curve of the defect size containing progressive local thinning obtained in step S4;

[0018] When p l As it approaches 1.0, the elbow with progressive local thinning defects approaches its ultimate internal pressure, at which point the pipe will fail.

[0019] The above method can also be used to evaluate the ultimate bearing capacity of elbows containing local thinning defects under bending moment, torque and axial force loads.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The method of the present invention is also applicable to evaluating the ultimate bearing capacity of bent pipes containing gradual local thinning defects under the action of bending moment, torque and axial force loads. Different load action forms require calculation and processing of the ultimate bending moment, torque and axial force curves varying with defect size in the first step, and dimensionless bending moment, torque and axial force are required for judgment in the fifth step.

[0022] The method for evaluating the ultimate bearing capacity of a pipe bend containing a progressive local thinning defect of the present invention determines the ultimate bearing capacity of a pipe bend with a known local thinning size through finite element ultimate load analysis; combines the wall thickness fixed-point thickness measurement result, uses computational fluid dynamics and a moving grid method to analyze the variation of the erosion and wear area (including the axial length and the circumferential length) and the maximum erosion and thinning depth of the pipe bend with time, and obtains the variation of the actual local thinning morphology defect with time; determines the area range and depth of an equivalent local thinning defect equivalent to the ultimate load of the actual local thinning morphology defect, and obtains the variation of the actual erosion and thinning area range and the maximum erosion and thinning depth with time, thereby realizing the ultimate bearing capacity evaluation of the pipe bend containing a progressive local thinning defect.

[0023] The method of the present invention avoids the complicated process of modeling the actual situation, proposes an engineering feasible method, and expresses the actual morphology with the calculation results within the CFD range, making it easier to evaluate the actual process. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the purpose of the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The drawings described below are only embodiments and verifications of the present invention. For ordinary technicians in this field, other drawings can be obtained according to the provided methods and drawings without making any creative work. In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the drawings in the embodiments of the present invention. All other embodiments obtained by ordinary technicians in this field without making any creative work are within the scope of protection of the present invention.

[0025] Figure 1 Flowchart of the method for evaluating the ultimate load-bearing capacity of elbows with progressive local thinning defects.

[0026] Figure 2 The geometric model of a pipe bend with a local thinning defect.

[0027] Figure 3 Finite element model of a pipe bend with local thinning defects.

[0028] Figure 4This is the finite element model of a 108mm elbow.

[0029] Figure 5 Meshing of curved pipes.

[0030] Figure 6 This is the erosion wear cloud diagram of 108mm diameter elbow under different erosion days.

[0031] Figure 7 This is the erosion wear cloud diagram of 55mm diameter elbow under different erosion days.

[0032] Figure 8 The bent pipe model with erosion and thinning defects after meshing, and the bent pipe model after applying boundary conditions, where (a) is the bent pipe model with erosion and thinning defects after meshing, and (b) is the bent pipe model after applying boundary conditions.

[0033] Fig. 9 Load-strain curves of the actual local thinning morphology defect elbow and the equivalent local thinning defect elbow, where (a) is the load-strain curve of the actual local thinning morphology defect elbow, and (b) is the load-strain curve of the equivalent local thinning defect elbow.

[0034] Fig.10 Curves of the change of the erosion thinning area range and the maximum erosion thinning depth of the elbow with time, where (a) is the curve of the change of the erosion area range with time, and (b) is the curve of the change of the maximum erosion thinning depth with time.

[0035] Fig.11 Dimensionless local thinning defect variation with time.

[0036] Fig.12 p l Curve of change with dimensionless time t.

[0037] Table 1 shows the pipe material parameters in the embodiments of the present application.

[0038] Table 2 shows the pipe dimensions calculated for erosion wear in the examples of the present application.

[0039] Table 3 is the impact angle function in the examples of the present application.

[0040] Table 4 is a calculation scheme for determining the equivalent local thinning defect elbow in the embodiment of the present application.

[0041] Table 5 is a comparison result of the ultimate internal pressures of the elbows containing actual local thinning defects and the elbows containing equivalent local thinning defects in the embodiments of the present application. DETAILED DESCRIPTION

[0042] To facilitate those skilled in the art to understand the technical content of the present invention, the present invention is illustrated in conjunction with the following examples.

