Analysis and Research on the service life of high

2022-10-18
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Abstract: This paper summarizes a series of research work carried out by Dongfang Boiler (Group) Co., Ltd. in the past 10 years in the life analysis and research of boiler high temperature components, that is, the systematic stress analysis of structure, the damage mechanism of creep, fatigue and their interaction, as well as the basic research work of high temperature performance of materials, and formulates the calculation method suitable for the actual situation of our country. This method is simple in calculation, with high accuracy, and tends to be safe in engineering application

key words: boiler high temperature parts; Creep; Fatigue; Interaction; life; Calculation method

1 preface

in recent 10 years, with the development of China's power industry, people have paid great attention to the life loss of high-temperature thick wall pressure bearing parts of utility boilers, such as steam drums, high-temperature superheaters, high-temperature reheater headers and their tees. These components work in the harsh environment of high temperature and high pressure, and bear the continuous and periodic alternating effects of pressure and temperature, which will produce fatigue, creep and the interaction between them. The working temperature of the drum is below 400 ℃, which only causes fatigue damage; However, other components work in high temperature environment above 500 ℃, and the mechanism of life damage is more complex. In addition to fatigue, they also bear the interaction of creep and fatigue creep. Because these components are difficult to replace, their service life also determines the overall service life of the boiler

in addition, users have gradually put forward clear requirements on the life loss of high-temperature components in the bidding document, which need the manufacturer's reply

up to now, the life loss analysis methods of high-temperature components in the world are still under research. The United States, which is in the lead, has only been treated as a case in the ASME standard (ASME code case N-47), and has not been put into use as a prescriptive standard; However, in China, there is no in-depth study in this field

in recent years, all boiler plants in the industry have worked together to formulate gb9222-88 Appendix D "calculation method of low cycle fatigue life of boiler drum", which fills the gap in the life calculation standard of power station industry and will provide conditions for further research on the life loss of high-temperature components

in order to maintain its industry advantages in this field and make its products more competitive in the fierce market competition, Dongguo company initiated a project in 1997 to analyze the life of boiler high-temperature pressure parts. Alan barcon, PolyOne market development manager, said in an interview with plastic news recently. This paper is the summary report of this work

2 preliminary preparation

when the project was approved, in order to make the research work have a high level and meet the needs of Dongguo boiler company for the life analysis and research of boiler high-temperature pressure parts, a lot of literature research work was done in the early stage

on the one hand, it has made a detailed understanding of the progress of life analysis and research work of boiler high-temperature pressure parts at home and abroad, and mastered the current level at home and abroad; On the other hand, the foreign standards are also compared and the similarities and differences between them are analyzed. Through this work, the goal and content of the company's life analysis and Research on boiler high temperature pressure parts are determined, and a detailed implementation plan is formulated

3 research objectives and main contents of the project

according to the actual situation of large power station boiler units of Dongguo company, the project selected high-temperature superheater, high-temperature reheater pipeline, tee and elbow as the main components of high-temperature life analysis. Through the calculation of three-dimensional elastic-plastic to creep stress and strain, and the life loss analysis of fatigue and fatigue to creep interaction, a set of life analysis and calculation method of high-temperature components suitable for engineering application is developed

On the basis of years of research, Dongguo company has carried out further cooperation with Tsinghua University and Southwest Jiaotong University, and digested and improved the technology introduced by ABB CE. In this project, the following aspects of research work have been carried out:

(1) analysis and calculation of stress concentration factors of boiler tees, elbows and other high-temperature pressure parts

(2) thermal elastoplastic to creep cyclic three-dimensional finite element stress analysis of boiler tee, elbow and other high-temperature pressure parts

(3) damage analysis and life prediction method of fatigue creep interaction of boiler tee, elbow and other high-temperature pressure parts

(4) digestion and improvement of ABB CE crepla analytical program

(5) study on the interaction between fatigue and creep of 12Cr1MoV

(6) examples of stress analysis and life evaluation of boiler high temperature components

(7) development of methods and procedures for drawing up boiler startup and shutdown curves

through research, new progress has been made in the stress analysis of high-temperature parts, the high-temperature characteristics of 12Cr1MoV materials and the life loss evaluation of fatigue creep interaction, and the life loss evaluation method of high-temperature pressure parts suitable for engineering application has been formulated, And put on a new package. Because there is a big gap between the actual available graphene and the ideal graphene, a practical software for calculating the life loss of high-temperature pressure parts has been developed

4 stress analysis

stress analysis is the basis of life analysis and research of boiler high temperature pressure parts. Therefore, the project has carried out a detailed three-dimensional finite element stress analysis of the high-temperature components of the boiler

(1) carry out a series of three-dimensional elastic finite element stress analysis on the tee and elbow, and determine the stress concentration factor of the tee

(2) detailed thermal elastoplastic to creep cyclic three-dimensional finite element stress analysis was carried out on the tee

(3) the simplified life calculation method of boiler high-temperature pipe joints is further developed, and simplified from the following two aspects:

1 structural simplification scheme, that is, the three-dimensional problem of pipe joints is simplified to one-dimensional problem

