recommended end bevel for GTAW root pass

[eneltalla]

Standard: ASME B16.25
Object of question: recommended end bevel for GTAW root pass
Context: ASME B31.1 material P92 steam pipeline (w.t. >60 mm)
Intended addressee: any expert in welding (piping)

ASME B16.26 specifies figure 6 for end bevel preparation with GTAW root pass with w.t. over 25 mm. I am a piping engineer with very little knowledge of welding and though investigating on welding manuals and books I couldn't find a proper explanation to the following:

Why is ASME B16.25 fig. 6 recommended (mandatory?) for GTAW root pass with w.t. >25mm?

Th obvious difference with other bevels is that there is a closer gap (20° as opposed to 37.5°) between facing pipes but how is this exactly related to GTAW technique and specifically to the root pass?

Further question would be inside fig. 6: how to select between fig. 6.A and 6.B?

Thanks in advance to anyone willing to reply.

Piping Engineer (Power Plants)

 

[metengr]

The bevel requirements listed in ASME B16.25 are required if this standard is being used in fabrication. The weld details in Figure 6 are those details which have been most successfully used by industry for the GTAW process on heavy wall pipe. The difference between Type A and Type B details in Figure 6 are the rounded inside corner to allow the welder flexibility in avoiding lack of fusion weld defects.

 

[eneltalla]

Thank you very much for replying.
Best Regards

Piping Engineer (Power Plants)