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IntroductionTo

ontrol

Valve

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Purpose of Presentation

By the end of this presentation, you will

be able to:

Identify types of Control valves

Control Valve’s Parts

Control Valve’s Characteristic 

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Flow of Presentation

What is a Control Valve?

Types Of Control Valve

Parts of Control Valves.

Control Valve Characteristics

Problems Effects Performance

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What is a Control Valve?

Control valves are valves usedto control following Processvariables.

Flow Pressure

Temperature

Level

By fully or partially opening or

closing in response to signalsreceived from controllers .

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Types Of Control Valves

Globe Valve Butterfly Valve

Ball Valve

Angle Valve

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Main Part of Control valve

Control Valve

Actuator

Valve Body

Positioner

Valve Body

Positioner

Actuator

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 Detail overview of Control Valve

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Control Valve Actuator  

Control Valve Actuator• Moves valve stem and plug relative to the

controller signal.

• Provide fail-safe position for the valve.• Typically operates on a pneumatic signal.

 Actuator Types

• Pneumatic spring diaphragm• Pneumatic spring return piston

• Double-acting piston

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Actuators

Spring and Diaphragm

(Reverse Acting)Spring Return Piston

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Types of Pneumatic Actuator

Actuator Types

Pneumatic

Piston

Diaphragm

Single acting

Double acting

Single acting

Double acting

Rack & Pinion

Piston

Diaphragm

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Positioner

Three Types

Pneumatic

Electro-pneumatic

Digital

Provides mechanical

feedback to ensure

accurate control.Increases reliability

Increases

dependability

Increases wear life

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Major Parts of Control Valve Body

TRIM

Plug

Cage

Seat

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Flow Characteristics

Flow characteristics fall into three major

categories: quick opening , l inear and

equal percen tage

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Flow Characteristics

Valve f low character ist ic curves are usuallyshown by plotting CV versus valve stem travel.

% Maximum Lift

00

10 20 30 40 50 60 70 80 90 100

10

20

30

40

50

60

70

80

90

100

IST-R00-(171A)

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Cv Equation

Liquid Sizing

Valve Flow Coefficient (Cv): 

The number of U.S. gallons per minute of 60°Fwater that will flow through a valve with a one poundper square inch pressure drop.

 P 

GQC    f   gpmV 

G f   = specific gravity at fluid temperature

 ΔP  = valve differential pressure in PSI

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Control Valve Sizing

Exercise

Qgpm

P2 P3P1

System Friction Drop

Gf = 0.8

100% 50% 110%

Qgpm 100 50 110

P1 130 130 130

P2 100 32.5 118.9

P3 10 10 10

 ΔPvalve  30 96.5 11.1

CV 16.3  4.6  29.5 

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Problems Effect Performance

1. Local Pressure

Drops Below Fluid

Vapor Pressure, Pv 

2. Vapor “Bubble”Forms

3. Pressure Recovers

 Above Pv 

4. Collapse of Vapor

Back to Liquid 

5. Addition of

Turbulence & Noise

P<Pv P>Pv

Flow 

Vena Contracta 

Cavitation

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Problems Effect Performance

Flashing:

Similar to the Cavitation, howevercollapsing of bubble at vena-contracta

does not occur due to downstream

pressure less than of vapor pressure.

Flashing may cause of erosion of Trim.

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Cavitation Phenomena 

P1 

V1 

V2

P2Pvapor  

Vvc

Cavitation Flashpoint  Pvc

Pressure 

Velocity 

Local Fluid Pressure Drops Below Fluid

Vapor Pressure

Formation of

Vapor Bubbles

Downstream Pressure Recovery

Above Vapor Pressure

BubblesCollapse

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Cavitation & Flashing Damage

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Problems Effect Performance

Packing friction

effects dead band and speed of response  Mechanical linkages

effects dead band 

Positioner design

effects dead band and speedof response 

(optional in some designs)

Actuator design

effects speed of response 

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