Hydraulic flow control valve are employed to modulate the fluid flow through a process system. They act upon the instruction from the signal from a process controller to make the necessary control activities. In any process station it is important to keep the variable that is controlled within the limits that are given. Therefore if the controlled variable increases above the limitations, signal from the process control mirror the valve to begin the action that is necessary.
The flow control valve is divided into the body and the trim. The flow control valve body contains casing for mounting connections and the actuator for attachment of the valve into a supply line and a delivery line. The trim generally features a valve seat, a stopper and a valve stem. The trimming is enclosed within the body. The actuator is driven by a current through I/P converter-provides a linear way – 20 ma current into 3 -15 psi signal. The actuator moves the valve stem as the pressure on a spring loaded diaphragm changes. The shank moves a plug in a valve seat to be able to change the resistance to flow control valve.
A flow control valve’s working principle is explained here. As plug and the stem move up and down the measurement of the opening between the stopper and the seat shifts so altering the flow rate. The direction of flow is essential for appropriate valve action. If the flow were reversed, force in the flow would tend to close the valve farther at small openings. There’ll be drop in pressure across this kind of limitation and the flow rate varies with the square root of the pressure drop.
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