Per ASME Sec IX, for a Procedure Qualification for most of the standard processes, joint design is a non-essential variable, meaning that one would cover the other, however, the deletion of backing is an essential variable for a Welder Qualification.
Weldtek is correct. I believe you misinterpreted his comment.
If you qualify your welder using a double V-groove, the performance test is considered to be "with backing". Thus, your welder would not be qualified for open root groove welds that require complete joint penetration.
If the welding procedure specification is supported by a PQR that utilized a double V-groove, you could weld grooves with or without backing per the WPS. However, remember that the PQR/WPS does not consider the welder's skill, only the mechanical properties. Thus, a welder that is tested using a double groove or a groove with backing is not qualified for single sided welds where backing is not used.
You must also consider the product form used for the welder performance / qualification test. In most cases, a performance test passed using pipe qualifies the welder for pipe and plate. However, the performance test that utilizes plate does not necessarily qualify the welder for pipe.
