I have heard of three alternate methods (vs the traditional one consisting of allowing 50 to 25% of the system frictional pressure drop excluding itself to the control valve) to determine the allowable pressure drop across a control valve during the design stage:1/ Connell's one using a formula that everyone knows2/ A method consisting of assigning a minimum pressure drop (10 or 15 psi) at its maximum expected design flowrate and at a upper opening limit 80%. This method has been presented by Frank Yu in "Easy way to estimate realistic control valve pressure drops" issued August 2000 in HP.Method 3 MIGHT work in selected applications, but it sounds more like drop expected with an on-off valve. 1/3 the system pressure drop at the valve at max flow is the rule-of thumb I was taught. That gives effective control. And "System pressure drop" is almost NEVER the same as the system pressure. JLSeagull's reference to a steam blowdown valve being the exception, where upstream is pretty much constant and downstream is atmospheric. Note that As the flow is dropped back the system pressure drop decreases as the square of the flowrate, and the pressure backs up on the pump curve, so you're not backing up the flow much before the valve is dropping essentially ALL of the system flow.

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