Questions: 

See output report, design calculation, Isolated Opening,  in operation Int.P., "Required thickness of the nozzle neck UG-45" per ASME BPVC VIII, Div I. Appears to be a mistake, variable Cav (Shell Corrosion + tolerance) is indicated in the equations when it should be Can (Nozzle Corrosion + tolerance):

Example beloiw:

Answer:

AutoPIPE Vessel calculations are correct as indicated above. 

Please have a look at actual code ASME BPVC.VIII.1, UG-45 NOZZLE NECK THICKNESS, tug-45, paying attention to the written definitions for Ta, Tb1, Tb2, and Tb3. In this case, Tb2 clearly says  "required for pressure (assuming E = 1.0) for the shell". 

Thus the equation is correct and should be using Cav and not Can.