cavitation n recirculation field problems

7/28/2019 Cavitation n Recirculation Field Problems

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TUTORIAL SESSIONonCAVITATION AND RECIRCULATION FIELD PROBLEMS

TUTORIAL LEADERBRUNO SCHIA VELLO

Engineering Manager, Fluid DynamicsDresser Pump Division/Dresser Industries , Inc.

Harrison, New Jersey

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136 PROCEEDINGS OF THE NINTH INTERNATIONAL PUMP USERS SYMPOSIUM

Bruno Schiavello is Engineering Manager for Fluid Dynamics at Dresser PumpDivision, Dresser Industries, Incorporated,inHarrison,NewJersey.Hereceiveda B.S.in Mechanical Engineering (1974) fromthe University of Rome, Italy, and an M.S.in Fluid Dynamics (1975)from Von Karman Institute for Fluid Dynamics, RhodeSt. Genese, Belgium. He was co-winner ofthe H. Worthington European TechnicalA ward in 1979, with a paper on pump suction recirculation. He has written several significant papers andlectured at seminars in the area of pump recirculation. He is amemberof he Italian T urbomachine1y Committee, Societe Hydrotechnique de France (SHF), International Association for Hydraulic Research (JAHR), and ASME.Mr. Schiavello spent seven years in the Research and Development Department ofWorthington Nord (Italy) dealing with pumphydraulic research and design. In 1982, he joined the CentralResearch and Development ofWorthington, McGraw Edison Company, at Mountainside, New Jersey, where he operated first asAssociate Director and later ( 1983) as Director for Fluid Dynamics. He was appointed to his present position in 1985.


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CAVITATION AND RECIRCULATION FIELD PROBLEMSby

Bruno Schiavel loEngineering Manager, Fluid DynamicsDresser Pump DivisionLiberty Corner, New Jersey

ABSTRACTHigh re l iab i l i ty an d large rangeab i l i ty a rerequired of pumps in exis t ing an d newplan t s which must be capable of r e l i ab leon-off cycl ing operat ions an d spec ia l ly lo wload dut ies .Th e r e l i a b i l i t y an d rangeabi l i ty t a r ge t i sa new t ask for the pump des igner / r esearcheran d i s made very chal lenging by th ecavi ta t ion and/or suct ion rec i rcula t ionef fec t s , f i r s t of a l l the pump damage. Thepresen t knowledge about the: a) designc r i t i c a l parameters and t he i r optimizat ion,b) f ie ld problems diagnosis an dt roubleshoot ing has much advanced, in thevery l a t e s t year s .The objec t ive of th e pump manufacturer i sto develop design solut ions an dt roubleshoot ing approaches which improvethe impel ler l i f e as re la ted to cavi ta t ionerosion and en large the r e l i ab le operat ingrange by minimizing the e f f ec t s of thesuct ion rec i rcula t ion. This paper gives ashor t descript ion of severa l f ie ld casescharacter ized by di f f e r en t damage pat te rnsan d other symptoms re la ted with cav i ta t ionand/or suct ion rec i rcula t ion. Th et roubleshoot ing methodology is described inde ta i l , also focusing on the ro le of boththe pump designer an d the pump user .INTRODUCTIONFrom the l a te 50 s u n t i l the ear ly 70 sthere was a continuous growth of th ei ndus t r i a l count r ies economy, which led tothe design an d in s ta l l a t ion of l a rger andl a rger plants ( fos s i l power plan t s andprocess plan t s ) .Both energy l eve l (head/stage) an d capac i tyof pumping equipment increased dras t i ca l ly .In the area of power plan t s th e head/s tageincreased by a fac to r 4 an d the impel lerper iphera l speed by a fac to r 2. In th earea of process plants pump maximumcapaci ty was more than doubled and also th e

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head increased to meet higher p iping lossesassocia ted with large an d complex plantsand high capaci ty as wel l .Moreover plan t engineering con t rac to r puthigh emphasis on low cap i t a l cos t . Plantava i lab i l i ty , which would requireredundancy and so severa l uni t s inpara l le l , received l ess considerat ion.Also pump users gave keen a t tent ion to lowenergy operat ing cos t a t fu l l an d maximumload of pumps.Therefore the pump designer was urged todevelop smaller an d consequently fas te rpumps and also maximize ef f i c iency a t highf lows. In add it on lower NPSHA levels ,which were dic ta ted by low p lan tin s ta l l a t ion cos t s , ha d to be met a tmaximum capac i ty (runout condi t ion).Impel ler designs were f ina l ized for highsuct ion spec i f ic speed, S, and alsoinducers were widely appl ied . Moreover itwas required to avoid sharp increase of theNPSHR curve (as based on 3% head drop) a thigh capaci ty , still mainta ining highef f ic iency. Th e design solut ion was thento open the th roa t area of the bladechannel a t th e impel ler i n l e t in order toreduce the impel ler sens i t iv i ty tocavi ta t ion blockage on through flow.consequent ly both the impel ler eye diameterwa s enlarged an d also the blade i n l e t anglewas increased, moving th e shocklesscapaci ty well above the b .e .p . capaci ty .As a r esu l t th e pump behavior was penal izeda t pa r t flows, in combination with moresevere requirement in terms of cav i ta t iona t dut ies above the best eff iciencycapaci ty .In th e l a te 70 's an d ear ly 80 ' s a slowdownof world economy fol lowed. Th e overa l lproduct io n car-abi l i ty , fo r which plan t s an dequipment were optimized, wa s exceeding themarket demand. Then process plan t s wereforced to operate a t reduced capaci ty . Inthe u t i l i t y area basic loads were picked bynuclear plants , while fos s i l power plantsmoved to cycl ing load along with t h e i rpumping equipment (namely feed water an d


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booste r ) .As a re su l t more and more high energy pumpswere operating in a broad range of capaci tyincluding long dut ies a t flowssubs tant ia l ly below the b .e .p . capacity,and, espec ia l ly , much lower than theimpel ler i n l e t optimum capacity orshockless capac i ty .Then frequent f a i lu res surfaced which werecharacter ized by heavy damage a t theimpel ler i n l e t an d ou t l e t , pressurepulsa t ions an d vibra t ions with wide bandand random frequency spec t ra .Key experimental research data on pumpbehavior a t low flows, which showed theoccurrence of in te rna l flow rec i rcula t ion(1 ,2 ,3 ,4 ,5 ,6 ,7 ,8 ,9) an d loca l cavi ta t ionwith assoc ia ted metal damage, weremeanwhile published and gave some c lea rins ights on the above f a i lu res . Also,other pump fa i lures were observed withincreasing frequency which presentedc las s ica l cavi ta t ion damage aspect , assurface p i t t i ng , even with NPSHA leve l wellabove the NPSHR. A basic study ofcavi ta t ion inception and growth ha d alreadybeen publ ished s ince 1941 (10) . However,extensive, exper imenta l research oncavi ta t ion in pumps has been carr ied outonly in the l a s t tw o decades mostlyconcentrat ing on cavi ta t ion v isua l iza t ion( t ransparent t e s t models an d s troboscopicl ight ) (1, 11, 12, 13, 14, 15) and acousticdetec t ion of high frequency cavi ta t ionnoise spec t ra (16) . Then it became fu l lyevident tha t cavi ta t ion inception occurs a tNPSHA l eve l (NPSHi) 5 to 20 t imes higherthan the conventional value of NPSHcorresponding to 3% head drop whilecavi ta t ion damage occurs for NPSH leve lbelow the incept ion point but s t i l l higherthan NPSHR, depending on various fac tor(17), f i r s t of a l l per iphera l veloci ty a tthe impel ler eye an d pump operatingcapaci ty as f rac t ion of the shocklesscapaci ty (11, 12).While new c r i t e r i a for es tabl i sh ingadequate NPSHA l eve l s ta r t ed to bedeveloped, ( l l , 12, 13, 17, 18, 19 ) pumpdesigners were more an d more cal led tosolve f ie ld t roubles re la ted withcavi ta t ion and/or suc t ion rec i rcula t ion .CAVITATION MODESBlade Attached (or Sheet ) Cavi ta t ionA large number of cavi ta t ion visua l iza t ions tudies in pumps can be found in thel i t e ra tu re . They are aimed a t detec t ingthe cavi ta t ion inception point , when thef i r s t cavi ta t ion bubble becomes v is ib le , byusing spec ia l t ransparent experimentalmodels and a st roboscope l i gh t . According

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to the class ic experiments of (1 ) with endsuc t ion pumps the curve of the NPSH a t th econdit ion of visual incept ion versus thepump capacity has a very pecul ia r shape, asshown by the top curves in Figure 1. TheNPSHi ( i "' inception) has a !ninimum a t acapacity which corresponds to shocklessi n l e t flow (Qsl) , i . e . , the re la t ive flowreaches the blade leading edge with anincidence angle around 0. The NPSHiincreases a t Q > Qsl an d Q < Qsl , withcavi ta t ion s tar t ing on the pressure(hidden) and suction (v i s ib le ) s ide of theblade respec t ive ly . At pa r t flow the NPSHipeaks a t a capacity s l igh t ly higher thanthe c r i t i c a l suction rec i rcula t ion onsetcapac i ty , Qcrs (cr - c r i t i c a l , s - suction)(1 , 20). A s imi l a r V-shape for theinc ip ient cavi ta t ion curve was also foundby using a small head drop c r i t e r ion (0 .5 \of the head of twice the impel ler ey eper iphera l ve loc i ty) for overhung pumps(10). Again, the curve exhibi ted a t partcapaci ty a peak, which i s at t r ibuted to ac r i t i c a l incidence angle causing flowseparation (10) or "s ta l l ing incidenceangle" ( 8 , 2 0 ) At the point of the visua l cavi ta t ioninception, the ra te of the erosion damagei s prac t i ca l ly zero . Visual s tudies ofcavi ta t ion show t ha t more an d more vapor i sgenerated while the suc t ion pressure orNPSH i s continuously decreased duringc las s ica l a t e s t s ( i . e . t e s t of head decaya t constant ro ta t iona l speed an d constantcapacity with decreasing NPSH). Th e vaportends to coalesce an d then forms a largecavi ta t ion bubble of increasing length.Pumps operate in the f ie ld witn a NPSHA (A= Available) which i s higher than the NPSHRbut s ignif icant ly below the NPSHi.Therefore they operate with bubble lengtha t the NPSHA which varies widely from 0.5inches to 4 inches or more depending uponthe operating point (speed, flow,temperature) an d the impeller design.In order to produce damage the vaporbubbles must col lapse in the v ic in i ty ofthe metal sur face . Normally it occurs forthe regime charac te r ized as "blade at tached(or sheet) cavi ta t ion" , which i s morecommon in the usual capacity operat ingrange. In t h i s cavi ta t ion mode the curveof the cavi ta t ion erosion ra te (ER =MDP/Time, where MDP Mean DepthPenetrat ion) versus capacity a t constantspeed/NPSHA has a pecul ia r v-shaJ;. .. , withminimum a t the shockless capacity (Figure2), which i s similar to the NPSHi curve.Th e damage develops as pi t t ing on the bladepressure s ide for f lowra tes above theshockless capac i ty . At f lowrates below theshockless one the cavi ta t ion damage occurson the v is ib le side of the vane. Recentresearch (21) has demonstrated tha t in th i scavi ta t ion regime the erosion ra te ,expressed as damage depth- to-operating time


