pelton turbines [compatibility mode]
DESCRIPTION
Sex Turbine, more like. IMO, TBH.TRANSCRIPT
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PELTONTURBINE
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Impulseturbine
Therelativeproportionofenergytransferobtainedbythechangeinstaticheadandbythechangeindynamicheadisoneoftheimportantfactorsy g y pforclassifyingfluidmachines.
Foranaxialflowimpulsemachine.Foranimpulsemachine,therotorcanbemadeopen,thatis,thevelocityV1 can, p , , y 1representanopenjetoffluidflowingthroughtherotor,whichneedsnocasing.Averysimpleexampleofanimpulsemachineisapaddlewheelrotatedbytheimpingementofwaterfromastationarynozzle
Degreeofreaction=0
Themachineforwhichthechangeinstaticheadintherotoriszeroisk i l hiknownasimpulsemachine.
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INTRODUCTIONINTRODUCTION
Named after the designer Mr A Pelton inNamedafterthedesignerMr.A.PeltoninCaliforniaat1810
Impulse turbine as there is no pressure drop Impulseturbine asthereisnopressuredropacrossthebucketsS i bl f hi h h d f b 100 500 Suitableforhighheadsofabout100500m
Requirescomparativelylessquantityofwater
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SCHEMATIC DIAGRAMSCHEMATICDIAGRAM
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WORKINGWORKING Watersuppliedisfromahighheadthroughalongconduitcalledpenstock.
Thewaterisacceleratedinnozzle andtheheadisconverted into velocity and discharges at high speed inconvertedintovelocityanddischargesathighspeedintheformofajetatatmosphericpressure
Thejetstrikesbuckets attachedtotherimofarotatingwheel (runner)wheel(runner)
Impactofwateronbucketsproduceaforcewhichcausesthewheeltorotatebysupplyingatorqueonh h ftheshaft
waterdischargedatrelativelylowspeedfallsintolowerreservoirortailrace
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NOZZLEASSEMBLY
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NOZZLEASSEMBLY PowerproducedbyturbineproportionaltoQxH
whereQ:VolumeflowrateH:effectivehead
SinceHcannotbevariedeasily,Q(crosssectionalareaxvelocityofflowisvaried)
Forwheelspeedtobemaintainedconstant,jetvelocityisnotvaried.
Crosssectionalareaisvariedusingaspearvalve Spearvalveisactuatedtoandfroasperthepowerdemand
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Deflector Sometimes,needarisesforimmediateclosureofnozzle.
Ifflowisreducedsuddenly,highbackpressurewaveswillbegeneratedongpenstockpipes
Deflector alters the jet trajectory Deflectoraltersthejettrajectory
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RUNNER WITH BUCKETSRUNNERWITHBUCKETS
Undercuts are provided so that the jet bye passes the inclined bladesUndercuts are provided so that the jet bye-passes the inclined blades which are obstructing the jet to strike the blade which is vertical in position
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RUNNERWITHBUCKETS Usually more than 15 buckets Usuallymorethan15buckets Eachbucketissplitverticallyintotwopartsbyasplitterthathasasharpedgeatthecentrep g
Strikingjetofwaterisdividedintotwopartsbythesplitter
eachpartofjetflowssidewaysroundthesmoothinnersurfaceofthebucketandleavesitwitharelativevelocity almost opposite in direction of the original jetvelocityalmostoppositeindirectionoftheoriginaljet
Notchattheedgeofouterrimofeachbucketensuresalossfreeentryofthejetintothebuckets.
Bucketofpeltonturbine
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RUNNERWITHBUCKETS Maximumchangeinmomentumisobtainedwhenthejetis180
But angle is restricted to 165 so that the fluid leaving Butangleisrestrictedto165 sothatthefluidleavingonebucketdoesnotstrikebackthesucceedingbucket
Bucketsaremadeofspecialbronze steelalloyswithp yNickel,Chromiumorstainlesssteel.
