What Is Control
Valve?
Control Valve is...
• Valves to control flow, pressure, temperature,
and liquid level by fully or partially opening or closing
in response to signals received from controllers that
compare a "setpoint" to a “process variable” whose
value is provided by sensors that monitor changes in
such conditions.
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Why Control Valves used?
• Process plants consist of hundreds of control loops
all networked together to produce a product.
• Each of these control loops is designed to keep
some important process variable (pressure, flow,
level, temperature)
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Three Main Parts
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How The Valve Is Controlled?
• The arm muscle and hand → actuator
• Positioning its movable part → Plug, ball, vane
• Accurately locate the valve plug in a position dictated
by the control signal.
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Control Valve Arrangement
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 GATE VALVE
Types
Of
Valve
 BUTTERFLY VALVE
 CHECK VALVE
 GLOBE VALVE
 DIAPHRAGM VALVE
 NEEDLE VALVE
 PISTON VALVE
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Types Of Control Valve
• a) Pneumatic
• b) Electrical
• c) Hydraulic
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a) Pneumatic actuators
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i. Piston actuators
• The compressed air is applied to a solid piston
contained within a solid cylinder.
• Piston actuators can be single acting or double acting
• withstand higher input pressures and can offer
smaller cylinder volumes
• act at high speed.
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ii. Diaphragm actuators
• Diaphragm actuators have compressed air applied to
a flexible membrane called the diaphragm.
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Valve Type
• Valve A are air to close type, indicating, if the air
fails, the valve will be fully open.
• Opposite for the case for valve B.
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Number of Plugs
• Control valves can also be characterized in terms of
the number of plugs present as :
i.
Single-seated valve
ii.
Double-seated valve
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Single Seated & Double Seated
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Advantages
&
Disadvantages
Single Seated
• Advantage :
• Fully closed
• Flow variation (0% to 100%)
• Disadvantage:
• Construction, pressure drop across the orifice, a
large upward force is present in the orifice area.
• Large force required to move the valve against
this upward thrust.
• Suitable for small flow rate.
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Double Seated
• Advantages:
• Flow moves upward in one orifice area, and downward in the
other orifice.
• Almost zero resultant upward or downward thrust.
• Less force required to move double-seated valve.
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• Disadvantage:
• Flow cannot be shut off completely.
• If one plug is tightly closed, there is usually a small gap
between the other plug and its seat.
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b) Hydraulic Control Valve
• Hydraulic actuators provide for semi-automatic or automatic positioning of
the valve (similar to pneumatic actuators).
• These actuators use a piston to convert a signal pressure into valve stem
motion
• A control valve slides in a cylinder and alters the flow of the fluid
• Hydraulic fluid is fed to either side of the piston while the other side is
drained or bled
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• Solenoid valves are typically used for automatic
control of the hydraulic fluid to direct either opening
or closing of the valve
• Manual valves can also be used for controlling the
hydraulic fluid thus providing semi-automatic
operation.
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Piston operated
control valve
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• The hydraulic control valve shown in figure above is
normally held in the CLOSED position by both a spring
force and by the main pressure acting against the disk
• When hydraulic pressure is admitted to the underside of
the piston a force is created to overcome both the spring
tension and the main pressure, causing the valve to open
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• When hydraulic pressure is released from under the
piston, the spring force the hydraulic fluid out of the
cylinder thus closing the valve.
• A ratchet lever is fitted to the valve so in an
emergency, the valve can be opened by hand
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c) Electrical Actuator Control Valve
1. Motor
2. Limit and
torque
sensor
3. Gearing
4. Valve
Attachment
5. Manual
Operation
6. Electrical
Connection
Working Principle
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MOTOR
• Three-phase AC motor are mostly used as the
driving force.
• Single-phase AC or DC motors
• Higher starting torque (asst by capacitors)
• NOT design to run for continues operation
• Robust
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LIMIT & TORQUE SENSOR
• The limit switch send signal when the end position
has been reached.
• Torque switching measures the torque present in
the valve.
• Remote position transmitter which indicates the
valve position.
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GEARING
• Excessive reduction ratio between gear stage is use
to reduce the high output speed of the electric
motor.
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VALVE ATTACHMENT
• The flange used to firmly connect the actuator to
the counterpart on the valve side.
• The higher the torque to be transmitted, the larger
the flange required.
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MANUAL OPERATION
• Power failure.
• Handwheel DOES NOT move during motor
operation.
• During manual operation :
i. Electronic torque limiting device will not be
functioning.
ii. Mechanical torque limiting device used.
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Electrical connection
• Supply cables of the motor and the signal cables
used.
• To transmit the commands to the actuator and
sending feedback signals on the actuator status.
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