Conical transition design

[johnnymist2003]

Hi all,

I'd greatly appreciate some specific assistance here.I would request that memebers refrain from providing "vague answers" or opinions.I have a kettle type exchanger with two fixed tubesheets.The layout is as follows : fixed tubesheet,conical transition attached directly to tubesheet hub tapering up from smaller diameter (hub inside dia) to large diameter cylinder,large diameter cylindrical shell section,conical transition tapering down again from large shell to smaller diameter at tubesheet hub, again, cone welded directly to tubesheet hub.TEMA RCB-7.3 (2) specifically lists this type of configuration as a special case.I intend to design the tubesheets to TEMA rules for fixed tubesheets, using the necessary paragraphs (no problem there).However, when I design the reinforcement of the cone/cylinder intersections, I think I need to consider expansion due to the fixed tubesheet arrangement.I was going to take the calculated tube-to-tubesheet joint load per tube (TEMA RCB-7.25) and multiply this by the number of tubes to achieve an axial load.I would then use TEMA T-4.5 to determine if this load is a tensile or compressive load.I am concerned that this may be an overly conservative approach (resulting in unnecessary increases in cone thickness).Does anyone have another proven method of calculating the axial loads for the design of the cone for this type of configuration?If more info is required, please ask.Thanks in advance.
John

 

[prex]

The use of the maximum tube-to-tubesheet joint load is really too much conservative, in my opinion: that load is valid only at the periphery of bundle (inner tubes having less) and it includes the effect of pressure.
I propose that you take the shell longitudinal stress of RCB-7.22 transforming it into a load. This can be done after observing that the formula is nothing else than P_s^*&[ignore]pi[/ignore];(D₀-t_s)²/4/(&[ignore]pi[/ignore];(D₀-t_s)t_s)
Also in that formula only the case P_s^*=-P_d should be used (assuming you have no equivalent bolting pressure).
This approach remains very conservative, as the longitudinal stiffness of the kettle is necessarily lower than that of a straight cylinder.
In determining P_d you'll need also a shell thickness: I think you can use the average thickness of cones and big shell portions of your kettle (averaged over respective lengths: this is because that thickness must represent the overall stiffness of a cylindrical shell). This again is on safe side.
Also don't forget to check the tube-to-tubesheet joint loads per RCB-7.25 calculated for a non kettle (straight cylinder) type of exchanger, but with a shell thickness taken equal to the above averaged thickness: this will represent the condition of the tubes in the lower portion of the tubesheet, that 'see' a straight cylinder wall with high thickness, hence high stiffness.

prex

http://www.xcalcs.com

Online tools for structural design

See

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[johnnymist2003]

Prex,

thank you very much for your insights. Having studied the problem a bit more since posting the question, I also "toyed" with the idea of using the the RCB-7.22 stress.I will need to think about it a bit more (also considering your insights)-but I feel a little happier knowing that I was heading in the right direction.You have also confirmed my gut-feel - that my initial intention to use the tube-tubesheet joint load was too conservative.
If you have any more thoughts on the matter, I'd appreciate the assistance.
Thank you once more.
John