Flare header have pressure at 1 barg and set pressure at 41 barg for relief LPG from pipeline. Dischage line have 2" length 500 m. pls recommend me to calculate back pressure.

Thank you

Start at the flare tip where you know the termination pressure and calculate backwards to the PSV outlet flange. Use the gas flow equations, adiabatic or isothermal, your choice. I usually use the Isothermal gas flow equation because it's simplier.