A method for calculating the creep life of a high-temperature pressure-bearing component

Abstract

The application provides a high-temperature pressure-bearing component creep life calculation method, which adopts primary stress under the action of component mechanical load to calculate the creep life loss of the internal component in the load action time period, adopts a fatigue life statistical calculation method to calculate the fatigue life loss of the component, combines the total action time to obtain the total creep life loss and the total fatigue life loss of the component, and comprehensively judges whether the high-temperature pressure-bearing component life is qualified or not. The application integrates and decomposes the total stress distribution of the high-temperature pressure-bearing component, decomposes the primary stress which directly contributes to the structural creep damage, and simultaneously considers the influence of the structural discontinuous effect; the creep life loss analysis and calculation process by using the method is simple and clear, the life loss evaluation result of the structure is more reasonable; the method is simple, convenient and practical in engineering application, and can be widely applied in practical engineering such as high-temperature life online management, structural optimization design, user operation regulation establishment and perfection and the like.

Description

Technical Field

[0001] This invention relates to the fields of power, petrochemical and pressure vessel, and in particular to a method for calculating the creep life of high-temperature pressure-bearing components. Background Technology

[0002] For high-temperature pressure-bearing components operating in the creep temperature range, fatigue life is lost due to alternating loads, and creep life is lost due to continuous high-temperature loads (mainly internal pressure). In addition to fatigue life loss, creep damage is also increased. The existence of creep loss will further promote the expansion of loss during the fatigue loss process. That is, fatigue loss and creep loss have a mutually reinforcing interactive effect.

[0003] In practical engineering, there are well-established statistical and calculation methods for calculating the fatigue life loss of components, based on the cyclic stress amplitude at the highest point of peak stress intensity (i.e., total stress intensity) or the maximum cyclic strain amplitude. However, there is still no universally accepted analytical calculation method for calculating high-temperature creep life loss, either domestically or internationally. The main reason for this lies in the appropriate selection of stresses for assessing creep life loss in high-stress areas of components. Generally, due to process requirements, components often have complex, irregularly shaped elements. High-stress areas are primarily located at these irregularly shaped elements, where the structure is geometrically complex, stress / strain concentration is high, and stress distribution gradients are large. Under alternating loads and continuous high-temperature loads, the fatigue life loss at these irregularly shaped elements is significant. Compared to continuous structural sections, the discontinuity effect also amplifies the creep life damage of these irregularly shaped elements, resulting in the fatigue life loss at these irregularly shaped elements being the most damaging location on the component. The service life of these irregularly shaped elements determines the service life of the entire component.

[0004] In creep life assessment calculations using fractional estimation methods based on endurance strength, various methods, including those from technologically advanced countries like the US, EU, and Japan, internal calculations by renowned international companies, and domestic standards for high-temperature pressure-bearing components in pressure vessels, power, and petrochemical industries, all have shortcomings or are highly unreasonable. For example, creep life calculations are often overly simplistic. Most methods only provide calculations for easily calculated low-stress areas, completely ignoring high-stress areas with discontinuous stress, or simply using stress relaxation methods to calculate total stress without considering the different effects of structural stresses on creep life and practical engineering applications. Currently, the only standard with limited engineering application that considers creep life calculations in high-stress areas is ASME BPVC.Ⅲ.5's non-mandatory Appendix T, which provides analytical calculation methods for fatigue life loss, creep life loss, and fatigue-creep interaction-induced life loss. This appendix provides detailed steps for calculations using elastic analysis methods. However, this analytical calculation method is provided as a non-mandatory appendix, and its analysis is specific to certain objects. If the lifespan of conventional high-temperature pressure-bearing components is calculated entirely using the elastic analysis method provided by ASME, the results will be extremely conservative compared to detailed three-dimensional nonlinear finite element analysis. This leads to the contradiction that components already in operation in the early stages would generally fail the lifespan assessment using the ASME method, while the actual components are still operating safely. Furthermore, the calculation method is extremely cumbersome in terms of operation and processing. Summary of the Invention

[0005] To address the problems existing in the prior art, a method for calculating the creep life of high-temperature pressure-bearing components is provided. This method can solve the problems of non-universality, overly conservative calculation results, and extremely cumbersome calculation operations in the calculation methods for high-temperature life of pressure-bearing components. It achieves the goal of making the calculation method more applicable, the calculation results more conservative and reasonable, and the analysis and calculation more convenient and simple.

