Lock-bottom joint
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I would like to ask the experts: Is a locked-bottom joint suitable for joining the cylinders or shells of compressive components?
Such a structure exists at the connection between the circular flat cover and the cylinder, as well as at the connection between the tube sheet and the cylinder. I have never seen butt joints for cylinders.
I really haven’t seen this type of joint on cylinders; pressure vessels do have welded joints with lining plates, but those lining plates are purely auxiliary elements and not load-bearing components. This type of joint is more common on storage tanks, where it resembles lap welding
Lock-bottom joints are permitted. It is shown in the schematic diagram of the connection weld between the flat head and the pressure-bearing element on page 300 of GB/T150; however, note on page 301 specifies the conditions under which this joint may be used. However, in air compressor units, such as separator components and air intake filters, this type of connector is widely used.
The bottom butt weld you mentioned can be used; our factory has manufactured containers of three types. This is one of the weld forms. Non-destructive testing is performed on Class C welds using magnetic particle or penetrant testing. Submerged arc welding is used. Such welded joints are very commonly used.
The attachment is the welding joint diagram for the bottom lock joint, used for the circumferential seam of the cylinder.
Bottom lock joints are certainly permitted to be used on pressure vessels, and they are specified in the standards; the question is whether they can be used on Class A and Class B welded joints.
According to Article 3.2.2.1 of TSG21-2016, Class A and Class B butt joints of pressure vessels manufactured by welding shall adopt a full-thickness penetration weld pattern. The lock-bottom joint clearly cannot meet this requirement. From the perspective of stress analysis, this form is not suitable for use in Type B joints, as the unequal thickness on both sides of the joint leads to structural discontinuity, thereby resulting in additional edge stresses in addition to the overall membrane stress at that location. Although UW13 in ASME Section VIII-1 allows the use of root-bound joints for the welds between the head and the shell, it imposes numerous structural and dimensional restrictions, all aimed at controlling additional edge stresses. There are no relevant regulations in the national standards; therefore, from this perspective, it is also not recommended to use such joints for joint B.