Hi
A couple of points here:
1) ASME IX does not give acceptance criteria for production welds, only for procedure and performance qualification. The acceptance criteria for production welding needs to be found in the referencing code. In the case of ASME IX that could be B31.3 for piping, or ASME VIII for PV's or B31.8 for pipelines. Interestingly, the acceptance criteria for NDT to B31.8 is API1104! Given that API1104 and B31.8 both deal with pipelines, the fact that they have the same acceptance criteria is probably not so crazy.
2) Because different codes have different assumptions built in, and also have different levels of engineering, (e.g. pipeline codes often require ECA's) it is not that surprising that they have different acceptance criteria for defects.
3) LOF is permitted by B31.3 in certain circumstances. (Mostly not.)
What we need to remember is that the principle job of the welding / fabrication team is to ensure that the design intent of the designer (and by implication the design code) is implemented during fabrication. Different design codes = different design intent.
Regards
Niekie