[0006] The technical solution adopted in this invention is as follows: a method for calculating the creep life of a high-temperature pressure-bearing component, which uses the stress under mechanical load on the component to calculate the creep life loss of the component within the time period of the load, uses the fatigue life statistical calculation method to calculate the fatigue life loss of the component, and combines the total time of the load to obtain the total creep life loss and the total fatigue life loss of the component, and then comprehensively judges whether the life of the high-temperature pressure-bearing component is qualified.

[0007] Furthermore, the specific calculation steps are as follows:

[0008] S1. Perform elastic stress analysis and calculation on the component under unit mechanical load, and perform stress decomposition on the calculation results;

[0009] S2. Based on the location of the stress decomposition lines, find the maximum stress intensity along the wall thickness on these two lines, and use this as the stress for calculating the creep life of the component.

[0010] S3. Based on the creep life calculation of the component, the creep strength of the component material at the operating temperature is obtained by looking up the stress table, and the maximum allowable continuous action time under the corresponding stress is obtained; then, combined with the actual continuous action time of the mechanical load, the creep life loss of the component during this action time period is obtained, and the total creep life loss is calculated based on the total action time.

[0011] S4. The fatigue life loss of the component and the total fatigue life loss during the stress cycle are calculated using the fatigue life statistical calculation method.

[0012] S5. Determine whether the component's lifespan is qualified based on the component's total creep life loss and total fatigue life loss.

[0013] Furthermore, in S1, the finite element method is used to perform elastic stress analysis and calculation on the component under a unit mechanical load.

[0014] Furthermore, the specific process of S1 is as follows: extract the maximum equivalent stress point and the maximum hydrostatic stress point on the component structure, draw stress integral decomposition lines along the wall thickness direction through these two points, and perform stress decomposition on the calculation results.

[0015] Furthermore, in step S2, the stress used to calculate the creep life of the component is the primary total film stress.

[0016] Furthermore, in step S2, the stress used to calculate the creep life of the component is the total film stress plus the bending stress.

[0017] Furthermore, in step S2, the stress used to calculate the creep life of the component is the primary local film stress.

[0018] Furthermore, in step S3, the creep life loss of the component is calculated by dividing the actual duration of the mechanical load by the maximum allowable duration.

[0019] Furthermore, in step S5, the known material fatigue-creep envelope is looked up based on the total creep life loss and total fatigue life loss of the component, and the structural life can be determined based on the location of the point.

[0020] Compared with existing technologies, the beneficial effects of adopting the above technical solution are as follows:

[0021] 1. Based on the analytical design method, the total stress distribution along the wall thickness of the high-stress zone in the high-temperature pressure-bearing component is decomposed by integral decomposition to obtain the primary stress that directly contributes to the creep damage of the structure. The influence of structural discontinuity effect is also taken into account in this component stress.

[0022] 2. The creep life loss analysis and calculation process using this method is simple and clear. Compared with the original creep life loss analysis and calculation methods, this method is particularly suitable for online analysis and monitoring of the high-temperature life of high-temperature pressure-bearing components;

[0023] 3. The creep life loss analysis and calculation results made using this method, combined with the fatigue life loss of the components, make the life loss assessment results of the structure more reasonable.

[0024] 4. This method is simple, convenient, and practical in engineering applications, and can be widely used in practical engineering projects such as high-temperature life online management, structural optimization design, and the formulation and improvement of user operation procedures. Attached Figure Description

[0025] Figure 1 This is a flowchart of the method for calculating the creep life of high-temperature pressure-bearing components proposed in this invention.

[0026] Figure 2 This is a schematic diagram of a three-way structure in one embodiment of the present invention.

[0027] Figure 3 This is a schematic diagram illustrating the life assessment of a three-way structure in one embodiment of the present invention. Detailed Implementation

[0028] The embodiments of this application are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar modules or modules having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application. Rather, the embodiments of this application include all variations, modifications, and equivalents falling within the spirit and scope of the appended claims.

