Selection of welding joints between the circular flat cover and the cylinder body and the K value

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Dear sea friends: As stated in the title, the diameter of the cylinder of this equipment is non-standard and relatively small; it uses a round flat cover as the end closure, and conditions for welding inside are not available. Therefore, the weld joint between the cylinder and the cover is constructed according to item 5 in Table 5-9 of GB/T150.3. The value of f must meet the requirements specified in the notes there, while the value of K is calculated in accordance with the requirements of that table. There are currently two opinions: 1. According to the schematic in item 5 of Table 5-9, this joint is of a fully welded-through structure for the head section; therefore, it should be designed in accordance with Figure a in Appendix D.4 of GB/T150.3, so that the K value can be calculated as per Table 5-9. 2. According to the definition in GB/T 150, page 130, regarding Table 5-9 of the flat cover coefficient table, compared with Table 7-7 of GB150-1998, connection structures with full cross-sectional penetration are moved to Table 5-10. Fillet weld or combined weld connections (numbers 2, 3, 4, 5) and root butt welds (numbers 6, 7) are structures where full cross-sectional penetration does not occur; therefore, the structural characteristic coefficient K takes a larger value. According to the definition, it can be seen that both item 4 and item 5 in Table 5-9 represent structures with incomplete weld penetration across the cross-section, and the requirements for the f value are the same; the choice is made based on whether internal welding conditions are available. Figure J10(b) in Appendix J of GB150-1998 shows this type of structure with incomplete weld penetration, and this structure is not listed in Appendix D of GB/T150.3 due to the difficulty of achieving complete weld penetration. When explaining the standards, the teacher also mentioned that the absence of this structure does not mean it cannot be used; it’s just that GB/T150-2011 does not recommend it. Moreover, Appendix D is a informational appendix and not mandatory. Please discuss this: the former is certainly safe, but it requires more welding work, which inevitably leads to greater welding deformation and poor cost-effectiveness. Is the understanding of the latter correct? Is it too rash and unsafe? Looking forward to your reply.