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How to conduct safety assessment of welding quality of pressure pipeline installation

Release time: 2015-08-08 17:25:00 Views: 1835

This paper introduces the research status and development trend of pressure pipeline safety assessment at home and abroad, and demonstrates the method of pressure pipeline safety assessment. The welding process is a key process in the construction of steel pressure pipelines. The safety assessment of the welding process plays a very important role in ensuring the installation quality of pressure pipelines.


At present, whether it is a domestic pressure pipeline or an imported pressure pipeline, there are inevitably defects to varying degrees, and during the use of pressure pipelines, new defects have arisen due to various factors such as loads and media. . The pressure pipeline installation project has the characteristics of one-time and complex and harsh site conditions. Therefore, its quality and safety assessment is very important. The quality and safety assessment of the welding process plays a vital role in ensuring the installation quality of the pressure pipeline project. .


Safety assessment method for pressure pipelines


1. Theoretical basis of safety assessment


Due to the characteristics of the welding process itself, the welded joint undergoes extremely complicated and uneven thermal processes during the formation process, resulting in greater residual stress and deformation of the welded joint. Discontinuities and non-uniformities in mechanical properties. At the same time, welding joints are also prone to various welding defects, such as cracks, slag inclusions, pores, undercuts, etc. The existence and development of these defects often lead to structural failure and fracture, but Defects are inevitable during the formation of welded joints. Engineering practice shows that not all defects will cause the failure of the structure, that is to say, the defects meeting certain conditions are not harmful to the structure.


2. Research Status of Safety Assessment of Pressure Pipelines at ⎝⎛炸金花⎞⎠ and Abroad


Foreign pressure pipeline safety assessment research started in the 1970s. It originated from nuclear power station pressure pipelines. On the one hand, due to the high safety requirements of nuclear power plants, on the other hand, accidents in nuclear power plants often occur in leakage or blasting of pressure pipes. Therefore, the United States, Germany, Japan and other developed countries have successively carried out research on pressure pipelines. With the deepening of research, the research cost is getting more and more huge. Many countries, including Japan, France, the United States, and Germany, have set up an "International Pipeline Integrity Research Working Group". The results were shared and some meaningful conclusions were obtained in the research. In China, in view of the long-standing common problems of pressure vessels and pressure pipelines in China, scientists from various scientific research institutes have summarized, inherited and absorbed the theory, technical practice and experience of boiler and pressure vessel safety assessment at home and abroad. The "Pressure Vessel Defect Evaluation Specification" (CVDA-84) was based on the COD theory and reflected the technological level at home and abroad at that time. In 2004, China formulated the "Safety Assessment of In-Use Defective Pressure Vessels" standard, which uses the failure assessment chart technology and standardizes the technical methods of pressure vessel safety assessment. While improving the overall level of pressure vessel safety assessment in China, it has also increased pressure. Container safety and economy.


3. Development Trends of Safety Assessment of Foreign Pressure Pipelines


At present, the international research work on the experimental research and safety assessment of pressure pipelines with defects is in the ascendant, and is continuously deepening. There are roughly three major trends in research in this field:


The first is engineering. Many scholars are working on the assessment methods suitable for the needs of engineering applications, from complex three-dimensional finite element calculations to the assessment of elastoplastic fracture mechanics. In the elastoplastic fracture mechanics evaluation, simplified induction and experimental verification are adopted to make it meet the engineering practicability.


The second is reliability. In engineering practice, neither the size of the pipe, the size of the defect, nor the material performance parameters are determined, but distributed in a certain law. With the application of reliability theory, the reliability assessment results of pressure pipelines are also presented with reliability indicators. For example, Hong, Y et al. Used fatigue crack propagation as the mechanism and considered the uncertainty of the parameters. The Paris formula was used to calculate the random expansion of cracks in the depth and length directions. Probability of leakage and blasting.


The third is intelligence. With the development of computer technology, the current international pipeline safety assessment methods are developing towards intelligence. For example: the ELBA expert system compiled by the German MPA for safety assessment of pipes with longitudinal cracks; the ESR expert system for high-temperature component pipes , Pipe system and component life prediction; American BMW expert system is used for boiler tube failure analysis.


