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Control Valves

What is a Control Valve?

Modern processing plants utilize a vast network of control loops to produce an end product for market. These control loops are designed to keep a process variable (i.e. pressure, flow, level, temperature, etc.) within a required operating range to ensure a quality end product is produced. Each of these loops receives and internally creates disturbances that detrimentally affect the process variable (PV). Interaction from other loops in the network also provide disturbances that influence the process variable. 

To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable (PV) and its relationship to some desired set point. A controller 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 the control 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 as possible to the desired set point. The control valve is a critical part of the control loop. Many people who talk about control valves are really referring to a control valve assembly. The control valve assembly typically consists of the valve body, 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 s, transducers, supply pressure regulators, manual operators, snubbers, or limit switches. There are two main types of control valve designs, depending on the action of the closure member: sliding-stem or rotary. Sliding-stem valves, use linear motion to move a closure member into and out of a seating surface. Rotary valves use rotational motion to turn a closure member into and out of a seating surface.

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