[0029] The life assessment results of non-nuclear Class I high-temperature pressure-bearing components according to the ASME analysis and assessment method are very conservative. This is because the analysis and assessment method is mainly designed for nuclear Class I equipment that operates at low temperatures and is stable under basic load but has extremely high safety requirements. In the calculation of this method, multiple adjustments and corrections were made to the calculation results of multiple processes, and excessive conservative and unreasonable treatments were made, such as setting high initial calculation values. For example, the fatigue strain amplitude calculation during each load cycle considers the creep strain increment caused by the continuous load during the current cycle (which is considered as an influencing factor of fatigue loss in this method), while the creep loss calculation considers the duration of this continuous load, thus repeatedly calculating the creep strain; the fatigue strain amplitude calculation includes the strain amplitude caused by alternating thermal loads, but the thermal load is a self-balancing force system, and the strain amplitude it causes will not increase creep loss; when using uniaxial iso-strain relaxation data to correct for multiaxial relaxation, an overly conservative lower limit value for stress relaxation is introduced; when checking the endurance strength curve, the equivalent creep stress is amplified by 25%, while creep life is inversely proportional to higher powers of stress, which will overestimate the creep life loss of the component.

[0030] In actual engineering projects, there are a large number of non-nuclear Class I high-temperature pressure-bearing components. Their design service life, actual high-temperature operating life, and total life loss are issues of great concern to design units and users.

[0031] To address the issue of the rationality of creep life calculation results in life analysis and evaluation, this paper re-explores the stress used for creep life loss calculation of high-temperature components by studying the creep damage mechanism of metallic materials and combining it with the perspective of pressure vessel stress classification. It determines that using the primary stress intensity in the high-stress region as the stress for component creep life evaluation is more reasonable. This embodiment presents the corresponding analysis process:

[0032] According to the relevant definitions of the analysis and design method, primary stress is the stress necessary to balance pressure and other mechanical loads. Primary stress is the actual bearer of external mechanical loads and is therefore an indispensable main stress in the structure. Primary stress is not self-limiting. According to the size of the stress range and the distribution along the wall thickness, primary stress is further divided into primary overall membrane stress, primary local membrane stress, and primary bending stress. Membrane stress refers to the average stress along the wall thickness direction. Primary overall membrane stress is far away from the discontinuous area of ​​the structure and its influence range covers the entire structure. Primary local membrane stress is located in the discontinuous area of ​​the overall structure where the geometry of the mid-surface changes abruptly. It is actually composed of two parts, namely the primary overall membrane stress and the membrane stress generated by the discontinuous effect of the structure. Its stress level is greater than that of primary overall membrane stress, but its influence range is limited to the local area of ​​the structure.

[0033] Secondary stress is the stress necessary to meet external constraints or the requirement for continuous deformation of the structure itself. Secondary stress refers to stress that is linearly distributed along the wall thickness. The basic characteristic of secondary stress is that it is self-limiting. Local yielding and small deformation can satisfy the conditions that cause this stress. Applying this stress once will not cause the structure to fail. Secondary stress is a self-balancing force system and is independent of the balancing external load.

[0034] Peak stress is the stress increment added on top of primary and secondary stresses to meet the deformation compatibility requirements of discontinuous parts of a local structure, and it also includes the effect of stress concentration. Peak stress is both self-limiting and localized. Peak stress may lead to fatigue cracks or brittle fracture, but it does not contribute to creep damage of components.

[0035] According to the definition of stress classification, there are only two types of stress in a structure under external load: primary stress that meets the requirements of balancing with the external load and secondary stress and peak stress that meet the requirements of deformation compatibility. When calculating creep life loss, only the stress components that affect creep life should be considered.

[0036] Based on the foregoing analysis, this embodiment proposes a method for calculating the creep life of high-temperature pressure-bearing components. The creep life loss of the component is calculated by using the stress under the mechanical load of the component during the load application period. The fatigue life loss of the component is calculated by using the fatigue life statistical calculation method. Combined with the total application time, the total creep life loss and the total fatigue life of the component are obtained. Finally, the component life is judged to be qualified based on the comprehensive loss.

[0037] like Figure 1 As shown, the specific steps of this calculation method are as follows:

[0038] S1. Perform elastic stress analysis and calculation on the component under unit mechanical load, and perform stress decomposition on the calculation results;

[0039] S2. Based on the location of the stress decomposition lines, find the maximum stress intensity along the wall thickness on these two lines, and use this as the stress for calculating the creep life of the component.

[0040] S3. Based on the stress table used to calculate the creep life of the component, obtain the creep strength of the component material at the operating temperature (in this embodiment, the lower limit of the dispersion band is taken) and obtain the maximum allowable duration of action T under the corresponding stress. Di Furthermore, considering the actual duration τ of the mechanical load... i The creep life loss D of the component during this operating period is obtained. ci Then, calculate the total creep life loss D based on the total action time. c ;

[0041] S4. The fatigue life loss of the component and the total fatigue life loss during the stress cycle are calculated using the fatigue life statistical calculation method.