[0043] The specific steps of the method for evaluating the ultimate load-bearing capacity of a pipe elbow containing a progressive local thinning defect of the present invention are:

[0044] The first step is to determine the law of change of the ultimate load of the elbow with local thinning defects as the size of the local thinning defects changes.

[0045] The model of the bent pipe with local thinning defect is as follows Figure 2 As shown in the figure, the elbow model size is a 90° elbow with a straight pipe. The straight pipe length L is 150mm, the pipe size is Φ108×8, and the inner radius of the pipe is R. i The outer radius is represented by R o To indicate the wall thickness, T g To indicate the bending radius R b The local thinning defect shape adopts ideal wall thinning, which is "rectangular" thinning. The local thinning defect position is at the center of the inner wall of the outer arch of the elbow. The local thinning defect size of the elbow includes the axial half-length angle γ, the circumferential half-length angle θ and the local thinning defect depth C. They are dimensionless, and the dimensionless geometric parameters are: dimensionless local thinning axial half-length a = γ / 45°, dimensionless local thinning circumferential half-length b = θ / 180°, dimensionless local thinning depth c = C / T g .

[0046] The material is assumed to be an ideal elastic-plastic material, the design temperature is 25°C, and the material properties used in this paper are shown in Table 1.

[0047] Table 1 Pipe material parameters

[0048]

[0049] The finite element model of the elbow with local thinning defect is as follows Figure 3 The internal pressure is applied to the inner wall of the pipe, and an axial equivalent surface load P is applied to the end of the pipe in the y direction. end The axial equivalent surface load is calculated according to formula (1), where P is the internal pressure applied to the model. Annular and axial constraints are applied to the ends of the pipe in the z direction.

[0050]

[0051] In order to more conveniently analyze the limit load of the elbow with local thinning defects, the limit load is normalized as follows: according to p LS =P LS / P L0 Obtain the normalized ultimate internal pressure p under a single load LS . Among them, P L0 Calculated by formula (2)

[0052]

[0053] P LS is the calculated ultimate internal pressure of the elbow with local thinning defects under a single load, P L0 is the calculated limit internal pressure of the defect-free elbow under a single load, R m is the average radius of the pipe, σ y is the yield strength of the material.

[0054] After finite element analysis and calculation, the empirical formula between the normalized limit internal pressure and the size of the local thinning defect was obtained by fitting with SPSS software.

[0055] The formula for the change of normalized limit internal pressure with the size of local thinning defect is:

[0056]

[0057] Among them, G1 is the geometric parameter of local thinning defect, G1 = a 0.4 b 0.3 c 0.7 , dimensionless local thinning axial half-length a = γ / 45°, dimensionless local thinning circumferential half-length b = θ / 180°, dimensionless local thinning depth c = C / T g .

[0058] By analyzing the ultimate load of elbows with various local thinning defects, the variation law of the ultimate load of elbows with local thinning defects with the size of local thinning defects (i.e., the normalized ultimate internal pressure and the dimensionless local thinning axial half-length a=γ / 45°, the dimensionless local thinning circumferential half-length b=θ / 180°, and the dimensionless local thinning depth c=C / T) is obtained. g The relationship between them is as follows: in the above, the normalized ultimate internal pressure is used to represent the ultimate load; the larger the defect, the smaller the ultimate load that can be sustained.

[0059] The second step is to determine the actual local thinning morphology defects.

[0060] Computational fluid dynamics (CFD) is used in combination with a certain working condition to analyze the erosion and wear of the pipeline by the fluid containing particles, and the time-varying law of the erosion depth and the erosion thinning range is given. In order to calculate the erosion and thinning range, the erosion and wear calculation results are explained by taking the water-sand system as an example. Taking the long radius elbow as an example, ANSYS Fluent is used to analyze the erosion and wear of the 90° metal elbow by the water-sand system, and the discrete particle model (DPM model) is used to simulate the erosion and wear. The flow field physical quantity information and the wall erosion and wear results are calculated on the basis of the dynamic grid erosion and wear calculation, and the limit load method is used to determine the equivalent local thinning defect range equivalent to the actual thinning morphology limit load under the water-sand system working condition, and further extract the erosion and thinning range (including depth and axial length, circumferential length) with time The curve is used to evaluate the ultimate bearing capacity of the elbow containing local thinning defects.

