PWHT After a Repair to a Boiler Superheater Header

[Sharik]

We have a new ASME Section I boiler under construction and have found an issue with the Secondary Superheater Outlet Header. One of the top support lugs was installed in the wrong location. The superheater header is a P5B material, 1.5" thick, 18" ID and was PWHT'd at header fabrication. We'd like to weld a new support lug/pad to the top of the header shell, at the correct location. The attachment fillet welds are much greater than 1/2" throat so PWHT would be required. The Contractor is proposing to use a 'donut' heating pad around the lug/pad.

Is this acceptable? How can this conform to the ASME Section I, Paragraph UG-40 where local PWHT must be done with a circumferential band all around the shell? How could they possibly do a circumferential band around the header shell when the bottom half of the header is all tube stub attachments?

Wouldn't it be better to use a high preheat and a temper-bead welding technique?

 

[metengr]

No. You must follow the rules of Section I until it is stamped and completed. Until that time, there is local PWHT permitted b y Section I, and that is a local circumferential band around the header. Temper bead is NOT permitted for materials other than P-No 1 and P-No 3 base materials. Please review Part PW-39.

 

[Sharik]

I know the requirements of PW-39 for a circumferential heating band, but how can this be done on a header where the bottom half of the header is all tube stubs? The way I read PW-39.3, the heating band would have to be width of the pad, including the fillet welds, plus 2" on either side (my typo in the original posting, the header is actually 2.5" thick). That would take us half way through some of the tube connections at the bottom of the header. So the way I see it, they would have to go 2" beyond the outer part of the affected tube stub welds. Correct? The tube ligaments are about 150 mm.

Where are the P1 and P3 limitations for temperbead welding? The only place I can find these limitations is in ASME Section VIII, Div. 1, UCS-56. There are no P number limitations indicated in ASME Section I or ASME B31.3, that I could find.

 

[metengr]

Are you sure you know the requirements in PW-39? First, the circumferential band would envelope the stub tubes on a finished header. You will need to ensure adequate thermal expansion and support during PWHT.

Second, please re-read PW-40 REPAIR OF DEFECTS, most important below

PW-40.3 Defects in P-No. 1, all groups (if not otherwise
exempted by Table PW-39-1), and P‐No. 3 Group
Nos. 1 and 2 materials, and in the welds joining these materials,
may be weld repaired after the final PWHT but
prior to the final hydrostatic test. The welded repairs shall
meet the requirements below.

The requirement for temper bead is listed in PW-40.3.4

 

[Sharik]

Metengr, I'm not trying to be argumentative but trying to understand the requirements. This boiler was designed and is being fabricated to ASME Section I, 2010 Edition with the 2011 Addendum. In this Code, PW-40.3 only indicates P3, Group 1 and 2 materials. I do not see where there are any limitations to using a temperbead welding technique for repairs involving P5B materials.

For the required width of the circumferential band, I remember completing alterations to pressure vessels in the past where we added nozzles to the shells of PWHT'd vessels. A circumferential band was used to PWHT the nozzle attachments. The width of the band included the nozzle and several inches beyond the edges of the nozzle attachment welds, depending on the shell thickness. If there was another nozzle in the shell, on a slightly different plane, the width of the band would be increased to include the other nozzle(s) and beyond. At times the band would be quite wide and temporary supports would be required.

My concern with the superheater header is, when we do the circumferential band around the shell, including all affected tube stub attachments at the bottom of the header, the band ends up being very close, but not including adjacent tube stub attachments. Is this going to cause issues with these tube stub attachments just outside the required band width? Should the width of the band be greatly increased so we can control the gradients better? Does anyone have experience performing local PWHT on boiler headers (waterwall, superheater, reheater, etc.)?

 

[metengr]

Sharik

In this Code, PW-40.3 only indicates P3, Group 1 and 2 materials. I do not see where there are any limitations to using a temperbead welding technique for repairs involving P5B materials.

No problem. First, temper bead repairs outlined in PW-40 applies only to P-No 1 and P-No 3 base materials because this is the introduction to PW-40. All of the rest of the paragraphs in PW-40 deal with specific repair requirements using welding again only applicable to P-No 1 and P-No 3 base materials.

I can assure you temper bead repairs under Section I cannot be applied to P-No 4 or P-No. 5 A or B or C base materials, period. I had to struggle to re-write the temper bead for PW-40 for P-No 1 and P-No 3 base materials as a TPM for Section I.

Since temper bead repairs are not permitted for materials above P-No 3, you have little choice other than using the circumferential band method. And yes, you can increase the band width to accommodate what you need.