Control Valve (Actuator)

8/11/2019 Control Valve (Actuator)

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What Is Control

Valve?


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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 partPlug, 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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Types

Of

Valve

GATE 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 doubleacting

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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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 orificearea.

Large force required to move the valve againstthis upward thrust.

Suitable for small f low rate.


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Double Seated

Advantages:

Flow moves upward in one orifice area, and downward inthe 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 andtorquesensor

3. Gearing4. Valve

Attachment

5. ManualOperation

6. ElectricalConnection


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Working Principle


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MOTOR Three-phaseAC motor are mostly used as the driving

force.

Single-phaseAC 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 thevalve.

Remote position transmitter which indicates thevalveposition.


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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 thecounterpart on thevalve side.

The higher the torque to be transmitted, the larger theflange 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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Control Valve (Actuator)