[0061] The erosion and wear analysis of elbows with different diameters and bending radii was carried out. Solidworks was used to build the inner wall of the pipe. The elbow model was a 90° elbow with a sufficient length. The pipe dimensions are shown in Table 2. The elbow model and mesh division (meshing is performed using ANSYS Fluent software) are now explained using a 108mm diameter pipe as an example. The model and mesh division are shown in the figure below. Figure 4 and Figure 5 The lengths of the inlet and outlet sections meet the lengths required for the turbulence to fully develop.

[0062] Table 2 Pipeline dimensions for erosion wear calculation

[0063]

[0064] Model selection: The Realizable k-ε turbulence model is selected as the turbulence model, which can effectively solve the boundary layer flow problem with directionality. The Generic model is selected as the erosion wear model. The erosion wear model is shown in formula (4). During the calculation process, the parameters in the erosion wear model need to be modified according to different working conditions so that the depth calculated by computational fluid dynamics is the same as the depth in the actual working condition. The erosion wear model needs to be adjusted according to the working conditions and is not fixed. The average mass flow rate of particles, particle size function, impact angle function, particle relative flow velocity, and exponential function of particle relative flow velocity in formula (1) all need to be actually selected according to different working conditions. At the same time, the working condition system parameters also need to be selected according to different working conditions.

[0065]

[0066] Among them, E R is the erosion rate (kgm -2 s -1);m p is the average mass flow rate of particles (kgs -1 );d p is the particle size function; f(α) is the impact angle function; v p is the relative flow velocity of particles (m / s); b(v) is v p The exponential function of A face is the erosion area (m 2 ); N is the number of impacts of the particle, C(d p ) is a function of particle size.

[0067] The parameters of the water-sand system in this embodiment are: the density of the particles is 1500 kg / m 3 , fluid density is 998.2kg / m 3 , viscosity is 0.001003kg / (m·s), pipeline material density is 7850kg / m 3 The relative flow rate of the fluid and particles is 3 m / s. The diameter of the particles is 200 μm, and the average mass flow rate of the particles is 0.1 kg / s.

[0068] Boundary conditions: select velocity inlet for inlet, pressure outlet for outlet, escape for both discrete phase inlet and outlet boundaries, no-slip solid wall condition for wall boundary, particle-wall collision behavior. Since particles have certain energy loss after colliding with the wall, it is necessary to determine the wall restitution coefficient to characterize the change in the motion state after the particle collides with the wall. The particle-wall collision restitution coefficient model is a model for calculating the particle rebound angle and rebound velocity after the particle collides with the wall. The wall collision restitution coefficient is composed of the normal component e n and the tangential component e t Since the pipeline model calculated in this embodiment is a metal alloy, the wall collision recovery coefficient calculation formula selected in this embodiment is:

[0069]

[0070] Here, α is the particle incident angle.

[0071] The impact angle function is defined in a nonlinear way. The data are shown in Table 3. The particle size function C(d p ) takes a fixed value of 1.8x10 -9 , b(v) takes a fixed value of 2.6.

[0072] Table 3 Impact angle function

[0073]

[0074] Fluent software was used to calculate the wear depth and wear area of ​​the elbow under erosion for different days, that is, the erosion calculation results of 108mm diameter pipes and 55mm diameter pipes at 90 days, 180 days, 240 days and 270 days, as well as the erosion calculation results at 60 days, 90 days, 120 days and 180 days, respectively. The erosion wear cloud diagram of 108mm diameter pipe at 90 days, 180 days, 240 days and 270 days is shown in the figure below. Figure 6 As shown in the figure, the erosion wear cloud diagram of 55mm diameter pipe at 60 days, 90 days, 120 days and 180 days is as follows Figure 7 shown.

[0075] It is worth noting that the prediction of erosion wear is related to many factors, such as material system, flow conditions, pipeline characteristics, etc. According to the actual situation and the pipeline fixed-point wall thickness monitoring data, the impact angle function value Value in Table 3 can be adjusted by trial calculation until the impact wear prediction result is consistent with the actual result, thereby obtaining the change law of the actual local thinning morphology defect over time.

