A single WPS will suffice this requirement , Mr.Williamns your undestanding was correct ,
ensure consuambels are using while production welding are meeting the requirment of Gr.70 plate weding in WPS as a note.
if you want to support this PQR for pressure vessels u need SA 516 Grade 70 65 60 MTLTV to SA 516 Grade 70 65 60 MTLTV (mostly prefered combination )- it can support for all the combinations
in other case you can have one side it is SA 516 Grade 70 65 60 MTLTV and other side SA 516 Grade 65 plate for plate to pipe /flange /fitting scope (better avode this ) - some clients wants this combination also.. u can concence them with code.
Regards,
Srinivas Rao
welding Engineer