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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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