Pu m Di v i s i o n Flowserve Pumps IDP Pumps
Cavi vita tatio tion n in Centr ntrif ifugal ugal Pump Pumps s and Predic redicti tion on There hereof of ran
.
sser ss
Flowserve Pump Division Etten-Leur,, The Netherlands Etten-Leur
Tutorial Presented at 2005 ASME Fluids Engineering Division Summer Conference, June 19 19-2 -23 3 2005 Houston Texas USA
Outline
• Part 1: What is cavitation and what does it mean for pumping machinery? • Pa Part rt 2: Pr Pred edic icti tion on of of cav cavit itat atio ion n in cent centri rifu fuga gall pump pumps s – Scaling laws – Thermodynamic effect effect (temperature (temperature depression) –
ec o
sso ve or en ra ne gases
NPSH ) from CFD – Calculating incipient cavitation ( NPSH
– Cavity length prediction
2
Outline
• Part 1: What is cavitation and what does it mean for pumping machinery? • Pa Part rt 2: Pr Pred edic icti tion on of of cav cavit itat atio ion n in cent centri rifu fuga gall pump pumps s – Scaling laws – Thermodynamic effect effect (temperature (temperature depression) –
ec o
sso ve or en ra ne gases
NPSH ) from CFD – Calculating incipient cavitation ( NPSH
– Cavity length prediction
2
ar t
–
at s c av t at o n
of the vapour phase of a liquid when it is subjected to reduced and subsequently increased pressures. The formation of cavities is a process analogous to boiling in a liquid, although altho ugh it is the result result of pressur pressure e reduction reduction rather rather than heat heat addition. Cavitation is a thermodynamic change of state with mass transfer from li uid to va or hase and visa versa bubble formation & collapse).
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ar
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a s cav a on con .
impeller vane leading edge (suction side) Speed = 2990 RPM NPSHA = 70 m(230 ft) (8015 gpm) intervals of 10 mm (0.4 in) (1.0 – 1.5 in) (from Visser et al, 1998) 4
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Cavitation causes or may cause: • er ormance oss ea rop • Material damage (cavitation erosion) • Vibrations • Noise • Vapor lock (if suction pressure drops e ow rea -o va ue (Visser et al, 1998)
Unfortunately, economic or operational considerations often necessitate , understand the (negative) effects of cavitation. Design optimization to minimize cavitation 5
ar
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a s cav a on con .
Typical cavitation damages
Centrifugal pump impeller cavitation pitting erosion @ inlet (from Dijkers et al, 2000)
Francis turbine runner cavitation damage @ discharge (from Brennen, 1994)
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Cavitation behavior is typically expressed in terms of cavitation parame ers. • Cavitation number:
p1 pV 1 2
2
U
; (Centrifugal Pumps : U U eye R1T )
• Net Positive Suction Head:
NPSH
p01 pV
g
• Thoma cavitation number:
TH
NPSH H 7
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In general, cavitation performance is related to some “critical” value: NPSHA (=available) > NPSHc or NPSHR (=critical or required) “ ” • Incipient cavitation ( NPSHi) • Develo ed cavitation causin 3% head dro NPSH3% • Developed cavitation causing complete head breakdown ( vapor lock). Choice of NPSHR is rather arbitrary, but usually NPSHR=NPSH3% Alternative choices: • NPSHR=NPSH1% or NPSHR=NPSH5% • NPSHR=NPSHi (cavitation free operation) 8
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Cavitation Phenomena 9
Cavitation Visualization Test Pump
Pump Division
Begin Visual Cavitation 3% head drop 1% head drop ea
rop Begin visual cavitation
Head (m) 4.05 4.00 3.95 3.90 3.85 3.80 . 3.70 0
10
20
30
40
50
60
70
80
90
100
NPSH (m) Pump Division
0% Head Drop 3% head drop 1% head drop Begin visual cavitation
Head (m) 4.05 4.00 3.95 3.90 3.85 3.80 3.75 3.70 0
10
20
30
40
50
60
70
80
90
100
m Pump Division
1% Head drop 3% head drop 1% head drop Begin visual cavitation
Head (m) 4.05 4.00 3.95 3.90 3.85 3.80 3.75 3.70 0
10
20
30
40
50
60
70
80
90
100
m Pump Division
3% Head drop 3% Head drop 1% head drop Begin visual cavitation
Head (m) 4.05 4.00 3.95 3.90 3.85 3.80 3.75 3.70 0
10
20
30
40
50
60
70
80
90
100
m Pump Division
Recirculation 3% head drop 1% head drop
Recirculation
Begin visual cavitation
Head (m) 4.05 4.00 3.95 3.90 3.85 3.80 3.75 3.70 0
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20
30
40
50
60
70
80
90
100
m Pump Division
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.
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icall
.
in ractice :
• NPSHA > NPSH3% • NPSHi > NPSHA (especially for low capacity)
Pum s run oka
BUT with some develo ed cavitation.
NPSHA > NPSHR No Cavitation
(This will only hold if NPSHR = NPSHi.)
