Control valves are usually referred to by their control function. If they control pressure, they are pressure control valves "PCV"; if they control flow, they are flow control valves "FCV"; if they control temperature they are temperature control valves, etc. Note that each type of valve is positioned based on a particular type of control signal, from a pressure signal, flow signal or temperature signal, respectively.
Since you asked "what outlet pressure should be used", I thought it was reasonable to think that was your intended contro function and you were sizing a "downstream pressure control valve".
Yes, of course you could size a "flow control valve" for a particular flowrate where the positioner is being driven by a flowrate signal from a flowmeter of some kind. In that case, you know the flow you want to hold, so you size the valve using that flowrate and using the Valve Cv at say about 75-80% open. That will give you a specific pressure drop at that flowrate. Then you check your piping pressures to find the minimum possible pressure at the flow control valve's inlet, subtract the pressure drop and you know the outlet pressure, assuming its not below 0 psia. If its below 0 psia, or its below some other minimum pressure you think you should be holding for some reason, product vapor pressure, or other reasons as per previous post, you resize the valve a little larger. If you valve becomes too large, you look at ways to increase the inlet pressure.
It is also sometimes possible to cross a particular desired control function, say flow rate control, with a different type of transducer signal, say pressure signal, but it is a more complex relationship and imprecise method for which you must be able to establish a very good relationship between flow and pressure that will hold for all pressures in your piping system. Your piping system must show relative stability between the cross-controlled variables. For example, when you increase a solar hot water flow to raise temperature of your bath water, you should ALWAYS get more hot water. If the water is cool before the sun comes up in the morning, your valve control function will not have the desired effect and you might flood the bathroom.
Say you want to have 10 gpm flowing from the end of a pipe open to atmosphere. You know atmospheric pressure is usually 14.696 psia with relatively small variations and your hydraulic analysis shows that by holding 30 psia at the inlet to the last pipe, you will discharge 10 gpm. There you can try to size a control valve to hold 30 psia, when it has a 15 psi pressure drop when flowing 10 gpm. Then you can add 14.696 + pressure drop of last pipe, say 1.5 psi = 16.196 psia (1.5 psig) and get the set pressure at the downstream side of the control valve, 1.5 psig. Then add the 15 psi to 16.196 psia to get the inlet pressure required at the valve = 31.196 psia (16.5 psig). Everything will be just fine, until your valve inlet pressure starts to change. If the inlet pressure goes up, more flow will start to move across the valve, which will increase the pressure drop across the valve. The valve sees the discharge pressure also goes up and starts closing to reduce it. If the valve closes the appropriate amount, it is possible that your flowrate is maintained at 10 gpm, but don't count on it closing an appropriate amount, because it doesn't really care about your 10 gpm. It cares about the 1.5 psig setting, so the flowrate will be whatever it must be to get the 1.5 psig outlet pressure.
So, after all is said and done, maybe it works, maybe it doesn't, but what is true is that you should have a very stable system to try to use cross control. It also requires a very good match of pressure drop and flow change. Its not easy to do. The system must be relatively stable. Then, you realize that, if your system is stable enough to do this, then you might not really need a control valve, as you could probably control flow just as well with a manual plug valve that you came around once a day or week to make some fine adjustments. That's a bit of an exaduration, but makes my point that a manual valve might do just fine if the system is stable enough to allow crossing control. You just have to check and make sure it will work, because if it doesn't, things can get bad quickly.
Going the Big Inch!
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