A control valve is the final control element, which directly changes the valve of the manipulated variable by changing the rate of flow of control agent.A control valve consists of an operator and valve body. The operator provides the power to vary the position of the valve plug inside the body. The plug is connected to the operator by a stem, which slides through a stuffing box. The air signal from the controller is applied above the diaphragm. The increasing air signal from the controller is applied above the diaphragm.
An increasing air signal will push the operator stem downwards against the force exerted by the spring on the diaphragm plate. The valve is adjusted in such away that the plug starts moving when 3 psi is applied to the diaphragm and touches the seat when 15 psi is applied to the diaphragm. Thus an increase in air pressure wjll close the valve. Hence the home “Air to Close”. Another type is “Air to open”, such that 3 psi on the diaphragm the value is closed and 15 psi air signal it in fully open.