Section VIII, Division 1, UHX Fixed tubesheet - Tube weight

[jmcy21]

Ref: ASME Section VIII, Division 1, Part UHX - Current 2021 code rules

I am hoping someone can provide some insight into the theory behind UHX as it pertains to design of fixed tubesheets in shell and tube heat exchangers - in particular that it does or does not consider the weight of the tubes in the calculations. UHX-10(e) states "These rules do not cover weight loadings..." I believe this statement is saying among other weights, tube weight is not considered. If so, why is that? All shell and tube HX's have tube dead weight, so its seems odd that the rules do not consider tube weight.

Consider a vertical fixed tubesheet heat exchanger with a tube total weight of 100,000lbs and a shell diameter of 8ft. Design and operating conditions are such that the required tubesheet thickness per UHX is quite thin (3/8" or less). This seems unreasonable if consideration of the tube weight is factored in. Adjusting the design pressure to include an equivalent for the tube weight results in an increased tubesheet thickness, but still thin (1/2" or less), but yet the practice of adding an equivalent pressure for the tube weight seems a reasonable approach.

UHX also has a note that tubesheet deflection, in particular when the thickness is less than the diameter of the tube, should be considered. For the example mentioned, I believe the tubesheet deflection will control in this case and therefore require what I consider a more reasonable tubesheet thickness for the given diameter. I am working on methods related to calculation of tubesheet deflection and have researched some documents out there to help, but find it to be scarce.

The whole topic is odd to me though in that UHX provides rules for design of tubesheets, but does not consider tube weight nor provide guidance on tubesheet deflection - both of which must be considered by engineers performing tubesheet design. How many actually do these additional calculations? PV design software that I am aware of does not include any guidance, discussion, or disclaimer. It is surprising that there are no published rules out there, ASME or otherwise, that provide guidance. I feel like I am missing something. These two topics seem to be skimmed over in UHX as well as any documents I have found out there.

If there is anyone out there with expertise on the theory behind UHX rules and is willing to provide some insight, I would greatly appreciate it. I apologize for the long winded post. Thank you for any help you can offer!

 

[BJI]

I would suggest you get a copy of AS 3857, it is an alternative method for the design of tubeplates. Quite similar in the calculations but gives you more flexibility in the application. You can directly extract stresses and displacements at any radial distance across the tubesheet.

A lot of the TEMA tubesheets have/had rotation issues under differential pressure, this is not as bad for the UHX designed tubesheets but can still cause an issue for joint integrity if not adequately considered. Weight may also be influential for vertical exchangers, as you mentioned.

 

[jmcy21]

Thank you BJI. I will look into this standard to see if it provides any insight.

 

[TGS4]

If you are looking for insight into the UHX rules, you need to review ASME PTB-7 - Criteria for Shell-and-Tube Heat Exchangers According to Part UHX of ASME Section VIII Division 1.