CE212-Part II - Hydraulic Machinery-Pumps.pdf

its a lecture of hydraulic machinery, a part of civil engineering subject

1. Hydraulic Machinery-Pumps
1
•Centrifugal Pumps
•Classification
•Specific Speed
•Characteristic Curves and
•Cavitation
Chapter #15

2. 2
•Machinesthataddenergytoafluidbyperformingworkonitare
calledpumps.
•Pumpsaregenerallygroupedintotwobroadcategories—positive
displacementpumpsanddynamic(centrifugal)pumps.
•Positivedisplacementpumpsuseamechanicalmeanstovarythesize
of(ormove)thefluidchambertocausethefluidtoflow.
•Ontheotherhand,centrifugalpumpsimpartmomentumtothefluid
byrotatingimpellersthatareimmersedinthefluid.Themomentum
producesanincreaseinpressureorflowatthepumpoutlet.
•Positivedisplacementpumpshaveaconstanttorquecharacteristic,
whereascentrifugalpumpsdemonstratevariabletorque
characteristics.
Pumps

3. 3
Centrifugal Pump
Thesearesocalledbecauseenergyisimpartedtothefluidby
centrifugalactionofmovingbladesfromtheinnerradiustotheouter
radius.
Themaincomponentsofcentrifugalpumpsare
(1)theimpeller,
(2)thecasingand
(3)thedriveshaftwithglandandpacking.
Additionallysuctionpipewithonewayvalve(footvalve)and
deliverypipewithdeliveryvalvecompletesthesystem.

4. 4
•Theliquidenterstheeyeoftheimpelleraxiallyduetothesuction
createdbytheimpellermotion.
•Theimpellerbladesguidethefluidandimpartmomentumtothe
fluid,whichincreasesthetotalhead(orpressure)ofthefluid,
causingthefluidtoflowout.
•Thefluidcomesoutatahighvelocitywhichisnotdirectlyusable.
Thecasingcanbeofsimplevolutetypeoradiffusercanbeusedas
desired.
•Thevoluteisaspiralcasingofgraduallyincreasingcrosssection.
•Apartofthekineticenergyinthefluidisconvertedtopressureinthe
casing.
Working of a Centrifugal Pump

5. 5
Main Parts of a Centrifugal Pump
h
s= Suction Head
h
d= Delivery Head
H = Static Head
NetPositiveSuctionHead(NPSH)isdefinedasthe
availabletotalsuctionheadatthepumpinletabovethe
headcorrespondingtothevapourpressureatthat
temperature

6. 6

7. •Specificspeedisanindexnumbercorrelatingpumpflow,headandspeedat
theoptimumefficiencypoint.
•Itclassifiespumpimpellerswithrespecttotheirgeometricsimilarityandis
definedasspeedofgeometricallysimilarpumpwhendelivering1m
3
/secof
wateragainstatotalheadof1meter.
•Twoimpellersaregeometricallysimilarwhentheratiosofcorresponding
dimensionsarethesameforbothimpellers.
•Thisindexisimportantwhenselectingimpellersfordifferentconditionsof
head,capacityandspeed.Usually,highheadimpellershavelowspecific
speedsandlowheadimpellershavehighspecificspeeds.4/3
2/1
H
NQ
Ns
SpecificSpeed
Where, Ns = specific speed rpm, N = pump speed rpm,
Q = Discharge in m
3
/sec H = Total Head (m)

8. Classification of Centrifugal Pumps
Centrifugal pumps may be classified according to,
1.Working Head
2.Type of Casing
3.Direction of Flow of Water
4.Number of Stages

9. Classification of CP based on Working Head
Centrifugal pumps may be classified in to low, medium and high-
head pumps.
•Low-HeadCentrifugalPumps
Theseareusuallysingle-stage-centrifugalpumpsandworkbelow15m
head.
•Medium-Head Centrifugal Pumps
Whentheheadliesbetween15and45m,thepumpsarecalled
medium-head-centrifugalpumps.
•High-HeadCentrifugalPumps
Whentheheadexceeds45m,thepumpsareknownashigh-head-
centrifugalpumps.Usuallythesearemultistagepumps,andare
providedwithguidevanes.Thesepumpsmayhavehorizontalor
verticalshafts.Verticalshaftsareusefulindeepwells.

10. 3.Types of Casing
Pumpscanbedividedintofollowingtypeaccording
totheircasing:
a)Volute-ChamberPump
b)Vortex-chamberPump
c)DiffuserPump
Classification of CP based on Types of Casing

11. (a) Volute-Chamber Pump
Suchtypeofcasingisofspiralform,andhasasectionalarea,which
increaseuniformallyfromthetonguetothedeliverypipeas
showninFig.10.1moreareaisprovidedtoaccommodate
increasedquantityofwaterasthewatermovestowardsthe
deliverypipe.

12. b)Vortex-chamberPump
Inavortexchamber,auniformlyincreasingareaisprovided
betweentheimpellerouterperipheryandthevolutecasingas
showninFig.10.2water,onleavingtheimpellerbecomesfreeto
adoptitspath.

13. c) Diffuser Pump
InadiffuserPump,theguidevanesarearrangedattheoutletofthe
impellervanes.Waterenterstheguidewithoutshock.Astheguide
vanesareofenlargingcross-sectionalarea,thevelocityofwater
decreasedandpressureincreasesSincethevanesprovidebetter
guidancetoflow,eddylossesarereducedwhichincreasesthe
efficiency.

14. a) Radial Flow
Radialflowisoneinwhichtheflowintheimpelleris
completelyinaradialdirection.Thisisusedwhenthe
requirementsarehighandlowdischarge.
b) Mixed Flow
Inamixed-flow,bychangingthedirectionofflowfrompure
radialtoacombinationofaradialandaxial,areaofflowis
increased.Thusmixed-flowpumpsareusedwheremedium
dischargeisneededtoraisethewatertomediumheads.
Thesearemostlyusedforirrigationpurposes.
c) Axial Flow
Thesepumpsfindtheirusewherehighdischargeatlowheads
isrequired,asinirrigation.
Classification of CP based on Direction of Water Flow

15. Direction of Flow of Water

16. •Acentrifugalpumpcanhaveasinglestagewithoneimpellerkeyed
totheshaftoritcanbeamulti-stagepumps.
•Amultistagepumphasanumberofimpellersmountedonthesame
shaftandenclosedinthesamecasing.
•Amultistagepumpishavingthefollowingtwoimportantfunctions
twoimportantfunctions:
1.Toproduceahighhead
2.Todischargealargequantityofliquid
•Ifahighheadistobedeveloped,theimpellersareconnectedin
series(oronthesameshaft)whilefordischarginglargequantityof
liquid,theimpellers(orpumps)areconnectedinparallel.
Classification based on Numberof Stages

17. 17

18. 18
•Asetoffourcurvesknownasthepump’scharacteristicscurvesis
usedtodescribetheoperatingpropertiesofacentrifugalpump.
•Thesefourcurvesrelatedhead,efficiency,powerandnetpositive
suctionheadrequiredtopumpcapacity.
•Pumpmanufacturersnormallypublishasetofcharacteristicscurves
foreachpumpmodeltheymake.
•Dataforthesecurvesaredevelopedbytestingseveralpumpsofa
specificmodel.
•Theoperatingpropertiesofapumpdependonthegeometryand
dimensionsofthepump’simpellerandcasing.
Pump’s Characteristic Curves

19. 19
Characteristiccurvesforasingle-stagecentrifugalpump