Thickness of Rectangular Tubesheet

I see that nobody reacts since no method is listed in the Wikipedia so that I try to give you a hint.

You should consider the square tube as a frame with rounded corners and determine the bending moments as well at corners as in the middle of the "beams".

This is valid quite far away from the frontal plates if you have any.

In the neighborhood of a wall to which the tube is welded you should use the equations from Timoshenco for long plates on all four side clamped. This because f the high inertia of the tube walls in their width (not thickness).

If it is not clear enough ask what you need for better understanding.

Complete details of design, fabrication, inspection and testing for this type of HX are contained in API-661.

You can get more information here:

http://www.api.org/publications-standards-and-statistics/publications-updates/publication-updates/new-refining-publications/api_std_661

No, I will not send you a free copy....

Sorry I did not look right my hints concerned a square TUBE only.