2. VALVES
VALVES Control valves are the most commonly used actuators or final control elements in process industries. They manipulate the flowing fluids to keep the variables being controlled in the desired positions. A control valve is known as the final control element because it is the element that ultimately manipulates the value of the variable in the control process. It is defined as a mechanism that alters the value of the variable being manipulated in response to the output signal from a controller, whether automatic, manual, or by direct human action. It is the element that implements the decision of the controllers. Controllers can be set in either automatic or manual mode control. A crosssectional diagram of a typical pneumatic control valve is shown in Fig. 2-1. The purpose of the valve is to restrict the flow of process fluid through the pipe that can be seen at the very bottom of the figure. The valve plug is attached rigidly to a stem that is attached to a diaphragm in an air pr...