Pressure Vessel Repair based on ASME VIII

[virjil]

We have a thinned down pressure vessel and as an immediate repair, we have installed a patch plate on the thinned down portion. Does ASME allow this repair method for a long term? Or shall I recommend sectional replacement? Where in the ASME code is this topic discussed? By the way, the operating conditions of the vessel is about 4 psig and 350 deg C.

 

[JohnBreen]

The ASME Boiler and Pressure Vessel Code does not address repairs to inservice vessels. You must go to the National Board Inspection Code (NB-23) to guide you in your repair. You would be wise to check with your local jurisdiction (State, Commonwealth or Province, Chief Inspector) before you do anything. If you do it wrong (technique or legal procedure) you could have a vessel that it is unlawful to operate.

Regards, John

 

[JohnBreen]

Where in the ASME code is this topic discussed?

Go here for the discussions on boilers and pressure vessels:

Forums > Mechanical Engineers > Activities
Boiler and Pressure Vessel engineering Forum

Regards, John

 

[metengr]

Just a note, the National Board Inspection Code (NBIC) does not permit external patch plates (see RD 2060). The proper repair is to either weld overlay or use a flush patch plate. For an in-service vessel, this work should be performed by a contractor that has a valid National Board Certificate of Authorization to repair boiler/pressure vessels.

 

[deanc]

If I read your post right----you are playing with fire!

A "scab patch" or surface patch plate is worse then having a hole in the vessel. If you hole thru under the scab you have made a new problem area. The pressure will extend to a large surface area that may be weaker then your vessel wall. Depending on what you have in this vessel it could let go when the fillet welds break and the contents under the patch flash.

People die from this kind of thing.

 

[waskillywabbit]

Is this even relevant since his vessel is operating at 4 psig? Was it designed to 15 psig or greater? Is it even ASME code stamped/designed now?

I concur w/ deanc, you are playing w/ fire.

Brian

 

[JoeTank]

No sure the "playing with fire" warning isn't premature at this point. Properly designed lap patches are permitted in API-510 and API-653. The 4 psi rating makes me less concerned.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[deanc]

SteveBraune

4psi yes-maybe not so much of a problem. 350 C-ouch!--whats in the tank?

Why take a chance? Still bad practice.

 

[panduru]

@virjil:
Did you really mean four psig? Just to make sure.

 

[JoeTank]

deanc,
Must confess. The 350C didn't sink in. At that temp, i would expect insulation, so delta issue might not be too bad.

I agree that lap patches could be poor practice, especially if mis-applied to a high hoop stress application. But, recognized Codes and Standards permit them when designed properly. Yeah, the high temp issues would still concern me on this one.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[stanweld]

In addition to the previous coomentary, I would not recommend scab patches as a long term repair for highly corroded areas. When corrosion breaks through the original pressure part, one should expect crevice corrosion to accelerate corrosion of the patch. This is one of the reasons why I have seen tanks with scab patch on scab patch.

 

[JoeTank]

Stanweld,
Why not put scab on side of wall that is corroding? Would that soothe your concerns any?

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[stanweld]

Steve,
That could work and has often been used to protect against errosion/corrosion. One may also wish to weld build up the corroded area and or apply a corrosion resistant barrier? The scab patches that I referenced have been on the outside surface and have been done so for the sake of expediency and initial lower cost so as not to require draining/cleaning the tank. Tank may remain in service during repair.

 

[deanc]

SteveBraune

Yes,I agree with you that in limited applications a lap patch could be OK. Water tank,oil tank,something that could not be drained and properly fixed. Small hole---maybe a coupling per UW-16(f)????

Back in the days of rivets,what else could one do? They were however required to be applied from the inside.

It would be a good idea if used,calc. for thickness,use plug or slot welds to attach and the outside fillet only to seal.

The big problem is that scab patches are misused,just sloppy work and are dangerous in many cases. Had to cut too many of them out and do a proper fix in the past.