NBIC - Maximum external weld build up

[stryker1080]

Hello,

Does anyone know the basis for the maximum external weld build up limitations.

Per NBIC 3.3.4.l:

For each repair, the maximum dimension compensated by a circular or oval weld build up shall not exceed the lesser of 1/4 the nominal outside diameter or the component of 8 in. The length of a rectangular patch is not limited.

Per ASME PCC-2: It has a similar statement as above, but a rectangular patch is not exempt from the size limitations.

Does anyone know if there is a technical basis for this criteria or was it just arbitrarily assigned?

We have some external corrosion underneath a saddle support, which is not accessible to build up externally. Plan is to do a weld build up from the inside and use the NBIC criteria (our province follows NBIC).

 

[metengr]

Yes, quite familiar with external weld build-up. Part 3, 3.3.4.3 (l) was inserted by the NBIC SC R&A to prevent one from re-building a complete circumference of a pressure retaining item by weld overlay.

 

[stryker1080]

Why is there limitation in size for oval or circular weld overlay, and no limit for rectangular?

 

[metengr]

Again, we were concerned with weld restoration of an entire circumference of a pressure retaining item. We selected 25% as a way to avoid this.

 

[KLee777]

Metengr,

I do not understand your above post regarding the entire circumference limitation. How does a circular/oval overlay on the shell of a pressure vessel have anything to do with restoring the circumference? When I think of a "rectangular" overlay, I think of one that "follows" the circumferential weld lines. When I think of an oval/circular restoration, I think of an oval/circular area on the shell of a vessel that would, for instance, be overlaid for local metal thinning.

What am I missing?

 

[metengr]

@KLee777
What you are missing is that the committee felt strongly that we did not want to re-build or reconstruct an entire pressure vessel circumference (360 degrees) or other geometry using only a repair weld overlay.

 

[jte]

Per ASME PCC-2: It has a similar statement as above, but a rectangular patch is not exempt from the size limitations.

I'm curious as to what the basis for this statement is?

Though I agree with the NBIC sentiment, I think the flexibility allowed by PCC-2 Article 2.2 is beneficial. It requires sound engineering judgment and probably involving multiple technical disciplines, but in theory one could perform a full 360° weld buildup. I don't think that I've ever seen any serious proposal which would be anywhere near that dimension, but on smaller diameter vessels - and particularly piping - I might be persuaded to endorse a design for a 120° to perhaps even a 180° buildup.

I think this highlights a basic difference in philosophy in the NB committee and the Post Construction Committee. Note that "different" does not imply "better" or "worse". The NB has a more focused scope (vessels) and can be - and is - more prescriptive in its rules. The PCC, and specifically the Subcommittee for Repair and Testing which is responsible for PCC-2, has a broader scope and is less prescriptive ("shall") and more good guidance ("should") oriented with more freedom given to the engineer to customize a solution to a particular problem. The NB had a representative on the PCC-SCRT for a while, but he left a few years ago and has not been replaced. This is unfortunate... It would be good for PCC-SCRT to have NB input on PCC issues on a routine basis. The added perspective would be valued.