[0042] S5. Determine whether the component's lifespan is qualified based on the component's total creep life loss and total fatigue life loss.

[0043] In this embodiment, the finite element method is mainly used to perform elastic stress analysis and calculation of components under mechanical loads (such as internal pressure and weight). The maximum equivalent stress point and the maximum hydrostatic stress point on the structure are extracted. Stress integral decomposition lines are drawn along the wall thickness through these two points, and stress decomposition is performed on the calculation results.

[0044] In step S2, the creep life of the component is calculated using either a single overall membrane stress, a single overall membrane stress plus a single bending stress, or a single local membrane stress, depending on the location of the maximum single stress on the structure. Specifically, the single overall membrane stress plus a single bending stress mainly corresponds to the center of the flat end cap under internal pressure; the single local membrane stress mainly corresponds to the discontinuous areas of the overall structure, while the remaining areas are calculated using the single overall membrane stress. It can be seen that the calculation process only considers the stress components that affect the creep life.

[0045] In this embodiment, creep lifetime loss D oi =τ i / T Di The total creep life loss of the component is D. o =∑D ci Wherein, parameter i refers to the time period corresponding to each creep calculation, and the creep life loss and fatigue life loss of the component are the sum of the creep life loss and fatigue life loss of all time periods.

[0046] In step S5, based on the total creep life loss and total fatigue life loss of the component, the known material fatigue-creep envelope is looked up, and the structural life can be determined based on the location of the point.

[0047] To verify the life calculation method proposed in this embodiment, this embodiment further verifies it using a compression tee on the outlet header of a 300MW unit natural circulation boiler in a power plant as an example. The tee structure is as follows: Figure 2 As shown, the specific process is as follows:

[0048] The tee operates at a pressure of 16.7 MPa and a temperature of 540℃. The tee's service life is estimated at 35 years, with a continuous internal pressure duration of 250,000 hours after deducting maintenance time. The tee material is 12Cr1MoVG, and its elastic modulus at the operating temperature is 1.754 × 10⁻⁶. 5 MPa.

[0049] Boiler start-up and shutdown cycle parameters and header operating parameters are shown in Table 1:

[0050] Table 1 Boiler Start-up and Shutdown Cycle Parameters and Header Operation Parameters

[0051]

[0052]

[0053] The relevant thermodynamic properties of 12Cr1MoVG material are shown in Table 2:

[0054] Table 2. Thermodynamic properties of 12Cr1MoVG material

[0055]

[0056] The stress of the tee structure under transient operating conditions during start-up and shutdown and under working pressure was calculated using MSC.Nastran software. Fatigue life statistical calculations were performed on the tee according to EN 12952-3:2011(E) standard. The calculation results are shown in Table 3.

[0057] Table 3 Fatigue life loss under various start-up and shutdown conditions

[0058]

[0059] The total fatigue life loss over 35 years is D f = 9.8208%.

[0060] Under working pressure, the local membrane stress of the tee is 66.33 MPa. Referring to the creep strength of 12Cr1MoVG material at 540°C, the allowable stress duration is 620,950 hours. Based on 250,000 hours, the creep life loss of the tee is D. c =31.25%.

[0061] The fatigue-creep damage envelope of 12Cr1MoVG material can be used for tee life assessment. (See attached image.) Figure 3 The assessment results show that the lifespan of the tee is nearing its end, which is consistent with the actual situation.

[0062] Example 1

[0063] This embodiment proposes a method for calculating the creep life of high-temperature pressure-bearing components. The creep life loss of the component is calculated by using the stress under the mechanical load of the component during the load application period. The fatigue life loss of the component is calculated by using the fatigue life statistical calculation method. Combined with the total application time, the total creep life loss and the total fatigue life loss of the component are obtained, and then it is determined whether the component life is qualified.

[0064] Example 2

[0065] Based on Example 1, this example proposes the following specific steps for lifetime calculation:

[0066] S1. Perform elastic stress analysis and calculation on the component under unit mechanical load, and perform stress decomposition on the calculation results;

[0067] S2. Based on the location of the stress decomposition lines, find the maximum stress intensity along the wall thickness on these two lines, and use this as the stress for calculating the creep life of the component.

[0068] S3. Based on the creep life calculation of the component, the creep strength of the component material at the operating temperature is obtained by looking up the stress table, and the maximum allowable continuous action time under the corresponding stress is obtained; then, combined with the actual continuous action time of the mechanical load, the creep life loss of the component during this action time period is obtained, and the total creep life loss is calculated based on the total action time.

