Increase Design Pressure Tube Side for External FV 4

[Mich_A]

Hello all

I'd like to ask you one particular point that the NoBo pointed out in a design of an hairpin heat exchanger, defined by the following data:

Calculation Code: ASME VIII Div.1 Edition 2023
Design Pressure SS/TS [barg]: 6.7 + FV / 6.9
Design Temperature [°C]: 111 / 111
Materials: Carbon Steel
MDMT [°C]: -12

The inspector asked to provide design calculation increasing the design pressure (tube side) by full vacuum shell side.
In ASME VIII Div.1 I haven't found any indication about that request. Could you please give me any tips about your experience or any indication if that is applicable?

Thank you in advance.

 

[iangineer]

Typically exchangers are designed for full differential pressure meaning that the tubes can handle the full external pressure of 6.7 barg on the shell side without any pressure inside, and full internal pressure of 6.9 barg with no pressure on the shell side. I haven't heard a request like that before, but it sounds like the inspector wants you to increase the design pressure to ~7.9 barg to account for shell side full vacuum. I'm not sure if that's what he wants, but Appendix 9 might have some information for pressure on the tubeside with full vacuum on the shell. It has to do with jacketed vessels.

 

[r6155]

The Inspector is correct.

Regards

 

[Christine74]

Shell & Tube heat exchangers have certain components (tubes, tubesheets, and floating head covers) that see pressure (or vacuum) from both the shell side and the tube side and need to be designed accordingly.

In some cases these pressures can be additive, for example if the shell side of the exchanger operates at a vacuum while simultaneously the tube side of the exchanger operates under positive pressure. In other situations these pressures can be subtractive, for example when you have a Feed/Effluent exchanger where the shell-side fluid exists and then gets routed through an exothermic reactor to be heated rerouted back through the tube side. In this system if there are no valves that can close, the tubes and tubesheets only need to be designed to support the maximum differential pressure between the shell side and the tube side, which would be equal to the maximum pressure drop through the system.

To summarize it's necessary for the engineer to understand all of the potential operating scenarios for the heat exchanger and identify them on the data sheet.


-Christine

 

[NRP99]

(1) Design requirements for shell-and-tube heat exchangers in Section VIII, Division 2, Part 4.18 shall be used in lieu of those previously listed in Part UHX. Table UHX-1.1 lists the new locations for all requirements formerly located in this Division.
(2) The Section VIII, Division 1 design requirements listed in Table UHX-1.2 shall be used in lieu of the corresponding Section VIII, Division 2 design requirements referenced in Part 4.18.

Refer Table 4.18.7/8/9 or you can consider the table 4.18.5 as guideline summary of the heat exchanger load cases. -1barg(FV on shell side/external pressure case) and 6.9barg(design pressure on tube side) is one such load case. IMO but not sure, this is what inspector might have suggested.

 

[Mich_A]

Thank you so much all! The inspector wanted exactly to increase the tube side pressure to 7.9 (design pressure + FV shell side), I've revised the design and effectively, as all of you suggested, that is a combination case that I should have considered before.
Thank you so much another time.