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– • Scalin laws • Thermodynamic effect • Effect of dissolved or entrained gases • Calculatin inci ient cavitation NPSHi from CFD • Cavity length prediction
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Predicting NPSH at speeds other than reference or test speed ( scaling laws) NPSHi:
N 2 NPSH i N NPSH i NPSH i REF
2
REF
(
NPSH
constant) 2
NPSH 3%:
N NPSH NPSH , N REF N N REF , f 1 ; N N REF , f 1
“Postulate”: Amount of developed cavitation depends on residence time f depends on size of the pump and ratio N/N REF 13
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NPSH
NPSH
,
N REF
1 2 Choice of is rather arbitrary and relies heavily on empiricism Conservative choice: N < N REF , = 1 N > N REF , = 2
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Thermodynamic effect (temperature depression)
depends on: • Tem erature of li uid • Type of liquid
NPSHR reduction (E.g. Stepanoff method, or chart)
(from Brennen, 1994) 15
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Predicting thermodynamic effect
NPSH 3% NPSH 3%, REF NPSH Equilibrium theory:
2
h fg
L
V 2 v fg g C p T
,
V V V L
1
2
L g C p T 1 1 ; [ ] [ ] B1 m or ft 2 V
NPSH
29 V
4
fg
B1 3 ; [m 1 ] or NPSH
64
4
B1 3 ; [ ft 1 ]
V
Non-equilibrium theory bubble dynamic (CFD) calculations, involving 16
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Influence of dissolved and/or entrained gases: “conceptual effective or artificial ” vapor pressure: P E = PV + P E = yP0
en,
Key characteristic: expansion, rather than classical vapor formation. Dissolved and/or entrained gases result in reduction of (effective) field NPSHA: * = 01 – E “Hidden danger”: NPSHA > NPSHR but NPSHA* < NPSHR 17
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Predicting incipient cavitation ( NPSHi) from CFD yp ca approac : Create 3D geometry model/grid of impeller passage Solve flow field with CFD code (non-cavitating) Calculate incipient NPSH from CFD pressure field (next slide)
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Streamline through point of minimum pressure
NPSH i
01,i
V
g
p01,i p1,i 12 U p1,i p1 ( pmin pV )
i
g
So: NPSHi follows from pmin and p01 of calculated pressure field, and does not require pV to be known! 19
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Running simulations for several flow rates produces NPSHi curve:
, 20
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Note: CFD calculated characteristic is for impeller flow! To project it on pump throughput one needs to account for volumetric efficienc
e e wear rin leaka e flow :
Qimpeller = Qpump + Qleakage Qpump = Qimpeller - Qleakage
=
Qleakage f( p, D, L, , , ) ~ 12 D u ; u
p L
laminar 24 / Re ; turbulent 0.2373 / Re 0.25 ; Re
u
2
It becomes particularly important to take Qleakage into account for low N S (specific speed) impellers. For high N S the relative influence is less. 21
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What if NPSHA < NPSHi ?
Find region on impeller blade surface where p < pV , • first “indication” of cavitation area, and •
bubble length will be underestimated
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To visualize p < pV region from non-cavitating flow simulation:
Plot isotimic surface for threshold value pV *
pV pV ( p1 p1, A ) *
p1 12 U 2 1 U 2
,
p01 g NPSHA 01
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Example: Plot of p < pV region NPSHA = 15.5 m (51 ft) NPSHi = 28 m 92 ft N = 2980 RPM Q = 400 m3/h
(1760 USGPM)
suction side
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m= 3 one can et some im ression of Puttin L =mL < expected cavitation erosion rate n
c
,
,
or with
L A U e CAV ,10
C : E
CAV
2
1
2
A 8T S 1
n = 2.83 for blade suction side and n= .
E uation * is es eciall owerful when com arin desi ns and evaluate susceptibility to cavitation erosion (in a relative sense).
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• Results and theory thus far do not require two-phase flow ca cu a ons. • Still it rovides im ortant information of an im eller desi n regarding cavitation performance.
with cavitation model. • Calculations with a cavitation model are time consuming and tend to be “CPU-expensive” • Several cavitation models exist to date, and development of cavitation models is still ongoing 26
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CFD Cavitation models Typically two approaches: • Equilibrium models – Barotropic or pseudo density models; = ( p) – Somewhat “sim listic”, et – Attractive since they can be used in single phase codes
• Bubble dynamic models – – – – –
Rayleigh-Plesset equation Vapor-liquid interaction (time-dependent mass & heat transfer) More complicated and more “CPU-expensive” E.g. Volume of Fluid (VOF) model 27
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Example: Plot of cavity bubble E uilibrium model CFX-TASCflow (CEV-model) . m NPSHA = NPSHi = 28 m (92 ft) N = 2980 RPM (1760 USGPM) m3 Cavitation on blade suction side
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Application: cav a on mo e s one can pre c calculated head drop curves
rom
from Visser 2001 CEV-model rediction 29
Concluding Remarks
• performance and operation of pumps. , not only for pumps, but for fluid machinery in general. useful. more and more common. •
u e ynam c me o s are emerg ng an promise for the future.
o
a
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Brennen, C.E. y ro ynam cs o umps. Oxford University Press (1994) , . . Cope with dissolved gases in pump calculations. Chemical Engineering, vol. 100 (1993), pp. 106-112. Dijkers, R.J.H., Visser, F.C. & Op De Woerd, J.G.H. Redesign of a high-energy centrifugal pump first-stage impeller. th , - , , North Carolina, USA.
,
, . . , . Quantitative Prediction of Cavitation Erosion in Centrifugal Pumps. Proceedings of the 13th IAHR Symposium (1986), Montreal, Canada. 31
. Gülich, J. F. and Rösch, A. Cavitation Erosion in Centrifugal Pumps. World Pumps, July 1988, pp. 164-168. Gülich, J. F. Guidelines for Prevention of Cavitation in Centrifugal Feedpumps. EPRI Final Report GS-6398, (1989). Gülich, J. F. Beitrag zur Bestimmung der Kavitationserosion in Kreiselpumpen auf Grund der . Thesis, Technische Hochschule Darmstadt, Germany, 1989. , . . Pumps and Blowers – Two-Phase Flow. John Wiley & Sons (1965), Krieger Publishing (1978) 32
