Normally you would want your dP to be as small as possible - while achiving the control that you aim for. What you are looking for is at what % open should a CV normally be operating at (required flow and dP). This would normally be around 70% open if you want to achive good control.
Your flow rate AND/or dP is given by your process - not the valve. So lets say you have a pump and you want to pump from one atm tank to the other and you want to control the flow rate to 1 m3/hr
In this case you want flow control - so you dP should be as low as possible while getting good flow control - because the CV is energy loss.
One way forward could be to
1) Determine your pump discharge pressure at your flow rate
2) Determine your pressure loss in piping and static head loss/gain at this flow rate
3) Tell valve vendor to size a valve based on this
He should then give you a valve that has your required flow rate at the required dP and be appriximately 70% open.
i mean the DP in the valve sizing equation (p1-p2) is DP across the valve only and not the total pressure loss of the system. valve DP is only a fraction of system DP.
i have a gate valve that is throttled that i want to replace with a control valve.
OP: "i have a gate valve that is throttled that i want to replace with a control valve." just to be clear on terminology:
A _control valve_ is not a _specific_ type of valve - its a valve that actively controls a process parameter - this might be flow or pressure but could also be level in a tank or temperature out of a heat exchanger.
It _seems_ to me that OP might be thinking of a specific valve type (most likely a globe valve) but still not actively controling the flow (could e.g. be set to a specific position by hand and then left "forever alone"?
You need to calculate required valve dP by determining the total available dP minus piping drops etc. This is to be done at max, min and normal flows. Based on this and other related data the CV supplier will offer you a suitable valve.
By the way when you are replacing a gate valve keep in mind that CVs normally do not have tight shut off.
Engineers, think what we have done to the environment !
I guess, if it is practicable for the valve size, you can create a test station with a pump for the pressure and flow, and check the real pressure drop under various openings. Most of the cases the manufacturers have this kind of facilities. You need to talk to them for this operation, but they may charge for it.
When you design for example a flow control valve, the differential pressure is determined by the process system in which the control valve will operate.
Imagine you have a source at a pressure of 5 barg and a receiver at a pressure of 1 barg. The source and pressure are connected by a process system composed of straight length piping, bends, reducers, a filter and a control valve. The differential pressure between the source and receiver is 4 bar and the flow control valve will 'consume' part of this differential pressure. It will do this by opening or closing and hence controlling the flow. The more the valve is closed, the les differential pressure is available for fluid flow and the flow rate drops. The more the valve opens, the more differential pressure is available for fluid flow and the flow rate increases. In order to determine the pressure drop across the control valve, you now calculate the operating pressure just up- and downstream the control valve for a certain flow rate. The remaining pressure drop will be taken by the valve.
This is typically done for 3 different flow rates (min., norm. and max). With the flow rates and calculated differential pressures, the Cv of the valve can now be calculated.
