Common Problems and Analysis in Pressure Vessel Manufacturing

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Abstract: Pressure vessels are special precision instruments widely used in industrial, agricultural, and other production fields; they come in various types and involve complex manufacturing processes and technologies. The safety of its use affects the entire industrial or agricultural area. At present, there are some issues in the manufacturing process of pressure vessels; this article briefly describes several common problems and analyzes them. 　　Since the 21st century, with the rapid economic development and advancements in technology in our country, the capabilities related to the design, manufacturing, and maintenance of pressure vessels have improved significantly, placing our country among the leaders in the world. Over the past decade or so, China has established and improved laws, regulations, and standard systems related to pressure vessels, thereby standardizing the entire process of their design, manufacture, and maintenance. Design and manufacturing methods for pressure vessels based on risk assessment were proposed and improved. To achieve a **sustainable development strategy, efforts have been made to promote and implement green design for pressure vessels, thereby gradually establishing a modern manufacturing model for such vessels. Although China has achieved independent design and manufacturing of pressure vessels in terms of materials, design, and production, there are still some problems and challenges associated with their manufacture. 　　1 Characteristics of pressure vessel manufacturing Pressure vessels are a type of special equipment used in various fields such as the petrochemical industry, agriculture, and national defense. There are many different types of them, which can be roughly divided into four categories: storage, reaction, heat exchange, and separation. Since most pressure vessels are used in harsh environments such as high temperatures and high pressures, it is essential to ensure their safety in terms of design and manufacturing. The material requirements for manufacturing pressure vessels are very strict. In practical operations, manufacturing pressure vessels is extremely difficult; it requires collaboration among various specialized fields, and the technology involved is very complex. For designers of pressure vessels, in addition to having expertise in pressure vessels, they also need to possess knowledge specific to the industry in which these vessels are used. They must design the pressure vessels in strict accordance with relevant standards and regulations to ensure their safety. 　　2 Common Problems and Analysis in the Manufacturing Process of Pressure Vessels As the manufacturing techniques for pressure vessels continue to evolve, various problems arising during this process have come to light. It is necessary to identify these problems promptly, avoid risks, and enhance the safety of pressure vessels. These problems include material issues, deformation issues, weld issues, stress corrosion issues, and hydrogen embrittlement issues, among others. 　　2.1 Material issues The problems that arise during the manufacturing of pressure vessels mainly include the use of substitute materials, replacing better materials with inferior ones and thicker materials with thinner ones – situations that are common in actual production processes. The control of material quality is a prerequisite for ensuring the safe use of pressure vessels. The quality of materials is strictly controlled throughout the entire process, from design and procurement to acceptance, storage, distribution, and use. When selecting materials, it is necessary to strictly adhere to **standards and various specifications. When selecting materials, it is essential to ensure that the materials produced by the manufacturing enterprises meet relevant standards and specifications. Before the materials arrive on site, they need to be inspected. First of all, it is necessary to verify the qualifications of the material suppliers; if substitute materials are used for the main pressure-bearing components of pressure vessels, this must be reported to the original manufacturer ； The transplantation of material markings is an effective means to prevent the mixing and misidentification of materials. Establishing a comprehensive management procedure for the transplantation of material markings ensures traceability of pressure-bearing components throughout their storage, distribution, cutting, and production processes ； Secondly, it is necessary to test various parameters of the raw materials and conduct inspections in strict accordance with relevant standards and specifications. After the materials arrive, the person in charge of materials assigns acceptance numbers to them. The management of welding materials must be carried out in accordance with specific regulations to avoid safety hazards and to ensure the quality of pressure vessels. During the production process, a secondary storage area must be established for these materials; they should be stored in designated areas, with clear labeling based on their grade, specifications, and material identification numbers. Special welding materials require separate storage. 　　