Variable thickness of cylinders and heads, as well as calculation issues related to heads
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1. When the thickness of the elliptical head is less than that of the cylinder, welding shall be carried out in accordance with GB150.3 P291 Figure e). 2. The thickness of the cylinder body is 22 mm, and the thickness of the head is 20 mm. This cylinder body is subject to external pressure, and a thickness of 20.6 mm is required to meet the requirements. May I ask: Does the strength verification calculation for the end cap include the straight edge sections? Is it necessary to thicken the head to 22 mm to ensure that the straight-edge section can withstand external pressure? Thank you!
First, does this situation you described actually exist? For ordinary elliptical heads, considering the amount of thinning, the material thickness required for such heads is greater than that of the cylinder. Secondly, if it is indeed necessary, I think it is required to thicken the straight edges of the head
This vessel is a jacketed vessel; during verification, the inner cylinder requires a thickness of 22, while the head only needs a thickness of 20. At this time, is the straight edge section of the end cap based on the thickness of the cylinder?
The head has a better capacity to withstand external pressure than the cylinder. However, the calculated thickness for external pressure on the head does not include the straight-edge sections; therefore, it is reasonable to make the head of the same thickness as the cylinder. The amount of material removal during processing also needs to be considered, as well as the convenience of preparing the materials.
The calculation for the end cap shows a requirement of 20.6; when considering thinning and negative material variations during pressing, it is necessary to use material in the range of 22–24. The straight edges will definitely be thicker than those of the cylinder, at 22 thickness.
Whether the thickness of the straight edge section of the head should be calculated based on that of the cylinder is an old issue, and no official explanation has been provided to date. In my opinion, it is not appropriate to calculate the thickness of the straight-edge sections using the formulas intended for the cylinder body; after all, the shape of the end cap also plays a role in strengthening this area. Moreover, in high-pressure equipment, it is not uncommon for the end cap to be 50 mm thinner than the cylinder body, and in such cases, the cylinder body is still cut and welded to the end cap. Otherwise, how can we explain those welding joints specified in GB150?
This post was last edited by zjq1962 on 2018-5-11 at 16:13. The situation you mentioned likely refers to a spherical head; spherical heads are often used in high-pressure vessels, as the calculated thickness for a spherical head is much smaller than that of a cylindrical shell. Although the thinning occurs on the cylinder, from the center of the head, the thinned area should be considered part of the head; the stress experienced by the thinned area is the same as that of the head, otherwise it would be incorrect.
I agree with you. The section where the cylinder wall is thinned is generally located within the radius of the spherical head.
For elliptical end caps, once the stamping of the end cap is complete, the straight-edge sections are typically the thickened areas
Thank you all for your discussions; in the end, the design was chosen such that the thickness of the straight edges is consistent with that of the cylinder wall.
Solution: 1. Increase the thickness of the head, with the minimum forming thickness being no less than the designed thickness of the cylinder. 2. By adding spiral guide plates to the cylinder, the length used in external pressure calculations is reduced, which lowers the design thickness of the cylinder; at the same time, the thickness of the end cap ensures that the minimum formability thickness is not less than the design thickness of the cylinder.