Per API 650 section 5.7.4.2 and 5.7.4.3, certain shell nozzle penetrations require PWHT depending on material group, shell thickness, and nozzle size.  When that is the case, "openings...shall be prefabricated into the shell plate or thickened insert plate and the prefabricated assembly shall be thermally stress relieved...prior to installation."  

This requirement is pretty cut and dry on a new tank but when API 653 is consulted regarding nozzle replacement on an existing tank, you are referred to the current standard of 650.  Does that mean that if I have an existing tank that requires a shell nozzle replacement, I need to remove a larger portion of the shell(say 3'x3'), weld the nozzle into that 3'x3' insert plate, PWHT, butt weld the insert assembly into the shell, and then weld a repad onto it?  It might be easier to do that for a thickened insert plate assembly but those are not preferred in my refinery.  The section of referenced above does not seem to make an allowance for repads in the even of replacement.

Any help or opinions on the matter would be appreciated.

Thanks!

The repad is part of the assembly that is stress-relieved.

On a new tank, this would normally be a full-height section of the shell ring, not just a square cut-out.

JStephen, I understand about the application of API in the event of new construction.  That seems very clear.  If you read the section though, it states that

"...all opening connections NPS 12 or larger in nominal diameter in a shell plate or thickened insert plate more than 25 mm (1 in.) thick shall be prefabricated into the shell plate or thickened insert plate, and the prefabricated assembly shall be thermally stress-relieved..."

It does not mention a repad, only a shell plate or thickened insert. Do you see my confusion?

I'd weld the entire nozzle with repad into a shell insert plate, PWHT the whole assembly then install it in the tank.

Also, look at using a thick shell insert plate to eliminate the repad welding & testing.  Just taper the insert-plate edges to meet the shell thickness.  Will be cheaper to do it this way if you can get milling [or large lathe] work done in-house and/or cheaply.

Thanks everybody.  It does seem like the whole assembly needs to be PWHT, shell, repad, and nozzles.  They try not to use thickened inserts out here but in this case I don't think there is a way aound it since it will be a low-type repad and we have to deal with the corner weld.

I read this with interest.  I don't think everyone has replied completly to tankerator's question.

Assuming his nozzle is not a flush type.   So, if he chooses to use a repad and his shell insert plate of Group I,II,III/IIIA is less than 1" thick, and the nozzle is not larger than 12", then he does not have to PWHT the assembly.  That's my interpreation of 5.7.4.2.  
 

Unless the shell material is group IV or above, then it needs to be PWHT no matter what the size if the shell is greater than half inch.

Since he referenced "certain nozzles", I assumed the nozzle in question fell into that category.  I think the last post is off, but would have to look it up later.

If it is my last post that is "off", please see 5.7.4.3

Does API 653 allow you to reference the As-built code of construction for a replacement nozzle?  If the tank is an older tank, you might be able to get away from the PWHT by refering to the edition of API 650 that the tank was built to.

IFRS- the opening "Unless" should be "When" in your statement per 5.7.4.3.