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A TECHNICAL PAPER PRESENTATION REPORT ON

INTRODUCTION TO CAVITATIONIN HYDRAULIC MACHINERY

BY

PRAMOD R. KHOT TE(MECH)- ROLL NO: TM 14

Under the Guidance of

Prof.M.S.Ramgir

DEPARTMENT OF MECHANICAL ENGINEERINGSINHGAD INSTITUTE OF TECHNOLOGY,LONAVALAUNIVERSITY OF PUNE

2006-07



CONTENTSCONTENTS

1. Introduction to Cavitations

2. Cavitation Testing

2.1 General Model Test

3. Cavitations in Different Hydro machines

3.1 Centrifugal Pumps

3.2 Francis Turbine

3.3 Kaplan Turbine

4. Method Of Measuring Cavitation Intensity

5. Conclusion

References



1. Introduction to Cavitations

Boiling (A->B)Boiling (A->B)

Change from liquid to vapor Change from liquid to vapor

due to increase indue to increase in

temperature.temperature.

Cavitation (A->C)Cavitation (A->C)

Change from liquid to vapor Change from liquid to vapor due to decrease in pressure.due to decrease in pressure.

BASICS OF CAVITATIONBASICS OF CAVITATION



Traveling of bubble from low pressure toTraveling of bubble from low pressure to

high pressure region.high pressure region.



The exact vapor pressure for any temperature of water.The exact vapor pressure for any temperature of water.

If you fall below this vapor If you fall below this vapor

pressure the water will boil.pressure the water will boil.

Fahrenheit

Centigrade

Vapour presslb/in2A

Vapour press(Bar) A

40 4.4 0.1217 0.00839

100 37.8 0.9492 0.06546

180 82.2 7.510 0.5179

212 100 14.696 1.0135

300 148.9 67.01 4.62



2. Cavitation Testing

test procedure consists intest procedure consists inmeasuringmeasuring

•• the overall hydraulicthe overall hydrauliccharacteristic of the machinecharacteristic of the machineand the correspondingand the correspondingefficiency;efficiency;

•• guaranteed operating pointguaranteed operating pointefficiencies and power output;efficiencies and power output;

•• cavitation characteristics;cavitation characteristics;

•• pressure fluctuations at the draftpressure fluctuations at the drafttube inlet, at the spiral casetube inlet, at the spiral caseinlet and on the head cover of inlet and on the head cover of

the machine;the machine;•• runaway speed ;runaway speed ;

•• index test and flow velocityindex test and flow velocitydistribution in various locations.distribution in various locations.

Reduced Scale Model of a Francis Turbine installed on the EPFLReduced Scale Model of a Francis Turbine installed on the EPFL

PF1 Test RigPF1 Test Rig



3. Cavitations in Different Hydro machines

Fig. 1Fig. 1 Traveling bubbleTraveling bubbledevelopment in a storagedevelopment in a storagepump impeller,pump impeller, QQ ==QQ00 , during, duringmodel testing.model testing.

For a lower discharge value,For a lower discharge value,the flow incidence is increasedthe flow incidence is increased

and then, a leading edgeand then, a leading edgecavity appears, as shown Fig.cavity appears, as shown Fig.2. For low values of σ,2. For low values of σ,cavitation vortices can becavitation vortices can beobserved at the inlet of theobserved at the inlet of therunner coming from therunner coming from theleakage flow through theleakage flow through theshroud seal,shroud seal,

Fig. 2Fig. 2 Leading edge cavityLeading edge cavitydevelopment in a storagedevelopment in a storagepump impeller,pump impeller, QQ<<QQ00 , during, duringmodel testing.model testing.

Fig. 1Fig. 1

Fig. 2Fig. 2

Centrifugal Pumps3.1



Fig. AFig. A Inlet edge cavitation,Inlet edge cavitation,

Francis turbine. This type of Francis turbine. This type of

cavitation is not very sensitive tocavitation is not very sensitive tothe value of the Thoma number the value of the Thoma number

and it can lead to a severeand it can lead to a severe

erosion of the blades.erosion of the blades. Fig. BFig. B

Traveling bubble cavitation in aTraveling bubble cavitation in a

Francis turbine runner.Francis turbine runner. .. ThisThistype of cavitation, see Fig. 10, istype of cavitation, see Fig. 10, is

very sensitive to the content of very sensitive to the content of

cavitation nuclei and to thecavitation nuclei and to the

value of the Thoma number value of the Thoma number

Fig. CFig. C Cavitation whirl at low andCavitation whirl at low andhigh discharge operation, in ahigh discharge operation, in a

Francis turbine discharge ring.Francis turbine discharge ring.

