any idea on prevent/minimize the distortion of shell(duplex) roundness during welding nozzle to shell?
For example: 1) put support.

With stainless steel vessels, distortion from nozzle penetrations is always an issue.

Our shop fabrication practice is to use smaller dia (GMAW or FCAW) wire, and use a welding sequence (skip or backstep) that minimizes heat input. Letting the weldment cool between passes will also help control distortion.

Supports or temporary stiffeners have be tried to no avail, besides we are concerned about residual stresses resulting from such.

Sorry I'm not much help,
Robert