I am trying to size a needle valve to make sure that it can allow the required flow rate at the fully open position (0.025 m^3/s). For a given Cv you should be able to calculate the maximum flow rate through the needle valve provided that you know the temperature of the gas and the pressure drop across the valve, but my problem is that I do not know what the outlet pressure of the needle valve is going to be ahead of time. Wouldn't P2 (outlet pressure) be dependent upon the needle valve itself? I seem to be confused on this point which is preventing me to get the correct size for the valve. The flow conditions are: Medium: Air Upstream Pressure: 2 atm Maximum flow rate required: 0.025 m^3/s T = 300 K Any help would be highly appreciated.