Valve flow with given Cv and Inlet Pressure
Valve flow with given Cv and Inlet Pressure
(OP)
I am trying to determine the flow rate of gas (Methane T=80degF SG=0.6) for a valve that I have selected for my system. I know the inlet pressure (10psig) and the valve Cv (22.5). Looking at the flow equations for gas, it seems I have to first determine if the flow will be critical (choked) or non-critical. Though, to determine this I would first need to know the pressure drop across the valve - which I do not know because I do not know what the flow will be. Is there a way to determine flow of a gas based solely of the inlet pressure and valve Cv?





RE: Valve flow with given Cv and Inlet Pressure
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Valve flow with given Cv and Inlet Pressure
Orifice Size (in): 1.25
Cv Flow Factor: 22.5
RE: Valve flow with given Cv and Inlet Pressure
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Valve flow with given Cv and Inlet Pressure
RE: Valve flow with given Cv and Inlet Pressure
RE: Valve flow with given Cv and Inlet Pressure
RE: Valve flow with given Cv and Inlet Pressure
GPSA = Gas Processors Suppliers Association
You may also find this procedure in the Fisher Control Valve Handbook ?
RE: Valve flow with given Cv and Inlet Pressure
Looking at this section of the GPSA, I would still need to know the pressure differential across the valve to size the valve.
RE: Valve flow with given Cv and Inlet Pressure
No. With 0 dP, you have no flow. With atmospheric pressure on the outlet, the dP is 10 psi and you have one flow.
With a full vacuum on the outlet, you would have choked flow likely through the valve and have the maximum flow possible through the valve.
RE: Valve flow with given Cv and Inlet Pressure
If P2 > 0.5 x P1, then flow is dependant on both P1 and P2.
If you dont know what P2 is, there may be clues to this by looking at the system downstream of the control valve ( you may need to work this out by trial and error).
If you still cannot figure this out, post a P&I D with a short process description of this system in this thread, and we will try to help.
RE: Valve flow with given Cv and Inlet Pressure
One way to get an idea is to try to set in 'guesstimate' ideas of max, minimum and avarage values and pre-calculate the flow data, adjust values and recalculate until you have an acceptable approximation of what you want.
RE: Valve flow with given Cv and Inlet Pressure
As above, look at the downstream system and see what is there.
First assume no pressure drop across the valve and calculate a flow. sub that into the valve Cv equation and find a pressure drop.
Half that figure, take it away from 10psig and repeat your flow calculation for the downstream system.
Repeat until you get reasonable convergence.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Valve flow with given Cv and Inlet Pressure
One part I am still stuck on is determining if the flow will be choked or not even considering this assumption. I have heard it stated that if the pressure drop across an orifice is greater than half of the inlet pressure, then the flow will be choked. However, I have seen this expressed as below.
If dP>(14.7+Po)/2, Then Flow will be choked.
With that equation, I could have a differential of 9 PSIG, an inlet of 10 PSIG (which would satisfy the statement in my second paragraph for choked flow bc 9>5), but according to the above equation this flow would not be choked bc 9 is not greater than 12.4 PSIG. Am I missing something??
RE: Valve flow with given Cv and Inlet Pressure
Hence for your case, p1 is 24.5 psia, hence half of that is 12.5psia, or less than atmospheric pressure at sea level.
To get choked flow at p2 is atmospheric needs approx 30psia or 15psig.
Hope that makes sense.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Valve flow with given Cv and Inlet Pressure