Our vacuum tower was 8 meters in diameter. The problematic weld appeared on a circumferential weld, i.e. perpendicular to the tower height and was longer than 2 meters. I don't remember exactly how much, but about one fourth of the tower perimeter.
The need of a new WPS and PQR was discussed with the client and finally abandoned. So, no new WPS nor PQR were written. The fact that the client was in a hurry to have his refinery operating worked in our favor.
I see no problem with NDE. Our tower had spot RT testing. The client required two radiographs instead of one on the repaired section and both were OK. Of course, special care was taken when buttering. Cleanliness, grinding, leaving the buttered surface smooth etc. deserved special attention.
As I told you, neither us nor the client thought of backing strip. Since the beginning, our discussion was centered in buttering and no one even mentioned backing strip.
This fact happened some 40 years ago, when GMAW was almost unknown in South America. Welding was performed by SMAW. Tower material was ASTM A-285 GrC. Thickness was 3/4 inches. First pass was given with E 7010 and subsequent ones with E 7015. For the buttering operation only E 7015 was used.

Any other question?
Giovanni S. Crisi