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The Concept of Control Valve

Hydraulic control valves are used to regulate the fluid flow through a process system. They act upon the instruction from the signal from a process controller to make the necessary control actions. In any process station it is necessary to keep the controlled variable within the specified limits. Hence if the controlled variable increases above the limits, signal from the process controller imitate the valve to initiate the necessary action. For example in level process station, if the water level increases above the limits, then the control valve is opened to drain the excess water and vice versa. van điều khiển điện

The control valve is divided into the body and the trim. The valve body consists of housing for mounting the actuator and connections for attachment of the valve to a supply line and a delivery line. The trim usually contains a plug, a valve seat and a valve stem. The trim is enclosed within the body. The actuator is driven by a current through I/P converter-gives a linear way of translating the 4- 20 ma current into 3-15 psi signal. The actuator moves the valve stem as the pressure on a spring loaded diaphragm changes. The stem moves a plug in a valve seat in order to change the resistance to flow through the valve.

The working principle of a control valve is explained here. As the stem and plug move up and down the size of the opening between the plug and the seat changes thus changing the flow rate. The direction of flow is important for proper valve action. If the flow were reversed, force from the flow would tend to close the valve further at small openings. There will be drop in pressure across such a restriction and the flow rate varies with the square root of the pressure drop.

 

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