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Safe and reliable: To ensure the safe and reliable operation of process equipment, pressure vessels must have sufficient capacity to withstand various loads that may occur during their design life. Therefore, the materials used to manufacture pressure vessels are required to have high strength and good toughness, be compatible with the medium, possess sufficient stiffness and stability against distortion, and offer excellent sealing properties. Strength, stiffness, toughness, and sealing performance are the main factors affecting the safety and reliability of process equipment. Strength is the ability of a pressure vessel to resist permanent deformation and fracture under load. In the design of pressure vessels, stresses or stress-related parameters are generally limited to allowable values based on different modes of strength failure, so as to meet the strength requirements. For example, gas storage tanks should not bulge, deform, or rupture under the pressure of the medium. Yield strength and tensile strength are commonly used criteria for evaluating the strength of steel. Under the same design conditions, increasing the material strength allows for an increase in the allowable stress, which enables thinner walls in process equipment, reduces weight, and simplifies manufacturing, installation, transportation, and setup – thereby reducing costs and improving overall economic efficiency. For large pressure vessels, the use of high-strength materials yields particularly significant benefits. However, the high strength of materials should not be overemphasized, as high-strength materials are often difficult to manufacture and process. Stiffness is the ability of a pressure vessel to maintain its original shape under load. Insufficient stiffness is one of the main causes of excessive deformation, instability, and leakage in pressure vessels. For example, in a connection structure made up of bolts, flanges, and gaskets, if the flange undergoes excessive deformation due to insufficient stiffness, it will lead to seal failure and leakage ； Vessels operating under vacuum and subjected to external pressure may experience unstable failure if the shell stiffness is insufficient. Therefore, the container equipment should have sufficient stiffness. Toughness refers to a material’s ability to absorb deformation energy before it breaks. Due to reasons such as raw materials, manufacturing (especially welding), and use (such as fatigue and stress corrosion), containers often have various defects, such as cracks, pores, and inclusions. Research shows that not all defects pose a threat to the safe operation of pressure vessels; rapid expansion leading to vessel failure occurs only when the defect size reaches a certain critical value. The critical dimension is related to the stress level at the location of the defect, the toughness of the material, as well as the size, shape, and orientation of the defect; it increases as the toughness of the material improves. The better the toughness of the material, the larger the critical size; thus, the container equipment becomes less sensitive to defects ； Conversely, under load, very small defects can rapidly propagate, leading to the failure of containment equipment. Sealing performance refers to the ability of a pressure vessel to prevent the leakage of media. Leakages in pressure vessels can be divided into internal leaks and external leaks. Internal leakage refers to the leakage between various chambers inside a container, such as the medium in the tube side of a shell-and-tube heat exchanger leaking through the tube sheet to the shell side. Such leaks can cause product contamination in mild cases, and explosion accidents in severe cases. External leakage refers to the leakage of a medium into the surrounding environment through detachable joints, or the entry of air into a container or equipment. The media contained within pressure vessels are often hazardous; external leaks can not only lead to accidents such as poisoning, fires, and explosions, but also cause severe environmental pollution. Therefore, sealing is one of the necessary conditions for the safe operation of pressure vessels. The strength of various components in a pressure vessel varies; the overall strength often depends on the strength of the weakest component. Making the strength of each component equal, that is, by adopting equal-strength design, allows for full utilization of the material’s strength properties, saving material and reducing weight. The toughness of materials generally decreases as strength increases. When selecting materials, special attention should be paid to the proper matching of material strength and toughness. While meeting the strength requirements, high-toughness materials should be selected as much as possible; it is very dangerous to prioritize strength at the expense of toughness. There have been numerous explosions of process equipment caused by insufficient resilience both domestically and internationally. The environment also affects the toughness of materials. A decrease in temperature, neutron irradiation, or operation under high temperature, high pressure, and hydrogen-containing conditions can all reduce the toughness of materials, making them brittle. Understanding the behavior of material properties under changing environmental conditions, and preventing the material from becoming brittle or keeping such effects within acceptable limits, is one of the effective measures to improve the reliability of process equipment. Meeting the requirements of the production process: Pressure vessels are first and foremost designed to meet the needs of production, with their main structural dimensions determined by the manufacturing processes. Equipment design can only be carried out based on the process condition data and site conditions provided by the process designers, in order to meet the functional requirements. There are also lifespan requirements. For example, in the petrochemical industry, it is generally required that high-pressure vessels have a service life of at least 20 years, while tower and reaction equipment should have a service life of at least 15 years. Corrosion, fatigue, and creep are the main factors affecting the service life of process equipment. During design, it is necessary to take into account factors such as temperature and pressure levels and their fluctuations, the corrosiveness of the medium, the impact of the environment on material properties, and the interaction between the fluid and the structure. Effective measures should be adopted to ensure that pressure vessels operate safely and reliably throughout their designed lifespan. With a favorable overall economic environment, the design of pressure vessels must ensure safety and reliability, while also striving for technical and economic Rationality in order to minimize the total cost of the products. To achieve this, when designing pressure vessels, it is necessary to select appropriate materials first. While ensuring that the production requirements are met, the structure should be as simple as possible with minimal material consumption. At the same time, factors such as manufacturing, inspection, installation, and maintenance must also be taken into account. For some highly advanced equipment, although the investment is higher, it should still be considered for adoption if it offers significant advantages in terms of processing capacity per unit, consumption metrics, and product quality. In short, pressure vessels should meet the requirements of high production efficiency, low consumption coefficients, reasonable structure, easy manufacturing, as well as ease of transportation and installation, as much as possible. In addition, pressure vessels should also be easy to operate, have good maintainability and repairability, be suitable for automatic control, adapt to changes in operating conditions, and meet environmental performance requirements. In general, the basic requirements for pressure vessel design are to ensure safety and reliability while also striving for economic efficiency. This requires accurate estimation of the operating conditions and loads on the container equipment, as well as a comprehensive analysis and evaluation of the overall stresses, local stresses, and thermal stress in pressure vessels, along with the possible modes of failure, in order to adopt appropriate design methods. At the same time, appropriate materials and a reasonable structure are selected based on the operating conditions and functions of the pressure vessel. Therefore, pressure vessel designers must not merely carry out a simple design in accordance with relevant standards and manufacturing conditions; they also need to take into account factors such as production conditions, safety requirements, and technical and economic feasibility in order to select the best design solution.