B31.4 is the code for the construction of liquid pipelines. B31.8 is for gas pipelines. The welding and welder qualification portions of these reference ASME Section IX and API 1104. If the contractor elects to use ASME Section IX for qualification of welding, welding procedures and welder qualifications the accept/reject weld dimensions and inclusion criteria for qualification of welders in Section IX would be used. Also the WPS and PQR's would need to follow ASME Section IX. Field welding would be tested and inspected as per ASME Section V
It is unusual for a pipeline contractor to use Section IX for pipeline, unless the piping is going to be used as station/terminal piping regulated by 49 CFR part 195. 49 CFR 192 covers gas pipelines.