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r a t io ( inches /year ) , i s propor t ional to thebubble length (exponent 2.7) an d the NPSHA(exponent 3 . 0 ) , for given f luid proper t ie sand impeller mater ia l . Therefore for givenimpeller l i f e , say 40000 hours, theacceptable cav i t a t ion bubble length i s verymuch shor te r for pumps running a t highimpeller eye speed an d so operat ing withhigh NPSHA than for small pumps runningwith lo w impel le r eye speed, which impl iesa low NPSHA. Correspondently, theacceptable cavi ta t ion bubble l ength canvary from 0.5 inches to 4 inches.Cavitation Induced by Suction Recirculation(Vortex Cavita t ion)Visual observa t ions with s t roboscopic l i g h tshow t h a t the cavi ta t ion bubble on theblade suct ion s ide becomes more an d moreunstable as the capaci ty i s cont inuous lydecreased below the suct ion rec i rcu la t ionpoin t towards shutof f . The bubble lengthchanges more or l e ss pe r iod ica l ly witht ime, even disappearing for a f rac t ion oft ime. Moreover, cav i t a t ing bubble cloudsseparate from the blade suct ion surface an dmove into the blade channel .Essent ia l ly a new flow regime takes placewhich i s character ized by "Stronglyi n t e r m i t t e n t c a v i t a t i o n - s u c t i o nr ec i r cu la t ion" (15) . As a genericindicat ion such very unsteady flow regimeoccurs in the capaci ty range from 0% toSO%, roughly. However, the upper capaci tyl imi t can reduce or increase even close tothe suc t ion rec i rcu la t ion onset capaci tydepending on the impeller design an dimpeller eye per iphera l ve loc i ty (Ueye) andNPSHA level .Experimental inves t iga t ion by means of ahigh speed movie camera along withstroboscope (22) c lea r ly shows tha t a t lowf lowrate tw o di f feren t pa t t e rns ofcavi ta t ion i . e . , shee t cav i ta t ion" an d"vortex cav i t a t ion occurred a l t e rna t ive lynear the leading edge of the impellerblades, as schematically shown in Figure 3.The cavi ta t ion s tar ted on the blade suct ionsurface fa r away from the leading edge an dmoved upstream with an abrupt s troke andcol lapsed on the pressure sur face of thenext blade. This cav i t a t ion ca l led "vortexcavi ta t ion" i s at t r ibu ted to the impellersuct ion rec i rcu la t ion . In f ac t , a vortexi s generated by the shear forces a t theinterface between the reverse flow leavingthe impeller near the f ront shroud and theordinary forward flow ente r ing in to theimpeller near the hub, as shown in Figure4a. Moreover s t reams of both backward an dforward flow can be suspected to occur alsoin the blade- to-blade plane in the i n l e tregion of the blade channel , as sketched inFigure 4b. Then shear forces componentsex i s t also in th i s plane an d cont r ibu te to

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generate a complex vortex in the th r ee dimensional space. When th e i n l e t pressure( the re fore , NPSHA) i s low enough and alsothe s t r ength of the vortex ( the re fore ,i n t ens i ty of the suc t ion rec i rcu la t ion) i shigh enough, then the pressure in thevortex core drops below the sa tura t ionpressure an d cav i t a t ion condi t ions arereached. A fi lament of cav i t a t ing flowdevelops which i s s ta r t ing on the suct ions ide of the blade an d i s ending on thepressure s ide of the next blade, as shownin Figure 4c. Moreover, th i s vortexo sc i l l a t e s in a di rec t ion normal to theblade eurface i . e . , more or l e ss in thedi rec t ion of the main flow as sketched inFigure 4d. Consequently , damage i s causedin form of a s ing le la rge c r a t e r a t themidspan of the blade on the pressure s ide .A t yp ica l curve of NPSHd (d damage) whichcan produce s ign i f ican t erosion damagethroughout th e whole range of operat ions i sshown in Figure 1. Th e NPSHd i s not uniquean d depends upon the des ired impeller l i f e ,the pump design, the mate r i a lcharac ter i s t ics , the f lu id densi ty an dtemperature. Th e basic engineering problemi s to determine how much erosion damage canbe allowed, under the operat ing condi t ions ,to ge t a reasonable impel le r l i f e ofseveral thousand hours an d so aneconomically acceptable pump r e l i ab i l i t y .Moreover, rangeabi l i ty and eff ic iency haveto be included in the balance.In le t Flow InfluenceA s t rong inf luence on cav i t a t ion incept ion(NPSHi) an d damage (NPSHd) i s produced bythe flow dis t r ibu t ion a t the impeller .eye,as induced by the upstream geometry i . e . ,in le t chamber and/or suct ion pip ing .For s ide suct ion pump the shape of NPSHicurve can be s trongly a l t e red by thesuct ion cas ing, which tends to displace an dsmooth the minimum an d the peak.The degree of dis to r t ion of the i n l e t flowbecomes s t ronger with increas ing capac i ty .Visual observations c lea r ly show (15) tha tboth the shape and the size of thecavi ta t ion bubble on each impel le r blade i sa per iodic function of t ime, as the bladecrosses flow zones with posi t ive flowswir l ing ei ther in the same direct ion ofthe tmpel ler ro ta t ion ( l e ss intensecavi ta t ion) or in the opposi te di rec t ion(more severe cav i t a t ion) . Thus pressurepulsa t ions and vibra t ions are induced bythe i n l e t flow dis to r t ion . They reach thehighest peak- to-peak va lues a t the runoutopera t ing poin t , which usual ly i s close tothe pump maximum brake-horsepower. Areasof damage can be produced on both thesur faces of the blade but the erosion i spreva i l ing on the pressure s ide .


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Moreover, f i e ld experience i nd ica tes tha twith double suct ion impel ler , thecav i t a t ion damage pa t t e rn ( l ike p i t t ing)may be d i f f e r en t fo r each impel ler eye,thus suggesting there i s a flow imbalanceforced by the upstream geometry ( suct ionpiping and/or pump i n l e t bay) .Cavi tat ion Due to Secondary Flow a t BladeFi l l e t s (Corner Vortex)In many cases cavi ta t ion damage has beenfound a t th e f i l l e t between th e bladesuct ion side an d th e i m p P ~ l e r hub sur f ace .The damage appears to be caused by a s trongvortex , which i s confined in the bladeroot- to-hub corner an d genera tes a dr i l l ingact ion leading to rapid perfora t ion of theimpel le r hub an d in many cases t o sha f tdamage. The flow sources of such cornervortex are the intense shear forcesassocia ted with the secondary flows pa t t e rndue to the in terac t ion of the blade sur faceve loc i ty prof i le an d the boundary layers ofth e impel le r hub sur faces . Flow separat ionmay be a contr ibutory source, but notnecessar i ly .CASE NO. 1 - BOILER FEED PUMP FOR 660 MWUNITCase Fai lure Analys isA cavi ta t ion erosion problem i s presentedwhich occurred in four fu l l capaci ty mainboi ler feed pumps opera t ing in the samepower plan t . Each pump was turbine drivenwith i t s booste r pump in closed loop.Th e plan t , having four genera t ing uni t s of660 MW each, ha d been planned for basic andhigh load dut i e s . Large fu l l capaci typumps were se lec ted as peak e f f i c i ency wasvalued more than r e l i ab i l i t y a t par t loadopera t ions . The pump design t a r g e t was: N= 5200 RPM, Q = 10,400 GPM, H = 12,850 FT,NPSHR = 32 5 FT, NPSHA = 560 FT, f lu idtemperature = 304F. Th e s tage speci f icspeed, se lec ted for high peak e f f i c i encywas around 1700 {US uni t s ) leading to 6s tages . Therefore, the BHP/stage was about6300 HP indicat ing a very high energy pump.Th e design suct ion spec i f i c speed of thef i r s t s tage impel le r was around 6,900 (USuni t s ) . The minimum continuous flow wasspec i f i ed a t 50\ b .e .p . capac i ty .Th e four uni ts s tar ted to operate in theear ly 80 ' s an d ha d di f feren t servicehistory also with inact ive per iods . InMarch 1986 the main feedwater pump of UnitNo. l with the longest service t ime wasin terna l ly inspected an d a severecav i ta t ion damage was noticed in the f i r s ts tage impel le r . Thereaf ter the inspect ionof th e main pump was extended to Units No.2 - 3 - 4 revea l ing s i m i l a r damage