[0076] Step 3: Determination of the equivalent local thinning defect range

[0077] In order to determine the risk of micro-leakage caused by failure of pipes with erosion and thinning defects, it is necessary to determine the relationship between the range of the erosion and thinning area and the erosion depth, as well as the time-varying law of the erosion and thinning area and the erosion depth. The limit load method is used to calculate the limit load of the elbow with actual local thinning morphology defects and the limit load of the elbow with equivalent local thinning defects using ANSYS APDL. The two limit loads are compared. If the two limit load values ​​are equivalent, the axial and circumferential range values ​​of the erosion wear area of ​​the elbow with equivalent local thinning defects at this time are determined to be the equivalent local thinning defect range of the elbow with actual local thinning morphology defects. In this equivalent process, the depth is not equivalently processed, and the law of the maximum erosion and thinning depth changing with time is always used as the depth law of the actual morphology.

[0078] Determine the calculation scheme: According to the erosion wear calculation results of different pipe diameters under different erosion times mentioned above, that is, the erosion calculation results calculated by Fluent software, Table 4 is a calculation scheme for determining the range of equivalent erosion thinning defects based on the erosion time and corresponding pipe diameter set when Fluent software performs erosion calculations, and calculates the ultimate internal pressures of pipelines containing different erosion thinning defects.

[0079] Table 4 Calculation scheme

[0080]

[0081]

[0082] The modeling and calculation were carried out according to the designed pipe dimensions in Table 4. The calculation model is the outer wall of the pipe, and in order to simplify the calculation, the inlet section and the outlet section are taken as 750mm respectively. The Shell181 shell unit is used for division, and the thickness extension direction is the Top plane. After modeling according to Table 4, meshing is performed. The meshing in this step will be the same as the number of mesh divisions of the outer wall of the elbow in ANSYS Fluent mentioned above. The coordinate information of the eroded pipe wall mesh unit and the erosion wear depth in Fluent are read out, and then the erosion and thinning depth of each unit is read into the unit corresponding to the calculation model in ANSYSAPDL according to the unit coordinates. The model of the elbow with erosion and thinning defects and the meshing are as follows Figure 8 As shown in (a). The maximum erosion thinning depth is selected as the equivalent local thinning defect depth. When the erosion thinning area is quantified, the area range is determined according to the maximum erosion thinning depth. The values ​​of the axial and circumferential range dimensions of the local thinning defect are determined according to the maximum range of the length and width of the erosion area corresponding to the edge depth of the equivalent erosion thinning range. At this time, the equivalent area range is determined.

[0083] The material properties of the pipeline are the same as those defined in Table 1. The internal pressure is applied to the inner wall of the pipeline, and an axial equivalent linear load P is applied to the end of the pipeline in the y direction. n ,like Figure 8 As shown in (b), the axial equivalent line load is calculated according to formula (6). Annular constraints and axial constraints are applied to the ends of the pipe in the z direction.

[0084]

[0085] Take the calculation condition of erosion and wear of a 108mm pipe diameter for 180 days as an example. The maximum erosion thinning depth is 3.86mm. Under this condition, all the depths of the erosion thinning area are obtained through CFD calculation. The maximum depth is expanded outward to form a curved surface area. Each point on the edge of the boundary of the area will have an erosion thinning depth. The erosion thinning depths at all points on the edge are compared with the maximum erosion thinning depth, and the average of all ratios is taken as the edge depth of the equivalent erosion thinning range. The maximum range of erosion area length and width corresponding to the edge depth of the equivalent erosion thinning range of 0.95mm is taken as the equivalent erosion thinning model for analysis and calculation. The internal pressure value applied is 100MPa, and the limit internal pressure of the elbow containing actual local thinning morphological defects and the elbow containing equivalent local thinning defects are calculated respectively. Take the load-strain curve at the maximum point of plastic strain. The load-strain curve of the elbow containing actual local thinning morphological defects is as follows: Fig. 9 As shown in (a), the load-strain curve of the elbow with equivalent local thinning defect is as follows: Fig. 9 As shown in (b), the horizontal axis of the curve is plastic strain and the vertical axis is the ultimate internal pressure. Fig. 9 The ultimate internal pressures corresponding to the last numerical calculation convergence point of the two figures are basically the same, that is, the ultimate internal pressure of the elbow with actual local thinning morphology defects is 27.67MPa, and the ultimate internal pressure of the elbow with equivalent local thinning defects is 27.31MPa. The ultimate internal pressure result of the elbow with equivalent local thinning defects is slightly smaller, and the relative error between the two is 1.30%, which is less than 5%. It is believed that the two can be equivalent at this time. The calculation results of the edge depth of the equivalent erosion thinning range and the ultimate internal pressure of each model under other pipe diameters and corresponding erosion days are shown in Table 5. According to the calculation results in Table 5, 24% of the maximum erosion thinning depth is taken as the edge depth of the equivalent thinning range, and the axial and circumferential range dimensions are obtained based on the edge depth of the equivalent erosion thinning range. The limit internal pressure of the elbow containing equivalent local thinning defects (equivalent local thinning defect circumferential range, axial range and maximum erosion thinning depth) is calculated, and its limit internal pressure is compared with the limit internal pressure of the elbow containing actual local thinning morphological defects. The average value of the relative error is less than 5%. In this embodiment, the edge depth of the equivalent erosion thinning range is taken as 24% of the maximum erosion thinning depth. At this time, the limit loads of the two are considered to be equivalent.

