Instrumentation Design

Control Valve: An Introduction

What is a Control Valve?

It is a final control element which is used to control fluid passage through a pipeline in order to control process parameters like Flow rate, Level, Pressure & Temperature.

Various parts or components of Control Valves:

#Valve Body: A housing for internal components of valve like plug, seat, cage. etc having inlet & outlet flow connections.

#Bonnet: It is an assembly through which a valve plug stem moves & is a means for sealing against leakage through packing.

#Actuator Diaphragm: It is a part of actuator which uses air pressure to move the valve stem and valve plug.

#Actuator Spring: It is a mechanical spring to move the actuator stem in a direction opposite to that created by diaphragm pressure. It helps to achieve the fail-safe condition of the control valve in case of air failure.

#Actuator Stem: A rod like extension of the diaphragm plate to permit convenient external connection.

#Trim: It includes internal parts of a valve like seat ring, stem, plug.etc which come in contact with the process fluid.

#Seat Ring: Portion of the valve seat which the valve plug contacts for closure.

#Valve Plug: A movable part which provides variable restriction in a port.

#Cage: The cage is a part of the valve that surrounds the plug and is located inside the body of the valve.

#Yoke: It is a structure by which diaphragm case assembly is supported rigidly on the bonnet assembly.

#Valve Stem: It serves as a connector to the actuator & provides necessary movement to the valve plug.

#Guide Bushing: A bushing in the bonnet or body to align the movement of the valve plug with a seat ring.

#Packing Box Assembly: It is a part of the bonnet assembly used to seal against leakage around the valve plug, stem.


Control Valve Body Types:

CV Body Types


Valve Actuator Types:

Actuator Types

Valve Characteristics:

Following is a graph which explains the basic characteristics of a control valve.

Valve Characteristics

  1. Linear Characteristics: Equal increments of valve stem travel will yield equal increments of flow. Ex. For a 40% valve opening there will be 40% of flow passing through the valve.
  2. Equal %: Each increment in valve lift increases the flow rate by certain percentage of previous flow. The relationship between valve lift & flowrate is not Linear but Logarithmic.
  3.  Quick Opening: Used mainly in case of ON-OFF valves where maximum flowrate is achieved with minimum stem travel.


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