Post on 02-Apr-2018
7/27/2019 7. Cavitation
1/12
SS 2012 Fachgebiet Strmungsmaschinen
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.
7/27/2019 7. Cavitation
2/12
SS 2012 Fachgebiet Strmungsmaschinen
Prof. Dr.-Ing. Martin Gabi
Machines und Prozesses Hydraulic FluidMachinery
7.
Cav
itation
7/27/2019 7. Cavitation
3/12
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.
7/27/2019 7. Cavitation
4/12
SS 2012 Fachgebiet Strmungsmaschinen
Prof. Dr.-Ing. Martin Gabi
Machines und Prozesses Hydraulic FluidMachinery
7.
Cav
itation
4
Cavitation damages (examples)
7/27/2019 7. Cavitation
5/12
SS 2012 Fachgebiet Strmungsmaschinen
Prof. Dr.-Ing. Martin Gabi
Machines und Prozesses Hydraulic FluidMachinery
7.
Cav
itation
5
Cavitation (examples)
7/27/2019 7. Cavitation
6/12
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.
7/27/2019 7. Cavitation
7/12
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
7/27/2019 7. Cavitation
8/12
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 >
7/27/2019 7. Cavitation
9/12
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):
7/27/2019 7. Cavitation
10/12
SS 2012 Fachgebiet Strmungsmaschinen
Prof. Dr.-Ing. Martin Gabi
Machines und Prozesses Hydraulic FluidMachinery
7.
Cav
itation
10
7/27/2019 7. Cavitation
11/12
SS 2012 Fachgebiet Strmungsmaschinen
Prof. Dr.-Ing. Martin Gabi
Machines und Prozesses Hydraulic FluidMachinery
7.
Cav
itation
11
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
7/27/2019 7. Cavitation
12/12
SS 2012 Fachgebiet Strmungsmaschinen
Prof. Dr.-Ing. Martin Gabi
Machines und Prozesses Hydraulic FluidMachinery
7.
Cav
itation
12