Design changes and material substitution

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Both design changes and material substitutions represent modifications to the original design in the pressure vessel manufacturing process, and they have a direct impact on the vessel’s usability, quality, and safety. Therefore, design changes and material substitutions require written approval from the original design agency. Sheet Design Change: A design change refers to altering the original design’s structure or dimensions, technical requirements, or technical conditions, etc. During the manufacturing of pressure vessels, it is sometimes difficult for manufacturers to avoid making changes to the original design documents. However, such design changes can give rise to various risks and potential problems, and may even lead to the failure of the vessel; for example, changing the structure could affect the safe use of the vessel or its operational efficiency ； Changing the size may affect the assembly of the container or the installation of the equipment ； Lowering technical requirements or specifications may affect the safety and quality of the container, as well as reduce its service life. Therefore, TSG 21 and GB/T 150.4 stipulate the following regarding design changes: if the manufacturing unit wishes to modify the original design documents, it must obtain written approval from the original design unit for such changes. Compared to material substitution, design changes involve a broader scope and carry greater risks and potential problems; therefore, under normal circumstances, any design change made by the manufacturing unit, except for the shape of the welding groove, must obtain the consent and approval of the original design unit. Substitution of materials: Due to reasons such as material preparation, procurement, and supply, it is not always possible to obtain the required materials, which leads to frequent substitutions of materials. To this end, the \"Regulations on Safety Inspection of Pressure Vessels\" (1999 edition) established procedures for substituting materials for \"major pressure-bearing components,\" which helped to standardize such substitutions, ensure product quality, and promote the development of the industry. However, in practice, issues related to material substitution led to situations such as unclear responsibilities, improper substitutions, and non-compliant use of pressure-bearing components. Only the designer is best aware of the risks and failure modes associated with the container, as well as the consequences of using substitute materials for the pressure-bearing components. Therefore, TSG 21 and GB/T 150.4 stipulate the following regarding material substitution: Pressure vessel manufacturers must obtain written approval from the original design unit before substituting materials for the pressure-bearing components. To determine whether one material can properly and effectively replace another, a comprehensive analysis and comparison of the chemical composition, mechanical properties, bending properties, weldability, as well as other hot and cold working properties and corrosion resistance of both materials are necessary, taking into account the design and operating conditions of the pressure vessel product, the associated risks, and the failure modes. When people say that one material is better than another, it essentially means that certain properties of that material are superior to those of the other material; it does not imply that all of its properties are better. Taking the comparison between Q345R and steel plates of the Q245R and Q235 series (including the corresponding steel pipes and steel forgings) as an example, the former possesses superior mechanical properties than the latter. However, steel plates of the Q245R and Q235 series have better resistance to stress corrosion caused by liquid ammonia than Q345R, a fact that has been confirmed by domestic and international experimental studies as well as numerous practical applications ； Taking S30403 as a substitute for S30408 and S31603 as a substitute for S31608 as examples, from the perspective of resistance to intergranular corrosion, substitution is generally feasible; however, attention should be paid to the corrosive effect of specific media (such as nitric acid) on the material. It is also necessary to consider whether the allowable stress of the substitute material at the design temperature meets the requirements of the original design. Another issue to consider when using substitute materials is that it may lead to corresponding changes in the manufacturing requirements. Taking the substitution of 14Cr1MoR with 12Cr2Mo1R as an example, it is necessary to pay attention to the changes in the proportion of radiographic or ultrasonic testing for Class A or Class B weld joints (such as those with a thickness of 16 mm). At the same time, changes in the welding materials and whether re-evaluation of the welding process is required must also be considered ； Taking the use of Q345R (δ40mm) in place of Q345R (δ38mm) as another example, it is necessary to pay attention to the changes in post-weld heat treatment for Class A or Class B weld joints in both cases. Additionally, factors such as the effects of butt joints with unequal thicknesses and increased weight on supports and foundations, as well as their impact on heat transfer, must also be considered. Another example is the use of 16MnDR in place of Q345R (for design temperatures of -20°C, in non-low-temperature containers, to address inventory issues); here, it is important to ensure compatibility and consistency between the welding materials and the base material, and to consider whether a new welding procedure qualification is required. Therefore, whether it is a \"primary compressive element\" or a \"compressive element\", replacing \"low\" with \"high\" or \"thin\" with \"thick\", etc., is not necessarily safe; each case should be analyzed on its own. In material substitution, there is another scenario: given the variety of operating conditions for pressure vessels, the selection of materials for vessel components (including those that are not under stress) is sometimes based primarily on corrosion resistance. In such cases, material substitution requires even greater caution. The safest approach is to explicitly state in the manufacturing technical requirements for the vessel’s design that \"any material substitution in this design must be approved in writing by the original design team\" in order to avoid future problems. There was an incident in which, in a certain unit featuring a condenser (a DN2100×6000 fixed-tube-sheet heat exchanger), the fluid flowing in the shell side was ethylene oxide, and any contact with rust had to be avoided. Therefore, the pressure-bearing components in the shell side were made of S31803 and 00Cr18Ni9, while the non-pressure-bearing components were made of 0Cr18Ni9. Upon completion of the vessel, it was discovered that the nuts on the tie rods had been replaced with 0Cr13, which led to the need to disassemble the equipment’s shell and replace the nuts, resulting in repair work. Material substitution is a special case of design modification, and it is a major factor contributing to safety hazards and management chaos. The design and manufacturing units of pressure vessels must conduct thorough consultations and, taking into account all factors that may affect the safe use of the vessels, make careful decisions regarding material substitution plans.