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ch a r ac t e r i s t i c s for a l l four pumps. Thef a i lu re ana lys i s ind ica ted t h a t (23): The eros ion area was local ized near thecorner between the suc t ion side of theblade root and impel le r hub, as shown inFigure 5. The t o t a l operat ion t ime was14,000 hours fo r Unit No. 1 an d 6,000 hoursfo r Unit No. 4. Both the locat ion an d thepa t t e rn of the eroded area were qui teunusual . Contrary to l i t e ra tu reindicat ions, the blade sur face was onlys l igh t ly damaged, while the cavi ta t ioneros ion perforated a l l th e way thru th e hubth ickness an d also penetra ted in to theshaf t . The impeller of Unit No. 4 opera ted forabout 70\ of the t ime around i t s b .e .p .capaci ty (c lose to 500 MW output ) , whilethe impel le r of Unit No. l operated formore than 50\ of th e t ime a t par t load(Figure 6). The impel ler eye speed wasranging from 170 FT/S to 200 FT/S. The NPSHA a t pump b. e . p. was about 1. 7t imes the NPSHR, which i s inadequate forth i s appl ica t ion according to some recentl i t e r a t u re ( 12) .Case Solut ionA new design impel le r ( impel ler B) whichwa s considered as th e most ef fec t ive s tepto reduce the cav i t a t ion damage, wasurgent ly developed and ins ta l l ed in theplan t in October 1986. Also a b e t t e rmater ia l from cav i t a t i o n resis tances tandpoint was used i . e . , CA6NM ins tead ofCA15 (damaged impel lers) . However, ap a r a l l e l program o f c a v i t a t i o nvisual iza t ion t e s t s on severa l designimpel le r variants models was planned tocover various hypothesis about the pecul iarcav i t a t ion p a t t e rn and so i den t i fy the bes tsolut ion for l a t e f ie ld implementat ion.This experimental invest igat ion was carr iedout in ear ly 1987.Several geomet r ica l conf igurat ions havebeen se lec ted and t es ted , including the one{impeller A) which experienced in the plan tthe cavi ta t ion eros ion pa t t e rn describedabove plus three impel le r var i an t s (B, BM,Bl) of new design. (23) .A ke y des ign goal was to reduce thesens i t iv i ty to cav i t a t ion eros ion in a widerange of p lan t loads from 300 MW to 660 MWcorresponding respect ively to 50\ an d 120\of pump b .e .p . capac i ty . Th e analys is ofthe va r i a t ion o f NPSHA and pump ro ta t ionalspeed with plant load, showed t h a t th e mostc r i t i c a l condi t ion was a t the maximum load.At par t loads more large margins developbetween the NPSHA an d the impel le r eyespeed, which s trongly determines cavi ta t ioneros ion r a te (12). Moreover, the future


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load spectrum versus operat ing hours, asexpected by th e user , wa s close to the onein Figure 6b. Then the shockless capaci tygiving minimum cav i t a t ion erosion wa sse lec ted around 110\ of the b .e .p . capaci tyfo r the impel le r B, which wa s planned fo rimmediate f ie ld ins ta l l a t ion (Conf. 2).Moreover, t he ove ra l l vane loading wa sreduced for the impel le r B, by lowering thehead coef f i c i en t an d also increasing theblade so l id i t y (over lap) .The cavi ta t ion bubble length (o n bladesuct ion s ide) Lc,b from model t e s t ssimulating the plan t NPSHA i s compared fora l l the var ian ts in Figure 7, assuming asreference the base l ine impel le r A a t b .e .p .capaci ty.For impeller B (Conf. 2) the bubble lengthLc,b wa s d r as t i ca l l y reduced across theen t i r e operat ing range. Moreover thesuct ion rec i rcu la t ion poin t wa s alsolowered (65\ Qbep) with respect to impellerA ( 90\ Qbep) Further reduct io n of Lc, bwa s obta ined wi th impeller BM (Conf. 4) bygrinding the vane suct ion surface a t i n l e t .The cavi ta t ion bubble length wa s alsoreduced with impel le r B1 (Conf. 2) but lessthan for impel le r B an d SM.The ana lys i s of the shape of the cav i t a t ionbubble a t the s i t e condi t ions from modelt e s t s (24) produced c lea r ins igh t s aboutdamage mechanisms. With reference toimpeller A, the bubble pat tern i s pecul iar ,showing a t r i angula r shape with highcav i t a t ion ac t iv i t y a t the blade roo t an dzero cav i t a t ion ac t iv i t y a t the blade t i p .This . indicates the exis tence of a highlythree-dimensional f low (even ex i s t ing a tb .e .p . capaci ty an d above), which produceshigh shear forces . Therefore, intenselocal vort ices are generated, whichcol lapse a t the impel le r hub and cause thepecul i a r e ros ion .pa t te rn (Figure 5). Withreference to impel le r B, the cavi ta t ionbubble showed in the model t e s t s arectangular shape with more or lessconstant length from hub to t ip which wasind ica t ing a r e l a t i v e ly tw o dimensionalflow f ie ld around th e blade leading edge.Some experiments with so f t pa in t ind icatedt h a t the damage was spread over a bandp ar a l l e l to the vane edge with zero damagea t the impeller hub. Thereaf ter , the bladewas cut back a t th e hu b causing a highpos i t ive inc idence angle an d then the shapeof the cavi ta t ion bubble became t r i angula rwith the l a rges t cav i t a t ion erosion a t thehub, as shown by so f t pa in t endurance t e s t .

Impeller Life Expeceancy (Theoreeical).The theore t ica l erosion ra t e (ER) has beenca lcu la ted by using a cor re la t ion based oncav i t a t ion bubble length , which wa s f i r s tpubl i shed in September 1986 (21) . The

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t rend versus capaci ty of the erosion r a tefo r the blade suct ion side (ER) has beender ived by using the v isua l cav i ty lengthLc , b from model t e s t s and i s plo t t ed inFigure 8 for a l l the four impel lers . I tappears t h a t : The erosion r a te has been reduced acrossthe fu l l range of cont inuous dut ies by a tl ea s t one order of magnitude with impellerB an d BM, an d also with impel le r Bl. Th e cav i t a t ion erosion on the bladesuct ion s ide i s remarkably increas ing a tpar t capac i t i e s as compare" to shocklesscapaci ty by a factor 3 to 5 for the bestimpel lers B an d SM. Then the impeller l i f ei s reduced for la rge boi l e r feed pumpsoperat ing for long t ime a t par t capac i t i e s .Therefore , the minimization of thecav i t a t ion erosion a t par t loads i s aninevi table but chal lenging task which mustbe accompl i shed by th e pumpdesigner /researcher in order to meet theneeds of widely cycling power plan t s .

ef f ic iencythe newOne t e s tef f ic iencyI t i s worthy to notice t h a t thehas been improved with a l limpel le rs a t par t capaci ty.var ian t has presented higherthroughout the whole opera t ing range.Moreover, a newly published method (25) forpredic t ing with probabi l i s t i c approach theexpected impel le r l i f e based on cavi ta t ionbubble length has been used for a l l thet h ree new design impellers (24, 26). Plantdata have been used fo r the load spectrumalong with other opera t ing condit ions .According to th i s theory the probabi l i ty ofreaching the t a r g e t impel le r l i f e of40,000 hours with the impel le r B i s a.bout65\ . Th e probabi l i ty ra i ses to about 80\i f a maximum erosion depth EDmax equal tothe fu l l blade th ickness i s allowed. Theimpel le r BM with very smooth th icknessdis t r ibu t ion a t the blade i n l e t i s evenb e t t e r than impeller B, with a t heore t i ca lprobabi l i ty of 90\ to reach a 40,000 hoursimpel le r l i f e . on the opposi te , theimpeller B1 shows only a probabi l i ty of 30\to 40\ , due to higher erosion ra t e (Figure8) as consequence of enlarged impeller eye(S = 10,100 US Units) even i f the NPSHA-toNPSHR margin i s higher across the operatingrange than for impel ler B (S 8150) an d BM{S .. 8100).The b o i l e r feed pump of Unit No. 1 has beenoverhauled (steam tu rb ine p r o b ~ e m s ) inSeptember, 1990 a f t e r 20,306 hours ofoperat ion with the new impel le r B. Theplan t operat ion prof i le r esu l ted to be veryclose to the on e ant ic ipa ted a t the designs tage. Th e inspect ion of the impeller hasshown t h a t : Some cav i t a t ion erosion has developed onthe suct ion s ide of each blade a t the


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midspan. The average of the maximumerosion depth (EDmax) i s 0.08 inches. Theaverage Lc,ed i s 0. 70 inches, while theaverage eros ion length Lc, ed i s 1 inch.Tota l absence of damage i s noticed on thepressure side . The sha f t damage has been t o t a l lyel iminated, while some minor cav i t a t ionerosion a t the f i l l e t between th e bladesuct ion sur face an d the impel le r hub i sex i s t ing . The average maximum eros iondepth i s 0.04 inches and the averageerosion length i s 0.15 inches . On the bas i s of the actual f ie ld erosiondepth developed in 20 , 306 hours th eprobabi l i ty to reach the t a r g e t l i f e of40,000 hours i s c lose to 95\ (EDmax equalto 75\ of the blade t h i ckness) , while thetheory (25) has ind ica ted a probabi l i ty of65%.More deta i l s about the comparison of th etheore t ica l predic t ion of the erosion r a tewith f i e ld data for t h i s pump case arepresented in another paper (26).CASE NO. 2 - BOILER FEED PUMP FOR 330MWUNITCase Failure AnalysisThis case history i s regard ing a s ing lemain boi ler feed pump (100\ capaci ty) of a330MW power plan t (26}. The pump, which i smotor dr iven, operates with variable speed.The pump suc t ion l ine i s di rec t ly fed byth e deaerator (open loop} and a doublesuc t ion impel le r i s used in the f i r s ts tage . The pump, which has e igh t s tageswas or ig ina l ly designed in 1965 for bas ican d high load dut i e s . The or ig ina l c .o . s .was: N = 3420 RPM, Q = 6250 GPM, H = 9140FT, NPSHR = 51 FT, NPSHA = 110 FT, f lu idt empera ture = 345 F. The design suct ionspec i f i c speed per eye of th e f i r s t s tageimpeller was about 9.900 (US uni t s} . Thepower/stage i s 1900 HP. Th e or ig ina l f i r s ts tage impel le r ha s eye per iphera l veloci tyaround 14 5 FTjS. Thus it can be consideredas r e la t ive ly high energy/high speed s tage .A f ield survey in August 1988 showed tha tthe f i r s t s tage impel le r has suf fered somemetal damage. The damage area was locatedon the pressure s ide (hidden s ide) of eachblade, but only on th e inboard impel le r eye(pump coupling side} . No damage wasnoticed on the impel le r eye a t the outboardside . A panel of exper ts concluded tha tthe damage was caused by the suct ionr ec i r cu la t ion du e to both operat ion at par tloads an d not equal r epar t i t ion of thet o t a l capaci ty between each impel le r eye.They recommended to redesign the f i r s ts tage impel le r to lower the suct ionr ec i r cu la t ion an d also reduce the tendency

