Flush Patch placement 5

[LittleEd]

Is is prohibited to install a flush patch across a girth seam?

Example: vessel is 10ft dia. carbon steel and has had multiple repairs at same area in circumferential girth seam weld. Repair company would like to install a 2ft x 2ft flush patch in shell at area of prior multiple repairs. Can the portion of the girth seam weld and some shell material on each side be cut-out and replaced by a single flush patch? Or would you have to have a 1ft x 2ft flush patch on each side of the girth seam forming new section of circumferential girth seam to be welded? Long seams and girth seams result in "T-joints" as would be created by the patch across the seam. When installing a nozzle in a weld seam there is NDE (RT) that must be performed on either side of the weld seam (but material is removed in that case). The NDE for this Repair will be 100% (RT or possibly UT)of the flush patch butt-welded seams.

 

[stanweld]

There is no such prohibition per NBIC 3.3.4.6.

 

[metengr]

LittleEd;
Since this is an in-service vessel you would be following either API 510 or the NBIC (NB-23). For either in-service repair code, a flush patch plate can be installed across existing shell weld seams (girth or long seam).

The flush patch requires full penetration welds and the plate should match the contour of the vessel.

Second, the Nondestructive Testing (NDT) requirements for the repair welds should follow the requirements of the original code of construction. If this is not practical or none was performed, or only partial NDT was performed based on the original code of construction, I would recommend you do the following;

1. After the flush patch plate has been tacked into position, deposit the root pass and perform a dry powder MT and visual.

2. After the weld joint deposit has reached 1/2 wall thickness, perform a dry powder MT and visual.

3. After the flush patch weld is completed and ground flush, perform a final wet fluorescent MT. This would be your final acceptance of the flush patch repair weld.

OR, if you prefer a volumetric examination, I would perform shear wave UT as final acceptance or you can RT. In either case the dry poweder MT across various points of weld deposition should be acceptable.



Run the above by your Inspector.

 

[LittleEd]

Original design specifications called for "Spot" RT (RT-3), and vessel is a large autoclave with access on both sides. It is doubtful vessel could be hydro tested in place after completing the repair, so contemplating 100% volumetric examination in lieu of pressure test. Is it permissible to perform UT exam in lieu of RT exam even though the welds to be examined are accessible? UW-51(a)(4) only stipulates wall be greater than 1/2" to perform UT in lieu of RT.

If RT must be performed, the owner would have to shutdown production as personnel working within a certain proximity of the vessel would have to stop and move out of the area? So, therefore it is more practical to perform volumetric exam using UT (shear wave).

 

[metengr]

Yes, UT is permissible with acceptance by the Inspector.

 

[Jfuego]

It is not prohibited as previously stated but you could save yourself a lot of trouble by encapsulating the area instead of removing it.

 

[LittleEd]

I have heard of "encapsulation" but not sure if that is recognized (accepted) repair method per the NBIC?

 

[metengr]

The NBIC does not endorse repair methods. Encapsulation or building a new pressure retaining boundary is not prohibited by NBIC rules. I sat through the presentation at the NBIC main committee about 5 years ago when this was presented for information only.

 

[pdiculous963]

Have you looked at ASME PCC-2? It is not a governing code, but it gives guidance for repairs on pressure vessels and piping. In terms of NDE for the seam you are crossing, ASME PCC-2 calls for full RT or UT, or MT or PT for 4 inches on both sides of the existing butt weld. Since the vessel is RT-3, I believe a MT would be sufficient for that seam.

In regards to the new patch, I would imagine that root and final MT or PT would be sufficient examination to avoid a hydro and full volumetric inspection.

Our AI recently accepted these inspection criteria on a flush patch crossing a seam in a RT-1 vessel without a hydrotest.

 

[LittleEd]

I am not familiar with ASME PCC-2. The Jurisdiction requires the use of NBIC for repairs and alterations, how do I get to ASME PCC-2 for repairs and alterations? ASME VIII-1 was original code of construction, not sure where ASME PCC-2 comes in?

 

[PMCap]

Restoration by encapsulation without removal of the degraded area is not prohibited by the NBIC per Interpretation 04-01. This method of restoration can be very cost effective as the degraded area does not have to be removed thus saving fit-up, welding, and examination installation time and avoiding entry of foreign matter into the vessel. The Encapsulation component attachment weld is typically classified as an ASME Category D weld that would not required volumetric examinations.

I suggest you search the internet using the keywords such as "asme vessel restoration by encapsulation" Let me now if you need any further help on this method of restoration for which I am extremely familiar.

 

[pdiculous963]

NBIC and PCC-2 can be used together. PCC-2 is a standard that was written to give a guideline on good engineering practice when it comes to pressure vessel repair and provide "methods for repair of equipment and piping within the scope of ASME Pressure Technology Codes and Standards after they have been placed in service." It is supposed to address repairs that are not covered in the original code book, such as an insert patch through a girth seam.

While NB-23 covers administrative and technical requirements for inspection repair and alteration for any construction code, ASME PCC-2 only provides technical methods for repair on ASME designed piping and vessels.

ASME PCC-2 can be used for technical reference on repairs under NBIC. While NBIC generally tends to tell you to repair to original code of construction, PCC-2 will give more detailed requirements for the repairs so that the repair is adequate for safe operation.

 

[LittleEd]

There must be some design work that must be done....how do you know how thick encapsulation material has to be? Would you use same thickness as original shell material? What about corner welds...do you have to create some sort of ogee type seam?

I have seen butt-weld caps used to cover pinhole leaks due to MIC in raw water (class 3) piping systems....so it is kind of the same philosophy I guess.

 

[Jfuego]

for the ones used at our plant they were thicker than the original shell - I think they were around 1 1/4". The thickness though all depends on the vessel design pressure and temp. The corners are rounded. They came with a supporting ASME code calculation - and they were U Stamp as a "PART".

 

[PMCap]

1. There must be some design work that must be done....

Encapsulation component are designed to satisfy ASME Code stress acceptance criteria. This can be done, depending on encapsulation component size and configuration, using hand calculations or finite element analysis.

2. how do you know how thick encapsulation material has to be? Would you use same thickness as original shell material?

Total thickness finished component, including material removed for the internal machined undercut, is dependent on the design pressure but an average thickness of the starting material would be in the range of 1 ¼” to 1 ½”. However, thickness of encapsulation component material where full penetration welded to shell is the same thickness as the shell material.

3. What about corner welds...do you have to create some sort of ogee type seam?

Typically the Category D Hub to shell full penetration corner weld (typically beveled at 45 or a J weld bevel) is the same thickness as the shell. This weld is typically reinforced with a fillet weld. Total weld deposition to attach and encapsulation component is generally less than that required for a Flush Patch and no removal of degraded shell material is required. Encapsulation components have no size limitations whatsoever and can be constructed to encapsulate large areas of degraded shell or nozzles – only being limited by the size of the milling machine.

4. I have seen butt-weld caps used to cover pinhole leaks due to MIC in raw water (class 3) piping systems....so it is kind of the same philosophy I guess.

They are undercut to form a hub (skirt) that results in an attachment weld configuration that is identical to that of a set-on nozzle – see Figure UW-16.1(a) of ASME Section VIII, Div. 1). Code NDE of the attachment weld would be that required for a Category D weld for which there is generally no code mandated NDE.