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Requirements for design in pressure vessel manufacturing – Basic requirements. Design is the foundation of manufacturing; during the design phase, consideration must be given to the feasibility of manufacturing processes as well as the ease of inspection. In other words, designers need to consider whether their designed products can be manufactured using reliable, simple, and cost-effective methods. From this perspective, the main requirements that manufacturing imposes on design are as follows. When selecting materials for surfaces, it is necessary to take into account not only factors such as the material’s mechanical properties, bending strength, corrosion resistance, and cost, but also its weldability and other properties related to hot and cold processing. The proper selection of materials is the main responsibility of the designer, as it directly affects the difficulty of manufacturing and the safety of the product. For carbon steels and low-alloy steels, as the minimum standard tensile strength value Rm increases, it is necessary to enhance the toughness and ductility of these steels. For carbon steels and low-alloy steels used in welding, limits are set on the contents of C, S, and P; whereas for carbon steels and low-alloy steels intended for pressure vessels, further restrictions on the contents of C, S, and P are imposed based on the minimum standard tensile strength value Rm and whether they are to be used in low-temperature vessels. When using materials with a higher strength grade in the design phase, it is essential to ensure that there are strict and feasible technical requirements, as well as to thoroughly assess the manufacturing capabilities and experience of the manufacturing company; the price quoted should not be the only criterion for making a decision. Furthermore, for steel, steel specifically designed for pressure vessels should be chosen whenever possible ； Materials of foreign brand names, new materials, and materials manufactured for the first time by material manufacturers shall comply with the relevant provisions of TSG 21 to facilitate manufacturing, procurement, and use. Technical requirements: The technical requirements for ordinary products can be specified on the drawings, known as drawing technical requirements; for complex and major products, separate product technical specifications can also be established. The technical requirements established by the designer should not only be accurate, comprehensive, and appropriate, but also take the following aspects into consideration: ① Technical requirements must be complete and clear; wherever there are requirements, there must also be means for inspection and acceptable criteria. For example, when requiring a leak test on pressure vessels, it is necessary to specify the type of leak test, the method standards, and the allowable amount of leakage. Otherwise, such leak test requirements remain nothing more than theoretical concepts. The level of technical requirements is closely related to the cost. It is the designer’s responsibility to consider the economic viability of the product, but this should not lead to a reckless reduction of technical requirements. For example, in some designs, the operating temperature for containers made of carbon steel or low-alloy steel is set at -20°C; this may be done in order to avoid using steels intended for low-temperature pressure vessels, which are more expensive. Such a seemingly legitimate approach can pose risks, as fluctuations during operation, along with the influence of ambient temperatures, increase the likelihood of brittle fracture at low temperatures. ②Parameters such as post-weld heat treatment and the percentage of non-destructive testing are often related to thickness; different materials have different thickness limits, commonly known as threshold values, and the technical requirements vary significantly above and below these threshold values. When the nominal thickness in the design approaches the threshold value, designers must be particularly cautious regarding the technical requirements specified, as manufacturers, in order to ensure that the product thickness is not less than the minimum formability thickness (or nominal thickness) indicated in the drawings and to take into account factors such as process-induced thinning, often round off the thickness (or perform a second thickness adjustment) to determine the actual thickness of the steel. As a result, the thickness chosen by the manufacturer may be greater than the nominal thickness, and may even reach or exceed the aforementioned threshold value. The thickness limits specified in standards (GB/T 150.2 and material standards) are based on the thickness of the steel. In terms of structure, structural design should be given equal importance to calculations; the structure is not only the basis for calculations but also has a direct impact on the quality of manufacturing. Wherever possible, structures that are easy to manufacture and easy to inspect should be used. Taking the welded joint type as an example, since the two weld surfaces allow for root welding on the back side, it is easier to ensure welding quality compared to single-sided welding, so it should be given priority ； Single-sided welding is divided into single-sided welding with gussets and single-sided welding without gussets. When performing single-sided welding without gussets, it is difficult to ensure that the bottom area is fully filled with metal; high current levels can lead to penetration through the material, while low current levels may result in incomplete welding at the root, making it difficult to guarantee quality. Therefore, this type of welding is used only for the final circumferential weld joints between pipes and pipe flanges, as well as containers. Standards (GB/T150.1) require full-penetration butt joints equivalent to those achieved through double-sided welding, that is, welding methods that ensure complete penetration, such as gas shielded welding for the root pass. Similarly, for large-diameter connections to important containers, embedded connections with integral reinforcement are preferred when conditions permit. These not only facilitate welding and improve the stress conditions but also allow for radiographic or ultrasonic testing, thus making it easier to ensure the quality of the welded joints. When using this type of structure, it is important to ensure that there is sufficient width at the connection point between the embedded connection and the container shell, so as to enable 100% radiographic or ultrasonic testing. During the design process, it is necessary to pay attention to the comparison of different structural options on one hand; on the other hand, for structures that are closely related to manufacturing processes, designers can merely specify requirements and leave the actual design of those structures to the manufacturing party, which is particularly important for designers in professional design firms who are not very familiar with manufacturing processes. Taking welded structures as an example, the welded structures specified in the standard (GB/T 150.3) are only indicative; they can be used as a reference during manufacturing but are not mandatory. In practice, many manufacturing companies have over the years developed their own effective sets of commonly used welded structures. Unless there are special requirements for the joint design, it is not necessary for designers to specify the type, size, and angle of the grooves – it is sufficient to state the main requirements in the drawings (such as full penetration or partial penetration). Manufacturing knowledge – The basic knowledge that designers should possess includes: (1) a preliminary understanding of the methods, fundamental principles, and key quality requirements related to critical manufacturing processes such as material cutting and shaping, welding, heat treatment, non-destructive testing, and pressure testing. This knowledge not only aids in structural design and the establishment of technical specifications, but also enables proper assessment of whether the manufacturer’s production capabilities and processing methods can ensure product quality. (2) Understand the requirements regarding manufacturing, inspection, and acceptance in the product manufacturing standards, to facilitate reference when needed during the design process. Having a certain understanding of the reasons behind these requirements can help in applying manufacturing standards to develop more cost-effective and reasonable designs. (3) Understand the main manufacturing processes of the designed product, as well as the key processes that have a significant impact on product quality, so as to comprehensively consider material selection, structure, and technical requirements from the perspective of the overall product quality. (4) Understand new technologies and processes related to manufacturing, inspection, and testing, and apply them appropriately to the products designed in order to drive technological progress.