6

to flow imbalance between the tw o impellereyes.Solution MethodologyStep 1 - Operator Input (Plant Data)The plan t operator was requested (August1989) to : Supply da ta on the expected pumpoperat ing prof i l e , as ke y input to theimpel le r design t a rge t . Such data includedthe base operat ing mode plus tw o poten t i a lal te rna t ives . Revise the or ig ina l c .o . s . an d pess iblyl imi t the highes t flow capaci ty (a t plan tfu l l load) t o the r ea l ly expected service ,in order to help th e designer in optimizingthe impel ler geometry for par t flowoperat ions . Th e pump operat ing l ine , asexpected , i s shown in Figure 9 along withthe pump performance map.Step 2 - Impeller Design StrategyAn impel le r design s t ra t egy wa s developedaimed a t achieving the spec i f i ed l i f e of40,000 hours with high probabi l i ty . Thes t ra t egy was focused on th e fol lowingaspects : The fu l l range of theparameters (Q, N, NPSHA} wasshown in Figure 10.

operat ionalanalyzed as

The erosion r a te (ER} predic t ion curvewas derived for a pre l iminary impel le rgeometry. A theore t ica l cor re la t ion basedon cav i t a t ion bubble length (21) was used,while the var ia t ion of the cav i t a t ionlength with capaci ty and NPSHA an d impel le rgeometry was inferred from ex i s t ingin ternal data base of cav i t a t ionvi sua l i za t ion model t e s t s . I t i s c lea rfrom Figure 10 , t h a t the tendency tocav i t a t ion erosion i s higher a t the baseload (300MW) and, espec ia l ly , the fu l l load(330MW}, while it i s lower a t par t load.However, the average l eve l of the erosionrate i s much lower than for case No. l(Figure 8), du e to lower impel le r eyeve loc i ty and also higher NPSHA-to-NPSHRmargins. This shape of the ER-curverapidly decreas i g with plan t load seems tobe pecul iar of a pumping conf igurat ion withvariable speed main feedwater pump an ddeaerator ( i . e . , es sen t i a l l y constantsuct ion pressure} which leads to theamplif icat ion of the NPSHA margin as theload i s reduced. I t i s also important tonote t h a t the e rosion r a te has tendency tosharp r i s e a t high f low/high speed. Thispecul i a r i ty suggests t h a t an overflow a tthe impeller eye, which i s du e to a flowimbalance between the inboard an d outboard


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ey e of the impel ler and produces a negat iveincidence angle , i s th e most l ike ly causeof cavi ta t ion damage over th e bladepressure s ide ("blade at tached cavi ta t ion")in th i s i n s t a l l a t i o n , ra the r than a suct ionrec i rcu la t ion re la t ed damage ("vortexcav i t a t ion" ) . Th e cumulative damage (EDmaxcorresponding to MOP Mean DepthPenetration) wa s compared for th e threecases of the pump operat ing prof i les i . e . ,base case , Alt . 1 ( fu l l load a t 330 MW) an dAlt . 2 (Unit Operat ing in Par t i a l LoadCondition) as shown in Figure 11. I t i sclear tha t the Al t . 1 i s the most severe interms of cavi ta t ion damage. Therefore th eshockless capaci ty for the new designimpel le r was se lec ted for th e fu l l loadopera t ion a t 330MW. However, th e expectedcumulative damage should not reach 75% ofthe blade thickness , even a f t e r 60,000hours. The probabi l i ty of achieving an impel le rl i f e of 40,000 hours (opera tor r ea l i s t i ct a rge t ) and even 60,000 hours (opera torul t imate goal) wa s also analyzed as shownin Figure 12. In general , the s i tua t ion i smore than sa t i s f ac to ry with lowerprobabi l i ty for Alt . 1/60,000 hours. (26) An addi t ion requirement (beside impellerl i f e ) from the operator was to extend pump r e l iab le continuous operat ions down to 100MW load. Then, th e suct ion rec i rcu la t iononse t was also lowered to 60% of theshockless capaci ty i . e . , Qcrs = 2650 GPM at2606 RPM (lOOMW load) . Then th e minimumcontinuous flow of 2146 GPM corresponds toBOt of the rec i rcu la t ion onse t capaci ty .This i s acceptable as the new impel ler hasmore mild rec i rcu la t ion in t ens i ty (smal le reye diameter and optimized blade geometryas enlightened by previous research data(8, 20 , 27 ) than for the exist ing impel le rwhich has Qcrs = 3200 GPM a t 2606 RPM. Addit ional key operat ing requirements a tfu l l load were considered too in optimizingthe impeller geometry ( e .g . acceptableNPSHA/NPSHR to withs tand t r ans ien ts causedby sudden loss of the suct ion pressure ) .The suc t ion spec i f i c speed wa s f ina l lyoptimized fo r 8,500 (US uni t s ) as the bes tcompromise between th e above conf l i c t ingrequirements a t high loads and p ar t loads.Step 3 - Suction Casing Modificat ionIn order to equal ize the capaci ty betweenthe inboard ey e an d the outboard eye of theimpel le r and mostly reduce the r i sk ofnegat ive incidence, it was necessary tos t reaml ine in a b e t t e r way the inflow tothe impel ler on th e inboard s ide of thei n l e t bay. This imposed to (Figure 13a ,b) :

7

Reshape by machining th e twin volu tei n l e t flange. Remachine the suct ion end cover formodifying the edge to match th e contour ofth e sha f t s leeve . Give a bet t e r contour to th e wear r ingsurface facing the suct ion channel . Produce a more smooth meridional contournear th e impel le r with shallowed shaf ts leeve .

Step 4 - Advanced Design Flow St ra igh tenerA flow s t ra igh tene r of advanced design wasalso included in the solut ion st ra tegy inorder to el iminate a poss ib le flowdis to r t ion due to the suct ion pipinglayout . However, the ins ta l l a t ion of theflow s t ra igh tene r wa s l e f t to future act ionby the operator , i f needed.Field ResponseThe boi l e r feed pump with the abovemodifications has s tar ted in July 1990.According to the p lan t opera tor , the pumpcan now be operated r e l iab ly from lOOMW toa fu l l load of 330 MW, while previously theopera t ing range wa s more narrow from 150 MWto 33 0 MW. Present ly, the operat ions a tpar t load down to 100 MW are sa t i s fac torywithout any ~ n d i c a t i o n of audiblecav i t a t ion , cont rary to th e previouss i tua t ion .

CASE NO. 3 - SCRUBBER RECYCLE PUMPSCase DescriptionThis case i s concerning tw o recyc le pumps(scrubber serv ice) operat ing in a powerplan t . The pump ( 6 inches suct ion J i ss ingle s tage with top- top f langes and s idesuct ion casing an d double suct ion impel ler .Each pump opera tes a t fixed duty (Figure14 ) i . e . , N = 3550 RPM, Q = 1350 GPM, H =520 FT, NPSHR = 12 FT with cold water ,which i s drawn from an open tank (NPSHA =36FT) . The power absorbed i s 253 HP. Theper ipheral speed a t th e impel le r eye i s 93FT/S.The operat ion mode inc ludes fu l l t imeservice with hot weat:.er (mostly summer)and no service a t a l l with cold weather(mostly winter ) .After about tw o years of se rv ice the pumpswere inspected due to high vibra t ion andthe impel le rs were found to be damaged.The erosion pat tern was character ized bythe following aspects :


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The metal damage was loca ted on the bladeauction aide (v is ib le aide) near the hu ban d also at the corner between the bladean d the hub. The damaged area over the blades show thepi t t ing aspec t which i s t yp ica l ofcav i t a t ion at tack . Heavy damagepenetra t ion was concentrated a t the cornerbetween the blade an d the hub. The above damage was present on a l l theblades a t both th e eyes of the impel ler .Bu t the ex tension of the damage was muchdi f feren t on each eye, being much worse onthe outboard a ide . This indicates tha t a)the capaci ty through the outboard impel le reye i s below 675 GPM ( i . e . , 0.5 x Qduty),b) the capaci ty through th e inboard i shigher than 675 GPM, but still producinghigh pos i t ive incidence angle and socav i t a t ion . There was no s ig n i f i ca t i v e damage on thepressure s ide of the blade, which a t par tflow (Qduty = 0.69 Qbep) usual ly i s causedby the suct ion r ec i r cu la t ion .Failure Analys isTh e f a i lu re analysis (pump an d systemgeometry, impel ler mater ia l , f luidproper t ies} enl ightened the fol lowingaspects : The suct ion piping l ayout (Figure 15 )with tw o elbows produces a flow dis to r t ionwith uneven dis t r ibu t ion of the t o t a lf lowra te a t the pump suct ion flange. Th e impel le r would have a t 675 GPM (0.5 xQduty) a r e la t ive flow incidence of 6 a tthe t ip and 15 a t the hub. Then the hubblade sec t ion i s prone to flow separat ion,which produces a cav i t a t ing vortex (cornervor tex) . A s trong sepa ra t ion /cav i t a t ioncan be expected fo r capac i ty below 675 GPM(outboard s ide ) . On the o the r hand,cav i t a t ion ca n occur , even l e ss in tens ivefor capaci ty above 67 5 GPM ( inboard side}due to pos i t ive incidence angle {bladea t tached or shee t cavi ta t ion) i . e . the flowangle is lower than the blade angle for theoperat ing condit ions , leading to highveloci ty peak in th e f i r s t por t ion of theblade on the suct ion side . This causes avery low pressure which ge ts local ly belowthe vapor tens ion. Consequently loca lconcentrated cav i t a t ion i s genera ted eveni f the margin NPSHA-to NPSHR i s apparentlyhigh (NPSHA/NPSHR = 3.0) an d the per ipheralspeed a t the blade i n l e t near the hub i sre la t ive ly low (57 FT/S). The NPSHR curve {Figure 14) measured withthe impel ler a t maximum diameter ( 13") doesnot l ike ly change a t the duty diameter(11 .25") , as the impeller blades have s t i l l

