Nozzle Design: Set-On vs Set-Through

[PVPEngr]

Here is an old familiar question on nozzle design: "Set-on" or "Set through"? Both are permitted by PV codes, but most company specs do not allow "set-on" design except for small nozzles. Can anyone advise what the pros and cons for both designs? What are we giving up in choosing "set-on" nozzles?

There was a thread in the Piping and Fluid Engineering Forum in 2003 on the same topic (

http://www.eng-tips.com/viewthread.cfm?qid=71234),

but I would like to see if there are any new advices from this forum. Thanks,

 

[codeeng]

Generally a set-thro will result in a stronger joint as the shell thk will be > nozzle wall. Any laminations in the shell (higher stressed part) would be exposed to service with a set-on. Repad welding detail is superior with set-thro. Double welding and NDE is probably more accessible with set-thro.

 

[juancito]

This subject is something I faced recently.
Design specification from a spaniard engineering company preferred the set-on design meanwhile the design spec. of a huge brazilean pretroleoum company forbides the set-on nozzles!!! (always speakig of nozzles <1.5in)
I believe the fear is the possibility that the nozzles falls in an area of the plate affected by lamellar-tearing. If this is the case, you can just UT the area and thats it. Set-on nozzles requires less welding and the hole preparation in the shell is faster that the set-trhough.

 

[PVPEngr]

Thank you codeeng and juancito. The spec I was given only allow set-on nozzles for nozzle size 2 inch and less AND only with parent metal ultrasonically tested. I believe the reason is exactly as you had mentioned - avoid the problem of lamination or lamellar tearing of the parent metal. I would think set-on design can be allowed for larger nozzles if similar UT is performed.

 

[jproj]

Just my 2 cents...

Fabricators like to use set through nozzles because ASME code allows you to use the internal nozzle projection in the nozzle reinforcement calcs. Set on nozzles therefore, will require either repads or a thicker nozzle wall in order to give the same amount of strength.

From a cost perspective, you may also want to think again about set-on nozzles, especially if UT is required. UT is really expensive compared to the costs of adding extra metal or weld.

Hope this helps.

Jproj

 

[pve5]

I would like to comment as follows :

In case thick vessel wall, attachment of small nozzle with set through type, requirement of FPW is difficult and therefore set on type is better option from fabrication point of view. However in case FPW is not required , then set through type can be also used.