2 scheme based on elastic stress analysis

through the stress analysis, the stress distribution and the change with time of the boiler structure (TEE, elbow, etc.) under the actual operating conditions are mastered, which also provides a basis for the formulation of the simplified method of life calculation

4.1 elastic stress analysis

referring to the series of tee products of Dongguo company, according to the design drawings, and according to the ratio of the outer diameter to the inner diameter of the tee main pipe, a detailed three-dimensional finite element analysis of the stress of the tee under internal pressure and temperature load is made; At the same time, the stress distribution of the elbow is also analyzed

4.1.1 stress analysis of tee and its stress concentration factor

the analysis results show that the stress distribution under internal pressure of tee is very close to the data of CE company, and the stress concentration factor of the maximum stress point on its inner wall is between 4.6 and 4.9; The stress concentration factor under the action of internal and external temperature difference is between 1.0 and 1.4

it should be noted that Dongguo company made electrical test and finite element stress analysis on the flanging tee from 1987 to 1993, and the stress concentration factor was about 3.1. The reason for the difference was that the actual wall thickness of the tee shoulder made by the company at that time was much greater than the design wall thickness

considering that the actual wall thickness of the tee has a great relationship with the control of the manufacturing process, and it can not be accurately determined, from the principle based on the drawing, it is suggested that the stress concentration factor should take the finite element stress analysis value, that is, 4.6 ~ 4.9, which is generally 4.9. Under the condition of the measured thickness of the tee wall, the finite element calculation can be carried out on this basis

the stress concentration factor of cylindrical open-ended pipe joints (including tees, drum downcomers, etc.) under the action of internal and external temperature differences was generally taken as 2.0 in early work. After years of research by the company, it was confirmed that its stress concentration factor was below 1.4. Appendix D of gb9222-88 adopts this result and takes 1.6 as a safe value. The recommended value here is still 1.6

4.1.2 elbow stress analysis and its stress concentration factor

u-shaped pipe under the condition of only internal pressure, β The change of the value has little effect on the stress concentration factor, which can be ignored. The stress concentration factors are all below 1.5, and it is recommended to take 1.5 as the stress concentration factor; Under the condition that the U-shaped pipe is only affected by the internal and external temperature difference, β The change of value has little effect on the stress concentration factor, which can be ignored. According to the calculation results, it is suggested that the stress concentration factor should be 1.2

4.2 three dimensional finite element stress analysis of plastic creep cycle

in order to master the stress distribution and change law of high-temperature pressure bearing parts such as boiler tee, a detailed three-dimensional finite element stress analysis of thermal elastoplastic creep cycle must be carried out. On the basis of long-term cooperation with Tsinghua University, Dongguo company commissioned it to develop a three-dimensional finite element stress analysis and life calculation program (TAP) for thermal elastoplastic creep cycle of boiler structure

The calculation of an engineering example shows that the development of the program is successful and has completely achieved the expected purpose; At the same time, a large number of calculations are carried out by using this program. Master the stress distribution state and its change law of high-temperature pressure bearing parts such as boiler tee and the life loss value in the 30-year operation period, and have a good idea of the safe operation of these parts; At the same time, it lays a foundation for the following simplified calculation scheme and the formulation of practical engineering calculation methods

4.3 simplified scheme and stress analysis of plastic creep cyclic stress of tee

plastic creep cyclic three-dimensional finite element stress analysis of tee is a very complex problem and takes a lot of time. In order to facilitate engineering application, it is very necessary to propose a simplified calculation method. To this end, two ways are provided

4.3.1 structural simplification scheme

simplify the structure, that is, through reasonable analysis and referring to the simplified scheme of creplan software of CE company in the United States, simplify the three-dimensional structure problem of the tee into one-dimensional problem, that is, treat it as a cylinder, and introduce the stress concentration factor to consider the influence of the opening

through this research, a software suitable for engineering application, DBC crepcyl, is compiled. The software inherits the idea of creplacyl software of ABB CE company in the United States, and improves its shortcomings. For example, the finite difference analysis of temperature field is improved to finite element analysis, and the plastic analysis process and iteration scheme of the software are improved, which greatly improves the convergence and convergence speed of the software

The example of DBC creplane shows that compared with the three-dimensional finite element stress analysis, the software has high accuracy, especially in the strength assessment point and life calculation point of engineering concern, that is, the position of the inner wall corner of the tee, that is, the inner wall of the simplified cylinder

4.3.2 based on the method of elastic analysis

simplify its mathematical model, directly use the results of elastic analysis for stress calculation, and correct the influence on plasticity and creep through correction coefficient

this method is used in the American ASME, boiler & Pressure Vessel Code Standard (ASME standard for short)

in the American ASME standard, some factors were not taken into account when modifying the stress considering the effects of plasticity and creep, so some defects were found and the limit of the lower limit of stress had to be introduced. There was still a large error between the modified stress and the results of inelastic stress analysis, and the results were very conservative

Tsinghua University has made a detailed study on this and proposed a new correction method to improve the defects of American ASME standard, which is also the method recommended in this research

however, because the simplified method based on elastic analysis is very conservative (as can be seen from the example analysis later), it is often

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