What material for tube and tube sheet?
How critical is seal?
How much thermal (and pressure) cycling?
In many applications rolled only is fine.
If you have a clad or overlayed tubesheet you must weld.
Acid coolers are welded because a leak is highly destructive.
And don't get me started on procedures, I can go on for a while......
Tube-to-tubesheet joints can be fully strength welded, partially strength welded, seal welded, expanded, or packed with a compression fitting. Tubes that are seal welded are normally expanded as well, but strength-welded tubes typically only require expanding to prevent crevice corrosion on the shell-side of the tubesheet.
Strength welds are typically specified to provide additional protection against leakage, but it some cases (e.g. fixed tubesheet exchangers, vertical floating-head exchangers) they can enhance the load-carrying capacity of the tubes.
Reasons for specifying welded tube-to-tubesheet joints include 1) to protect against interstream mixing of the shell-side and tube-side fluids, 2) to protect the base metal from corrosion for clad tubesheets, 3) to protect hazardous streams from leaking to the environment (first through the tubesheet joint then through a downstream gasketed joint), and 4) to enhance to load-carrying capacity of the tubes.