4. Development Trend of Domestic Pressure Pipeline Safety Assessment


The research on safety assessment of pressure pipelines in China started late, but in recent years, people have paid great attention to it. Because we have a solid theoretical foundation for fracture mechanics, and a lot of foreign research results can be used for reference, therefore, the starting level is higher and the development is faster. According to China's situation, its development can be deepened from the following aspects.


The first is localization. Make research plans based on our country's situation. Foreign research in this field is mainly focused on the nuclear power industry, and its materials are mainly used for nuclear pipelines. In China, the nuclear power industry has just begun. Of course, we can carry out research in this area as a technical reserve. However, the research should face the research of pipelines in petroleum, chemical and civil engineering, such as city gas, in order to greatly reduce the incidence of catastrophic accidents in pipelines in these areas.


The second is engineering. In addition to the simpler assessment method and more suitable for engineering applications, it is more worth pointing out that research based on engineering practice has drawn conclusions that are more in line with the actual situation of the project. Foreign studies are currently focused on pipelines with cracks, but from the large number of results of pipeline inspections in China, undercuts, incomplete penetration, slag inclusions and pores are more common. Therefore, research on pipelines with undercuts and pores The failure behavior is more in line with the actual engineering.


The third is intelligence. With the gradual popularization of computer applications, intelligence will inevitably become the development direction of pipeline safety assessment. China should keep pace with the world's development trend and keep pace with the world.


Safety assessment content


1. Welding management system


Application of welding process assessment to meet the needs of engineering The welding process assessment of pressure pipelines is generally carried out in accordance with the requirements of JB4708-2000 "Steel Pressure Vessel Welding Process Assessment". The storage, drying, receiving, distribution and recovery of welding materials shall meet relevant regulations Welders shall have corresponding qualification items, and during the validity period, welding procedure specifications shall be prepared according to the welding procedure assessment. Nozzle processing and groups shall comply with the process requirements. The welding process parameters shall comply with the welding procedure specifications. Good welding records, the appearance of the weld shall be qualified. In the assessment, whether the process assessment file is complete, the number of test specimens, the bending angle and the bending center diameter of the bending test, whether there is an evaluation number mark on the test specimen, whether the evaluation can be effectively covered, whether the welder has super welding, welding materials library Whether the temperature and humidity meet the requirements and whether the electrode is dried and recorded should be checked.


2. Welding process file


For the welding process documents of pressure pipelines, in addition to the engineering construction technical plan, a special welding operation guideline should also be prepared. The instructions for welding of pressure pipelines shall be prepared by professional welding technicians and approved by the welding and heat treatment responsible division. Pressure pipe welding operation instructions shall be prepared according to the drawing requirements and the corresponding welding process assessment completed in advance according to the relevant construction and acceptance specifications. The corresponding bevel requirements shall be specified according to the various welding connection forms encountered in the entire pressure pipeline project. , Welding position, preheating before welding, welding material selection, interlayer temperature, number of welding layers, number of welding passes, welding current, welding voltage, post-weld heat treatment, welder qualification, inspection and non-destructive testing requirements, and fixed welding 2. Special process measures for obstacle welding.


3. Licensed Welder


Welders engaged in the welding of pressure components of pressure pipelines shall comply with the requirements of the boiler and pressure vessel welder examination rules and the requirements of the welder examination management system of the unit, and after passing the prescribed basic knowledge and operation skills examination, the welder issued by the municipal quality and technical supervision department Welding of the corresponding project is carried out within the validity period of the certificate, and "overdue" or "unlicensed" posts are not allowed. Pressure pipe installation units are generally not suitable to temporarily hire external welders. In special cases, the above requirements should also be met when necessary, and a written contract should be signed with them to specify quality responsibilities.


All in all, during the production and installation of large pressure pipelines, welding defects are unavoidable. Welding defects have a significant impact on the static load strength, fatigue strength, brittle fracture and stress corrosion cracking of the structure. Therefore, it is particularly important to perform safety assessment on pressure pipelines with defects.


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