cavitation in centrifugal pumps and prediction thereofpump division flowserve pumps idp pumps...

Pump DivisionpFlowserve PumpsIDP Pumps

Cavitation in Centrifugal Pumpsand Prediction Thereof

F k C ViFrank C. Visser

Flowserve Pump DivisionEtten-Leur, The Netherlands

Tutorial

Presented at 2005 ASME Fluids Engineering Division Summer Conference, June 19-23, 2005, Houston, Texas, USA, , , ,

Outline

• Part 1: What is cavitation and what does it mean forPart 1: What is cavitation and what does it mean for

pumping machinery?

• Part 2: Prediction of cavitation in centrifugal pumps

– Scaling laws

– Thermodynamic effect (temperature depression)

Eff t f di l d t i d– Effect of dissolved or entrained gases

– Calculating incipient cavitation (NPSH) from CFD

– Cavity length prediction

2

P t 1 Wh t i it tiPart 1 – What is cavitation

Cavitation is defined as the process of formation and disappearanceCavitation is defined as the process of formation and disappearance 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 it is the result of pressure reduction rather than heat addition.

Cavitation is a thermodynamic change of state with mass transfer from liquid to vapor phase and visa versa ( bubble formation & q p p (collapse).

3

P t 1 Wh t i it ti ( t )Part 1 – What is cavitation (cont.)

Sheet cavity on pumpSheet cavity on pump impeller vane leading edge (suction side)

Speed = 2990 RPMNPSHA = 70 m(230 ft)Flow rate =1820 m3/hFlow rate =1820 m3/h(8015 gpm)

Vane marker stripes atVane marker stripes atintervals of 10 mm (0.4 in)

Cavity length = 25-40 mmCavity length = 25-40 mm (1.0 – 1.5 in)

(from Visser et al, 1998)

4

Part 1 What is cavitation (cont )Part 1 – What is cavitation (cont.)

Cavitation causes or may cause:P f l (h d d )• Performance loss (head drop)

• Material damage (cavitation erosion)• Vibrations• Noise• Vapor lock (if suction pressure drops

b l b k ff l )below break-off value)(Visser et al, 1998)

General Advice: TRY TO AVOID CAVITATION (under normal operation)General Advice: TRY TO AVOID CAVITATION (under normal operation)

Unfortunately, economic or operational considerations often necessitate operation with some cavitation and then it is particularly important tooperation with some cavitation, and then it is particularly important to understand the (negative) effects of cavitation.

Design optimization to minimize cavitation

5

P t 1 Wh t i it ti ( t )Part 1 – What is cavitation (cont.)

Typical cavitation damages

Centrifugal pump impeller Francis turbine runnercavitation pitting erosion @ inlet

(from Dijkers et al, 2000)cavitation damage @ discharge

(from Brennen, 1994)

6


Cavitation behavior is typically expressed in terms of cavitation tparameters.

• Cavitation number:RUU

Upp

TeyeV

12

211 )(; :PumpslCentrifuga

• Net Positive Suction Head:ppNPSH V

01

• Thoma cavitation number:g

NPSH

HNPSH

TH

7


In general, cavitation performance is related to some “critical” value:gNPSHA (=available) > NPSHc or NPSHR (=critical or required)

Typical “critical” characteristics identified for centrifugal pumps:Typical critical characteristics identified for centrifugal pumps:• Incipient cavitation (NPSHi)• Developed cavitation causing 3% head drop (NPSH3%)p g p ( )• 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


Cavitation Phenomena

9

Cavitation Visualization Test PumpCavitation Visualization Test Pump

Pump Division

Begin Visual CavitationBegin Visual Cavitation

0% h d d1% head drop

3% head drop

4.05Head (m) Begin visual cavitation

0% head drop

3.95

4.00

3.85

3.90

3 75

3.80

3.85

3.70

3.75

0 10 20 30 40 50 60 70 80 90 100

Pump Division

NPSH (m)

0% Head Drop0% Head Drop

0% head drop1% head drop

3% head drop

4.05Head (m) Begin visual cavitation

0% head drop

3.95

4.00

3.85

3.90

3.75

3.80

3.700 10 20 30 40 50 60 70 80 90 100

NPSH ( )

Pump Division

NPSH (m)

1% Head drop1% Head drop

0% head drop

1% head drop

3% head drop

4.05Head (m)

Begin visual cavitation 0% head drop

3.95

4.00

3.85

3.90

3.75

3.80

3.700 10 20 30 40 50 60 70 80 90 100

NPSH ( )

Pump Division

NPSH (m)

3% Head drop3% Head drop


3% Head drop

4.05Head (m)

Begin visual cavitation0% head drop

3.95

4.00

3.85

3.90

3.75

3.80

3.700 10 20 30 40 50 60 70 80 90 100

NPSH ( )

Pump Division

NPSH (m)

RecirculationRecirculation


3% head drop

Recirculation

4.05Head (m)

Begin visual cavitation0% head drop

3.95

4.00

3.85

3.90

3.75

3.80

3.700 10 20 30 40 50 60 70 80 90 100

NPSH ( )

Pump Division

NPSH (m)


10


Typically (in practice):Typically (in practice):

• NPSHA > NPSH3%

• NPSHi > NPSHA (especially for low capacity)

Pumps run okay, BUT with some developed cavitation. u ps u o ay, U t so e de e oped ca tat o

General misconception:General misconception:

NPSHA > NPSHR No Cavitation

(This will only hold if NPSHR = NPSHi.)