2.2 Deformation Issues The deformation of pressure vessels significantly affects their integrity and reliability, which in turn impacts their safety during operation. It is therefore essential to carefully establish and strictly adhere to manufacturing processes and standards, to ensure that the quality of these vessels meets the requirements of relevant specifications and regulations. The deformation of pressure vessels during the manufacturing process is mainly caused by errors in the cutting dimensions, the shape of the molds, and errors during the assembly process. To solve this problem, it is also necessary to focus on these three aspects. First and foremost, it is important to improve the accuracy of material cutting by carefully calculating the amount of material required; computer software can be used to assist with such calculations and management ； Secondly, the shape of the mold must be strictly controlled, taking full account of the changes that occur during and after the forming process of the pressure vessel. Additionally, during the assembly of the pressure vessel, positioning fixtures are used to prevent deformation, and a theodolite is employed to check for straightness. In actual operations, methods such as symmetric cutting or machining, as well as flattening and straightening after cutting the blank inclined plates, are employed to prevent deformation caused by flame cutting ； For unstable deformation, stiffening plates attached to the shell can be used to reinforce the open areas; once the shell is in a stably integrated state after resistance welding of the tubes, these stiffening plates can be removed. To avoid deformation due to heat treatment, a fire wall is installed at the flame nozzle on the furnace wall; where necessary, pressure vessel components that are prone to losing their temperature at high temperatures are reinforced. 　　2.3 Welding issues During the manufacturing of pressure vessels, many components are joined together by welding. The arrangement of welds and their quality have a direct impact on the quality of the pressure vessel; special attention must be paid to weld defects, as they can lead to stress concentration. During the welding process, pay attention to the arrangement of welds. The crossing and overlapping of welds are inevitable; therefore, a stress analysis of the pressure vessel must be conducted prior to operations. Computer software can be utilized to assist with these calculations. The key factor affecting welding quality is the professional competence of operators. Therefore, it is essential to manage welding personnel properly, verify their qualifications and relevant certifications, and ensure that they work only after obtaining the necessary permits. 　　2.4 Stress corrosion issues Stress corrosion cracks arise as a result of the combined effect of stress and a corrosive environment. It is impossible to avoid stress during actual operation, nor can one escape a corrosive environment. Therefore, it is necessary to address this issue starting from the material itself. When designing pressure vessels, it is necessary to fully consider the issue of stress corrosion and select appropriate materials. Additionally, the structure can be subjected to processing, heat treatment, anti-corrosion treatment, etc., to minimize stress corrosion ； Possibility of cracks occurring. 　　2.5 Hydrogen embrittlement issue
Hydrogen embrittlement occurs when hydrogen dissolved in steel aggregates to form hydrogen molecules, causing stress concentration. This exceeds the strength limit of the steel, resulting in the formation of tiny cracks within it; these cracks are also known as white spots. Hydrogen embrittlement can only be prevented, not cured. Once hydrogen embrittlement occurs, it cannot be eliminated. The higher the temperature, the more severe the hydrogen embrittlement; the higher the carbon content in steel, the worse the phenomenon of hydrogen embrittlement becomes. To prevent hydrogen embrittlement, metal elements such as chromium, titanium, and vanadium can be added to steel. Corrosion inhibitors can also be added. Methods such as electroplating and sandblasting for rust removal can likewise reduce the occurrence of hydrogen embrittlement to a certain extent. 　　3 Non-destructive testing 　After the manufacture of pressure vessels is complete, quality inspection must be carried out before the products leave the factory. Currently, the main testing methods used in China are radiographic testing (RT) and ultrasonic testing of welds (UT). With the innovation and development of non-destructive testing technologies, the results of such tests have become more objective, accurate, and reliable. 　　4 Conclusion
There are many issues that need attention during the manufacturing process of pressure vessels; there are still some problems in aspects such as design and inspection. Every stage, from design and manufacturing to acceptance, must be carefully managed, with strict adherence to **standards and various regulations. Additionally, computer software is utilized as a tool to minimize quality issues in the manufacturing of pressure vessels, thereby ensuring their safe operation after they leave the factory.