One of the most common final control elements in industrial control systems is the control valve. A “control valve” works to restrict the flow of fluid through a pipe at the command of an automated signal, such as the signal from a loop controller or logic device (such as a PLC). Some control valve designs are intended for discrete (on/off) control of fluid flow, while others are designed to throttle fluid flow somewhere between fully open and fully closed (shut), inclusive. The electrical equivalent of an on/off valve is a switch, while the electrical equivalent of a throttling valve is a variable resistor.
Control valves are comprised of two major parts: the valve body, which contains all the mechanical components necessary to influence fluid flow; and the valve actuator, which provides the mechanical power necessary to move the components within the valve body. Oftentimes, the major difference between an on/off control valve and a throttling control valve is the type of actuator applied to the valve: on/off actuators need only position a valve mechanism in one of two extreme positions (fully open or fully closed). Throttling actuators must be able to accurately position a valve mechanism anywhere between those extremes.
Within a control valve body, the specific components performing the work of throttling (or completely shutting off) fluid flow are collectively referred to as the valve trim. For each major type of control valve, there are usually many variations of trim design. The choice of valve type, and of specific trim for any type of valve, is a decision dictated by the type of fluid being controlled, the nature of the control action (on/off versus throttling), the process conditions (expected flow rate, temperature, pressures, etc.).
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Instrumentation Basics 