Stress Nozzle - thickness of reinforcing pad

[Matrix_93]

Hello,
I was studying the nozzle junction stresss in coincident of external PAD outside diameter and I have seen something I don't understand.
I have a cylinder NPS 24 thk. 10 mm with a nozzle opening of 400 outside diameter thk. 14 mm by plate.
Around the opening I have a reinforcing pad O.D. = 560 mm x 14 mm. The opening is at the centre of the vessel (Lenght = 4000 mm).
Nozzle has internal pressure and external loads (forces and moments) as per specification.
With FEA analysis It seems that a PAD of 10 mm thickness leads to smaller stresses at Outside pad diameter junction compared to pad thickness of 14 mm.
In my opinion this could depends on because with a pad of 10 mm thickness, nozzle is more deformable than 14 mm, is it right?

 

[TGS4]

When you say "smaller stresses", are you referring to peak stresses from a FEA, or are you talking about linearized membrane or membrane-plus-bending stresses?

 

[JStephen]

I can visualize that a repad of infinite stiffness, ie, rigid plug, could give you higher stresses in the shell at the edge of the repad than a flexible repad, so it doesn't seem unreasonable in that regard.

 

[r6155]

And the Code is?
Why not a reducing outlet tee NPS 24 x 24 x 16? (ASME B16.9 2018)

Regards

 

[DriveMeNuts]

Reinforcing Pads are suitable for nozzle diameters less than 50% of the cylinder diameter.
Putting a Re-Pad around your huge nozzle is possible but doesn't sound great. The stresses start to get unpredictable even with FEM, especially with external applied loads.
A cleaner more predictable option would be to increase the thickness of the cylinder and nozzle and make it an integrally welded nozzle (i.e. no Re-Pad) or even better follow r6155's advice of using a Tee.

 

[r6155]

See ASME VIII Div.1 Appendix 1 1-7 LARGE OPENINGS IN CYLINDRICAL AND CONICAL SHELLS

Regards

 

[KevinNZ]

Yes will often see higher stress in the shell at the edge of a thick pad. The change in stiffness between the pad+shell to just shell is a stress riser.