[0086] Table 5 Comparison results of limit internal pressure

[0087]

[0088] Step 4: Extract the time-varying curves of the circumferential range, axial range and maximum erosion thinning depth of equivalent local thinning defects

[0089] According to the edge depth value of the equivalent erosion thinning range (24% of the maximum erosion thinning depth), the size range, erosion wear calculation results and maximum erosion thinning depth within the range are determined. The equivalent local thinning defect circumferential range, axial range and depth change curves over time in the actual local thinning morphology defect change law over time are obtained within the equivalent local thinning defect range, and the time change curve of the size of the progressive local thinning defect is obtained. When the erosion time is 270 days, the changes of the erosion area range and the maximum erosion thinning depth of the elbow with time are shown in the figure. Fig.10 shown.

[0090] Step 5: Evaluation of the ultimate bearing capacity of the elbow with gradual local thinning defects.

[0091] On the premise of obtaining the local thinning defect size data in the fourth step, the normalized limit internal pressure p is obtained using formula (3) in the first step: LS , and then further use p LS =P LS / P L0 Obtain the ultimate internal pressure of the elbow with local thinning defects under a single load, and finally use the pipe system design internal pressure P TThe ultimate internal pressure P of a pipe elbow with local thinning defects under a single load LS Perform dimensionless processing. The processing method is: p l =P T / P LS When p l As it approaches 1, the closer the elbow with local thinning defects is to its ultimate internal pressure, the pipe will fail. p1 is the load state point, which is used to judge the risk of the elbow with local thinning defects.

[0092] The defect size is dimensionless, and the time is dimensionless according to 360 days, and the dimensionless local thinning defect size versus dimensionless time t (here t is a dimensionless parameter, which is dimensionless according to years (360 days) in the process of processing the defect size versus time curve) is obtained. The dimensionless local thinning defect size versus dimensionless time t curve is as follows: Fig.11 shown.

[0093] Further, we can get the limit internal pressure p l The curve of the change with dimensionless time t is as follows Fig.12 As shown in the figure, when the time t = 0.661, that is, 238 days, the pipeline will fail.

[0094] The method of the present invention can evaluate the ultimate bearing capacity at different moments during the erosion process. As time progresses, the method is combined with on-site erosion data to perform effective evaluation. This application combines erosion and wear analysis with the local thinning defect depth and operating parameters to give the temporal variation pattern of the local thinning defect size range (including axial length, circumferential length, and depth). According to the pipeline operating conditions, material properties, and conveying medium, the local thinning area range and erosion depth are reasonably given in combination with wall thickness fixed-point thickness measurement and computational fluid dynamics erosion and wear calculation, and then brought into the ultimate load formula to evaluate the ultimate bearing capacity of the elbow according to the ultimate load.

[0095] Any matters not described in the present invention are applicable to the prior art.

