Shell repair 1

[nickelkid]

Has anyone had experience removing a nozzle from an ASME Section VII Div I vessel and patching the shell?

My thoughts are proposing rolling plate to match shell radius/thickness and have a full penetration weld with 100% radiography to the Authorized Inspector.

 

[metengr]

robinchandy;
Yes. The repair you speak of would be called a flush patch plate (referenced in the National Board Inspection Code, NB 23). Just make sure you use equivalent material in addition to the contour and shell thickness. Also, if you use a rectangular plate, make sure you have rounded corners to reduce stress concentration. Follow Section VIII, Div 1 requirements for preheat, and PWHT requirements.

 

[lostsailor]

robinchandy,
Consult the National Board Inspection Code and contact the National Board to obtain of copy of the Manufacturers Data Report, if you do not have it.
As this will be considered an alteration, your AI is going to want to see the data report, ASME code calculations supporting proposed alteration, material test reports for the patch plate, WPS/PQR, alteration drawing, and a hydrostatic test procedure or proposed NDE in lieu of.
Patching a shell isn't uncommon, your basic premise is sound.
Regards,
RLS

 

[deanc]

One hint: If your vessel wall is not thick and you do not want a sunken spot in the wall,cut your patch just a little large and dome it about half the material thickness. Then grind/cut for your root pass spacing. The weld shrink will pull it flat.

 

[metengr]

lostsailor;
Regarding alteration, yes, Appendix 6-3000, (d) refers to a change in contour or dimensions of a pressure retaining item. However, in the content of adversely affecting the pressure retaining capability, which is normally the approach where one would change the shell thickness or OD or head geometry or adding a nozzle or appurtance that would increase stress concentration, I would argue removing a nozzle would serve to strengthen the vessel, and should be considered part of a flush patch plate repair. I am not sure this would necessarily be an alteration to the vessel because one is not introducing an opening that would constitute a zone of weakness (like adding a larger nozzle) that would adversely affect the pressure retaining capability. Nevertheless, the AI would make this final call.

 

[deanc]

metengr: Never thought of it that way...makes good sense. Will be useful,another star.

Another hint for robinchandy, If you do a square patch,use a drill or hole saw in the corners of your vessel cut out. Then finish cutting the straight lines,torch/grinder/gouge.

 

[metengr]

deanc;
We are in the midst of re-structuring and revising the NBIC into 3 parts - Inspection, Installation and Repair/Alterations that will be published next December. I don't want to give away my alias on this web site, but this is a major undertaking for the main committee.

However, your response has prompted me to consider as an action item for the next meeting to propose to revise the current example (d) of an alteration in Appendix 6-3000. If you have a chance, I would encourage you to attend the next NBIC meeting in Tucson, AZ.

 

[lostsailor]

metengr,
Points well taken.
Regards,
RLS

 

[robsalv]

RobinChandy, we've used the flush plate patch repair method several times and works a treat.

We had no choice the first time we employed it when the vessel fabricator put a nozzle in 180degrees incorrectly... right where some auxilary structure had to be. D'oh.

As it was our first, we moved forward fairly cautiously.

The nozzle was removed. The resulting hole was enlargened to remove heat effect zone. The patch plate was rolled from the same plate material to suit the curvature. We went ballistic on the NDE. Our steps were: UT parent plate for laminations, weld prep edge MT (both plate and vessel), a lowish stress LH weld procedure (notionally lots of preheat over an extended area and slow cool down), root pass MT and MT, 100% RT after final capping pass, followed by blend grinding and further MT inside and out. We didn't notice much suckback or plate distortion.

Today, it's hard to find the patch insert unless you know exactly where to look.

Cheers

Rob