Design Factor for ASME Section VIII Div. 2

[Meck91]

The design factor for ASME Section VIII Div. 2 is 3. But when we try to back calculate this factor of safety by dividing specified min. yield stress with the allowable stress for low to moderate temperature range, the actual ratio comes as 2.82, 3.08, 2.97 etc. not exactly 3. So it means that we rounded-off the figure to nearest number or is there any other reason for these random numbers for factor of safety.
I also read that this number was previously 4 and revised to 3 in near past.

Any thoughts?

Thanks,

Meck91

 

[TGS4]

What year are you dealing with? Since the 2007 Edition of VIII-2, the design margins have been the lesser of:
a) 2.4 on room temperature minimum specified tensile strength
b) 1.5 on minimum specified yield strength
c) 1.5 on yield strength at design temperature

If you are the creep regime, then additional factors apply.

Prior to the 2007 Edition, a) was 3.0 - ALL previous Editions of VIII-2 had the above-noted values.

For additional reference, please review ASME Section II, Part D, Appendix 10.