It's not exactly a flange that's welded on a vessel. The flange is welded on a piece of pipe, which in turn is welded to the vessel wall, in which a hole was made to accomodate the pipe. In the majority of cases, there's also a round reinforcing plate around the pipe to compensate for the loss of strength that the hole has produced on the vessel wall.
The weld between the pipe and the vessel wall is called nozzle weld.
Giovanni S. Crisi
Sao Paulo - Brazil