Cavitation In Centrifugal Pumps And Prediction Thereof

Pumpp DivisionFlowserve PumpsIDP PumpsCavitation in Centrifugal Pumpsand Prediction ThereofF k C.FrankC ViVisserFlowserve Pump DivisionEtten-Leur, The NetherlandsTutorialPresented 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 forpumping machinery? Part 2: Prediction of cavitation in centrifugal pumps– Scaling laws– Thermodynamic effect (temperature depression)– EffectEff t off dissolveddil d or entrainedt i d gases– Calculating incipient cavitation (NPSH) from CFD– Cavity length prediction2

P t 1 – WhatPartWh t iis cavitationit tiCavitation is defined as the process of formation and disappearanceof the vapour phase of a liquid when it is subjected to reduced andsubsequently 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 heataddition.Cavitation is a thermodynamic change of state with mass transferfrom liquidqto vaporp pphase and visa versa (( bubble formation &collapse).3

P t 1 – WhatPartWh t iis cavitationit ti(cont.)(t)Sheet cavity on pumpimpeller vane leadingedge (suction side)Speed 2990 RPMNPSHA 70 m(230 ft)Flow rate 1820 m3/h(8015 gpm)Vane marker stripes atintervals of 10 mm (0.4 in)Cavity length 25-40 mm(1.0 – 1.5 in)(from Visser et al, 1998)4

Part 1 – What is cavitation (cont(cont.))Cavitation causes or may cause: PerformanceP flossl(head(h d ddrop)) Material damage (cavitation erosion) Vibrations Noise Vapor lock (if suction pressure dropsb lbelowbbreak-offk ff value)l )(Visser et al, 1998)General Advice: TRY TO AVOID CAVITATION (under normal operation)Unfortunately, economic or operational considerations often necessitateoperation with some cavitationcavitation, and then it is particularly important tounderstand the (negative) effects of cavitation. Design optimization to minimize cavitation5

P t 1 – WhatPartWh t iis cavitationit ti(cont.)(t)Typical cavitation damagesCentrifugal pump impellercavitation pitting erosion @ inlet(from Dijkers et al, 2000)Francis turbine runnercavitation damage @ discharge(from Brennen, 1994)6

Part 1 – What is cavitation (cont(cont.))Cavitation behavior is typically expressed in terms of cavitationparameters.t Cavitation number:p1 pV; (Centrifugal Pumps : U Ueye R1T ) 122 U Net Positive Suction Head:p01 pVNPSH g Thoma cavitation number: THNPSH H7

Part 1 – What is cavitation (cont(cont.))In ggeneral, cavitation performance is related to some “critical” value:NPSHA ( available) NPSHc or NPSHR ( critical or required)Typical “critical”critical characteristics identified for centrifugal pumps: Incipient cavitation (NPSHi) Developedp cavitation causingg 3% head dropp ((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

Part 1 – What is cavitation (cont(cont.))Cavitation Phenomena9

Cavitation Visualization Test PumpPump Division

Begin Visual Cavitation3% head drop1% head drop0% hheaddddropBegin visual cavitationHead (m)4.054.003.953.903.853.803 753.753.700102030405060708090100NPSH (m)Pump Division

0% Head Drop3% head drop1% head drop0% head dropBegin visual cavitationHead 8090100NPSH ((m))Pump Division

1% Head drop3% head drop1% head drop0% head dropBegin visual cavitationHead 8090100NPSH ((m))Pump Division

3% Head drop3% Head drop1% head drop0% head dropBegin visual cavitationHead 8090100NPSH ((m))Pump Division

Recirculation3% head drop1% head dropRecirculation0% head dropBegin visual cavitationHead 8090100NPSH ((m))Pump Division

