Not sure where else to post this question, but I know a lot of pipe welders pass thru here so here goes;

We are reworking some spool pieces by removing flanges and replacing with like kind, typically will be A105 flange onto A106 pipe or onto B16.9 elbows and tee's; applicable Code is B31.1; joints will be x-ray quality; pipe sizes are ranging from 3"NPS to 6"NPS and are XS.

In cases where we work into a Tee or Elbow, sometimes we cannot remove all of the previous weld metal or else we would be too short (as in the case of a flange welded directly to the tee or elbow). We are removing just enough of the previous weld metal to maintain dimension and allow us to cut a new bevel.

I am concerned that because we can't remove all the previous weld that we may inherit pre-existing flaws. Additionally we have no history of the inspection of the prehious weld or the fillers used, we can only assume it to be a correct 70xx filler and that they passed x-ray because the spools were in service for several years.

QUESTION: Are there any known Code rules that I can't find that disallow me from welding over existing unknown weld?