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Prof. Dr.-Ing. Martin Gabi

Machines und Prozesses Hydraulic FluidMachinery

7.

Cav

itation

1

Cavitation

What is cavitation?

Formation of local vapor areas and vapor bubbles in the inside

of flowing fluids.

When does cavitation occur?

When the static pressure drops below the vaporpressure of the liquid

What are determining factorscavitation in centrifugal pumps?

Operating state, rotor/blade geometry, fluid properties.

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Prof. Dr.-Ing. Martin Gabi

Machines und Prozesses Hydraulic FluidMachinery

7.

Cav

itation

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SS 2012 Fachgebiet Strmungsmaschinen

Prof. Dr.-Ing. Martin Gabi

Machines und Prozesses Hydraulic FluidMachinery

7.

Cav

itation

3

Cavitation (2)

Why is cavitation potentially dangerous?

If the steam bubbles are carried by the flow into areasin which the static pressure rises again above the

vapor pressure, then the vapor bubbles implode. At the sametime extremely high flow velocities are locally present around thevapor bubbles.If this takes place in the vicinity of a wall, then the impulse

forces of the high flow velocities can cause material damages.

Is it sensible to avoid cavitation?

No. Cavitation is only dangerous if an intensive interactionwith the walls occurs. Modern centrifugal pumps operate with a partial andcontrolled amount of cavitation.

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SS 2012 Fachgebiet Strmungsmaschinen

Prof. Dr.-Ing. Martin Gabi

Machines und Prozesses Hydraulic FluidMachinery

7.

Cav

itation

4

Cavitation damages (examples)

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SS 2012 Fachgebiet Strmungsmaschinen

Prof. Dr.-Ing. Martin Gabi

Machines und Prozesses Hydraulic FluidMachinery

7.

Cav

itation

5

Cavitation (examples)

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SS 2012 Fachgebiet Strmungsmaschinen

Prof. Dr.-Ing. Martin Gabi

Machines und Prozesses Hydraulic FluidMachinery

7.

Cav

itation

6

Harmful effects of cavitation inhydraulic FM

Two-phase flowThe vapor bubbles cause two-phase flow. Sincethe specific volume of the vapor phase is much more larger, theeffective cross-section of the flow is narrowed. This leads to

blockage and reduction of performance and efficiency.

Noise and vibrations

The implosions of the vapor bubbles cause high-frequencynoises. Through blockage effects, unstationary flowphenomenon can occur.

Material removal, destructionThrough the intensive and frequent pressure peaks, due to theimplosions near the wall, local material strains can appear, whichcan lead to erosion and corrosion.

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SS 2012 Fachgebiet Strmungsmaschinen

Prof. Dr.-Ing. Martin Gabi

Machines und Prozesses Hydraulic FluidMachinery

7.

Cav

itation

7

Evap

oration

curve

K

avitation

p T diagram

Evaporation

Temperature T

Absolu

te

pressur

e

p

fluid

solid

gasTP

CP

CP critical point

TP Triple pointMelting

curve

p > pD

p < pDS

ublim

atio

n

c

urve

Solid

ifica

tioncurve

M

elting

curve

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SS 2012 Fachgebiet Strmungsmaschinen

Prof. Dr.-Ing. Martin Gabi

Machines und Prozesses Hydraulic FluidMachinery

7.

Cav

itation

8

avoiding cavitation?

Nowhere in the fluid machine the pressure shall be lower than the vaporpressure of the fluid.

Criteria:

Problem: Where is pmin?Only the manufacturer has this information.

Solution: Manufacturer must give the permissible minimal pressure ata reference point (usually at the inlet flange)

min Dp p>

, , mintot E D tot E

p p p p >

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SS 2012 Fachgebiet Strmungsmaschinen

Prof. Dr.-Ing. Martin Gabi

Machines und Prozesses Hydraulic FluidMachinery

7.

Cav

itation

9

Cavitation index

Definition:

(Net positive suction head)

Difference of the total pressure to the vapor pressure at theinlet (E)

Given by manufacturer: NPSHerf , NPSHreq

( ) ( ),1

tot E DNPSH p pg=

2

, ,2

tot E I I Verl S I p p gz p c

= + +

To be figured out by operator: NPSHvorh , NPSHav

Energy balance at inlet part of facility (I E):

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Prof. Dr.-Ing. Martin Gabi

Machines und Prozesses Hydraulic FluidMachinery

7.

Cav

itation

10

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7.

Cav

itation

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2

,1 ( )

2= + +vorh DI I Verl S INPSH p gz p p c

g

2

, , 2tot E I I Verl S I p p gz p c

= + +

( )( ),

1tot E DNPSH p p

g=

vorh erf NPSH NPSH

Criteria for free of cavitation

Inlet facility Machine

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SS 2012 Fachgebiet Strmungsmaschinen

Prof. Dr.-Ing. Martin Gabi

Machines und Prozesses Hydraulic FluidMachinery

7.

Cav

itation

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