Never, ever weld on the outside world portion of a vacuum situation!!!! Live by this maxim and you are way ahead of the game. All welds should be internal only!!
The pros never weld on the outside unless ordered to by someone holding their mother and children hostage.
The obvious reason is no interior gas traps and better control over the weld which in these cases is almost always a flush weld. (easy)
On the outside, you have to dance around rising tubes, worry about punch through on thin sections, work thick cross sections needlessly, etc.
Remember the key word is "weld lip".
All vacuum joints should come to this. All flanges have them in place already for common sizes and diameters. Flush mount flanges must have a weld lip machined out of the base face the thinkness of the vessel.
You will never injure the mating surfaces as all flanges are welded internally well away from this lip.
Still, if some non-vacuum weld clown does your work, impress upon him that the mating surfaces and knife edges are crucial,and if damaged or scratched by him in his vise to hold the work, he will owe you a flange.
Richard Hull
Created on Tuesday, May 08, 2001 11:27 AM EDT by Richard Hull