Basic requirements for selecting the welding joint coefficient

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The selection of the weld joint coefficient is related to the weld pattern of the joint, the proportion of non-destructive testing, and the requirements for the container. The main issues are as follows: ① When the structure of longitudinal joints and circumferential joints differs, as well as their respective proportions of non-destructive testing, for example, if longitudinal joints require double-sided welding along with 100% RT or UT testing, while circumferential joints involve single-sided welding without the possibility of RT or UT testing, the appropriate welding joint coefficient must be used in the design calculations of the container. The formula for calculating the thickness of a pressure cylinder is derived from the strength of the hoop stress (primary) in the cylinder,; therefore, the corresponding welding joint coefficient should be that of the longitudinal welding joints in the cylinder. Ring (circumferential) film stress also exists in the minimum cross-section of the cylindrical circumferential joints. Additionally, although circumferential joints have only half the ring stress in the axial direction of the cylinder, as a complete pressure vessel, to ensure the strength and safety of the entire cylinder, it is generally required that circumferential joints possess the same quality level as longitudinal joints, that is, they should have the same weld joint factor. If there are manufacturing difficulties, the relevant provisions in GB150—2011 may be followed. At this point, although the quality of the circumferential joint (weld joint coefficient) may not be exactly the same as that of the longitudinal joint (weld joint coefficient), the weld joint coefficient of the longitudinal joint is still used when calculating the cylinder thickness. At this point, the designer should specify the technical requirements for that welded joint, in order to remind the manufacturer to use welding process measures to ensure the quality of the weld. ②Joint coefficient for the head joint. GB150.4—2011 specifies that 100% UT or RT testing shall be carried out on the head jointing seams, but it does not specify how to select the joint coefficient for such seams. The non-destructive testing requirements for the head jointing seams are primarily aimed at addressing the significant deformation that occurs during head forming, which facilitates the propagation of defects; they are unrelated to the calculation of the head thickness. Therefore, although the end cap joints require 100% UT or RT testing, such testing is still only part of the inspection of the entire container, and its acceptance criteria are still determined based on those required for the whole container. Therefore, the weld joint coefficient for the head splicing joints is generally still taken as that of the longitudinal joints of pressure vessels. ③Relationship between the joint coefficient and the non-destructive testing ratio. Generally speaking, when the weld joint factor is set at 1.0, such containers must undergo 100% non-destructive testing and meet the specified testing quality requirements ； Conversely, when a vessel, particularly one of its components such as the head mentioned in ②, or a specific part of it such as a joint covered by gaskets, is subject to full non-destructive testing, the welding joint factor for that vessel (including that component or part) is not necessarily 1.0; rather, it depends on the testing requirements for the entire equipment.