If I used a flued opening and then butt welded a pipe to the flange end, would the attached pipe need UG-45 calculations? It wouldn't be a rated part. Here's a picture to better show my situation:
The attached/welded pipe to the flued opening is just a normal 'nozzle neck', hence this first weld is part of pressure vessel design and is under jurisdiction of your design code, including the nozzle neck thickness (your pipe wall thickness). Also, part of the pressure vessel code is the next weld if you attach a flange, otherwise it will be a piping weld if you attach another pipe.