11

Part 2 Cavitation predictionPart 2 – Cavitation prediction

• Scaling lawsScaling laws

• Thermodynamic effect

• Effect of dissolved or entrained gases

• Calculating incipient cavitation (NPSHi) from CFDCa cu at g c p e t ca tat o (N S i) o C

• Cavity length prediction

12

Part 2 Cavitation prediction (cont )Part 2 – Cavitation prediction (cont.)

Predicting NPSH at speeds other than reference or test speed ( scaling laws)

NPSHi:2

,2

NNNPSHNPSHNNPSH REFiii

(

,

HNPSH

N

TH

REF

constant)

NPSH3%:

(

2

%3%3

NNPSHfNPSH

H

REF

TH

1,;1,

%,3%3

fNNfNNN

f

REFREF

REFREF

“Postulate”: Amount of developed cavitation depends on residencetime f depends on size of the pump and ratio N/NREF

13

REF


Alternative approach to account for deviation from affinity law:Alternative approach to account for deviation from affinity law:

%3%3

REF NNNPSHNPSH

21

%,3%3

REF

REF N

Choice of is rather arbitrary and relies heavily on empiricism

Conservative choice:N < NREF , = 1N > NREF , = 2

14


Thermodynamic effecty(temperature depression)

Cavitation performanceCavitation performance depends on:• Temperature of liquidTemperature of liquid• Type of liquid

NPSHR reduction(E.g. Stepanoff method, orHydraulic Institute correctionHydraulic Institute correctionchart)

(from Brennen, 1994)

15

Part 2 Cavitation prediction (cont )Part 2 – Cavitation prediction (cont.)Predicting thermodynamic effect

Equilibrium theory:VfgL

REF

VBh

BNPSH

NPSHNPSHNPSH

;2

%,3%3

Stepanoff (1965 1978):

LpfgV VB

TCgvBNPSH ;2

NPSHBBStepanoff (1965, 1978):

][][; 112

2

1

1

ftormh

TCgB

NPSHBB

pL

][;64][;29 11

11

34

34

ftBH

NPSHormBH

NPSH

hfgV

Non-equilibrium theory bubble dynamic (CFD) calculations, involvingtime-dependent two-phase flow calculations

HH VV

16

time dependent two phase flow calculations


Influence of dissolved and/or entrained gases: “conceptual effective or artificial” vapor pressure:

PE = PV + P P (Ch 1993)PE = yP0 (Chen, 1993)

Key characteristic:Performance (breakdown) comes from gas evolution and gasPerformance (breakdown) comes from gas evolution and gas expansion, rather than classical vapor formation.

Dissolved and/or entrained gases result in reduction of (effective) field NPSHA:

NPSHA* = (P01 – P ) / gNPSHA (P01 PE) / g

“Hidden danger”: NPSHA > NPSHR but NPSHA* < NPSHR

17


( ) f CPredicting incipient cavitation (NPSHi) from CFD

T i l hTypical approach:

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)

18

Part 2 Cavitation prediction (cont )Part 2 – Cavitation prediction (cont.)Streamline through point of minimum pressure

ppg

ppNPSH Vi

i 21

,01

ppppUpp

Vi

ii

min1,1

221

,1,01

)(

ppNPSH min01

gppNPSHi

min01

So: NPSHi follows from pmin and p01 of calculated pressure field, anddoes not require pV to be known!

19


Running simulations for several flow rates produces NPSHi curve:

(from Visser 2001)

20

(from Visser, 2001)


Note: CFD calculated characteristic is for impeller flow!

To project it on pump throughput one needs to accountfor volumetric efficiency ( eye wear ring leakage flow):y ( y g g )

Qimpeller = Qpump + Qleakage Qpump = Qimpeller - Qleakage

Computed curve shifts left by amount Q = Qleakage Computed curve shifts left by amount Q Qleakage

;~),,,,,f( 2

1leakage L

puuDLDpQ

2Re;Re/2373.0;Re/24 25.0

turbulentlaminaru

L

It becomes particularly important to take Qleakage into account for lowNS (specific speed) impellers. For high NS the relative influence is less.

