Dec 2014, AutoPIPE V8i 09.06.01.10
I understand the value for the allowable displacement stress range within ASME b31.3 to be sa = f(1.25sc + 0.25sh). if my piping has the same high temperature 640c everywhere, and the same ambient temperature 20c everywhere, why is the computed allowable displacement stress range (allowable expansion stress value) different along the pipe?
There are 3 things that would control the displacement stress allowable:
1. Tools> Model Options> Results> Sustain Margin (Y/N/E)note: See AutoPIPE online help for information on this feature.
2. From the Model Consistency check report the following warnings are displayed:
W2143-121 Sc > 20 ksi (138 MPa). In calculation Sc will be set to 20 ksi (138 MPa) for expansion allow for pipe(s)
W2143-123 Sh > 20 ksi (138 MPa). In calculation Sh will be set to 20 ksi (138 MPa) for expansion allow at following point(s)
3. Different material properties - confirm same material properties are being used for piping to have the same allowable calculation.
AutoPIPE uses the expansion Allowable Stress Range calculation equation 1A. However, whenever accounting for sustain load margin. Both Sh and Sl are taken from the Sustain stress results. Another words, equation 1B is not used in AutoPIPE, use equation 1A with the requirements as mentioned in the paragraph between equation 1A and 1B.
ASME B31.3: 2018 SA = f(1.25Sc + ((0.25Sh)+(Shsus – SLsus)))
ASME B31.3 Piping Code Calculation Issues