Pressure stiffening rings for Internal pressure design. 1

You don't use or need stiffening rings for internal pressure, that's why you're not finding anything on it.

If it's open-top, it wouldn't normally be called a vessel or require design to the ASME code. Why the stiffeners are there, I can't say. It's possible that the usage or configuration has been modified and that at one time it was subject to external loading. It's possible that the stiffeners were for stiffening the shell due to stresses from leg or lug supports. It's possible they were for heat transfer purposes or for insulation supports. Perhaps the specs said "design for full vacuum" and by golly, they designed an open top tank for full vacuum.

The key here is that the ring is located at the cone-cylinder junction.

That location, due to the internal pressure (in your case from the static head), the bending moments necessary to ensure compatability will cause the come and cylinder to bend inwards. Additional moment of inertia is required to resist that and the classical solution is to provide a ring.

Check out any thin-shell theory textbooks.! I've done this type of calculation by hand myself - balance the forces and enforce compatability.

As TGS4 mentions, the area that normally requires a ring on a vessel with a conical section is at any cone/cylinder junctions. Codes often address this issue in one way or another. In ASME this requirement is covered in Appendix 1, paragraphs 1-5(d), 1-5(e).

Given that you're working with an open-top tank, I would suspect that API620 would be a more appropriate code to consider. I don't have a copy in front of me at the moment, but I'm pretty sure there is an equivalent section that covers this requirement.

Otherwise as JStephen mentions, there could be many other reasons that a ring is specified...

Hi Everyone,

I'd like to add some clarifications.
This is used, in service vessel. It is 35' high and 10' in diameter. Although it is open on the top, there is filter element in the middle (like tube sheet) which is feed from bottom part of the vessel and can plug. Liquid passes through filter and flows out through overflow nozzle in the top portion of the vessel.

It looks like main purpose of eight (8) 3/8" x 3" rolled flat bar rings welded outside is to keep vessel in shape because when it is filled it would distort otherwise. There are four (4) support pads around middle of it.

The main reason to call it a vessel is it is feed from pump, and filter may plug which will increase pressure in the bottom part of the vessel above 30 psig.
There is no PRV in the feed line or in the vessel, and the vessel should be able to withstand 30 psig near filter and 30 psig + 16 psig static head = 46 psig in the bottom.

In UG-27 Thickness of Shells Under Internal Pressure there is paragraph (e) which reads:

"When necessary, vessels shall be provided with stiffeners or other additional means of support to prevent overstress or large distortions under the external loadings listed in UG-22 other than pressure and temperature".

In this scenario how should I account tension effects of stiffening rings? Does ASME Section VIII Div 1 allows any credit for calculation of tmin? Or in order to have credit for stiffening rings it should be Div 2? Any thoughts about that?

Thank you,

Curtis

Hi All,

No new ideas? Does anyone has S value for SA-240 316L from Boiler and Pressure Vessel Code 1988 Edition?

Thanks,

Curtis

Under ASME VIII you can't take any credit for stiffening rings to internal pressure. As in your quoted sentence, stiffening rings may be adopted (and it is probably your condition) to distribute support reactions.
S at room temperature for SA-240 316L (plate) in VIII-1 was 16.7 ksi in 1992 (most likely the same in 1988)

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Prex,

Thank you for your response.

Are you sure about 16.7 ksi? I've got 16.7 for 2010 edition. Can you check for me one more time if you have 1992 edition?

Thank you again,

Curtis

It is 16.7 in 1992, 2003 and 2011a editions.

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Hi all,

I have checked myself and can confirm that S = 16.7 ksi for SA-240 316L in the code's 1986 edition.

Thank you all,

Curtis

Curtis-

Seems you were expecting a lower allowable stress in a pre-1999 edition. That would make sense if the allowable stress was ultimate limited. But it would seem that for this stainless steel it is not limited by the design margin against ultimate. Instead it is most likely governed by a yield criteria, which did not change in 1999.

JTE,

Yes, you are 100% correct! These values do not exceed 90% of the yield strength. That's why it did not changed after 1999.

Thanks,
Curtis