21


What if NPSHA < NPSHi ? Find region on impeller blade surface where p < pV

• physically unrealistic but it gives• physically unrealistic, but it gives• first “indication” of cavitation area, and• first approximation of cavity bubble lengthfirst approximation of cavity bubble length

Note: The actual cavity will be biggerNote: The actual cavity will be bigger bubble length will be underestimated

22


To visualize p < pV region from non-cavitating flow simulation:

Plot isotimic surface for threshold value pV*

pppp AVV ,11* )(

pUp

Up

VA

2

21

1

221

1

)(

NPSPApNPSHAgp

pp VA

01

2,1 )(

NPSPAp 01

23


Example:

Plot of p < pV region

NPSHA = 15.5 m (51 ft)NPSHi = 28 m (92 ft)( )N = 2980 RPMQ = 400 m3/h (1760 USGPM)

Cavitation on bladeCavitation on blade suction side

24


Putting LCAV = m L(p<pV), m=O(3), one can get some impression of g CAV (p pV), ( ), g pexpected cavitation erosion rate

Güli h (1986 1988 1989) n

CAVL 23632

Gülich (1986, 1988, 1989):

(*)

nn

SeACAV

CAV

LLEELE

TAULLCE 23632

10,

8

or (*)

with n = 2.83 for blade suction side and 2 6 for blade pressure side

nnCAV LLEELE 1212

n = 2.6 for blade pressure side

Equation (*) is especially powerful when comparing designs and q ( ) p y p p g gevaluate susceptibility to cavitation erosion (in a relative sense).

Design optimization studies

25

Design optimization studies


• Results and theory thus far do not require two-phase flow l l ticalculations.

• Still it provides important information of an impeller design p p p gregarding cavitation performance.

• Next level of improvement has to come from CFD calculations• Next level of improvement has to come from CFD calculations 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


CFD Cavitation models

Typically two approaches:

• Equilibrium models– Barotropic or pseudo density models; =(p)– Somewhat “simplistic”, yetp , y– Attractive since they can be used in single phase codes

• Bubble dynamic modelsy– Rayleigh-Plesset equation– Vapor-liquid interaction (time-dependent mass & heat transfer)– Closer to realityCloser to reality– More complicated and more “CPU-expensive”– E.g. Volume of Fluid (VOF) model

27

Part 2 Cavitation prediction (cont )Part 2 – Cavitation prediction (cont.)Example:

Plot of cavity bubble

Equilibrium modelqCFX-TASCflow(CEV-model)

NPSHA 15 5 (51 ft)NPSHA = 15.5 m (51 ft)NPSHi = 28 m (92 ft)N = 2980 RPMQ = 400 m3/hQ = 400 m3/h (1760 USGPM)m 3

Cavitation on blade suction side

28

Part 2 Cavitation prediction (cont )Part 2 – Cavitation prediction (cont.)Application:

With CFD it ti d l di t NPSH3% f CFDWith CFD cavitation models one can predict NPSH3% from CFD calculated head drop curves

(from Visser, 2001; CEV-model prediction)

29

( , ; p )

Concluding Remarks

• Cavitation is a phenomenon which can seriously impact• Cavitation is a phenomenon which can seriously impact performance and operation of pumps.

• Predicting cavitation performance is an important topic• Predicting cavitation performance is an important topic, not only for pumps, but for fluid machinery in general.

• Traditional (scaling) methods are still important and• Traditional (scaling) methods are still important and useful.

• CFD methods provide further insight and are becoming• CFD methods provide further insight and are becoming more and more common.

Bubble dynamic (CFD) methods are emerging and hold a• Bubble dynamic (CFD) methods are emerging and hold a promise for the future.

30

ReferencesReferences

Brennen, C.E.H d d i f PHydrodynamics of Pumps. Oxford University Press (1994)

Chen C CChen, C.C.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.Proceedings of the 20th IAHR Symposium August 6-9 2000 CharlotteProceedings of the 20 IAHR Symposium, August 6 9, 2000, Charlotte,North Carolina, USA.

Gülich J F and Pace SGülich, J. F. and Pace, S.Quantitative Prediction of Cavitation Erosion in Centrifugal Pumps.Proceedings of the 13th IAHR Symposium (1986), Montreal, Canada.

31

References (cont )References (cont.)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 derBlasenfeldlänge und des KavitationsschallsBlasenfeldlänge und des Kavitationsschalls.Thesis, Technische Hochschule Darmstadt, Germany, 1989.

Stepanoff A JStepanoff, A.J.Pumps and Blowers – Two-Phase Flow.John Wiley & Sons (1965), Krieger Publishing (1978)

32

References (cont )References (cont.)

Visser, F.C., Backx, J.J.M., Geerts, J., Cugal, M. & D. Miguel Medina TorresPump impeller lifetime improvement through visual study of leading-edge cavitation.Proceedings of the 15th International Pump Users Symposium, TurbomachineryLaboratory,Texas A&M University, College Station, Texas, USA, pp. 109-117. y, y, g , , , ppAlso in: Pumping Technology, vol. 2 (1998), pp. 149-157.

Visser F CVisser, F.C.Some user experience demonstrating the use of CFX-TASCflow computational fluiddynamics for cavitation inception (NPSH) analysis and head performance predictionof centrifugal pump impellers. FEDSM2001-18087Proceedings of the 4th ASME International Symposium on Pumping Machinery,May 29 – June 1 2001 New Orleans Louisiana USAMay 29 June 1, 2001, New Orleans, Louisiana, USA.

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