8

large over lap a t the t r im diameter. The impel le r mater ia l (CA15) has lowre s i s t ance to intense cav i t a t ion a t t ack ,al though it i s an adequate choice in mostcases for pumps of t h i s s ize running a t3550 RPM. Th e suct ion chamber geometry (nozzle plusi n l e t bay) i s not di rec t ly responsible forthe unequal dis t r ibu t ion of th e t o t a lf lowra te , because of i t s symmetry, asclear ly shown by the pump cross sect ion.However, it might enhance an y flowdis to r t ion presenr a t the suct ion f lange .Solu t ion S t ra t egyThe fol lowingimplemented: design changes were

Eliminate/reduce the flow dis to r t ion inthe suct ion piping by ins ta l l ing a flows t ra igh tene r of advanced design. The ex i s t ing design impel le r withappropr ia te modif ica t ions was used asurgent temporary f ix . The modif ica t ionswere derived from cav i t a t ion v i sua l i za t iondata which were previously obta ined with aboi ler feed pump old design impellercharac te r i zed by inc idence angledis t r ibu t ion , simi lar to the one of theimpel le r of th e recycle pumps i . e . lowincidence a t the t ip but much higher a t thehub. The cavi ta t ion bubble i s shown inFigure 16a, ind ica t ing a very intensecav i t a t ion zone from th e hub to the blademidspan. By using a spec ia l th ro t t l ingcone a t the impeller hub it was possibl.e todras t i ca l ly reduce the cav i t a t ion length ,espec ia l ly in the hub region, as shown inf igure 16b. This r e su l t i s du e to a netimprovement of the flow pa t t e rn an d areduc t ion of the incidence angle a t thehub. Therefore, a hub th ro t t l ing cone wasrecommended as temporary f ix for t h i sspeci f ic case and appl ied in the outboardimpel le r eye along with gr inding theimpel ler blades (Figure 17) on the suct ions ide to reduce the blade angle . As ult imate solut ion, it was s t ronglyrecommended to i ns t a l l a new designimpel le r an d also upgrade the impel le rmater ia l for higher - cav i ta t ion re s i s t ance .Moreover, the ins ta l l a t ion of some baf f l e sinside the suc t ion nozzle andfor the i n l e tvolute was suggested in order to improvethe flow dis t r ibu t ion , although, they mighthave only marginal impact.CASE NO. 4 - SMALL RAW SEWAGE PUMPScase Descr ipt ionThis f ie ld case i s concerning t h ree raw


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sewage pumps operat ing in a sewage pumpings ta t ion . Each pump (6 inches suct ion by 4inches discharge) , cons i s t s of a singlesuct ion impeller and a single volute . Thethree pumps are v er t i ca l l y mounted,connected to individual hor izontal suct ionl ine s tar t ing from the wet wel l and endingwith a 90 elbow j u s t a t the impel le r eye.The pumps have a discharge l ine to a commonheader for p ar a l l e l operat ion. Th e r a tedc.o.s. for each pump were spec i f ied for : N= 1770 RPM, Q = 75 0 GPM, H = 78 FT, NPSHR =12 FT, NPSHA = 3 5 FT approx. Th e powerabsorbed i s 21 HP. Th e per ipheral speed a tthe impeller eye i s 42 FT/S. Impel lermater ia l i s cas t i r on .During normal p l a n t opera t ion only a singlepump was running a t the above duty .However, when increased flow requirementswere occurring, tw o pumps were opera t ing inp ar a l l e l . At t h i s condi t ion each pumpopera ted a t reduced capaci ty around 45 0GPM. The operat ion for each pump wasi n t e rmi t t en t , cal l ing for a se rv ice of 8hours/day.Field data taken by the operator a t thep lan t f i r s t s t a r t up in January 1989indicated t ha t the pump operat ion was quie tand smooth with a s ing le pump running a tthe duty capaci ty ( 750 GPM), while thepumps became noisy during p ar a l l e lopera t ions (about 40 0 to 50 0 GPM). Thenoise l eve l increased progress ively in thefol lowing months, also extending in morewide capaci ty range . In November 1989, thepump user complained with the manufacturerabout the pump noise problem, while thepumps were still under warranty . Th e userexpressed h is concern t h a t the pump noisewith associa ted increase in vibra t ionlevels may lead to premature fa i lu re ,inc luding damage of impel ler as well aswear on the pump sha f t an d bear ings, withul t imate e f fec t of reduced l i f e .A subsequent f i e ld repor t in March 1990from the manufacturer service man ind icatedt h a t : Noise, charac te r ized as "gravel sound"was present , which could be generated bythe check valve a t the pump discharge. The noise l eve l was around 82db to 83dbfor a l l the t h ree pumps and var iousse t t ings of the check valve. The vibra t ion leve l a t the bear ings(lower/ top) was ranging from 0 .5 to 1 .0mils peak-to-peak a t maximum for a l l thethree pumps an d var ious se t t ing of thecheck valves . No damage (p i t t ing o r o ther aspect) couldbe seen in an y area of the impel ler , a f t e rapproximately on e year of operat ion.

9

In September 1990, the pump user i ns i s t edt h a t the pump wa s not acceptab le , inconsiderat ion of the expected l i f e span ofthe bearings an d ro ta t ing pa r t s , and askedfo r a t ime extension of warranty . Then,the pump manufacturer agreed with the userto commit the so lu t ion of the t rouble tocorporate engineering spec ia l i s t , s tar t ingfrom a f resh an d deep f a i lu re ana lys i s .Failure AnalysisThe ac tua l parameter of the pumps in thef i e ld which were measured during a newf ie ld inspect ion in November 1990, areshown in Table 1 . Only on e pump wasopera ted across a wide range of capaci ty.The i so la t ion valve, which was loca ted fa raway from the pump, wa s used fo r t h r o t t l i n gthe pump ins tead of the check valve, inorder to c lar i fy the source of the noise ,e i th e r the pump or the check valve.The f ie ld ind ica t ions (Table l ) showedtha t : The noise wa s occurr ing across the en t i r erange of capac i t i e s , even a t the ra ted one.Clear ly , the noise was or ig ina t ing from thepump. Th e noise reached a maximum l eve l a tcapaci ty of 40 0 GPM an d near ly disappeareda t shutof f , thus ind ica t ing the presence ofa noise peak a t par t capac i t i e s . The NPSHA l eve l wa s about 3 t imes higherthan the NPSHR. Such a ra t io i s apparentlymore than adequate for t h i s s i ze of pumpaccording to usual prac t i ce . Th e noise was audible only in the pumproom an d r e la t ive ly close to th e pump (5 to6 f ee t ) . Th e noise sound was as "crackl ing" ast yp ica l fo r cav i t a t ion noise .The ana lys i s of the pump performance a t thedesign impel le r diameter (10 inches)indicated t h a t the bes t eff ic iency poin t ,was corresponding to : N = 1770 RPM, Q =1050 GPM, H = 73 FT, NPSHR = 16 FT (Ns =2300 and s = 7200). The r a ted capaci ty(750 GPM) i s a t 71\ of the b .e .p . capaci ty ,while the second duty capaci ty of 45 0 GPM(two pumps in para l l e l ) i s a t 43\ of theb .e .p . capaci ty. Therefore the pump i srunning a t capaci t ies well below the designone.Th e ana lys i s o f the impeller geometrypoin ted out t h a t : The shockless capaci ty was s l i g h t ly abovethe b . e . p . capaci ty. The impel le r ha d tw o long vanes withlarge t h roa t areas ( i n l e t / out l e t ) between


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the b lades , as dic ta ted by so l ids p ar t i c l ehandling requirement fo r raw sewage pumps.However 1 t h i s impl ies a smal l over l apbetween vanes, which promotes high NPSHi -to-NPSH r a t i o ( inc ip ien t cav i ta t ion)espec ia l ly a t Q < Qsl an d a lso high suct ionrec i rcu la t ion onse t capac i ty (50\ to 70\ ofb . e . p . capac i ty) . Th e impel ler wa s only s l i g h t ly tr immed a t9.8:2 inches .Then the group of the f i e ld observat ionsalong with the ana lys i s of the pumpperformance an d geometry poin ted c lea r lyout t h a t the noise wa s due t o cav i t a t ioni n s ide the pump, because th e NPSHA l eve lwa s below the NPSHi correspond ing thei nc i p i e n t acous t i c cav i t a t ion . Moreover,for operat ions around 450 GPM suct ionr ec i r cu la t ion wa s a l so p resen t i n t e rac t ingwith the cavi ta t ion (vortex cav i t a t ion) .Th e cavi ta t ion noise wa s no t or ig ina l lyobserved for s ing le pump operat ion a tmaximum capaci ty dur ing th e p l a n t s t a r t upbecause the NPSHA wa s higher . In . fact , thewater l eve l in the wet wel l wa s org ina l lymaintained about 1 FT higher , an d a lso thehead los s in the suct ion p ip ing wa s lowerwith new an d c lean p ipe sur face.

Case Solut ionTh e var ious concerns of the pump user aboutpump dis t r ess an d l i f e needed to beaddressed .A. Cavit.at.ion Damage and Impel ler Li f eAt the poin t of the visua l o r acous t i ccavi ta t ion incept ion the r a te of theeros ion damage i s p r a c t i c a l l y zero .Therefore the r i g h t quest ion i s "what i s adet r imental c a v i t a t i on i n t e n s i t y l eve l?" .In f ac t most of th e pumps operate in thef i e ld with a NPSHA (A = ava i lab le ) which i shigher than NPSHR but s i g n i f i c a n t l y belowthe NPSHi i . e . they operate withc a v i t a t i on .There i s exper imental ind ica t ion t h a t theerosion ra t e du e t o c a v i t a t i on i s func t ionof the impel ler eye per iphera l speed 1 Ueye,and the length of the cav i t a t ion bubblelength at tached to the blade Lc,b (12 ,21) .On the o ther hand the cav i t a t ion bubblel ength Lc,b i s dependent from the impel lereye per iphera l speed. For s imi la rhydraul ic condi t ion (roughly, same marginNPSHA/NPSHR) the cav i t a t ion erosion ra t evar ies with about th e s ix th power of theper iphera l ve loc i ty a t the impel ler eyeUeye ( 19 1 :21). Therefore for thec a v i t a t i on erosion an d the impel ler l i f ethe impel ler eye per iphera l speed i s the

10

most importan t f ac tor .Moreover , the re i s some s t a t i s t i c a lexper imenta l ev idence t h a t c a v i t a t i onerosion i s p r a c t i c a l l y zero i f Ueye i sbelow 60 FT/S to 70 FT/S ( th resho ld va lue) .Such a t h resho ld value i s probab ly r e la tedwith the phys ica l mechanism of cav i t a t iondamage along with the amount of the impactenergy which i s du e to bubble col lapse an di s r e la ted with the pump energy l eve l .Th e impel ler eye per iphera l ve loc i ty forthe spec i f i c case i s Ueye 4:2 FT/S, whichi s very low and below the s t a t i s t i c a lt h resho ld l eve l for cav i t a t ion metaldamage. Thus impel ler damage is veryunl ike ly , as a lso al ready ind ica t ed by thef i r s t f ie ld i n spec t ion a f t e r more than on eyear operat ion an d a lso confirmed inNovember 1990, a f t e r l l, years operat ion .There i s high probabi l i ty (above 90\) t ha tthe impel ler l i f e expectancy an d the pumpr e l i ab i l i t y w i l l not be impaired .