AA

BB

CC

Francis Turbine 3.2



Fig.Fig. Hub cavitation development for a Kaplan runnerHub cavitation development for a Kaplan runner

Fig.Fig. Tip clearance and hub cavitation for a Kaplan Tip clearance and hub cavitation for a Kaplan

RunnerRunner

3.3 KaplanTurbine



Kaplan Turbine

This type of cavitation is very sensitive to the ThomaThis type of cavitation is very sensitive to the Thoma

number. Any effect of the water cavitation nuclei contentnumber. Any effect of the water cavitation nuclei content

is observed for this type of cavitationis observed for this type of cavitation

the air entertainment can have a great influence on thethe air entertainment can have a great influence on the

extent of this cavity. Since the blades are adjustable, theextent of this cavity. Since the blades are adjustable, the

runner is not shrouded and, then as shown Fig. 21, tiprunner is not shrouded and, then as shown Fig. 21, tip

clearance cavitation takes place in the gap between theclearance cavitation takes place in the gap between the

blades and the machine casing, leading to an erosionblades and the machine casing, leading to an erosion

risk even though the head could be low. This type of risk even though the head could be low. This type of cavitation is driven by the flow shear layer in this gap andcavitation is driven by the flow shear layer in this gap and

it is not very dependent of the Thoma cavitation number it is not very dependent of the Thoma cavitation number



Typical eroded areas of a pump impeller (A), Francis runner (B),Typical eroded areas of a pump impeller (A), Francis runner (B),

Kaplan runner (C).Kaplan runner (C).

A B

C



4. Method of measuring cavitation intensity4. Method of measuring cavitation intensity

Blue tack paint methodBlue tack paint method

Pressure sensitive paper Pressure sensitive paper

methodmethod

Impulse pressure sensor Impulse pressure sensor

methodmethod



Effect of cavitationEffect of cavitation

The metallic surfaces are damaged and cavities areThe metallic surfaces are damaged and cavities are

formed on the surfaces.formed on the surfaces.

Due to sudden collapse of vapour bubble, considerableDue to sudden collapse of vapour bubble, considerable

noise and vibration are produced.noise and vibration are produced.

The efficiency of a turbine decreases due to cavitation.The efficiency of a turbine decreases due to cavitation.

Due to pitting action the surface of the turbine bladesDue to pitting action the surface of the turbine blades

becomes rough and the force exerted by water on thebecomes rough and the force exerted by water on the

turbine blades decreases. Hence the work done by water turbine blades decreases. Hence the work done by water

or output horse power becomes less and thus efficiencyor output horse power becomes less and thus efficiencydecreases.decreases.



Precaution against cavitationPrecaution against cavitation

The pressure of the flowing liquid in any part of theThe pressure of the flowing liquid in any part of the

hydraulic system should not be allowed to fall below itshydraulic system should not be allowed to fall below its

vapour pressure. If the flowing liquid is water, then thevapour pressure. If the flowing liquid is water, then the

absolute pressure head should not be below 2.5 m of absolute pressure head should not be below 2.5 m of

water.water. The special materials or coatings such aluminium –The special materials or coatings such aluminium –

bronze and stainless steel, which are cavitation resistantbronze and stainless steel, which are cavitation resistant

materials should be usedmaterials should be used..



CONCLUSIONCONCLUSION

A survey of the different types of cavitation featured by hydraulicA survey of the different types of cavitation featured by hydraulicmachinery has been carried out. This survey finally emphasizes themachinery has been carried out. This survey finally emphasizes theimportance of model testing for defining the proper setting level of theimportance of model testing for defining the proper setting level of themachines. The determination of the plant NPSE of a machine is amachines. The determination of the plant NPSE of a machine is asubtle process, which should include:subtle process, which should include:

The type of cavitation developments the runner or the impeller isThe type of cavitation developments the runner or the impeller is

experiencing over the operating range of interests;experiencing over the operating range of interests;The risk of cavitation erosion associated to this type of cavitation;The risk of cavitation erosion associated to this type of cavitation;

The risk of performance alteration.The risk of performance alteration.

However, the assessment of those cavitation developmentsHowever, the assessment of those cavitation developmentscan take benefits in a large extent by developing monitoringcan take benefits in a large extent by developing monitoringinstrumentation for the free stream cavitation nuclei, which caninstrumentation for the free stream cavitation nuclei, which can

influence the cavitation development, Moreover, with respect to theinfluence the cavitation development, Moreover, with respect to thecavitation erosion, relevant indirect method, such as measurement of cavitation erosion, relevant indirect method, such as measurement of vibratory levels, can be very useful for quantifying the risk during modelvibratory levels, can be very useful for quantifying the risk during modelteststests



REFERENCESREFERENCES

[1] Francois AVELLAN “Cavitation In Hydraulic Machinery”.Proc.The[1] Francois AVELLAN “Cavitation In Hydraulic Machinery”.Proc.The66thth

International Conference On Hydraulic Machinery andInternational Conference On Hydraulic Machinery andHydrodynamics,Romania,October 21-22,2004.Hydrodynamics,Romania,October 21-22,2004.

[2] Masatake Maekawa, “Study Of Cavitation Erosion On Hydraulic[2] Masatake Maekawa, “Study Of Cavitation Erosion On HydraulicTurbine Runners”. 5Turbine Runners”. 5thth International Symposium on CavitationInternational Symposium on Cavitation(CAV2003) Osaka, Japan, Nov 1-4,2003(CAV2003) Osaka, Japan, Nov 1-4,2003[3] P.N.Modi and S.M.Seth. “Fluid Mechanics and Hydraulic[3] P.N.Modi and S.M.Seth. “Fluid Mechanics and HydraulicMachinery.Machinery.

[4] R.K.Rajput “Fluid Mechanics and Hydraulic Machines”.[4] R.K.Rajput “Fluid Mechanics and Hydraulic Machines”.

[5] R.K.Bansal “Fluid Mechanics and Hydraulic Machines”.[5] R.K.Bansal “Fluid Mechanics and Hydraulic Machines”.

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