Is the fatigue analysis required ?

[jtseng]

I have a Div 1 vessel(1 1/4Cr-1/2Mo material) in cyclic service, with pressure and temperature flutuated at every 48 hours (600psi-100psi-600psi with 100degF-600degF-100 degF). It will have 3650 cycles for 20 years.

I try to use Div 2, AD-160.2, Condition B & Appendix 5 to see if the fatigue analysis is required. Can I substitue Sm in Condition B with the actual calculated stress intensity to get a higher allowable cycles and higher allowable range of pressure to waive the fatigue analysis ? In other words, can I increase the thickness more than required so it has lower stress intensity to avoid the fatigue analysis ?

Jeff

 

[JoeTank]

Fatigue is driven by the applied loads and resulting stresses. Reducing the loads or stress or both are effective solutions. So your thoughts about increasing thickness to reduce stress is of course one of the possible solutions. I have used this appproach frequently. But, I have encountered a case or two wherein that will not work for cyclic loads due to thermal cycling.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com