JrValveEng,
The ASME B31.1 Power Plant Piping code material allowable strength had a 4:1 design factor for decades, until it was recently 'reduced' to a more economical 3.5 :1 design factor that is consistent with the same design factor in ASME Section I Fired Pressure Vessels.
The B31 Code allowable strengths are tabulated in the different Codes appendices with de-ratings for temperatures and different material grades. The B31.3 Process piping code has a 3:1 design factor. The design factor includes fatigue 7,000 cycles allowance for one complete thermal cycle per day to result in approximately 20 years of life. The piping is not designed just for the presure hoop stresses to equal the material ultimate tensile strength.
There are calculations that summarize the sustained stresses of pressure, dead weight, live loads, bending, cyclical stresses from thermal expansion, occasional loads of excess pressure or temperature variation, wind, seismic, and even wave action. Pressure design is the starting point.