Intermediate stiffeners in jacketed vessel ?

[Mandeep1]

Hello,

I have a question on the attachment of intermediate stiffeners in a jacketed vessel.

Lets assume we have a cylinder inner shell with a jacket (ASME Sec. VIII Div. 1 App. 9). The inner shell can experience a full vacuum. So, for the design the inner shell is subjected to an external pressure equal to the jacket pressure + 15 psig. How the intermediate stiffener is typically connected to the inner shell.

1) Is the stiffener welded to both the inner shell and the jacket? So, the stiffener extends from the outside of the inner shell to the jacket. I think in this case the stiffener would be designed only for the full vacuum.

2) Is the stiffener only welded only to the inner shell? So, the stiffener only extends partially from outside of inner shell to the jacket. Should the stiffener be analyzed for an external pressure = Jacket Pressure + 15 psig.

Stiffener should also be subjected to the requirements of the closure bar.

I would appreciate feedback on the above.

Thanks,
Mandeep Singh

 

[gr2vessels]

Mandeep,
My understanding is that you design the intermediate stiffener for vessel type 4 of Fig 9-2, as a vacuum ring to reduce the effective length requirements;- then the ring has to be designed for vacuum condition only. However, if your ring is also part of the jacket closures, then the ring is checked for the jacket pressure also, depending on the jacket design pressure / temperature conditions. Ironically, the jacket design contitions could prevail over the inner vacuum conditions.
I hope this helps,
gr2vessels

 

[Tomtation]

Hello Mandeep

The stiffener can be welded either way, but must be rigid enough to support the shell against the external pressure loads that act upon it.

For both cases the stiffener and inner shell must be designed for the combined pressures. The unsupported length of the inner shell is reduced due to the intermediate stiffener and this will reduce the required thickness of the inner shell.

Regards

Tom Timmons

 

[vesselfab]

I think that you will find, the code has no mention of vacuum, just external pressure. In the case of a jacketed vessel, a stiffener ring for external pressure should be designed for the total external pressure, which is in this case is jacket MAWP plus the 15 psi for vac.

what we do is utilize these as jacket closures as well, using wider shorter bars if possible, and gain an easier joint for jacket shell.

we try as much as possible to get any appurtence on on a vessel to serve double duty.

 

[Mandeep1]

Thanks for feedback. This is my reasoning.

If the stiffener is welded to both inner shell and the jacket, then the jacket will experience tensile load from the internal pressure and will pull the stiffener ring outward.

While, the inner shell will experience compressive stress from the jacket pressure and will pull the ring inward.

So, as long as there is no vacuum in the inner shell then stiffener ring will not experience net load on it.

That is why I was thinking that if ring extends from shell to jacket then only design for vacuum condition of the inner shell. If we design the ring for the combined pressure it may be very conservative. Space is also limited between the shell and jacket.

But, if the ring does not extend from shell to jacket then I think the combined pressure would act on the ring.

Is my thinking on track?

Mandeep

 

[doct9960]

Mandeep,

If the jacket closure bar meets the requirements of paragraph 9-5(c) of Appendix 9, then the closure bar is adequate as stiffening ring. Refer to UG-29(f).

 

[Mandeep1]

Thank you to all the posters and to doct9960 for the code reference.

Mandeep