Discussion on the issue of the calculated thickness of tube sheets sealed with octagonal gaskets
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This post was last edited by Douwan on 2017-10-26 at 14:49. I haven’t posted for a long time; I’ve been quite free lately, so I have the time to share various technical problems I’ve encountered in the past with everyone. I hope I can persevere, and I also hope that fellow sailors will interact and communicate more. I encountered a case with a rather unique structural design before, and I discussed it with expert teachers; therefore, I would like to share it with everyone here. Note: 1) An Type A clamped tube sheet structure is adopted ； 2) Adopt an octagonal gasket sealing structure ； 3) Design pressure: 9.8 MPa; the specific configuration is shown in the figure below. Question: When calculating the thickness of the tube sheet, should the groove depth of the gasket be taken into account when determining the thickness? For example, in the case of the octagonal gasket shown in the image, whose groove depth is 17, does this need to be considered? The calculated thickness plus corrosion allowance is 152; what is the total depth of the grooves on both sides of the gasket, 35? Everyone shared their opinions, and a summary reply was given on the 10th floor. Hehe
This is a good question. GB/T151 only specifies the depth of structural grooves, but it does not explain what exactly constitutes the depth of such grooves, nor does it indicate which factors affect the calculation of thickness. Generally, what we take into consideration are the depth of the partitions and the depth of the stress-relief grooves, both of which are typically within the range of the nominal diameter. Anything beyond the nominal diameter is generally considered as the difference between the tube sheet thickness and the flange thickness. The structure shown by the original poster has not been used by us yet. If there are partition grooves, that would make things simpler; otherwise, I tend to consider it as part of the difference in thickness between the tube sheet and the flanges. However, no research has been done on the mechanical model of the tube sheet, so I’m not sure about the exact theoretical explanation.
Yes, this structure is quite special; it’s not something I’ve encountered before, nor have I built it myself. But this case was indeed done by a friend
Except for the U-shaped tube sheets with type A connections (clamping), grooves need to be considered. Since the maximum (bending) stress in a U-tube sheet is at the center, the peripheral grooves can generally be ignored in engineering design. For other types of tube sheets, it is necessary to consider grooves and handle them carefully during engineering design, as it is not possible to determine them using current engineering methods. For the cases mentioned above, due to the high pressure, it is recommended to leave an appropriate margin. Reason: 1) Deeper grooves cause higher local stresses (the shear stress around the U-shaped tube sheet is higher than at the center) ； 2) The seal must have sufficient stiffness; in particular, the hydrostatic test pressure after backpressure at a design pressure of 9.8 MPa cannot be ignored, as leaks often occur under such hydrostatic conditions. Due to the deep grooves, it is a special case and it is recommended to give appropriate consideration
I think this should be taken into consideration, because DG in the formula for calculating the thickness of the tube sheet clearly represents the diameter of the circle at the center of the gasket compression force; therefore, the thickness of the tube sheet must cover at least the area occupied by the gasket.
Are you satisfied with the answer for floor 4?
I wonder if that ring on the left side is a flexible gasket