Easy,
I am assuming that this is on a power plant. It would depend on the WPS. Most P91 WPS's I have seen and used require purging. There are now companies qualifying P91 procedures without purging. ( mainly using the Lincoln STT process or the Miller Pipe Pro). You would need to view the WPS. If the WPS requires purging then you have to purge. ASME Sec. IX QW 408 says that purging is an essential variable. You or the QC can't just deside not to do it, because it's "just a waste of time and money". ( In the big picture argon is cheap) building purging dams or using purge bladders is easy.  If it is required and your not doing it your violating that WPS. If something was to happen and the weld failed and it is proved that the company did not follow their own WPS ( even if the failure was not tied to not purging) then they are in a bind. If it is a power plant and this weld is in the code boundry of Sec I or B31.1 BEP then their in a bind with the A.I.
Jim

Below is not ammo but may give you some ideas about what you are "shooting at". I amy be missing something so someone else feel free to step in if I missed something required by the code.

The ASME Boiler and Pressure Vessel B31.1 code does not:

1) Address purging at all
2) Indicate that oxidation at the root is a "defect"

ASME Sec IX does not

1) Require purging of any material, it only indicates some instances in which requalification is required if it is ommitted.
2) Indicate root oxidation as a defect
3) Indicate that P number 5 B requres requalification of the procedure if backing gas is ommitted. QW 404.9 Essential says

For groove welds in P-No. 41 through
P-No. 49 and all welds of P-No. 10I, P-No. 10J,
P-No. 10K, P-No. 51 through P-No. 53, and P-No. 61
through P-No. 62 metals, the deletion of backing gas or a
change in the nominal composition of the backing gas from
an inert gas to a mixture including non-inert gas(es).

This does NOT mean that purging is not a good idea. That is an engineering decision that would have to be made by a qualified party. IN addition customer specs may require purging. And as stated above, if the WPS says purge, then purge. Check with the QC guy :) Or better yet, let him look in a root that you do WITHOUT purge. Root contour will be harder to control without purge.

Gerald

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