Is there any published standards for straight-pattern valve body flange face parallelism? When flanges are welded to a butt weld glove valve body, there is no assurance of inherent parallelism as there would be if the facings were machined on a cast or forged body.
I would apply the same tolerance criteria as in Code for normal flange alignment tolerance.  If you are mating up two flange then the allowable misalignment should be the same whether welded to pipe or if one of them is on a valve.  Actually, I'd think there would be a tougher standard for the valve but if not then I'd default to your piping code. How about Para 335 of B31.3 Item (c) "before bolting up, flange faces shall be aligned to the design plane within 1 mm in 200 mm measured across any diameter; flange bolt holes shall be aligned within 3 mm maximum offset".
If I have a perfectly straight and true piping installation and the flanges on the valve were such that it could not be dropped into a square and true set of piping flanges, within the tolerance specified in B31.3, I would reject it.

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