Hi,
I am new to pipe stress analysis and been confused with this,
In the calculation of pipe thickness, the formula used is:

t= PD/[2(SEW+PY)]

where:
S=guaranteed minimum yield strength of the material per Apendix A of the code

Is it really safe?Because it goes that the factor of safety is around 1 with the above equation.

Also,I read somewhere that the difference between ASME B31.1 and B31.3 is the factor of safety where the first has 4 and the latter 3.Is this true?I try scanning the code but can't find this matter?

I hope you can give some of your time to share your idea on this.
Thank you

The basic allowable stress S in B31.3 is typically based on lesser of one third tensile strength and two thirds yield strength. So the 1/3 tensile at least gives a safety factor of three for S.