TABLE OF CONTENTS Control valves are a continuously varying orifice in a fluid flow line that changes the value of a process variable by changing the rate of flow. The typical control valve consists of three main components: the body, the trim (usually consisting of a plug and seat), and the actuator.
In most applications, a control valve is the final element in a control loop. It provides the power needed to translate the controller's output to the process, either in a two-position (onoff) or proportional (throttling) control mode. Of the three basic components of a typical control loop (sensor, controller, and valve), the valve is subject to the harshest conditions and is the least understood. To complicate matters, the valve is also the most expensive and most likely to be selected incorrectly.
A successful control valve installation requires both knowledge and experience. First, personnel must gather the process information and then select the appropriate valve. Additional information on control valves is available from the ISA-75 series of standards.
Selecting the right valves involves the following factors:
Process requirements: The type of fluid passing through the valve, the inlet pressure and differential pressure (dp) across the valve, the maximum and minimum flows, the flowing temperature, and the degree of shutoff.
Correct sizing of the valve: The valve must be able to handle its maximum design flow (say, at 75% fully open). However, the designer must avoid oversizing or undersizing since they degrade the valve's operation. Typically, a properly sized valve should not...
