Automatic Card Shuffler and Dealer Brian Cody Matt Erhard Justin Madigan.
ERHARD Kavitation En
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Transcript of ERHARD Kavitation En
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ERHARD Regelarmaturen Kavitation
Understand
Recognize
Avoid
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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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What is Cavitation?
liquid
vaporous
liquid
Proceeding in two steps:
Transforming from liquid into vaporous
Transforming fromvaporous into liquid
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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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Impacts of Cavitation
Loud, crackling Sound
Rough vibrations
Material loss (Cavitation damage)
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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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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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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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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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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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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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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