If the welders are working for the company you represent, then you should have the procedures. B31.3 qualify welders to ASME Section IX. If the welders are working for a contractor, then they have to be qualified by that companies procedures. IN that case, all you would do is witness the test. The test would need to be performed under the supervision and direction of the contractor's QC. You would review the WPS and PQR's and check the test coupons to be sure the material is right. But you would not sign the test papers for that welder unless you work for the contractor.
BABRT's
fertilizer plant.. the temtation to mention the farm code was inescapable. With that out of the way, a quote from B31.3 2006.
B31.1 under scope:
300.1.3 Exclusions. This Code excludes the following:
(a) piping systems designed for internal gage pressures
at or above zero but less than 105 kPa (15 psi),
provided the fluid handled is nonflammable, nontoxic,
and not damaging to human tissues as defined in 300.2,
and its design temperature is from −29°C (−20°F)
through 186°C (366°F)
If your going to have an AI to begin with, most likely it will be B31.3 etc, however; B31.3 has specific criteria for application. Something you may need to be aware of in the future.