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# PSV crtical vs. sub sonic

## PSV crtical vs. sub sonic

(OP)
Hello all,

I am recent graduate engineer. I am currently working on the PSV sizing project. I want to understand between the crtical vs. subsonic flow calculation while sizing PSV. When you have critical flow or sub-sonic flow,what is actually hapening?

Do i have to worry about critical flow and sub-sonic flow both for a gas and liquid ( oil and water)?

My problem
Line coming from 3 phase seperator ( natural gas, water and oil) at 1000 psig. Water and oil goes to water and oil tank which is at atmospheric pressure. Th oil and water line has choke, pipe and fittings. The fittings are only good for 285 psig that is why want to install a relief valve to protect flanges.
I am trying to use Promax to simulate what orifice size PSV. I want to set PSV at 285 psig + 10% 0f 285= 313 psig
On promax i input pressure and flowrate, by the time the flow gets to the point where i want to install my PSV, the pressure is 313- great. However the problem is we want to relief this PSV in the tank which contian water. However, i do not understand is that can orifice of PSV reduce so much pressure which is from 313 to atomsperic when it relief back to tank?

Choke is 0.5"
Flowate IS 70700 LB/HR
iNLET PESSURE = 1000 PSIG

Thank you

### RE: PSV crtical vs. sub sonic

Critical flow means the velocity is sonic/at the speed of sound/Mach 1/choked at the outlet of the PSV's flow nozzle. It's not really an orifice; it's modeled as an isentropic nozzle.

Subsonic flow means the velocity is less than sonic/at the speed of sound/Mach 1/choked at the outlet of the PSV's flow nozzle.

Uploading a P&ID, PFD or sketch would be very helpful understanding the various flows and pressures you mentioned.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

(OP)
Latexman,

### RE: PSV crtical vs. sub sonic

It looks like you are on the right track. Scenarios are dump valve fails open with liquid flow and dump valve fails open with gas flow. Use API 520/521 as Dejan said to size flow area for both cases, and pick the larger one. Size inlet (3% rule) and outlet (10% rule) for PSV's rated flow (which will probably be > 70,700 pph). Attach your calcs and maybe someone will check them.

Be sure the vent on atmospheric tank can handle the possible reliefs without the backpressure exceeding it's design pressure.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

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