Explain why is it not possible to split the shell in 3 parts? And why it is failing in strength at shell/channel joint?

Example: 

Case 1: When we use single shell with 25mm thickness, there is no error.

Case 2: When we split the shell in three parts with 25mm thickness at near both tubesheets and  20mm shell thickness at middle shell component we are getting below error.

Answer: 

Note that, in case we have the different shell thicknesses in the heat exchanger then Shell axial stiffness and Shell-Tube Rigidity coefficients will change and that will affect the stresses in the integrated shell/Tubesheet as per ASME Section VIII Div.1, UHX-13.

Code Reference:

Lengths of shell of thickness t s ,1 adjacent to tubesheets :

Shell axial stiffness & Rigidity Stiffness: 

Stress in the integrated shell/Tubesheet : 

In Case 1 stresses are within the Allowable limit but in Case 2 allowable stresses are exceeding the allowable limit hence you are facing the error.

To avoid the error you need to limit the stresses by changing the design parameters.