Boundary Condition at Nozzle for Piping Loads FEA (ASME Sec VIII Div 2 Part-Nozzle Shell Junction)

[Vicker85]

Hello Experts,

I have external nozzle loads provided by piping group and this needs validation by FEA (elastic analysis for now) for mechanical integrity (beyond WRC).
The piping guys have considered the actual Shell-Nozzle stiffness for arriving at the imposed loads.

My first question is -
1. When putting boundary conditions in FEA model at 'nozzle end', do we need to know the piping stiffness/flexibilities to apply that in our model OR we need to leave the nozzle end completely un-restrained (with imposed loads and applicable thrust) ?

My second question (not related to above) -
2. Do we follow the same ASME allowable stress values when checking for different modes of failures (S, S_PL, S_PS etc. as in ASME Div 2, Part 5) for the following :

(a). Design and Sustained Loads​

(b). Occasional Loads​

(c). Upset conditions like Start-up and Shutdown etc.​

Note : The loads in Occasional and Upset conditions are substantially high compared to design and sustained loads.

Thank you!

 

[FEA way]

If you have a model of the whole or larger part of the structure (not only the nozzle) then you can use submodeling technique to apply boundary conditions that prescribe displacements from the global model to the refined nozzle model. Otherwise you will have to play with values of support’s stiffnes.

 

[Vicker85]

FEA way,

Thank you for your prompt response. My query arises because I understand that if the piping group has considered the nozzle-shell flexibilities in their analysis then we do NOT need to impose any other flexibility condition at Nozzle End when applying piping loads.

What would be your opinion on that ?

Also, any guidance on question 2 in my original post ?

 

[IdanPV]

Basically, you need to satisfy each and every requirement in VIII-2, Part 5.
Table 5.2 shows the load combination and loads that need to be consider.
Table 5.3 show the stress assessment and allowable stress for each case.

(a) - Table 5.3.
(b) - Table 5.3.
(c) - Start-up and Shutdown etc. - see 5.5.6 & 5.5.2.

 

[Vicker85]

IdanPV,

Thank you for your guidance on the code related clarifications!

- Could you please suggest - under which Design Load Parameter (Table 5.2), shall we consider Occasional piping loads and Upset (Start-up/Shutdown) piping loads ?

- Also, from Table 5.3, is there no classification provided for these upset loading conditions (only Design, Operating and Pressure test conditions covered) or am I missing something here ?

From your response on Start-up/Shutdown (upset condition), I understand we need to check for Failure mode due to Cyclic Loading (Ratcheting and Fatigue) ONLY - is that correct ? By the way, this particular equipment is not operating under cyclic service conditions; in which case, I understand, we need to check for Ratcheting only.

Any guidance on Question 1 ?

 

[IdanPV]

Occasional Piping Loads - See Table 5.3, lines (6) and (14).
For the upset conditions (Start-up/Shutdown) you will have to demonstrate protection against plastic collapse (5.2.2 if using elastic-stress method) and than, you will have to do a ratcheting assessment (5.5.6).

As for Question 1, I think that "FEA way" already answered that question.

 

[Vicker85]

IdanPV,

Thank you again for your prompt response!

I now understand the Code requirements based on your suggestions.

However, for Question 1, I did see FEAway's response. But (as written earlier)- my query arises because I understand that if the piping group has considered the nozzle-shell flexibilities in their analysis then we do NOT need to impose any other flexibility condition at Nozzle End when applying piping loads.

Why do we need to apply any other restrictions or stiffness if the generated loads are already based on response of the Shell-Nozzle-Piping flexibility ?