Nozzle fatigue analysis for external loads as per ASME VIII Div.2 Ed.2021, Annex 3F

[pvros]

thread794-336380
Dear All,
I am reviewing a fatigue analysis of a vessel with an agitator on the top nozzle which is subjected to external loads fatigue (same situation of the quoted thread).
No pressure or temperature fatigue is expected.
Vessel is analyze with a FEA considering the the external cyclic loads like static, P=0, T=0. Temperature corrected stress amplitude is considered, max stress on welds are multiplied by the correct penalty factor as per table 5.13 and endurance limit is guaranteed.

My "issue" is that with this analysis you are just considering the cyclic stress range, but you are completely ignoring the mean stress (no pressure and temperature) which is much higher that what evaluated by the FEA.

Protection against ratcheting is also verified considering stress given only by cyclic external loads, again ignoring the higher max stress given by the static pressure and temperature + external loads.

Is the fatigue analysis as per annex 3f the correct approach for the described problem?
If yes, at least local failure shall be evaluated considering overall loads?

Can someone provide me some literature reference specifically related to fatigue analysis in ASME/EN code?

Thanks,

 

[TGS4]

As discussed in ASME PTB-1, there is a mean-stress correction factor built-in to the fatigue curves.

Certainly, check Protection Against Plastic Collapse. And demonstrate Protection Against Local Failure. Likewise with buckling.

As as I mentioned in the referenced thread, make sure that the welds are handled appropriately.

 

[DriveMeNuts]

I assume that the agitator cyclically operates with steady state pressure and temperature greater than zero.
Annex 3f doesn't have an approach. It just has fatigue curves.
Are you using the smooth bar fatigue curve? If so, margin has been implicitly added to the curve to take care of high mean stress, cyclic stress.

 

[pvros]

Thanks both, answers are clear.

To sum up:
- Agitator cyclically operates with steady state pressure and temperature greater than zero.
- FEA for stress evaluation has been modeled considering agitator mechanical cyclic loads and null static loads.
- Smooth bar fatigue curve has been used
- Fatigue curves incorporates a mean-stress correction factor to consider the mean stress given by the static load ignored in the FEA
- Temperature and welds have been properly analyzed considering adequate correction/penalty factor