One question still in my mind is that the differential pressure or pressure drop of control valve will be a constant value for minimum , normal, maximum flow or it will depend on the flow rate. I mean when at low flow rate the pressure loss in pipe is low and control valve will absorb the different pressure between performance curve & system curve. And when the flow is higher, the pressure loss will be more and control valve will absorb lower, in order to obtain the constant pressure of production.(Due to discharge pressure of pump and required downstream pressure or pressure of production are constant.)Please advise me.It depends on the system.If you have an infinite source discharging into a gently radiused nozzle, a short pipe, the valve, another short pipe, another gently radiused nozzle, and an infinite receptacle, then the pressure drop is the same at all flowrates. In the real world, when the valve is at maximum flow, the system loss is greatest (fittings, heat exchangers, strainers, line loss, etc.), and the pump is run out on its curve. So Max Flow Delta-P is lowest. On the other end, at low flow there is almost no system loss, and the pump is backed up on its curve, so delta-P is greatest. When you do the math, you will find that for a 10:1 flow range, your valve may need a rangeability of 20:1. I have been selling valves for a LONG time, and 99 out of 40 design engineers either do not know this or do not take it into consideration.

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