12_Control_Valves_Chap9S10_4

Valve positioners pneumatic control valves can be

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Unformatted text preview: siderations. … a few more pictures 8 9 Valve Positioners Chapter 9 Chapter 9 Pneumatic control valves can be equipped with a valve positioner, a type of mechanical or digital feedback controller that senses the actual stem position, compares it to the desired position, and adjusts the air pressure to the valve accordingly. Specifying and Sizing Control Valves A design equation used for sizing control valves relates valve lift to the actual flow rate q by means of the valve coefficient Cv, the proportionality factor that depends predominantly on valve size or capacity: q = Cv f () ΔPv gs (9-2) 10 11 • Here q is the flow rate, f ( ) is the flow characteristic, ΔPv is the pressure drop across the valve, and gs is the specific gravity of the fluid. • Specification of the valve size is dependent on the so-called flow of the valve size is dependent on the so flow (or valve) characteristic f. • Three flow characteristics are mainly used for control valves. • For a fixed pressure drop across the valve, the flow “characteristic” f ( 0 ≤ f ≤ 1) is related to the lift ( 0 ≤ ≤ 1) , that is, fraction of valve opening, by one of the following that is, fraction of valve opening, by one of the following relations: Linear: f= Quick opening: Equal percentage: f= f =R (9-3) Chapter 9 Chapter 9 • This relation...
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This note was uploaded on 04/17/2010 for the course CHE 461 taught by Professor Staff during the Winter '08 term at Oregon State.

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