What is control Valve and Working Principles

What is control Valve and Working Principles

Control valves automatically regulate pressure and/or flow rate, and are available for any pressure. Control valves are used in many processes to control flow, pressure, temperature or other variables. The type of valve used will depend on the size of the pipe, the overall pressure that the system operates, the flowing media, process conditions, and other factors. There is also a balance between the cost of the valve and the cost benefits associated with tighter control.

Principles of Operation

The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.

Control valves may be the most important, but sometimes the most neglected, part of a control loop. The reason is usually the instrument engineer’s unfamiliarity with the many facets, terminologies, and areas of engineering disciplines such as fluid mechanics, metallurgy, noise control, and piping and vessel design that can be involved depending on the severity of service conditions.

Any control loop usually consists of a sensor of the process condition, a transmitter and a controller that compares the “process variable” received from the transmitter with the “set point,” i.e., the desired process condition. The controller, in turn, sends a corrective signal to the “final control element,” the last part of the loop and the “muscle” of the process control system. While the sensors of the process variables are the eyes, the controller the brain, then the final control element is the hands of the control loop. This makes it the most important, alas sometimes the least understood, part of an automatic control system. This comes about, in part, due to our strong attachment to electronic systems and computers causing some neglect in the proper understanding and proper use of the all important hardware.

  • Body
    It is a type of a pressure vessel containing an orifice or an opening. The controlled liquid is allowed to flow through the body of the valve. It helps to monitor the flow regulation behaviour.
  • Trim
    Besides the body, trim is one such part of the valve that comes directly in contact with the fluid. It consists of the seat, disc, plug, and stem.
  • Actuator
    It consists of electric or pneumatic mediums to provide the force required to operate  the control valve.
  • Bonnet
    It provides a mounting for the guide and actuator and a medium for the stem to pass through. It is made of the centrepiece, packing, packing nut and guide. The packing acts as a fastener between the bonnet and stem. It helps to avoid any leakage.

 Working Principles of a Actuator ?

  • Pneumatic Actuated
    Pneumatic Actuators use an air or gas signal from an external source to produce a modulating control action. The actuator receives the force of the pneumatic signal through a top port. Then, it distributes the signal across the actuator’s diaphragm. As a result, the diaphragm exerts pressure on the diaphragm plate. This moves the valve stem downward in a way that strokes the control valve.
  • Electric Actuated
    They are motor-driven devices. They use an electrical signal that can help create a motor shaft rotation. This movement is converted into a linear motion, which helps to drive the stem in the valve for flow modulation.
  • Hydraulic Actuated
    Hydraulic actuators are similar in operation to pneumatic actuators except they use a fluid, hydraulic oil, as the signal fluid to control the action of the valve. They are used, in place of pneumatic or electric actuated valves when the force required to move the valve stem is high.

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