Is the thinning rate for pressure vessel head forming taking into account the negative deviation of the sheet metal?

The thinning rate is a practical value that should be determined based on the actual thickness of the steel plate. The negative deviation of the steel plate is the theoretical value specified by the standard. It’s not the same thing.

For example, for the head of EHA600X6, the thinning rate is 13%; thus the minimum thickness calculated is 5.22 mm. The manufacturer provides a thickness of 5.3 mm – does this 0.7 mm of thinning include the -0.3 mm negative deviation in the sheet thickness?

The actual thickness of the steel plate multiplied by 13% is the amount of thinning.

I believe that the 13% thinning rate for the head of the EHA600X6 is a design control value used to determine the minimum thickness after forming as per the design requirements; it is calculated based on the nominal thickness, without taking into account the positive or negative thickness variations of the steel plate. The control value for head forming is the thickness after forming, rather than the reduction rate. The calculated thinning rate after shaping should be based on the measured thicknesses before and after shaping, but it is only used for reference and not as a criterion for evaluation.

There is no relationship between the minimum thickness of a pressure vessel head after forming, the degree of thinning during forming, and the negative deviation of the sheet material; it is sufficient to ensure that the thickness after forming is not lower than the minimum thickness achieved after forming

Given that there is no concept of thinning rate on SW6, during calculation, it might be possible to add the thinning rate and the negative deviation together, and then use this sum as the negative deviation value to be entered into SW6. Perhaps this represents some kind of relationship between them; actually, however, the two are not related to each other. I’m not sure if my understanding is correct

When actually using SW6 for calculations, I noticed that the thinning factor wasn’t taken into account in many head calculations. Still taking the EHA600X6 head as an example (the head manufacturer specifies a minimum thickness of 5.3 mm for such heads), with a negative deviation of 0.3 mm and a thinning rate of 13%, what should be the actual minimum thickness of the head after it is formed? Should it be (6–0.3) × (1–13%), which gives a value of 4.96 mm? What if the calculated minimum thickness is 5.1 mm? How should this issue be resolved? I understand that the 5.3 mm provided by the head manufacturer certainly meets the requirements, but the manufacturer’s dimensions clearly do not take into account a negative deviation; that’s why there are doubts in this regard. Does the minimum thickness of the head after shaping, as given by the manufacturer, already include the amount of thinning corresponding to the negative deviation? (The previous expression may not have been very clear.)

Negative deviation refers to the negative deviation of the steel plate before head forming, not the deviation after the head is formed; they are not the same thing. When calculating the strength of the head, it is sufficient to check whether the actual thickness of the head after formation meets the minimum thickness specified in the design documents.

A negative deviation doesn’t matter; the head must be specified with its minimum thickness, and design calculations are also based on this minimum thickness, not on the thickness obtained by subtracting the negative deviation from it. The final forming thickness of the end cap should be no less than the minimum thickness specified on the drawings; if it is lower, it is considered unacceptable. Negative deviations cannot be taken into consideration here, as these are two completely different concepts.

Since it is our company that provides the materials and the head manufacturer handles the processing, I overthought things. Thank you all for your advice.