PWHT

[sksmechanical]

After inspecting a vessel (tower) it was decided that the vendor was required to remove a downcomer bar and re-install in another location. The tower was already post-weld heat treated. shall the vendor be required to do another post weld heat treat after this alteration? Their A.I. has already signed off on the order which states that no PWHT is required.

 

[stanweld]

It depends on the material, thickness of the weld and service (Lethal) or other SCC environment.

 

[vesselfab]

from a code issue....normally if welds are small and proper preheat is maintained....then normally no PWHT

if there is a service issue as noted above then it may be required.

This is all up to the end user.

 

[sksmechanical]

material is 516-70
weld sizes are 1/4" and 5/16"
lethal service

 

[stanweld]

Review Table UCS-56 Note (2)(c). Based on your description, PWHT would not be required.

 

[sksmechanical]

i have read the exemptions and have a follow up question....would internal welding of downcomers for trays be considered a nonpressure part?

 

[stanweld]

If the downcomers are not pressure retaining, they can be considered non-pressure parts.

 

[vesselfab]

This is not considered a pressure containing weld.

but....lethal service requires PWHT per UW-2
but....UCS-56 says it is not mandantory for these size welds if welding is done per UCS-56 and procedure.


BUT....when dealing with service requirements and PWHT...
It is always better to let the end user make the final decision. If it's just one downcomer bar, a local PWHT is not that expesive and if it makes the customer happy....then everbody is happy.

 

[sksmechanical]

Good info. Thanks to all for the help

 

[jamesl]

Check out code interpreations VIII-77-104 and VIII-1-98-108. You will see additional PWHT is not required after minor weld repairs on lethal service vessels as long as the repairs are done per notes in UCS-56.

 

[rneill]

I think the right answer depends on the specific service environment and if this is a service environment where PWHT is required in order to prevent possible cracking (e.g., sour service). In such a service, the end user may well want PWHT regardless of code requirements (and may have a spec that requires it).