PSV Model with sonic flow
PSV Model with sonic flow
(OP)
I've been giving a model of PSV, it's connection sizes, orifice size, flow rate, and set pressure. I'm attempting to model the discharge piping in AFT Arrow.
This particular PSV has a 3" outlet and is rated for 18,780 lb/hr of steam with a set pressure of 205psig. When it's run with just 3" piping discharged to atmosphere, there's sonic choking at the exit and the pressure drop is greater than 10% of the set pressure, so I tried to increase the piping to 4".
From what I've read online, this is called expansion choking, and just seems to make the situation worse in the model (increases pressure drop and seems to exacerbate sonic choking).
Any recommendations on how to eliminate sonic choking or reduce the pressure drop across the discharge piping so it's < 10%?
Thanks in advance.
This particular PSV has a 3" outlet and is rated for 18,780 lb/hr of steam with a set pressure of 205psig. When it's run with just 3" piping discharged to atmosphere, there's sonic choking at the exit and the pressure drop is greater than 10% of the set pressure, so I tried to increase the piping to 4".
From what I've read online, this is called expansion choking, and just seems to make the situation worse in the model (increases pressure drop and seems to exacerbate sonic choking).
Any recommendations on how to eliminate sonic choking or reduce the pressure drop across the discharge piping so it's < 10%?
Thanks in advance.





RE: PSV Model with sonic flow
a) assume a pressure at PSV outlet
b) model the PSV to calculate the conditions (temperature, density, speed of sound) at PSV outlet (I use PRODE PROPERTIES which includes a specific procedure to model a PSV)
c) calculate a line diameter for a mach number about 0.5
d) verify pressure drop and density in different sections of piping to avoid mach numbers > 0.7 , if required (mach number > 0.7) increase line size.
RE: PSV Model with sonic flow
It doesn´t matter to have choking velocity at the end of the discharge pipe if there is enough capacity to discharge the required flow.
casflo
RE: PSV Model with sonic flow
differently you may follow a procedure as that described by apetri
RE: PSV Model with sonic flow
I think I got my model sorted out. I didn't stop to think the placement of fittings could have such a large impact, but putting two elbows on the 4" section of piping after the area change made things much better.
RE: PSV Model with sonic flow
RE: PSV Model with sonic flow