Cavi tat ion Syct ion Rec i rcu la t i onInteract ionAs discussed above when a pump opera t es a tpar t capac i t i e s suc t ion rec i rcu la t ionoccurs a t the impel ler i n l e t which cancause cav i t a t ion ( "vor tex cavi ta t ion" or" rec i rcu la t ion cav i t a t ion" ) .Th e i n t ens i ty of such "vor tex cavi ta t ion"i s re l a t ed with both the ons e t capaci ty an dthe i n t ens i ty of t he suc t ion rec i rcu la t ion ,which are very much dependent from theimpel ler design (8, 20, 27)t an d mostly thepump energy l e ve l (capaci ty , head, brakehorsepower) . According to an i n t e rna ls t a t i s t i c a l char t der ived from f i e ld da taan d cover ing thousands of pumps no f i e ldproblem has been caused by suct ionrec i rcu la t ion with smal l pumps below thecombined l imi t of 1000 GPM an d 10 0 FT forcapaci ty an d head, r espec t ive ly , a t thebes t ef f ic iency poin t .Therefore it i s very l i ke ly t h a t ther e l i ab i l i t y of t hese spec i f i c pumps i s notimpaired, because the energy l eve l of thepump i s very lo w (brakepower below 30 HP).Moreover, the pump head of 70 FT a t thebes t ef f ic iency po in t i s below thes t a t i s t i c a l head boundary, above whichf i e ld problems cou ld be expected accord ingto the above s t a t i s t i c a l i n t e rna l char t fo rt r o u b l e s r e l a t e d w i th s u c t i o nrec i rcu la t ion .

cav i ta t ion Pressure Pul sa t ions , Noise,Vibrat ionsWhen cavi ta t ion bubbles co l l apse theyproduce pressure pulsa t ions , which i nc reasethe sound pressure l eve l {or "cavi ta t ion


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noise") and also may induce somevibra t ions . I f the bubbles col lapse ins idethe l iquid stream fa r away from the metalsurface (b lade , shrouds) , t he re i s no metaldamage but only noise , which does not havean y impact on pump r e l i ab i l i t y .Moreover, the i n t ens i ty of the pressurepulsa t ions induced by the cav i t a t ion i sdetermined by the pump energy level (power,head) and the suct ion pressure . For lowenergy pumps (roughly brake horsepowerbelow SO HP and head below 10 0 FT) thei n t ens i ty of pressure pulsat ions inducedfrom the cavi ta t ion i s pract ica l lynegl ig ib le an d not harmful for the pump.The dynamic load which i s re la ted with suchpressure pulsa t ions i s very low andconsequently the leve l o f vibra t ions i sonly very marginal ly increased . In f ac tthe key dynamic load for bearings (designload) i s determined by: The pressure di s t r ibu t ion a t the impel lerdischarge, du e to the impel le r - volu tei n t e rac t ion . Such a pressure leve l i spropor t ional to the pump head and so i shigher than the suct ion pressure . The surface a t the impel le r oute rperiphery. This area i s much l a rge r thanthe impeller eye area , which i s i n t e res t edby the suc t ion ? ressure pul sa t ions .Therefore the ex t ra dynamic load du e tocav i t a t ion pressure pulsa t ions i s only afew percent of th e bear ing des ign load. I ti s more or l e ss comparable to the accuracydegree of the bear ing l i f e ca lcu la t ions .The vibra t ion l eve l a t the pump bearinghousings, which i s ranging from 0.5 to 1.0MILS P-P a t maximum, i s wel l below theacceptable l imi t s given by var ious char t s ,including Hydraulic Ins t i tu te Standards .Such a low vibra t ion leve l indicate thatthe cavi ta t ion occurring in the impellerhas zero or marginal impact on the bear ingloading. Therefore the probabi l i ty toreach the spec i f ied bear ing l i f e i s veryhigh (above 90%).The overa l l pump noise l eve l of 82-83db i slow an d ful ly acceptable according tos tandards . Moreover, the pump s ta t ion i scompleted i sola ted an d far from habitedareas , while no noise i s audib le fromouts ide the s t a t ion and even in the pumproom a t a dis tance of 5 to 6 fee t .Final ApproachAs a general conclus ion, some cavi ta t ion i soccurring in the pump, which produces someacoust ic noise but a t acceptable level . onthe other hand such cav i t a t ion in tens i ty i snot detr imenta l for both the impeller l i f e(damage) and the bear ing l i f e ( addi t iona l

11

dynamic loading an d vibra t ion inc rease ) .Therefore an y act ion aimed a t el iminat ingthe cav i ta t ion / rec i rcu la t ion would beunnecessary and only cost ly an d possib lydetr imental for the e f f i c i ency .The above considerat ions were basical lyaccepted by the pump user . Th e case wa sse t t l ed ~ i t h o u t an y modif icat ions to thepumps.The l a t e s t f ie ld informat ion a f t e r yearsof operat ion confirmed both the absence ofimpel le r damage an d the f u l l r e l i ab i l i t y ofthese smal l sewage pumps.

CASE NO. 5 - CHEMICAL PLANT PUMP SERVICECase DescriptionThis case his tory i s regard ing four uni tsof the same cen t r i fuga l pumps operat ing inthe same chemical plan t for tw o di f feren tservice (one pump operat ing an d one pump ons tand-by) . Th e pump i s a s ing le s tage endsuc t ion type (2 0 inches suct ion an d 16inches discharge) with a discharge volu tecasing . The suct ion piping of eachi n s t a l l a t i o n s t a r t from a tank an d has tw oelbows close to the pump i n l e t . The tw oservice condi t ions are:Duty A BN (RPM) 980 980Q (GPM) 10570 8800H (FT) 98.5 105BHP {HP) 32 6 30 3NPSHR (FT) 16.6 16.1NPSHA (FT) 22.8 22.8

(20.5) (20.5)Process f lu id water + water +Na2Co3 CaOHFluid temp (F) 192 192Spec i f ic g r av i ty 0.97 0.97Basical ly each service ca l l s fo r a singleoperat ion a t near ly fixed capaci ty.According to the plan t opera tor , a l l thefour pumps exhibi ted t yp ica l cav i t a t ionnoise s ince s tar t -up (December 1982). Thenthe NPSHA was increased from the or ig ina ll eve l of 20.5 FT up to 22.8 FT by r i s ingthe f lu id l eve l in each tank . However, thenoise wa s noticed even during a cold waterf ie ld t e s t with an approximate NPSHA l eve lof 29 FT.

T ~ e n a f t e r a few months of operat ion(February 1983) the pumps were opened forinspect ing the impel lers . Th e inspect ionrevealed tha t : Absolutely no s ign of damage wa s presenton both the tw o impel lers of the pumps forduty A (d i lu te water solut ion of sodiumcarbonate a t the sa tura t ion po in t ) , a f t e r1100 hours of continuous se rv ice .


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Some damage was a l ready evident on boththe two impel lers of the pumps opera t ingfor duty B (d i lu te water so lu t ion ofcalcium hydroxide a t the sa tura t ion point)a f t e r 1000 hours (pump B-1) and 300 hours(pump B-2) of continuous service . Thedamage which was still a t an ~ a r l y s tage ,c lea r ly showed th e t yp ica l aspect ofcav i t a t ion p i t t i n g . The erosion waslocated on the vis ib le side ( suct ion s ide)of each blade, s ta r t ing from the leadingedge with length increas ing from the t ip tothe root of the blades .The highest concern of the p lan t operatorwas about the cav i t a t ion erosion of th epumps on duty B.Failure Analys isAll the four uni t s used the same pump s i zewith the impel le r trimmed a t 20.1 inches( 85% of the design diameter) Theper iphera l veloci ty a t the impel ler ey e wasthe same (Ueye = 80 FT/S). Also, theimpel le r mater ia l was the same (cas t i ron) .Basical ly, the only s ign i f ica t ivedifference between duty A an d duty B wasthe operat ing capaci ty. This was higherfor the pumps on duty A, which showedcav i t a t ion noise but not damage. TheNPSHA-to-NPSHR r a t io i s 1.37 for duty A(cavi ta t ion noise but no damage) an d 1.40for duty B (cavi ta t ion noise and damage)but ca n be considered pra t i ca l ly the same.The audib le cav i t a t ion noise in the f ie ldwas not cont inuous l ike per s i s ten tcrackl ing indicat ing intense cav i t a t ion ,but r a ther the noise was i n t e rmi t t en t l ikerandom bubble col lapse more charac ter i s t icof small degree of cav i t a t ion , close to theincept ion poin t . Moreover, th e locat ion ofthe damage on th e blade suct ion (v i s ib le )s ide an d visua l appearance as pi t t ingc lea r ly indicated t ha t the pumps wereoperat ing in the regime of "blade at tachedcavi ta t ion" corresponding to high posi t iveincidence angle (Figure 1).The ana lys i s of th e pump performance a t thefu l l (design) diameter (23.6 inches) showedfor the bes t e f f i c i ency poin t : N = 980 RPM,Q = 15200 GPM, H = 118 FT, BHP = 530 HP,NPSHR = 2 3 F T , Ns = 3380, S = 11,500. Thenthe duty A corresponds to 70% of the b .e .p .capaci ty (Qbep-des, des = design diameterof the impel ler ) , which i nd ica tes pa r t flowoperat ion. The duty B corresponds to 58%of Qbep-des, thus a fur ther reducedoperat ing capac i ty .The ana lys i s of the impel le r geometryind ica ted tha t : The shockless capac i ty , Qsl, i s a t 17350GPM i . e . at 114% Qbep-des, which i s a

