Control valves are instruments that control the flow of fluids through passages by varying the size of the passageway. They have a body, actuator, and positioner. The actuator directly controls the opening and closing of the valve body in response to signals from the positioner. Common actuator types include pneumatic, hydraulic, and electric. Pneumatic actuators typically use compressed air and a diaphragm to move the valve stem and vary the flow opening. Positioners receive control signals and ensure the actuator moves the valve to the desired position.

2. It is an instrument that controls the flow of a
fluid in a passage by varying the size of that
passage.
Main parts:
1- Body.
2- Actuator.
3- Positioner.
Control valve

3. It increases or decreases fluid flow path size in
order to control flow rate.
Control system in the plant will send a control
signal to operate the control valve to a status
between fully open or fully close. There are
feedback mechanisms to tell the current status
of the control valve to the control system.
Some control valves have a manual override
option, which is useful when there is a problem
in automatic position control.
Usage

4. It is the part which the fluid will
flow through, and it is the one
which will move to change fluid
passage size.
Valve body
Globe Butterfly
GateDiaphragmBall

5. It is a mechanical assembly
that directly controls the
opening or closing of valve
body.
Main types:
1- Electrical: electric motor.
2- Hydraulic: many types
such as piston.
3- Pneumatic: piston or
diaphragm.
Valve actuator
Hydraulic (Piston) Pneumatic (Diaphragm) Electric

6. It is a mechanical assembly that directly controls the
opening or closing of valve body.
It is the most famous control valve actuator used in
the industry.
It has a diaphragm material that is being pushed by
compressed air which causes valve movement from
fully open to fully close depending to air supplied.
Once air stops, a spring returns the diaphragm to its
original state. This is for single acting actuators, but
there are double acting ones which use 2 air
supplies on the two sides of the diaphragm so it
controls the opening and the closing of the valve.
Diaphragm pneumatic actuator

7. It is the device that gives control signals for the
actuator, and tells makes sure to move it to the
desired position and maintains it there.
It has an air supply input (ex. 6 bar) and a pneumatic
control signal input (3-15 psi), with an air output
which goes to the actuator. So it takes the 3-15 psi
control signal and tries to produce air that matches
it to the actuator. The 3-15 psi air signal is take from
an I-to-P converter which in turns takes the 4-20mA
control signal from the PLC (or any control system)
and converts it to 3-15 psi.
Some modern positioners have the IP built-in, so
the user feeds the 4-20mA signal to it directly.
Positioner

8. Control valve connections with double acting actuator