Control Valve

Control valve is a fundamental component of any process. It is a type of final control element, which is a crucial instrument used to control different process parameters like flow, level, temperature, pressure etc.
A process consists of various loops which may or may not be dependent on each other. Each loop is expected to maintain a specific value of the process parameter passing through it. This value may be defined while designing the process or may be provided by the operator in real time or may be obtained from some other process variable in another loop. We call this value as Set Point of our loop and we need to maintain the value of our process variable at this set point. To maintain this set point we may need to reduce or increase the value of our loop's inputs or outputs in order to nullify the error by altering the Process Variable (PV). This value is defined by the Controller. The controller processes the error and sends the signal to control valve. This action of controlling the process variables on receiving signal from a controller is done by a final control element called Control Valve.

This PV value is sent to the controller. The controller also receives a value from the operator. This is the expected value of the parameter and is known as Set Point (SP). Any deviation of PV from SP results in an error. The controller is responsible to minimize the error and it does so by sending a signal to partially or completely open the control valve concerned with the value of parameter in the process. The opening and closing of the valve changes the PV value of the parameter and brings it closer to SP, thus reducing error in the process.Hence the control valve plays a crucial role in smooth functioning of a process. Control Valve is sometimes termed as Final Control Element as it is the final element in a control loop.

The opening and closing actions of control valves are initiated by equipment known as actuator. Actuators can be of electrical, pneumatic or hydraulic type.
They receive the manipulated signal from the controller and initiate the mechanical movement of the valve parts, thus resulting in opening or closing of the valve. Opening and closing of the valve is controlled by positioners based on electric or pneumatic signals.
Following are the uses of Control Valves:

  • To Stop and Start a flow
  • To Reduce or Increase a flow
  • To Control the direction of Flow
  • Regulating a flow on Process Pressure
  • Relieving a Pipe system on certain pressure