Loading History for ASME Sec. VIII Div. 2 Fatigue Screening

[Int22]

Hello,

As I work through the screening criteria for fatigue outlined in Section 5.5 of ASME Sec. VIII, Div. 2 I am wondering whether I should be considering the life of the vessel or a given period. I am familiar with remaining life assessments of flaws, but I have never screened for fatigue while designing before. Additionally, as I do not have any actual operating data, I am finding myself making some very conservative assumptions. Would it be reasonable to assume a time period of 1 inspection cycle (say 5 years)?

 

[TGS4]

You definitely need to consider the life of the vessel. As far as inspection intervals go, the facility that the vessel will be installed in will have a well-defined turn-around schedule. Base it on that.

 

[NRP99]

Normally assuming 2 startup and shutdown cycle/year is conservative. That becomes then 25 years of life*2=50 cycles besides any pressure-temperature or any other load transients. But if you have continuously working vessels like those in refineries, their startup-shutdown cycles will be limited and hence designed rigorously for load transients.

You can make conservative assumptions initially and finalize after you have data backing.

 

[davefitz]

An alternative approach to the fatigue damage issue is in EN 12952-3 annexes B and C. If actual operating data is avaialble then the better estimate for fatigue life damage is by cycle counting using the rainflow cycle counting algorithm. Likewise a lower stress concentration factor can be used if the welded joints are modeled using 3D finite elements and design efforts are expended in selecting the best weld geometry which would lower the local stress concentration factor. Nozzle reinforcement, full penetration welds , and improved surveilance of all welded surfaces can lead to a much longer fatigue life, and newly available robotic welders become valued investments.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick