ALLOWABLE STRESS SUSTAINED LOAD AND THERMAL EXPANSION

Hello guys, i have some question, i have piping system that ASTM A335 P22 for material, and it can be operate at 1305 Psi and 789 Fahrenheit in pressure and temperature design. can you show me about the calculation formula that  i have reached to solve this. BTW its a pipe for Powerplant. because my own calculation its not same with autopipe result. thankyout, appreciate it.