Post Weld Heat Treatment

[etrobal]

we need to install (asap!) a tee connection. however, the tee welds will require pwht (as the thickness is 25.4mm) but we don't have the time as the plant shutdown cannot be extended.

what we did is that we tapered the ends of the tee connection (three ends) to a thickness where pwht will not be needed and proceeded with the weld connection. needless to say, we did not do any pwht on the three welds.

is the tapering of the thickness to forego the pwht allowed? with the tapered butt ends of the tee, the heat affected zone (haz) is thus reduced to a thickness where pwht is no longer needed. am i correct on this haz?

the resulting tapered thickness was 12.5mm or half an inch while the length of the taper from all butt ends was approximately 50mm.

is there a code or a standard that says so? is this good engineering practice? what might be the consequences of our action?

our design pressure is 2.14 mpag while design temperature is 216degC. the tee size is 30-inch diameter with a wall thickness of 25.4mm.

 

[davefitz]

I suggest you have a qualified design engineer determine the required minimum wall thickness for the tee weld end based on nat'l board documented design pressure and temperature for that specific piping section of that specific plant.

You cannot arbitrarily reduce the wall thickness based on avoiding PWHT, the wall thickness wa provided for the purpose of keeping the fluid inside of the pipe .

While these observations may seem obvious, it often occurs that the Code permits use of a wallthickness much less tahn implied by allowable stress considerations , as in cases of butt welding P91 piping to a F22 weld end fitting or valve- and I the same reservations about that loophole as I do with Etrobal's approach.

 

[stanweld]

What is the thickness of the mating pipe?

 

[etrobal]

sir stanweld, the remaining thickness of the taper was 0.625 of an inch to match the mating pipe wall thickness of 0.625 of an inch. my earlier report of a tapered thickness of half an inch was not correct.

the shutdown was such in a short notice with short duration, so we made do with the available tee material, a 30-inch straight tee with a wall thickness of 25.4mm.

am afraid the taper would affect not only the integrity of the double longitudinal welds of the tee but also the structure of the tee itself.

thanks again sir stan. appreciate your replies indeed.

etrobal

 

[stanweld]

In short the weld thickness is 5/8"; PWHT is not required under B31.3 or B31.1.

 

[stanweld]

Tees require reinforcement at the branch opening. The thickness of that reinforcement is based on the diameter of the branch and pressure rating. That reinforcement does not extend to the ends at the butt welds to the mating pipe. I would recommend that the long seams of the tee be examined by MT after tapering (tapered length only).