Dear friends
I would like to add further add to Mr.Mandeep Singh's reply that increasing the tubesheet thickness result in decreased deformation (deflection or slope) at the junction and thus the resulting shear force / bending moment (arising out of compatibility consideration) decreases. That is why the shell stress in the junction decreases.
With regards
_________________________
R.SRINIVASAN
Senior VICE PRESIDENT,
Head DESIGN and Engineering
ISGEC
YAMUNANAGAR
HARYANA INDIA