Part 1 – What is cavitation (cont(cont.))10

Part 1 – What is cavitation (cont(cont.))Typically (in practice): NPSHA NPSH3% NPSHi NPSHA (especially for low capacity) Pumpsu ps runu ookay,ay, BUTU witht someso e dedevelopede oped cavitation.ca tat oGeneral misconception:NPSHA NPSHR No Cavitation(This will only hold if NPSHR NPSHi.)11

Part 2 – Cavitation prediction Scaling laws Thermodynamic effect Effect of dissolved or entrained gases CaCalculatingcu at g incipientc p e t cacavitationtat o (N(NPSHi)S i) fromo CCFD Cavity length prediction12

Part 2 – Cavitation prediction (cont(cont.))Predicting NPSH at speeds other than reference or test speed( scaling laws)2 N 2 NPSHNNPSHNPSHNPSHi:iii , REF N REF NPSH( TH constant)H2NPSH3%: N NPSH 3% f NPSH 3%,% REF N REF N N REF , f 1 ; N N REF , f 1“Postulate”: Amount of developed cavitation depends on residencetime f depends on size of the pump and ratio N/NREF13

Part 2 – Cavitation prediction (cont(cont.))Alternative approach to account for deviation from affinity law:NPSH 3% N NPSH 3%,% REF N REF1 2 Choice of is rather arbitrary and relies heavily on empiricismConservative choice:N NREF , 1N NREF , 214

Part 2 – Cavitation prediction (cont(cont.))Thermodynamicyeffect(temperature depression)Cavitation performancedepends on: Temperature of liquid Type of liquid NPSHR reduction(E.g. Stepanoff method, orHydraulic Institute correctionchart)(from Brennen, 1994)15

Part 2 – Cavitation prediction (cont(cont.))Predicting thermodynamic effectNPSH 3% NPSH 3%, REF NPSHEquilibrium theory:h fgV L NPSH B 2;B V V v fg g C p TVLStepanoff (1965(1965, 1978):2B B1 NPSH2 L g C pT 1 1 B1 ;[m]or[ft]2 V h fg29 4 364 4 3 NPSH B1 ; [m 1 ] or NPSH B1 ; [ ft 1 ]HVHVNon-equilibrium theory bubble dynamic (CFD) calculations, involvingtime-dependenttimedependent two-phasetwo phase flow calculations16

Part 2 – Cavitation prediction (cont(cont.))Influence of dissolved and/or entrained gases: “conceptual effective or artificial” vapor pressure:PE PV PE yPP0(Ch(Chen,1993)Key characteristic:Performance (breakdown) comes from gas evolution and gasexpansion, rather than classical vapor formation.Dissolved and/or entrained gases result in reduction of (effective)field NPSHA:NPSHA* (P01 – PE) / g“Hidden danger”: NPSHA NPSHR but NPSHA* NPSHR17

Part 2 – Cavitation prediction (cont(cont.))Predicting incipient cavitation (NPSHi)() fromfCFDCT i l approach:TypicalhCreate 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(cont.))Streamline through point of minimum pressureNPSH i p01,i pV gp01,i p1,i 12 U 2p1,i p1 ( pmin pV ) NPSH i p01 pmin gSo: NPSHi follows from pmin and p01 of calculated pressure field, anddoes not require pV to be known!19

Part 2 – Cavitation prediction (cont(cont.))Running simulations for several flow rates produces NPSHi curve:(from VisserVisser, 2001)20

Part 2 – Cavitation prediction (cont(cont.))Note: CFD calculated characteristic is for impeller flow!To project it on pump throughput one needs to accountfor volumetric efficiencyy ( ( eyey wear ringg leakageg flow):)Qimpeller Qpump Qleakage Qpump Qimpeller - Qleakage Computed curve shifts left by amount Q QleakageQleakage p f( p, D, L, , , ) D u ; u L 12 laminar 24 / Re ; turbulent 0.2373 / Re0.25u ; Re 2 It becomes particularly important to take Qleakage into account for lowNS (specific speed) impellers. For high NS the relative influence is less.21

