Measurement and Control Basics Fourth Edition

Chapter 10 - Final Control Elements

Introduction

In Chapter 1, we introduced the concept of process control and defined its
three elements: measurement, evaluation, and final control. The final control
element is probably the most important because it exerts a direct influence
on the process. Final control devices contain the essential pieces of
equipment to convert the control signal (generated by a process controller)
into the action needed to correctly control the process.

In this chapter, we discuss the fundamentals of final control devices, such
as control valves, motors, and pumps. Because control valves are the single
most common type of final control element in process control, we will
discuss them first and in the greatest detail.

Control Valve Basics


A control valve is simply a variable orifice that is used to regulate the flow
of a process fluid according to the requirements of the process. Figure 10-1
illustrates a typical globe-type control valve body in both the fully open
and fully closed positions. In a control valve, an actuator that is connected
to the valve's plug stem moves the valve between the open and closed
positions to regulate flow in the process. The valve body is mounted in the
process fluid line and is used to control the flow of fluid in the process.

The body of a control valve is generally defined as the part of the valve that
comprises the main boundary, including the connecting ends. Valves are
classified into two general types based on the movement of the valve's closure
part: linear and rotary.

Figure 10-1. Typical globe-type control valve

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