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design choice qui te usual for highe f f i c i ency and reasonable suct ion spec i f i cspeed. Then, with comparison to t h i s veryimportant capaci ty (zero incidence angle,minimum NPSHi) the duty A is a t 58% an d theduty B a t 51%. At these condit ions theincidence angle i s very high, producingi nc ip ien t cavi ta t ion (v i sua l and acoust ic )for NPSHA-to-NPSHR ra t io much higher thanthe 1.4 value of th i s speci f ic case (16). The theore t ica l suct ion rec i rcu la t ioncapaci ty i s around 50% of Qbep-des. Thenthe operat ing capac i ty fo r duty B i s c loseto the one a t which both the curve ofinc ip ien t cav i t a t ion , NPSHi, and the curveof damaging cav i t a t ion , NPSHd, reach apeack (Figure 1 ) .The comparison of the duty A and duty Bwith Qbep-des an d Qsl i s shown in Figure18a. Moreover, the comparison i s extendedto the best eff ic iency capaci ty a t theac tua l f i e ld impeller diameter, Qbep-dd (d d= duty diameter) . When the impeller i strimmed, the shockless capaci ty which i sexclus ively determined by th e impellergeometry a t the i n l e t , does not reducewhile the b .e .p . capaci ty decreases withl inear proport ion . Then duty A an d duty Bcorrespond to 82% an d 68% of Qbep-dd. Suchvalues , even i f indicates par t flowoperat ions , are apparently not c r i t i c a lwith reference a lso to the NPSHA-to-NPSHRra t io o f 1.40 (Figure 18b), which i scommonly considered more than adequate fort h i s re la t ive ly low speed pump (980 RPM,Ueye = 80 FT/S). Also, these values ofQduty/Qbep-dd may erroneously suggest tha tthe pump operat ion i s for both the dt ieswell above the suct ion r c i r cu la t ion point .In f ac t , t h i s i s customarily defined asf rac t ion of the b .e .p . capac i ty an d simplyind ica ted Qrs/Qbep, but in the cur r en tprac t i ce (pump se lec t ion , operat ions ,t roubleshoot ing) i t i s tendencial lys t ra ightway referred to the b .e .p . capac i tya t the impel le r diameter selected for theduty .The NPSH t e s t curve a t the capac i ty of dutyA i s shown in Figure 17c. There anindicat ion t h a t the cav i t a t ion incept ionpoin t i s a t NPSH l eve l about 2 t imes theNPSHR, i . e . much above the NPSHA ava i lab lein the f i e ld (NPSHA/NPSHR 1 .4) .Moreover, the vi rbra t ions were measured onthe ex te rna l s ide of the volu te dischargecasing . The RMS (Root Mean Square) l eve lof the casing vibra t ions i s shown versusdecreasing NPSH for the ra ted capac i ty B inFigure 18c. The vibra t ion leve l s t a r t s toincrease a t NPSH = 1.4 x NPSHR which givesa very rough ind ica t ion t h a t the cav i t a t ionin tens i ty has grown enough to i n i t i a t e somedamage in the impel le r .Then i t was concluded tha t the NPSHi curvewas above the f ie ld NPSHA for both duty A


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and duty B, thus generating noise for a l lthe pumps. On the other hand, thecavi ta t ion damage curve NPSHd, which s t a r t sto r i se a t Q


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The inducer blades were heavi ly damaged.The loca t ion of the eros ion wa s on thesuct ion (visible) s ide of each blade a t SO\of the chord and also on the pressure(hidden) s ide of the next blade a t 25\ ofthe chord. The damage length was a fewinches on both the blade suct ion side(more) an d the blade pressure side ( l ess ) .The damage areas were located a t the oute rperiphery of the blade ( suct ion andpressure s ides) extending r ad ia l ly downwardfrom the t ip for about 20\ of the bladeheight . In the ax ia l di rec t ion ( ro ta t ionaxis) th e zone of the deepest e rosion onthe ~ l a d e suct ion side wa s near ly in f ron tof the corresponding maximum depth oferosion on the pressure side of the nextblade. The visual aspec t of damaged areaon each sur face of the blades was l ike asponge cloth with innumerous an d i r r egu laran d deep minicraters in the cen t ra l region,while presented a more a l e ss uniformpi t t ing in the pe r iphera l zone. The oute r s leeve was also damaged aspi t t ing , but much l ess severely, in areasfacing in the axia l di rec t ion the damagedzones of the inducer blade. Th e main impel le r was to ta l ly f ree of an ydamage a t the i n l e t an d also the o u t l e t ofthe blades.I t was immediately clear tha t each inducerwould have very shor t re s idua l l i f e an d hadto be replaced. Failure AnalysisTh e audible noise was simi lar to cavi ta t ioncrackl ing sound, but i n t e rmi t t en t withrandom loud bubble col lapse . Th e visua lappearance of the damaged areas c lea r lyindicated the presence of cav i ta t ion . Ther ad ia l an d ax ia l locat ions of the damageareas on each side of the blade channelsuggested t h a t the cavi ta t ion was occurr ingblade- to-blade in th e t ip stream tube .The pump performance a t the f u l l (design)impeller diameter had best e f f i c i ency poin ta t : N = 2980 RPM, Q 2 4900 GPM, H = 569 FT,BHP = 845 HP, Nsimp = 1790, NPSHRimp = 57FT, Simp = 10,000, Hind - 32 FT, Nsind =15500, NPSHRind = 25.0 FT, Sind = 18660.Then the opera t ing capaci ty i s a t 76% ofthe b .e .p . capaci ty, indicat ing tha t boththe impeller and inducer were runninga t par t flow.The ana lys i s of the inducer geometryindicated tha t : The shockless capaci ty was well above thedesign on e of 4900 GPM. This i s apecula r i ty of constant pi t ch inducers ,which use a f la t (not combered) a i r f o i lprof i le fo r the blade geometry.

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At the duty capaci ty the incidence anglea t the t ip wa s +11. This i s a very highvalue which can eas i ly promote cav i t a t ionon the suct ion s ide of the blade . Th e per iphera l veloci ty a t the inducert ip was 13 4 FT/S, which i s high andsuscept ible of developing cav i ta t ion evena t very high NPSHA. Th e vane loading of the blade a t the t i psect ion was too high a t the duty capaci_tyof 3720 GPM, indicat ing t h a t flowseparat ion was more than probable withconsequent reverse flow inside th e vanechannel (wsuction crec i r cu la t ionw) .Th e ana lys i s of the main impel le r geometryindicated t h a t a t the duty poin t theincidence angle was still below thes ta l l ing inc idence . on the other hand anexperimental i nves t iga t ion about suc t ionrec i rcu la t ion wa s in course in the samet ime, which included th i s spec i f i c impeller{7, 8) . Th e experimental data ha d c lea r lyshown t h a t th e suct ion r ec i r cu la t ion wasoccurring below the duty capaci ty of 3720GPM.Therefore it was concluded (Summer 1978)from the above ana lys i s t h a t the induceralone was subjec ted t o cav i t a t ion on thesuct ion side of the vane along with flowseparat ion. Then the t ip f lu id streamf i l l ed with vapor bubbles was turned by thei n t e r n a l r ev e r s e flow ( s u c t i o nrec i rcula t ion) toward the pressure side ofthe blade where the residual vapor bubbleswere col laps ing . The same mechanism wasvisual ized in cen t r i fuga l flow pumps in thesame per iod by other researchers ( 22 h asalso shown in Figure 4.Then the pecul iar damage pa t t e rn appearedas a combined act ion of both the bladeshee t cavi ta t ion (damage on the inducerblade suc t ion s ide) an d also the vortexcavi ta t ion due t o the suct ion r ec i r cu la t ion(damage on the inducer blade pressures ide ) . The inducer was working jus t belowthe capaci ty giving the peak of NPSHi(Figure 1) .I t was thought tha t the damage r a te wasqui te intense because: The per ipheral veloci ty Ueye wasremarkably high. Th e high pos i t ive .:.ncidence angle wasproducing a l a rge cavi ta t ion cloud, withmany col laps ing bubbles. Th e NPSHA-to-NPSHR ra t io around 1.35 wasvery marginal . The spec i f i c gravi ty around 1. 3 was anaggravating fac tor . In fac t , it should beexpected tha t the implosion of the vaporbubbles would produce high impinging


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pressure in high densi ty l i qu id , as l a t e lypublished (21, 25).Solution StrategyI t wa s c lea r from the above ana lys i s t h a t anew design inducer with lower shocklesscapacity would a l l ev i a t e the damageproblem.However as urgent an d temporary f ix it wasconsidered to make new cas t ing of th e samedesign of the inducer in the f ie ld an doverlay the blades with S t e l l i t e Grade 6 bywelding. But the s t e l l i t e coa t ing did notr e s i s t too long because of i t s in t r ins icb r i t t l en es s an d mainly an inadequate bond(welding) with the base metal . Moreover,it happened t h a t the hot welding processgenerated a dis to r t ion of the b ladegeometry thus changing the blade angle a ti n l e t which i s extremely c r i t i c a l for theinducer cavi ta t ion charac ter i s t ics of NPSHian d NPSHd. In fact these curves (minimuman d peak value from Figure 1) aredras t ica l ly changed by a deviat ion of th ei n l e t blade angle even l e ss than on edegree.Th e design st ra tegy fo r the new inducer wasfocused on the fol lowing c r i t e r i a : Reduce the incidence angle a t the t ip forthe duty capacity from 11 down to a fewdegree. Then the shockless capaci ty wa schosen around 4500 GPM (20\ higher than theduty capac i ty) . Inerease the NPSHJ\-to-NPSHR margin bylowering the NPSHR and thus increas ing thesuct ion spec i f i c speed of the inducer . Atthe inducer shockless capaci ty the S valuewas 24,400 (US uni t s ) with NPSHRdes = 16.5FT. Then the NPSHR a t the duty capaci tywas 12.5 FT, thus giving a margin NPSHA-toNPSHR = 1.60 approximately . Keep the same NPSHR for the impeller a tthe duty capaci ty. Then the head generatedfrom the inducer needed to be s l igh t lyincreased . As a consequence, the t i pdiameter of the inducer wa s unchaged,al though a reduction would have beenbenef i c i a l for lowering th e cav i t a t iondamage r a te . Also the ex i t blade angleneeded to be increased. Change the blade shape by using acombered prof i l e a t the t i- ; to meet theabove design needs for low inc idence anglean d lower shockless capaci ty an d higherinducer head. Then a var iab le pi tchinducer was se lec ted . Increase the blade th ickness from t i p tothe hub. Impose very narrow to le r ances on

