Fatigue Analysis in Cyclic Service 1

[weeeds]

For ASME Div. I vessels, what are the most common methods used for fatigue analysis in cyclic service?
Does the Code specify anywhere a preferded analysis method?
Thank you

 

[eliebl]

Section VIII Div 1 does not address fatigue. It is essentially covered under U-2(g).

In the cases that I was familiar with we used the old Division 2 to do the fatigue analysis, called for a few special weld details meeting the Div 2 radii and also required that small nozzles be calculated and did not per UG-36(c)(3)(a) to be applied.

EJL

 

[LSThill]

widla (Mechanical)


Review API-ASME FFS-1

 

[weeeds]

Does anyone know which components usually control the number of cycles that a vessel is good for, and why?
Also, I have been told that fatigue analysis is more an art than a science. Does this make sense?

 

[eliebl]

If you use the old Div 2 approach applied to a div 1 vessel it will typically be the inside radius at the nozzle connection due to the larger thickness of shell and nozzle required to meet the Div 1 rules.

As I understand the fatigue rules in the old Div 2 may have not been conservative in some cases. Fatigue life can be affected by the small details, the type of corrosion being one of them. I have not applied the two methods for calculating fatigue life provided in the new Div 2 but I believe that it will give different results.

Cyclic service requires a thorough understanding of the entire pressure excursions of the vessels, including the expected hydrostatic tests throughout the lifetime of the vessel.

I hope that I am being helpful.
Regards,
EJL

 

[jrjones]

You can use the new Section VIII Div. 2 to perform a more detailed fatigue assessment of a Div. 1 vessel. The new S8D2 (along with the new API 579-1/ASME FFS-1) cover off the fatigue design better especially for welds where the structural stress method is used.

As for the controling part...depends on the vessel type and the loading (thermal cycles, pressure cycles, etc). Most of the assessments that I have done have been on vessels with significant hot to cold cycling atthe cold to hot wall transitions (nozzles). Other than nozzles, highly constrained internal attachments may cause problems.

Anyway, hope that adds to the discussion.

jrjones

 

[weeeds]

Thank you eliebl and jrjones. Your input is enlightening.
Sure seems to agree with one of my colleagues who described the process as "more art than science".