Pipe Length Affecting Design Thickness 3

[JordanB13]

Hi all,

I am looking into designing a stabilizer tower for a process application using ASME VIII Div 1 (first time doing so) and have a curiosity question regarding the correlation between design thickness for a shell and the length of said shell.

Some info:
Design pressure is 200PSI@400F, 24" OD, Sa-333-gr6, .118" CA, 50' tall, Full radiography, tower is split into a top and bottom section at 120" from bottom.

My concern is when I run the bottom shell (~120") versus the top shell (~480") the design thickness for the shells differ (.2755" versus .3322") and the only thing I can accredit this to is the length of the top shell.

So i'm assuming that Compress is taking into account the length of the top shell, but if I were to do this calculation by hand I would end up with a min. thickness much less than what Compress is requiring.

I guess my ultimate question is at what point does Compress start to increase the required thickness of the shell?
Is there somewhere in code that requires the length of the shell at some point to be taken into consideration?
Am I potentially just missing something in general?

Any info is appreciated, if I have forgotten to mention a factor that requires more information please let me know and I can provide more info.
Any feedback is welcome.
Thank you!

 

[Afox88]

Couple of things it can be:
- Length of the shell is a factor in external pressure calculations. SO if you have full vacuum, it would affect the thickness.
- Also, if you set the liquid level of the vessel to be the top of the tower, the corresponding weight would translate to a pressure that is highest at the bottom. This extra pressure would require a thicker shell.

 

[Afox88]

I just opened compress and ran this case with the information you provided. It is most definitely the external pressure because I also came up with exactly 0.3322" as well. If you want reduce the thickness so that you can use Std-Wt pipe, try using a vacuum ring (stiffener ring) or two to get the thickness down.

 

[JordanB13]

Afox88, you are correct it is most definitely the external pressure. Thanks for the quick reply i knew i was overlooking something very simple. Again much appreciated!

 

[DandB]

With the "Cylinder Dimensions" dialog open, you may consider enabling the "Details >>" tab at the bottom. This will reveal thickness requirements due to internal "t(int)" or external "t(ext)" pressure.

 

[JordanB13]

DandB, as always the Details tab is very useful, and 90% of the time I do run everything with it open. The issue for me is I overlooked this as I usually don't end up designing towers, mostly just scrubbers and suction discharge bottles. But you are correct, the "t(ext)" is very clearly shown as the contributing factor to my issue here.
Thanks for the input!