Looking for PWHT exemption for butter pad on P5 pipe.

[timsch]

I have an existing ASME CODE-U stamped vessel that I need to weld an Alloy 625 butter pad on 4” Sch.80 P5 pipe using GTAW for later welding onto them tubeskin temperature sensors.

Referring to ASME BPVC SEC. VIII, DIV.1 table UCS-56-4, general note (b)(1), PWHT is not required for circumferential butt welds in pipe. Could this also apply to a buttering layer? If not, is there another exception I may take to performing PWHT? Is looking at this section irrelevant since this is not new construction?

 

[DVWE]

There are two options that I'm aware of:

1. ASME PCC-2, which will refer you to the original code of construction for PWHT requirements, so VIII, Div. 1 is not irrelevant.

2. If the vessel is going to be R-stamped, then it will fall under the umbrella of NBIC. I believe NBIC only covers P5A materials exempt from PWHT, and the WPS used must be qualified as a temper bead procedure.

The devil is in the details; she also wears prada.

 

[timsch]

Revisiting this, The P5 pipes are 4" Sch.80, with a nominal wall thickness of 0.337". ASME BPVC SEC VII, DIV.1 requirements for thickness given our process conditions is around 1/8", so less than nominal thickness.

The pipes in question are in the heater fixture, which was Code-U stamped. It is used solely for testing of temperature sensors, which must be welded to the OD surface pipes. The heat transfer fluid inside the pipe is limited to 800F before it starts to degrade from heat, which poses a problem for PWHT @ 1200F (fluid cannot feasibly be evacuated between each test).

I've read nothing definitive regarding the depth of the HAZ for welds such as these (i.e. surface welds of up to 1/4" fillet, roughly), but have seen references that for TIG welding, it could be up to 2mm. If I remove 2mm from the wall thickness of my pipe, I'm still left with sufficient thickness to pass my pressure calculations.

Is there any leeway in this predicament regarding the PWHT?