ASME VIII div 2 part 5.6 Nozzle necks 1

[JVT_Stress]

Hello,

Concerning classification of stresses according to ASME VIII div2 part 5.6 "Supplemental requirements for stress classification in nozzle necks", I have read this part 5.6 multiple times but but i'm still find this part very confusing.

When i model nozzles in my FEA software, and performing elastic analysis with shell elements (R/t > 4) for protection against plastic collapse: (I'm also considereing ratcheting but I leave this out of the discussion for now)
I only model the cylindrical shell /nozzle interface (cylinder/cylinder with sufficient length, including repad if requierd) and apply the calculated nozzle loads from pipe stress software (as described in table 5.3). And additionally apply pressure thrust on the shell and nozzle.

I understand the classifiication of stresses in the shell according to part 5.2.2.2,
(with only load controlled loads considered for the shell for protection against plastic coollapse), I typically use the calculated midplane (avarage) VM stress results as PL near the nozzle and the Top/bottom VM stress of the shell elements as the Pl+Pb stress (or Pl+Q if bending stress is purely from pressure load and discontinuity between shell/ nozzle).

However, For nozzle necks,
within the limits of reinforcement a Pm classification is used for stresses "resulting from pressure-induced general membrane stresses as wel as stresses, other then discontinuity stresses, due to external loads and moments including those attributable to restrained free end discplacements of the attached pipe."
And a Pl classification for stresses "derived from discontinuity effects plus primary bending equivallent stress due to combined pressure and external loads and moments including those attributable to restrained free end discplacements of teh attached pipe."

If I understand correctly a different meaning is given to the Pm adn Pl classification whitin the limits of reinforcemnts in the nozzle neck, as comaperd to the shell?

So my approach with my FEA software for the evaluation of stresses in the nozzle neck would be as follows:
Include all nozzle loads (load controlled aswell as displacement controlled loads) on the nozzle.
-use the calculated midplane (average) VM stress of the elements within the limits of reinforcement of the nozzle neck and classify these stresses as Pm
-compare Pm < S requirment and this almost always fails, so i use elastic plastic-analysis to assess the stresses further.

Is my understanding correct, because i'm always ending up with elastic-plastic analysis for evaluation of stresses in nozzle necks. (when it is not possible to make the nozzle thicker, for ex: during the evaluation of existing pressure vessels)

Best regards,

JVT_stress

 

[TGS4]

A quick rule-of-thumb: if the Code assigns a stress categorization of Pm, then you should be performing a hand calculation. Form the equivalent stress from a hoop stress of P*R/t, a longitudinal stress of P*R/(2*t) + F/A + M/Z, a through-thickness stress of -0.5*P and the shear stresses.

Your FEA will also include the discontinuity stresses, the membrane of which will be compared to Pl - just like you would in the shell.

Your approach is excessively conservative.

 

[JVT_Stress]

Thank you very much for your answer, this makes completely sense now.

Indeed, we can calculate Pm stresses by hand.
I should have known this answer, it was lingering in the back of my head but by reading your post the pieces of the puzzle fall togheter.

Best regards,

JVT_Stress