Pump Division Flowserve Pumps IDP Pumps Cavitation in Centrifugal Pumps and Prediction Thereof F kC Vi Frank C. Visser Flowserve Pump Division Etten-Leur, The Netherlands Tutorial Presented at 2005 ASME Fluids Engineering Division Summer Conference, June 19-23, 2005, Houston, Texas, USA
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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)
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
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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)
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Part 1 What is cavitation (cont )Part 1 – What is cavitation (cont.)
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
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Part 1 What is cavitation (cont )Part 1 – What is cavitation (cont.)
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)
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Part 1 What is cavitation (cont )Part 1 – What is cavitation (cont.)
Cavitation Phenomena
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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
0% head drop1% 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
0% head drop1% head drop
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)
Part 1 What is cavitation (cont )Part 1 – What is cavitation (cont.)
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Part 1 What is cavitation (cont )Part 1 – What is cavitation (cont.)
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.)
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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
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:
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
• 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
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)
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( , ; 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.
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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.
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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)
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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.