3 replies to this topic

[LeoLeo]

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.

 

 

 

 

[latexman]

How about, from inlet (point 1) to throat (point 2) -> fully isentropic AND from throat (point 2) to outlet flange (point 3) constant stagnation temperature/enthalpy. At least that’s what Shapiro and NASA recommend to use across a normal shock wave.

https://www.grc.nasa...ane/normal.html

[breizh]

Hi,

You may wat to take a look at documents attached.

Breizh

[snickster]

I agree it is basically a reversible isentropic expansion in the nozzle of the relief valve, and across the relief valve an irreversible constant stagnation enthalpy (not constant enthalpy because velocity changes) expansion process where stagnation temperature remains constant but stagnation pressure is reduced, and velocity decreases.  On the downstream of the relief valve the pressure is decreased to a pressure equal to the pipe end of line exit pressure (considering the end of line exit pressure can be above the end of line discharge pressure if sonic flow exists at that point) plus pressure drop back to the relief valve, velocity is decreased from sonic velocity in the orifice (assuming velocity on downstream is lower than the sonic velocity in the orifice) and temperature is increased due to the decrease in velocity.