Bucketsareproperlypolishedforsmoothchangeindi ti d th f ti f l k tdirectionandtheformationoflowpressurepocketscausedduetoerosionareavoided
Buckets can be bolted construction( for large turbines)Bucketscanbeboltedconstruction(forlargeturbines)orofsinglecast
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CASING
Preventssplashingofwater Guideswatertoflowthroughthetailrace Provides housing and support to the thrustProvideshousingandsupporttothethrustbearingsandshaft
Since the buckets are symmetrical side or Sincethebucketsaresymmetrical,sideoraxialthrustsproducedbywaterineachhalfisbalancedbalanced
Providessafetyagainstaccidents
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ANALYSIS
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Velocityofwaterjet,
1 2vV C gH=where,
vC =0.95to0.99
Histhenetheadavailableatthenozzleexit
Coefficientofvelocity
214
Q d V=Rateofdischarge, distheleastdiameterofthejet;variesfrom20to30 variesfrom30to45
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All fluid is assumed to be striking the bucket at radius r( pitch circle radius)=D/2Allfluidisassumedtobestrikingthebucketatradiusr(pitchcircleradius)=D/2
60DNu =Peripheralvelocityisgivenby60
WhereNisthespeedofthewheelinrevolutionsperminute
2 1r rV KV= WhereKislessthanunityRelativevelocities 2 1r r yForceexertedbythefluidjetinthedirectionofrotationis
1 2( )x w wF Q V V= + (DirectionofVw2 isoppositetoVw1)
Powerproducedbythewheel
( )P Q V V u= +1 2( )w wP Q V V u= +Inputiskineticenergy.Sohydraulicefficiency,
( )Q V V u +1 221
12
( )w wh
Q V V uQV
+= =
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1 cos (1 cos )KV u K = +2 2 1 1cos cos [( ) cos ]w r rV V u KV u K V u u = = =
Wehave
121
2 (1 cos )( )hu K V u
V = +
1
ForagivenvalueofV1,efficiencyismaximumwhen
0 (2 4 )d V u = = 10 (2 4 )V udu = =
1
2Vu =2
max1 cos
2K +=
Andmaximumefficiency
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Efficiencyisbetween85and90% Power loss occur due toPowerlossoccurdueto
Frictioninbearing Frictionbetweenwheelandatmosphere
L i l Lossinnozzle
u=V1 iscalledrunawayspeed,speedatnoload Volumetric efficiencyVolumetricefficiency
Q Qv
Q QQ
=
Sincesomewatermaysliptotailracewithoutdoingusefulworkefficiencyspeedratio
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Q1.Apeltonturbineproduces25MWwhilerunningat750rpmunder an effective head of 1770m. Calculate (i) least diameterunderaneffectiveheadof1770m.Calculate(i)leastdiameterofthejet(ii)meandiameteroftherunner(iii)numberofbuckets:
Letthecoefficientofvelocitybe0.98.Theabsolutevelocityofwater jet will then bewaterjetwillthenbe
1 2 0.98 2 9.81 1770 182.63 /vV C gH m s= = =Themaximumpossibleoutputfromtheturbineis.QgHp pAssuminganoverallefficiencyequalto0.85,thegivenoutput
Qg625 10 0.85watts QgH =
Thevolumeflowrate,6
35
25 10 1.694 /Q m s= =50.85 10 9.81 1770Q
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Since 214
Q d V= 4
wheredisthediameterofthejet,
4 4 1 694Q
1
4 4 1.694 10.87182.63
Qd cmV
= = =h i ffi i i b i d d ithemaximumefficiencyisobtainedatspeedratio,
0.462
ugH
=Therefore,themeanspeed
0.46 2u gH=
0.46 186.360DNu = =
and
60
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therefore 0.46 186.3 60 2.18750
D m = =750
the ratio D/d is called the jet ratio and is an importantparameter in the design and selection of Pelton turbine. Theparameter in the design and selection of Pelton turbine. Thenumber of buckets in any Pelton wheel is fixed by theconsideration that the buckets do not frequently interfere withthe path of incoming ater jet If the n mber of b ckets isthe path of incoming water jet. If the number of buckets issmall, they would be widely spaced and a part of the water jetmay not strike a bucket. If the buckets are closely spaced, thepath of water jet will get distributed by the incoming buckets.Dr. Taygun of Zurich (Austria) has suggested an empiricalrelation for obtaining the number of buckets. According to him,relation for obtaining the number of buckets. According to him,the number of buckets
2.180 5 15 0 5 15 25Dz = + = + 0.5 15 0.5 15 250.1089
zd
= + = +
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Q2.Apeltonwheelproduces15MWunderaheadof400mwhenrunning at a speed of 500rpm. If the diameter of the jet is notrunningataspeedof500rpm.Ifthediameterofthejetisnottoexceedonetenthofthewheeldiameter,calculatethenumberanddiameterofthejets,diameterofthewheelandquantity of flow Assume overall efficiency =0 88 Cv=0 98 andquantityofflow.Assumeoverallefficiency=0.88,Cv=0.98andspeedratio=0.46
Theoverallefficiencyis0.88andtherefore,thepowerthatcouldhavebeendeveloped,
6
0
15 10QgH =
615 10 23
15 10 4.344 /0.88 10 9.81 400
Q m s= = Thevelocityofthejet,
31 2 0.98 2 9.81 400 86.817 /vV C gH m s= = =
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Peripheralspeedofwheel,
0.46 2 40.75 /u gH m s= =Wheeldiameter,
60 40.75 60 1.56500
uD mN = = =
fThejetratioshouldnotexceed10themaximumdiameterofthejetis0.156m
and2
1 . .4Q d V no of jets=
and,
Noofjets 2 21
4 4 4.344 2.18(0.156) 86.817
Qd V
= = = Or3jets
Jetdiameterd=1
4 4 4.344 0.1457 145.73 3 86.817Q m mmV
= = =
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Specificspeedalsoindicatesnumberofjets
/4 34.2N PN = =5/4 34.2sN H
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Quiz
The mean blade speed in a Pelton turbine is 14 m/s. The rate of flow of water supplied by the jet under a head ospp y j450 m is 800 L/s. If the jet is deflected by the blades at an angle of 165o and the friction factor at the blade is 0.9, find the theoretical power output and hydraulic efficiency of thethe theoretical power output and hydraulic efficiency of the turbine.
Hint: the blade speed is not optimumAdditonal question
Compare the current efficiency with the optimum condition and discuss the physical reason for change in efficiency
BITS Pilani, K K Birla Goa Campus
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