Fatigue Evaluation as per ASME B&PV Code Section VIII Div 1

[Abhayk]

I am designing a pressure vessel with internal and external pressure loads with ASME B&PV Code Section VIII Div 1. I wish to know following things:

1. How can I do the fatigue evaluation? Whether there is any need to qualify for fatigue if I satisfy conditions given for secondary stresses in UG 23 (e). Can I put even peak stresses also in the secondary stresses category for this purpose. Won't it render the design too conservative?

2. What are the additional constructional feature that should be implemented in case of external pressure design?

3. Whether it is correct to use Div.2 for doing Fatigue evaluation only if the vessel is not designed as per the rules of Div.2?

4. Can I follow fracture mechanics approach for fatigue evaluation? If yes then which code shall I use?

Regards
Abhay Kumar

 

[jte]

Abhayk-

1. As Div. 1 provides no mandatory rules for fatigue evaluation (other than to mandate some kind of evaluation), you can evaluate the fatigue life of the design any way you want. What you must accomplish, however, is to convince the AI that your approach is valid. This is why a Div. 2 approach is often used. An AI would have a hard time dismissing a Div. 2 approach as not reasonable.

2. Read the Code. "Features" that you'd usually see are vacuum rings and/or thicker shells, depending on economics and other design issues. For a vessel in fatigue service, I'd push for the thicker shell approach to avoid the stress caused by the rings.

3. Huh? If you have a Div. 2 vessel, it would usually be correct to use a Div. 2 approach. If you have a Div. 1 vessel, see my response to your question 1 above.

4. See my response to your question 1 above.

See also thread292-71740, and thread292-51689 and perhaps others you'll find when doing an advanced search of this site.

jt