Pressure drop across a relief valve
Pressure drop across a relief valve
(OP)
Hello,
Could someone clarify what is the process occuring across the RV orifice: some times hundreds of bars are dropping across the RV, to what kind of energy this process is transformed? is it friction (heat)?
regards,
roker
Could someone clarify what is the process occuring across the RV orifice: some times hundreds of bars are dropping across the RV, to what kind of energy this process is transformed? is it friction (heat)?
regards,
roker





RE: Pressure drop across a relief valve
RE: Pressure drop across a relief valve
Being isentropic, it is usually treated as frictionless.
Good luck,
Latexman
RE: Pressure drop across a relief valve
Good luck,
Latexman
RE: Pressure drop across a relief valve
Thank you, as I understand the DP across the PSV is converted to kinetic energy at the PSV outlet and than to friction loss on the outlet piping to the flare.
What disturbs me is that a DP of sometims hundreds of bars is acomplished on a "disk" of a few milimiters.
regards,
roker
RE: Pressure drop across a relief valve
To find the conditions in the PSV "nozzle throat", you do a isentropic flash from the inlet.
To find the PSV outlet (tail pipe) conditions, you do a isenthalpic flash from the inlet.
RE: Pressure drop across a relief valve
Thanks. I should have said, RVs are modelled, NOT as an orifice, but, as a flow nozzle.
Good luck,
Latexman
RE: Pressure drop across a relief valve
I'm wondering what you mean by "a DP of sometims hundreds of bars is acomplished on a 'disk' of a few milimiters."
The disk of a PSV sits on top of the full nozzle and has the spring force pushing it down so it opens at the "set pressure". Most disks have a diameter larger than the nozzle diameter and, possibly a lip at the outside diameter making what is called a "huddling chamber". Once open, the pressure and flow of the fluid act on this larger area to fully open the PSV. This is what "pops" the valve open and keeps it open until the pressure is somewhat less than set pressure.
But, I don't believe the thickness or diameter of the disk comes into play with determining the pressure drop. Yes, it has to be thick enough to have the mechanical strength it needs. The majority of the pressure drop comes from the fluid no longer being confined and expanding.
Good luck,
Latexman
RE: Pressure drop across a relief valve
Correct me if I'm wrong but by flash did you mean expansion?
Latexman,
It will be helpful to understand at the end of the relief process what is the product of the energy release.
regards,
roker
RE: Pressure drop across a relief valve
Good luck,
Latexman
RE: Pressure drop across a relief valve
I have written a FAQ on the issue of what is the process occurring across the PSV. It is a derivation of the numerical integration technique I use for sizing PSVs. It is FAQ1203-1293 I have found that if you understand the derivation of a solution method, you most likely understand the problem and it's solution. Take a look.
Good luck,
Latexman