I had come across an ASME WPS (no notch toughness reqs) with "no pass shall exceed 1/2" thickness for SMAW, GTAW, GMAW or FCAW" and was just wondering if that was a requirement per Sec IX or any other Code or Standard that I was aware of.
ASME IX thickness ranges are restricted when you deposit any pass thicker than 1/2", so if they want to take advantage of the full ASME range (normally 2t) then they need to place this restriction on the WPS. You should see that statement on almost all ASME IX WPS's.
QW403.9 addresses the 1/2" Pass as referenced as an essential variable in QW-253 which limits the WPS to 1.1 Times the base metal thickness qualified on the PQR as opposed to 2 Times the thickness.
403.9 which limits the thickness of single pass weld is only mentioned for SMAW, SAW, GMAW-P. But not for GTAW process, 403.9 is not mentioned as essential parameter, does it means we can deposit more than 13mm Single pass weld with GTAW process??
You are permitted to deposit a single pass thicker than 1/2 inch with any welding process if you are willing to qualify the procedure to demonstrate you can do it and meet pass all the requisite test requirements.
In this case, when welding with the GTAW, it is not an essential variable, thus you are free to deposit a single pass weld without the need to demonstrate the ability to pass all the tests needed to qualify the WPS. However, it is unlikely one would be able to do so.