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Control Valves

A control valve is defined as a variable orifice in a fluid flow system. It can be adjusted manually by hand wheel or lever, or it can be operated automatically by controller outputsignals. The final operator in a control loop directly changes the value of the manipulated variable by changing flow rate.
A complete automatic control valve consists of three major components:1. The actuator transforms the controller signal into motion, providing power to vary the orifice.
2. 2. The valve body assembly consists of a pressure-tight fitting that is threaded, flanged, orwelded in a fluid flow line and contains on or more internal orifices through which fluid flow is controlled.
3. A plug, damper or louver is positioned in the orifice by actuator to control thepressure drop and rate of flow

To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired setpoint. A controller then processes this information and decides what must be done to get the process variable back to where it should be after a load disturbance occurs. When all the measuring, comparing,and calculating are done, some type of final control element must implement the strategy selected by the controller. The most common final control element in the process control industries is thecontrol valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close aspossible to the desired set point.
Many people who talk about control valves or valves are really referring to a control valve assembly. The control valve assembly typically consists of the valvebody, the internal trim parts, an actuator to provide the motive power to operate the valve, and a variety of additional valve accessories, which can include positioners, transducers, supply pressure...
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