Selection of welding joint coefficient for pressure vessel heads

Correct; the welds at the joint of the head must undergo 100% radiographic or ultrasonic testing, with a grade of II as the acceptable standard. This constitutes complete non-destructive testing, and the welding factor is 1.0. Otherwise, it is local; 100% detection of type III in the local area, with a welding coefficient of 0.85.

This post was last edited by fzujunru on 2019-2-19 at 14:27. In most cases, even if the head undergoes 100% non-destructive testing, its acceptance level is set to the same as that of the longitudinal seams of the cylinder; there has been no instance where the acceptance levels for non-destructive testing of the cylinder and the head were different. For the poster’s situation, a cylinder diameter of 0.85 and a head diameter of 1.0 is likely not suitable. As for the effect of the ring weld connecting the head to the shell on the weld coefficient of the head, GB 150 does not take this into account, whereas ASME VIII-1 does. If the ring weld connecting the head to the shell is not inspected, even in the case of a head that is formed from a single sheet of metal, its weld coefficient cannot be set at 1.0; see ASME VIII-1 UW-12. The head is not an isolated component; its load-bearing capacity should be considered within the context of the entire equipment. ASME’s approach is quite comprehensive.

For conventional containers, the calculated thickness of the elliptical head is always less than that of the cylinder wall; what’s the situation in the case mentioned by the poster?

From the calculation formula, it is indeed true that the thickness of the head should be less than the calculated thickness of the cylinder； However, the forming thinning amount must be taken into account for the end caps; meanwhile, the effective thickness of standard end caps is required to be no less than 0.15% of the inner diameter of the end cap to ensure stability.

Factors such as different thicknesses resulting from varying thinning amounts, different processing methods, and different manufacturers all lead to differences; generally, the design only specifies the minimum formable thickness.

Don’t forget that the head has straight edges; the thickness of these straight-edge sections should not actually be calculated using the head formula

The original poster’s understanding is correct; the end caps must indeed undergo 100% testing. If the passing standard for this testing is at least grade II, then a joint factor of 1.0 can be used.

Actually, this issue has also been taken into account in national standards, just in a different way. Now, consider a standard-compliant device with seamless end caps; however, if such a device falls under the scope of Chapter 4 of TSG21-2016, partial non-destructive testing must be carried out if full non-destructive testing is not done, let alone full non-destructive testing. For local testing, GB150.4-2011 10.3.2 specifies that the testing length shall be no less than 20% of the length of each weld joint; therefore, the welds between the head and the cylinder must be tested. Furthermore, in ASME standards, non-destructive testing is aimed at welders, with the primary purpose of assessing the quality of their welding; the performance of the welded joints is ensured by weld qualification. Our national standards, on the other hand, do not provide any clear guidelines; since there is no trust in the competence of workers in factories, the testing requirements set forth are stricter than those specified by ASME.

Local detection in TSG can also be understood as 100% detection, resulting in a grade of III being considered acceptable; after all, factors such as the difficulty level of grading films of grades II and III have a significant impact.

Regarding the issue of qualification levels, my understanding is as follows: Level II qualification applies to those containers that require 100% testing, as containers that need such testing are relatively dangerous; whereas Level III qualification applies to containers with a relatively lower degree of danger. The current wording of the standards gives the impression that the acceptable level is determined by the testing ratio, which leads to the question you raised: for containers where 100% testing is not mandatory, is it acceptable if I conduct 100% testing and still get an IIII rating? In its standard literal meaning, no! From the original intention behind standard drafting, okay!