Dear all,
I have a question regarding fluid condition at outlet of a safety valve.
If we have saturated steam at the relief condition (44 barg) at the inlet of safety valve which is reliving to ATM (PSV back pressure of 2,5 barg), based on API 520 throat/orifice size will be calculated considering isentropic path (100% isentropic, I guess to simplify the calculation). In reality, we are not 100% isentropic.
To estimate the fluid condition at the outlet of safety valves (outlet flange), I have seen three different approaches:
1. From inlet (point 1) to outlet (point 3) -> fully isentropic. Since this approach will result in 2-phase flow at discharge, it is considered as worst case considering the momentum of 50,000 for 2-phase flow in the pipe;
2. From inlet (point 1) to throat (point 2) -> fully isentropic AND from throat (point 2) to outlet flange (point 3) isenthalpic. This approach also leads to 2-phase flow however the amount of liquid will be less compared to approach 1;
3. From Inlet (point 1) to outlet flange (point 3) -> fully isenthalpic. This approach results in single phase (superheated steam) at the outlet.
Unfortunately, I couldn't find any reference in API 520 explaining which approach should be used to estimate the fluid condition at the outlet flange.
I would appreciate if you share thoughts on this matter or if you know any reference which I can use.