Control Valves

BINIT GOENKA2ND YearElectronics and InstrumentationBITS Pilani, Goa Campus

Control Valves

     A Control Valve is a power-operated device used to modify the fluid flow rate in a process system.

There are two basic types of control valvesa) Rotary motion valves having ball, butterfly or plug type closures.

b) Linear motion valves having globe , diaphragm or pinch type closures.

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Control Valve Classification

Control Valve

Linear Motion

Globe

Globe

Single Seated

Double Seated

Angle

3 way

Diaphragm Pinch

Rotary Motion

Ball Butterfly Plug

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Linear Motion

Types Of Closure1. Globe

1.1. Single Seated1.2. Double Seated1.3 Three way

2. Diaphragm3. Pinch

Linear Valve Features

i. TORTUOUS FLOW PATH ii. LOW RECOVERY iii. CAN THROTTLE SMALL FLOW RATES iv. OFFERS VARIETY OF SPECIAL TRIM DESIGNS v. SUITED TO HIGH-PRESSURE APPLICATIONS vi. USUALLY FLANGED OR THREADED vii. SEPARABLE BONNET C

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Single Seated Globe Valve

Featureso Large amount of force

required to drive the stemo Tight shut offo Used in small diameter

applications Applicationso Used for controlling large

range of process parameters hence used in oil and gas production, power generation

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Double Seated Globe Valve

Featureso Stem can be driven with small

amount of forceo Can not achieve perfect closure

resulting in leakageo Used in larger diameter

applicationso Body parts wear out due to

erosion and cavitations which results in annoying noises

Applicationso Used for high flow and high

pressure services

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Three Way Valves

Featureso High flow capacityo Valve used for mixing and

diverting liquids.o High rangeabilityo Leakage capability

consistent with normal single seated globe valve

Applicationso Used in textile industries

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Diaphragm Valves• Featureso Consists of a saddle

or seat upon which the diaphragm closes the valves.

o Can handle corrosive fluids.

Applicationso Used as shut off

valves in food and beverage , pharmaceutical and biotech industries

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Pinch ValveC

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• Featureso Tight shut offo The flexible sleeve allows the valve to close drop tight around solids , solids that would typically be trapped by the seat or stuck in crevices in globe, diaphragm, butterfly, gate or ball valves.• Applications•Best used in controlling corrosive fluids, steam and thermic fluids.

Rotary Motion

Types Of Closure1. Ball2. Butterfly3. Plug

Rotary Valve Features

i. STREAMLINED FLOW PATH ii. HIGH RECOVERY iii. MORE CAPACITY iv. LESS PACKING WEAR v. CAN HANDLE SLURRY AND ABRASIVES vi. FLANGELESS vii. INTEGRAL BONNET viii. HIGH RANGEABILITYC

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Ball Valve

Featureso Tight Shut Offo Minimum obstruction for the

flowing media.o High flow capacityo Can withstand high pressure and

temperatures

Applicationso Used for hazardous and corrosive

fluids

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Butterfly Valve

Featureso High flow capacityo Unlike ball valve the

disc is present in the flow of the liquid and hence a pressure drop is always induced in the flow.

o Tight shut offApplicationso Used in Steel , Sugar

and textile industries

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Actuators

In control valve we have seen that , the operation of a control valve involves positioning its movable part (the plug, ball or vane) relative to the stationary seat of the valve. The purpose of the valve actuator is to accurately locate the valve plug in a position dictated by the control signal.

The actuator accepts a signal from the control system and, in response, moves the valve to a fully-open or fully-closed position, or a more open or a more closed position

The major types of actuators are :1. Pneumatic type2. Electric type

Pneumatic Actuators

They are basically of two types1. Piston actuator2. Diaphragm actuator

These actuators are designed so that with a specific change of air pressure, the spindle will move sufficiently to move the valve through its complete stroke from fully-closed to fully-open.

Piston ActuatorThey can withstand

higher input pressures.

Can offer small cylinder volumes.

They are generally used where the stroke of a diaphragm actuator would be too short or the thrust is too small.

Diaphragm Actuators

• They have compressed air applied to a flexible membrane called the diaphragm

• They are single acting i.e. air is supplied from single side of the diaphragm

Diaphragm Actuators

Electric Actuators

Electric actuators use an electric motor with voltage requirements in the following range: 230 Vac, 110 Vac, 24 Vac and 24 Vdc.

There are two types of electrical actuator 1. VMD (Valve Motor Drive)

2. Modulating.

VMD ( Valve Motor Drive ) This basic version of the

electric actuator has three states:

I. Driving the valve open. II. Driving the valve closed. III. No movement

• The controller positions the valve by driving the valve open or closed for a certain time, to ensure that it reaches the desired position. Valve position feedback may be used with some controllers.

Modulating In order to position the control valve in

response to the system requirements a modulating actuator can be used. These units may have higher rated motors (typically 1 200 starts/hour) and may have built-in electronics.

A positioning circuit may be included in the modulating actuator which accepts an analogue control signal (typically 0-10 V or 4-20 mA). The actuator then interprets this control signal, as the valve position between the limit switches.

To achieve this, the actuator has a position sensor (usually a potentiometer), which feeds the actual valve position back to the positioning circuit. In this way the actuator can be positioned along its stroke in proportion to the control signal.

Thank You

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