ASME DIV 1 & B31.3 Factors Of Safety

[stryker1080]

Hello,

Was just wondering if someone could help clarify something for me. Can someone help provide me the details as to where the various factors of safety come from?

In B31.3/Div 1, people in general would say that you have a factor of safety of 3/3.5 for post 1999, and 4.0 for pre-99.

Lets look at two different examples.

1) For A 106 Gr. B seamless Pipe.

For A 106 Gr B from B31.3, we got a min yield of 30,000 psi and allowable of 20,000 psi. Usually it is lower of 1/3 ultimate, or 2/3 yield at lower temperatures. This gives us a factor of safety of 3/2 or 1.5
For A 106 Gr B ASME Div 1, we got a min yield of 35,000 psi and allowable of 17,1000 psi. This gives a factor of safety of 2.0.

Where does the rest of the factor of safety come in? I believe by using the allowable stress in B31.3, the piping is actually designed for 7,000 cycles. And for fatigue alone the FOS was 2? So using B31.3 does that give us 1.5*2 = 3? What about ASME Div 1?

The actual design calculations don't give you any additional factor of safety. Can someone please pop in and help me out a bit to understand? Did a quick search and did not find the answer.

 

[XL83NL]

Review B31.3 302.3.2(d) (not sure where in VIII-1 it's located). I believe you got it wrong;

Usually it is lower of 1/3 ultimate, or 2/3 yield at lower temperatures. This gives us a factor of safety of 3/2 or 1.5

Please explain why you're doubling the reasoning on safety factors.

Also see this thread.

 

[TGS4]

XL83NL - for VIII-1, it's located in Section II, Part D, Appendix 1.

I agree with the question to the OP - what's your real question? There are different failure modes, and each failure mode has a design margin. Please don't mix up plastic collapse with fatigue with other failure modes.