I have something very basic to ask about CONTROL VALVE selection. Hope i will someone who could reply in the best and easiest way.

1. What is Valve Authority (In layman' term) and how to calculate it. What is the formulae. Usually we have flow rate only and we assume system DP as 5psi. From Flow and DP i calculate Cv and basis of CV globe valve is selected. Now how i can calculate Valve authority out of it. If someone can explain.

2. What is Valve Rangeability (usually it is 50:1 for Globe valves) so what does this 50:1 means.

Please reply shall be very very grateful.