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ERHARD Kavitation En

Apr 03, 2018

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    ERHARD Regelarmaturen Kavitation

    Understand

    Recognize

    Avoid

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    ERHARD Regelarmaturen Kavitation 2

    What is Cavitation?

    Cavitation is the description for a physical process, where liquid medium

    can occur and is very detrimental to plants and their components.

    liquid

    vaporous

    liquid

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    ERHARD Regelarmaturen Kavitation 3

    What is Cavitation?

    liquid

    vaporous

    liquid

    Proceeding in two steps:

    Transforming from liquid into vaporous

    Transforming fromvaporous into liquid

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    ERHARD Regelarmaturen Kavitation 4

    Where is caviation possible?

    Cavitation is possible in all medium, where transformation from liquid tovaporous state is possible.

    Cavitation is possible, when medium has high speed of velocity changes.

    Moving parts:

    Turbine blades

    Pump impellers

    Ship propellers

    Static parts:

    Abrupt cross-sectional constrictions

    in pipes

    Throttling with apertures

    Throttle operations on valves

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    ERHARD Regelarmaturen Kavitation 5

    Impacts of Cavitation

    Loud, crackling Sound

    Rough vibrations

    Material loss (Cavitation damage)

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    ERHARD Regelarmaturen Kavitation 6

    Typical damages of cavitation

    Butterfly valve

    Operating conditions

    Inlet pressure 1,2 1,4 bar

    Counterpressure: 0,1 bar

    Velocity of the flow 2,2 m/s(related to DN)

    Time of use 2 years

    Opening angle of the discapprox. 30

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    Typical damages of cavitation

    Gate valve

    The valve is not completelyclosed, so that there occurea very high velocity of flowin the remaining gaps, and

    the valve body is clearlydamaged after 3 months.

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    ERHARD Regelarmaturen Kavitation 8

    Typical damages of cavitation

    Angle valve

    The valve was used to filla tank.At ventil outlet there wasfalsely flanged a pipe,

    that dipped under thewater. This occuredcavitation at the restrictedbody, which had a

    damage, as illustrated, asa result.

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    ERHARD Regelarmaturen Kavitation 9

    Typical damages of cavitation

    Needle valve

    With wrong kind ofdimensioning is itpossible that even

    Control valves arecavitiering so strongly,that cavitation damagehas occured.

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    ERHARD Regelarmaturen Kavitation 11

    When is water vaporized?

    Temperature

    Pressure

    0.01

    0.10

    1.00

    10.00

    0 20 40 60 80 100 120

    liquidliquid

    vaporvapor

    Vaporpressure curve for water

    Temperature

    [C]

    Pressure

    [bar]

    The transforming of thephase from liquid to

    vapor depends on twoparameters:

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    ERHARD Regelarmaturen Kavitation 12

    When is water vaporized?

    In the pressure of theatmosphere (1 bar) is

    water vaporized intemperature of 100C.

    If the pressure sinks, isthe vaporising processstarting at a lowertemperature.

    Example:

    With a pressure of 0,02bar is the water vaporisingat a temperature of 18C.

    0.01

    0.10

    1.00

    10.00

    0 20 40 60 80 100 120

    liquidliquid

    vaporvapor

    Vaporpressure curve for water

    Temperature

    [C]

    Pressure

    [bar]

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    ERHARD Regelarmaturen Kavitation 13

    Underrun of the vaporpressures

    Flowing water in a pipe is effected by pressure, either from apump or by a geodetic height difference (elevated tank) and is farhigher than the vapor pressure.

    In order to clarify why at the restricted area of a valve of the vapor

    pressure can still be reached, the energybalance must beconsidered as a flow.

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    Energy content of the medium

    Position or potential energy

    Pressure energy

    Velocity energy

    Energy loss

    +

    constantconstant

    Law of Bernoulli

    The total energy of aflowing mediumconsists essentially ofthe following specificforms of energy:

    The sum of thesevarious forms of

    energy is constant!

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    Processing of energy in the system

    Po

    tential

    energy

    Velocity energy

    Pressure energy

    Energyloss

    In the existing storage pooltotal energy of a stationaryflow is stored as potentialenergy.

    This available potentialenergy is converted to flowthrough a horizontal line in:

    Velocity energy

    Pressure energy

    Energy loss

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    The processing of the energy restricted area

    Druck-Energie Geschwindigkeits-

    Energie

    Verlust-EnergieDruck-

    Energie

    Lossenery

    vena

    contracta

    Velocity energy

    Pressureenergy

    Due to the narrowing of the

    flow cross section at therestricted area (venacontracta), there is increasingthe flow velocity and thus thecorresponding amount ofenergy sharply.

    The percentage of losses isincreasing sharply due to the

    restriction.

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    Processing of energy at the restricted area

    Pressure

    energy

    Velocity energy

    Lossenergy

    Wird an dieser Stelle der Dampfdruck des Mediums unterschritten, soverdampft dieses.

    An der engsten Stelle nimmt wegen der Konstanz der Gesamtenergie dieverbleibende Druckenergie und damit der rtliche Druck stark ab.

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    Proceeding of the energy at the restricted area

    Vapor bubbles are formed

    The deform of bubbles withincreasing pressure

    And imploding

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    The implosion of the vaporbubbles

    Middle ofthe pipeline

    Pipe wall

    Direction of the flow

    Direction of the flow

    Fullydeveloped

    bubble

    Flatteningand

    indentation

    Implosion Mikrojet

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    When does it become cavitation?

    As the steam bubbles in the transitionfrom vapor to liquid suddenly collapse(implode), the water vapor bubblessurrounded are accelerated inwardwithin a split second.

    The resultant microjet hits the body orpipe wall with very high velocity,which causes pressure peaks up to10 000 bar, breaking the area of the

    molecular material.

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    ERHARD Regelarmaturen Kavitation 21

    When does it become cavitation?

    Higher differential pressure

    Less counterpressure

    Higher velocity of the flow

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    Measures against cavitation

    Cavitation is a physical process that can occur according to theprevailing operating conditions.

    The plant development should be trying to keep cavitation as far awayas possible from the plant.

    If this is not possible, it is possible to make the plant more reliable byselecting suitable valves.

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    What causes Cavitation?

    Geschwindigkeits-Energie

    Valves are only used fully opened or closed.

    Needle valves are control valves; but must be selected according tothe operating conditions (such as vaned rings - or slotted cylinder

    design).

    For extreme operating conditions, that are no more controllable withspecial control valves, must the restriction be staged (such as a gatefor counterpressure) or the restricted area must be ventilated.

    Notes for planning

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    Thank You

    ERHARD Water is our passion