Claims

1. A method for evaluating the ultimate load-bearing capacity of a pipe bend with a progressive local thinning defect, the method comprising the following contents: S1: Taking the elbow with local thinning defect as the research object, the variation law of the ultimate load of the elbow with local thinning defect with the size of the local thinning defect is obtained; S2: Use computational fluid dynamics (CFD) and the dynamic grid method to simulate and analyze the erosion wear of the elbow to obtain the time-varying curve of the local thinning defect size, and use the fixed-point wall thickness measurement results to correct the erosion wear model parameters in the CFD according to the actual working conditions until the erosion wear prediction results match the actual ones, and determine that the time-varying curve of the local thinning defect size at this time is the time-varying law of the actual local thinning morphology defect; the local thinning defect size includes the axial length, circumferential length and depth of the local thinning defect; S3: at different times, the actual local thinning morphology defects are subjected to equivalent processing. If the average value of the relative error between the limit loads of the elbow containing the actual local thinning morphology defects and the elbow containing the equivalent local thinning defects is less than 5%, the axial and circumferential range values ​​of the erosion wear area of ​​the elbow containing the equivalent local thinning defects at this time are determined to be the equivalent local thinning defect range of the elbow containing the actual local thinning morphology defects, and the equivalent local thinning defect range is obtained; S4: obtaining the time-varying curves of the circumferential range and axial range of the equivalent local thinning defect in the time-varying law of the actual local thinning morphology defect in step S2 within the equivalent local thinning defect range of step S3, and obtaining the time-varying curve of the maximum erosion thinning depth, and then obtaining the time-varying curve of the size of the progressive local thinning defect; S5: Substitute the values ​​at different times corresponding to the curve of the change of the size of the progressive local thinning defect with time obtained in step S4 into the law of the change of the ultimate load of the elbow with local thinning defects with the size of the local thinning defect obtained in step S1, and evaluate the ultimate load-bearing capacity of the elbow with progressive local thinning defects during the erosion process.

2. The method for evaluating the ultimate load-bearing capacity of a pipe elbow containing a progressive local thinning defect according to claim 1, characterized in that: In step S2, the method for determining the variation law of the actual local thinning morphology defect over time is: Computational fluid dynamics is used to establish an erosion wear model. Based on the dynamic grid erosion wear calculation, the flow field physical quantity information and wall erosion wear results are calculated to obtain the wear depth and wear area range under different erosion times. According to the actual situation and the pipeline fixed-point wall thickness monitoring data, the average mass flow rate of particles, particle size function, impact angle function, particle relative flow velocity, and exponential function of particle relative flow velocity in the erosion wear model are adjusted through trial calculation until the impact wear prediction results are consistent with the actual results. The time-varying laws of erosion depth and erosion thinning area range are given at this time, thereby obtaining the time-varying laws of actual local thinning morphological defects; the pipeline fixed-point wall thickness monitoring data are measured data.

3. The method for evaluating the ultimate load-bearing capacity of a pipe elbow containing a progressive local thinning defect according to claim 1, characterized in that: The process of equivalent quantification in step S3 is: through the limit load equivalence, the size and depth of the equivalent local thinning defect range are determined, and the time variation law of the size and depth of the equivalent local thinning defect range is obtained; the maximum erosion thinning depth is selected as the equivalent local thinning defect depth; when the erosion thinning area is equivalent, the area range is determined according to the maximum erosion thinning depth, and the values ​​of the axial and circumferential range sizes of the local thinning defects are determined according to the maximum and minimum values ​​of the area boundary coordinates corresponding to the edge depth of the equivalent erosion thinning range at the time when the limit internal pressure is equal to the maximum erosion thinning depth.

4. The method for evaluating the ultimate load-bearing capacity of a pipe elbow containing a progressive local thinning defect according to claim 3, characterized in that: The process of determining the regional range is: taking a fixed percentage of the maximum erosion thinning depth as the edge depth of the equivalent erosion thinning range; the values ​​of the axial and circumferential range dimensions of the local thinning defects are determined according to the maximum range of the erosion area length and width corresponding to the edge depth of the equivalent erosion thinning range, and the equivalent regional range is determined at this time.

5. The method for evaluating the ultimate load-bearing capacity of a pipe elbow containing a progressive local thinning defect according to claim 1, characterized in that: In step S5, the limit load is calculated based on the load state point p l Whether it is close to 1.0 is used as the criterion for risk assessment of pipe bending, and the load state point p is obtained. l Curve of variation with defect size; p l =P T / P LS , where P T is the design internal pressure of the piping system, P LS is the ultimate internal pressure of the elbow with local thinning defects under a single load, P LS Obtained by using the time variation curve of the defect size containing progressive local thinning obtained in step S4; When p l As it approaches 1.0, the elbow with progressive local thinning defects approaches its ultimate internal pressure, at which point the pipe will fail.

6. The method for evaluating the ultimate load-bearing capacity of a pipe elbow containing a progressive local thinning defect according to claim 1, characterized in that: This method can also be used to evaluate the ultimate bearing capacity of elbows with local thinning defects under bending moment, torque and axial force loads.

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