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CONTROL VALVES

INDUSTRIAL VISIT BY UOB - 10 MAY 2010

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TOPICS COVERED

1. WHAT IS A CONTROL VALVE

2. VARIOUS TYPES

3. VALVE CHARACTERSTICS

4. VALVE TYPES AND APPLICAITON

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A Control Valve is a power-operated

device used to modify the fluid flow

rate in a process system (either gases,

fluidised solids, slurries orliquids) by

opening, closing, or partially obstructingvarious passagewaysor

It is a variable orifice in a line.

WHAT IS A CONTROL VALVE

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Air in

1 23 4

5

6

7

Fluid in Fluid out

The plug 1is placed in the orifice 2of the valve andattached to the end of the stem 3. The orifice isplaced inside the body of the valve 4made of castiron, alloy steels, alloy steels plus corrosion-resistant

alloys, or bronze. The upper part of the final controlelement is an actuator5.

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TYPES OFVALVES

There are two basic types of control valves:

LinearRotary

In addition to linear and rotary, controlvalves are also classified according to their

guiding systems and the types of servicesthey are used in.

http://www.maintenanceresources.com/referencelibrary/controlvalves/cashcolinearvalves.htmhttp://www.maintenanceresources.com/referencelibrary/controlvalves/cashcorotaryvalves.htmhttp://www.maintenanceresources.com/referencelibrary/controlvalves/cashcorotaryvalves.htmhttp://www.maintenanceresources.com/referencelibrary/controlvalves/cashcolinearvalves.htm

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Control Valve Classification

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WHAT IS Cv

Cv IS A CAPACITY INDEX(VALVE CO-EFFICIENT)

THE NUMBER OF US GALLONSPER MINUTE OF

WATER AT 60 DEG F WHICH PASS

THROUGH A

GIVEN FLOW RESTRICITON WITH A

PRESSURE

DROP OF 1PSI

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Control Valve Flow Characteristics

THE RELATIONSHIP BETWEEN VALVE CAPACITY ANDVALVE TRAVEL IS KNOWN AS THE FLOW

CHARACTERISTIC OF THE VALVEQuick opening :

large increase in flow with a small change in valve stroke

For frequent on-off services

Instantly large flow is needed.

Linear :Valve travel is directly proportional to the valve stoke

In liquid level or flow loops

Pressure drop across valve is almost

constant.

Equal Percentage:Equal increments of valve travel produce an equal

percentage in flow change.

Large changes in pressure drop are expected

Minimum pressure drop due to valve

In temperature & pressure control loop

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VALVE SELECTION AND APPLICATION

Gate Valve

Best Suited Control: Quick Opening Recommended Uses:

Fully open/closed, non-throttling

Infrequent operation

Minimal fluid trapping in line

Advantages:

High capacity

Tight shut off, Low cost, Little resistance to flow

Disadvantages:

Poor control Cavitate at low pressure drops

Cannot be used for throttling

Applications: Oil, Gas, Air, Slurries, Heavy liquids, Steam,

Non-condensing gases, and Corrosive liquids

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VALVE SELECTION AND APPLICATION

Ball valve

Best suited control

Quick opening linear . Recommended uses

Fully open/closed limited throttling

Higher temperature fluids

Advantages

Low cost

High capacity

Low leakage & maintenance

Tight sealing with low torque

Disadvantages Poor throttling characteristics

Prone to cavitations

Applications Most Liquids, high temperatures, slurries

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VALVE SELECTION AND APPLICATION

Globe valve

Best Suited Control: Linear and Equal percentage Recommended use-

Throttling services/flow regulation

Frequent operation

Advantages: Efficient throttling

Accurate flow control valves

Available in multiple ports

Disadvantages: High pressure drop

More expensive than other

Applications : Liquids, vapors, gases, corrosivesubstances, slurries

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VALVE SELECTION AND APPLICATION

Butterfly valve Best Suited Control: Linear, Equal percentage

Recommended Uses :

Fully open/closed or throttling services

Frequent operation

Minimal fluid trapping in line

Advantages : Low cost and maint.

High capacity

Good flow control

Low pressure drop

Disadvantages

High torque required to control

Prone to cavitation at lower flows

Applications : Liquids, gases, slurries, liquids withsuspended solids

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A heart valve regulates blood flow through the

heart in many organisms.

A pressure reducing valve reduces pressure to apreset level downstream of the valve

A pressure sustaining valve maintains pressure

at a preset level upstream of the valve

A regulator is used in gas cooking equipment toreduce the high pressure gas supply to a lower

working pressure

A safety valve orrelief valve operatesautomatically at a set differential pressure to

correct a potentially dangerous situation, typically

over-pressure

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As you work with control valves always keep in mind that aControl Valves Do What They Are Told!

Fail-safe mode"Well, what happens if the power is cut off?

When a Control Valve is sized or selected to do a particularjob, one of the first questions you should consider is how

that valve will respond in the event of a loss of signal orpower.

This is called its "fail-safe mode"

Knowing the fail-safe mode is the key to

troubleshooting it.In most applications (about 80%), it is desirable for valves tofail closed.

In other applications, you might want a valve to fail openor

fail in place. Safety concerns and process requirements willmandate the fail mode of the valve.

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`

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Thanks

Any Questions?

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