[0069] S4. The total fatigue life loss of the component during the cycle is calculated using the fatigue life statistical calculation method.

[0070] S5. Determine whether the component's lifespan is qualified based on the component's total creep life loss and total fatigue life loss.

[0071] Example 3

[0072] Based on Example 2, this example proposes that in step S1, the finite element method be used to perform elastic stress analysis and calculation on the component under a unit mechanical load.

[0073] Example 4

[0074] Based on Example 2, this example proposes that the specific process of step S1 is as follows: extract the maximum equivalent stress point and the maximum hydrostatic stress point on the component structure, draw stress integral decomposition lines along the wall thickness direction through these two points, and perform stress decomposition on the calculation results.

[0075] Example 5

[0076] Based on Example 2, this example proposes that in step S2, the stress used to calculate the creep life of the component is either a single overall membrane stress, a single overall membrane stress plus a single bending stress, or a single local membrane stress. The single overall membrane stress plus a single bending stress mainly corresponds to the central part of the flat end cap under internal pressure; the single local membrane stress mainly corresponds to the discontinuous area of ​​the overall structure, while the remaining areas correspond to the single overall membrane stress.

[0077] Example 6

[0078] Based on Example 2, this example proposes that in step S3, the creep life loss of the component is calculated by dividing the actual duration of the mechanical load by the maximum allowable duration.

[0079] Example 7

[0080] Based on Example 2, this example proposes that in step S5, the known material fatigue-creep envelope is looked up based on the total creep life loss and total fatigue life loss of the component, and the structural life can be determined based on the location of the point.

[0081] It should be noted that, in the description of the embodiments of the present invention, unless otherwise explicitly specified and limited, the terms "set" and "connection" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in the present invention based on the specific circumstances. The accompanying drawings in the embodiments are used to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0082] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A method for calculating the creep life of high-temperature pressure-bearing components, characterized in that, The specific steps are as follows: S1. Perform elastic stress analysis and calculation on high-temperature pressure-bearing components under unit mechanical load, extract the maximum equivalent stress point and the maximum hydrostatic stress point on the component structure, draw stress integral decomposition lines along the wall thickness through these two points, and perform stress decomposition on the calculation results. S2. Based on the location of the stress decomposition lines, find the maximum stress intensity along the wall thickness on these two lines, and use this as the stress for calculating the creep life of the component. S3. Based on the creep life calculation of the component, the stress table is used to obtain the endurance strength of the component material at the operating temperature, and the maximum allowable duration of action under the corresponding stress is obtained. Then, by combining the actual duration of the mechanical load, the creep life loss of the component during that period of time is obtained, and the total creep life loss is calculated based on the total duration of the load. S4. The fatigue life loss of the component and the total fatigue life loss during the stress cycle are calculated using the fatigue life statistical calculation method. S5. Determine whether the component's lifespan is qualified based on the component's total creep life loss and total fatigue life loss.

2. The method for calculating the creep life of high-temperature pressure-bearing components according to claim 1, characterized in that, In S1, the finite element method is used to perform elastic stress analysis and calculation on the component under a unit mechanical load.

3. The method for calculating the creep life of high-temperature pressure-bearing components according to claim 1, characterized in that, In step S2, when the maximum primary stress intensity is located far from the structural discontinuity region, the stress used to calculate the creep life of the component is the primary overall film stress.

4. The method for calculating the creep life of high-temperature pressure-bearing components according to claim 1, characterized in that, In step S2, when the maximum primary stress intensity is located at the center of the flat end cap under internal pressure, the stress used to calculate the creep life of the component is the primary overall membrane stress + primary bending stress.

5. The method for calculating the creep life of high-temperature pressure-bearing components according to claim 1, characterized in that, In step S2, when the maximum primary stress intensity is located in the discontinuous region of the overall structure, the stress used to calculate the creep life of the component is the primary local membrane stress.

6. The method for calculating the creep life of high-temperature pressure-bearing components according to claim 1, characterized in that, In step S3, the creep life loss of the component is calculated by dividing the actual duration of the mechanical load by the maximum allowable duration.

7. The method for calculating the creep life of high-temperature pressure-bearing components according to claim 1, characterized in that, In step S5, the known material fatigue-creep envelope is looked up based on the total creep life loss and total fatigue life loss of the component. The structural life can be determined based on the location of the point.

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