How to calculate the valve on the pipeline fluid flow interception?

Valve is installed in the pipeline, the valve is working with the entire pipeline system, the valve opening changes in the impact on traffic flow must be determined by the entire system of hydraulic calculations. In fact, when the water flow through the valve has a partial resistance role, set the valve local resistance coefficient ζ, the pipeline of the water area F, water density ρ, when the valve fluid pressure p1, valve fluid pressure p2, p1, p2 The flow rate through the valve is Q = (F / √ζ) √ [2 (P1-P2) / ρ] The valve coefficient of local resistance ζ changes with the valve opening, so the flow rate is Opening changes, this change can only be obtained experimentally. If the flow is Qmax at full open (100% opening) and the flow at open X% Q = f (X%) * Qmax, where the function is experimentally determined and given as a curve for future reference . The simplest case is f (X%) is a straight line, that is proportional to the flow and opening, then the valve opening at 25% of the flow Q = f (X%) * Qmax = 0.25Qmax