Part 2 – Cavitation prediction (cont(cont.))What if NPSHA NPSHi ? Find region on impeller blade surface where p pV physically unrealisticunrealistic, but it gives first “indication” of cavitation area, and first approximation of cavity bubble lengthNote: The actual cavity will be bigger bubble length will be underestimated22

Part 2 – Cavitation prediction (cont(cont.))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 ( p1, A 12 U 2 pV ) p01 g NPSHA p01 NPSPA23

Part 2 – Cavitation prediction (cont(cont.))Example:Plot of p pV regionNPSHA 15.5 m (51 ft)NPSHi 28 m ((92 ft))N 2980 RPMQ 400 m3/h(1760 USGPM)Cavitation on bladesuction side24

Part 2 – Cavitation prediction (cont(cont.))Puttingg LCAV m L(p p(p pV), m O(3),( ), one can getg some impressionpofexpected cavitation erosion raten LCAV 2 36 32 Güli h (1986Gülich(1986, 19881988, 1989)1989): E C U A8TeS L A CAV ,10 nor(*)E Ln E E L L CAVwith 21 21n 2.83 for blade suction side andn 2.62 6 for blade pressure sideEquationq((*)) is especiallypyppowerful when comparingpg designsg andevaluate susceptibility to cavitation erosion (in a relative sense). Design optimization studies25

Part 2 – Cavitation prediction (cont(cont.)) Results and theory thus far do not require two-phase flowcalculations.l l ti Still it pprovides importantpinformation of an impellerpdesigngregarding cavitation performance. Next level of improvement has to come from CFD calculationswith cavitation model. Calculations with a cavitation model are time consuming andtend to be “CPU-expensive” Several cavitation models exist to date, and development ofcavitation models is still ongoing26

Part 2 – Cavitation prediction (cont(cont.))CFD Cavitation modelsTypically two approaches: Equilibrium models– Barotropic or pseudo density models; (p)– Somewhat “simplistic”,p, yety– Attractive since they can be used in single phase codes Bubble dynamicymodels–––––Rayleigh-Plesset equationVapor-liquid interaction (time-dependent mass & heat transfer)Closer to realityMore complicated and more “CPU-expensive”E.g. Volume of Fluid (VOF) model27

Part 2 – Cavitation prediction (cont(cont.))Example:Plot of cavity A 15.515 5 m (51 ft)NPSHi 28 m (92 ft)N 2980 RPMQ 400 m3/h(1760 USGPM)m 3Cavitation on bladesuction side28

Part 2 – Cavitation prediction (cont(cont.))Application:With CFD cavitationit ti modelsd l one can predictdi t NPSH3% fromfCFDcalculated head drop curves((from Visser,, 2001;; CEV-model pprediction))29

Concluding Remarks Cavitation is a phenomenon which can seriously impactperformance and operation of pumps. Predicting cavitation performance is an important topictopic,not only for pumps, but for fluid machinery in general. Traditional (scaling) methods are still important anduseful. CFD methods provide further insight and are becomingmore and more common. Bubble dynamic (CFD) methods are emerging and hold apromise for the future.30

ReferencesBrennen, C.E.H d dHydrodynamicsi off PPumps.Oxford University Press (1994)Chen, CChenC.C.CCope 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,Symposium August 6-96 9, 20002000, CharlotteCharlotte,North Carolina, USA.Gülich, JJ. FGülichF. and PacePace, S.SQuantitative Prediction of Cavitation Erosion in Centrifugal Pumps.Proceedings of the 13th IAHR Symposium (1986), Montreal, Canada.31

References (cont(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 KavitationsschallsKavitationsschalls.Thesis, Technische Hochschule Darmstadt, Germany, 1989.Stepanoff, AStepanoffA.J.JPumps and Blowers – Two-Phase Flow.John Wiley & Sons (1965), Krieger Publishing (1978)32