Different thickness for Appendix 2 Flg and Shell 1

[LY.Yan]

Good Day to all,
For Section VIII Appendix 2 Figure 2-4 sketch 6(welding neck), is there anything in code that states the small hub thickness has to be the same as the shell attached? The small hub end is 16mm (sa-105)and shell plate is 14mm (sa-515 70). Also, I did not quite understand why the intentional difference in thickness for design? Thank you.

Yours Sincerely,
L.Y

 

[SnTMan]

LY.Yan there a couple of reasons why this might be the case.

Per Apx 2, 2-1(a), the hub must meet the minimum thickness requirements of Subsection A. Functionally this means it must meet the thickness required by UG-27 or UG-28 if applicable.

In some cases, the details of allowable stress, joint efficiency and corrosion allowance can result in a greater UG-27 / UG-28 thickness for the flange "go" dimension than for the attached shell thickness.

Some designers may choose to increase the flange thickness above the minimum required to produce a beneficial effect on the flange design, such as reduced hub stress or decreased flange thickness.

If you have the calculations available for the flange and shell you should be able to determine the reason.

Regards,

Mike





The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Christine74]

While I agree that it's theoretically possible that the flange hub may need to be thicker than the cylinder to which it attaches, as a practical matter I think it was almost always done as a way to decrease the required flange thickness in order to cut costs.

I'm too lazy to research it right now but I think ASME Section VIII Div. 1 Appendix 2 used to allow you to take credit for using a flange hub that was thicker than the actual cylinder, but they revised the code 10-15 years ago so you can no longer take credit for using a thicker hub.


-Christine

 

[SnTMan]

Christine74, larger go dimension can occur when stainless, lower allowable forgings are mated with higher allowable plate. Not terribly rare

The problem with sloppy work is that the supply FAR EXCEEDS the demand