15

geomet r ica l devia t ions . Then the inducerwas made by using a S-axis NC machine. Maintain the inducer mate r i a l .Th e new design inducer wa s i n s t a l l ed in theear ly 1979 an d opera ted for more than tw oyears . The noise was el iminated while thecav i t a t ion erosion r a te was d r as t i ca l l yreduced, but not completely e l imina ted .Thereaf ter , in 1982, the inducer des ign wasf ina l ly optimized to poss ibly e l iminate thecav i t a t ion eros ion. Th e f i n a l inducer wasstill of var iab le p i tch type with reducedt ip diameter giv ing a per iphera l ve loc i tya t the t i p of 124 FT/S (previously 134FT/S). Th e shockless capaci ty fo r theinducer wa s maintained a t 4500 GPM whilethe suct ion speci f ic speed wa s reduced to21,500. The NPSHR duty capaci ty was about13.5 FT giving a NPSHA-to-NPSHR r a t ioaround 1.50, which appeared to be adequate.Also the f ina l inducer was made by NCmachining in the same mater ia l as theprevious one. This inducer was ins ta l ledin the summer of 1982 an d i s stillopera t ing a f t e r more than 9 years .Inducer Rel iab i l i t y and Suct ion Speci f icSpeedThe inducer i s a spec ia l device purposelydesigned fo r very lo w cav i t a t ionrequirements (NPSHR) an d so high S valuefrom 15,000 to 25,000 (or higher) fori n d u s t r i a l appl icat ion (29, 30) .Th e high S value i s obtained by combiningspec ia l blade geometry and very low head(and thus low energy level) and ax ia l ,flowconf igurat ion (speci f ic speed Ns from15,000 to 20,000 u.s.). Typica l ly , thebrake horsepower of the inducer i s rangingfrom only 5% (a t b .e .p . capac i ty) up to 10\(a t SO\ b .e .p . capacity) of the fu l l pumphorsepower.Some published ana lys i s an d conclus ionsbased on a s t a t i s t i c a l survey ofcen t r i fuga l flow pumps (28) about a l imi tc r i t i c a l s value of 11,000 do notabsolu tely apply to inducers, which havemuch higher speci f ic speed ( ax ia l f low) an dmuch lower brake horsepower than the valuescharac ter i s t ic of the pump population usedfor the survey.Rather, the inducer cav i t a t ion erosionl imi ts are s t rongly re l a t ed to both theinducer design an d the ro ta t iona l speed asclear ly shown in Figure 20 (31) , which wasobta ined for water . Field exper ience hasshown t h a t S l imi ts up to 20,000 u.s. forwater an d even 25,000 u.s. for hydrocarbonf luids can be reached with negl igible orzero cavi ta t ion erosion .


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Extensive research both theore t ica l an dexperimental on inducers since 1930 's( f i r s t inducer patent his to r ica l ly ) to1990 's (cav i t a t ion v i sua l i za t ion an dacoust ic measurements plus in ternal flowmeasurements by Laser Doppler Anemometry(32) {Figure 21) , has generated goodinsides about inducer design optimizat ion .Moreover a very la rge indus t r ia l populationof inducers (around a few thousands) havegained a depth of knowledge and wideexperience in inducer technology.Only a few inducer f a i lu res {quickincidence damage) were reported to theau thor ' s knowledge in the l a s t 15 yearswhich were caused by misappl ica t ion ofinducer under c r i t i c a l dut ies i . e . highinducer t ip speed (around 100 FT/S) plusla rge size inducers (10" and 14") plus highspecif ic gravi ty (S.G. = 1 .0 to 1.3) pluslow flows dut ies {60% to 80% of b. e . p.capacity) plus inadequate NPSHA/ NPSHRmargins (from l . lS to 1.30) plus inducersmater ial with low re s i s t ance to cavi ta t ionat tack. All these f ie ld cases weesuccessfu l ly fixed by using a new designinducer optimized for par t capaci ty(inducer B) and a more res i s t an t mater ial(CA6NM, Duplex S.S. ) .The key fac tors determining the inducerr e l i ab i l i t y as r e la ted to cavi ta t ion and/orsuc t ion r ec i r cu la t ion are : Impeller ey e periphera l speed (Ueye). Duty capacity as percent of the"shockless capaci ty (which ca n bedi f feren t from the b .e .p . capaci ty) . Impeller design (Deye, blade geometry). NPSHA (and so NPSHA-to-NPSHR margins) . Fluid densi ty . Fluid thermodynamic proper t i e s . Fluid temperature. Impeller mater ia l .I t should be remarked tha t the abovefac tors do not show any di rec t inf luence ofthe suction speci f ic speed on thecav i t a t ion damage ra te and so on theinducer l i f e ,CONCLUSIONSVarious pump f ie ld problems re la ted withcav i t a t ion and/or suct ion rec i rcu la t ioncharacter ized by di f feren t degree of damagehave been widely discussed . The damagepat tern has been clear ly in ter re la ted withthe cavi ta t ion mode an d the flow mechanisman d the key geometrical parameter of the

16

impel le r and the opera t ing conditions plusthe t ime spectrum of the plant load.The f ie ld problems were ful ly clearedapplying new design cr i t e r ia for theimpel le r along with other per t inen tmodif ica t ions an d recommendations assuggested by a thorough f a i lu re analysis ,inc luding system aspects an d operatorinput .Th e se lec t ion of the shockless capaci tywith r espec t to both the pump opera t ingrange an d a l so the expected plant loaddis t r ibu t ion versus operat ing hours i s akey design choice. In t h i s regard a closecooperation between the pump designer an dthe pump user i s fundamental in order toharmonize the hydraulic design c r i t e r i a ofthe impeller or inducer with the expectedoperat ing mode of the plan t , as bas i s forreaching the impeller / inducer l i f e t a rge t .Inducers ca n opera te with suction speci f icspeed very high, i f they are properlyse lec ted an d t h e i r design i s optimized forthe actual dut ies with adequate NPSHA.NOMENCLATUREBHPDEDERILLcHNPSHANPSHRNspQRLsTbuw1:

Subscr ip tsb bubble

brake horsepowerdiametereros ion deptherosion ra teimpel le r l i f ecavity lengtht o t a l dynamic headnet pos i t ive suct ion headavai lablenet pos i t ive suct ion headrequired {3\ head drop)specif ic speed (US unit)power plan t loadcapaci tyl i f e fac torsuct ion speci f ic speed {USuni t s )blade th icknessperiphera l speedprobabi l i tyo p e r a t i n g t ime (g ivenload)-to-total s e r v i c et ime ra t io

bep best eff ic iencycal ca lcu:a ted poin td damagedd duty diametered eros ion maximume l erosion lengthey e impel le r eyei inc ip ien timp impel le rin d inducermax maximum

depth


19/27

re f

req

reference (a tpoin t of impel lercase No.1)requi red

bes tA - ef f ic iency

REFERENCES1.

2.

3.

4.

s.

6.

7.

8.

9.

Minami, s . , Kawaguchi, K., and Homma,T ., "Experimental Study on cav i t a t ionin Cen t r i fuga l Pump Impel ler" , Bul l .JSME, Vol. 3, No. 9, September 1960,pp. 19-29.Murakami, M., and Heya. N., "Swir l ingFlow in Suct ion P ipe of Cen t r i fuga lPumps (l s t - 3rd Report)" , Bul l . JSME,Vol. 9, No 34, September 1966, pp. 328-352.Toyokura T ., and Kubota, N., "Studieson back-flow mechanism of turbomachines(Par t I , Axial-f low Impel le r Blades)" ,Bul l . JSME, Vol. I I , N. 43, 1968.Toyokura, T ., Kubota, N ., an d Akaikes., "Studies on back-flow mechanism ofturbomachines (Par t 2, Mixed flowImpel ler Blades)" , Bul l . JSME, VoL 12,N. SO, 1969.Grianko L.P . , an d Papi ra , A.N. ,"Determining Flow Parameters a t theImpel ler I n l e t of a Mult i s tageCentr i fugal Pump dur ing HydraulicStagnat ion" , Lopastn ie Nasosi (BladedPumps), Mashinost roen ie - LeningradDept . 1975 pp. 50-66 ( in Russ ian) .Fer r in i , F . , "Some Aspects of Sel fInduced Prero ta t ion in the Suction Pipeof Cen t r i fuga l Pumps", Pumps an dPumping Systems for Liquids in Singleor Mult i s tage Flow, H. WorthingtonEuropean Technical Award - 1973, Vol.I I I , Hoepli , Milan 1974, pp. l -27Janigro , A., and Schiave1 lo , B .,"Pre ro ta t ion in Centr ifuga1 Pumps.Des ign C r i t e r i a " , O f f - D e s i g nPerformance of Pumps, LS 1978-3, VonKarman I n s t i t u t e , Rhode-Saint-Genese,Belgium, March 6-8, 1978.Schiavel lo , B ., and Sen, M. "On thePredict ion of the Reverse Flow Onseta t the c e n t r i f uga l Pump I n l e t " ,Performance Predict ion of Cen t r i fuga lPumps and Compressors, ASME 22ndAnnual Fluids Engineering Conference,New Orleans Louisana, March 9-13,1980,pp . 261-272.Fraser , W.H., "Reci rcu lat ion inCentr i fugal Pumps", Mater ials ofConstruct ion of Fluid Machinery andTheir Relat ionship to Design andPerformance, ASME Winter Annual

17

Meet ing, washington D . c . , u . s .A . , Nov.15-20, 1981, pp. 65-86.10. Gangwer, c .A . , "A Theory of Cavi tat ionFlow in Cen t r i fuga l -P u m p Impel lers" ,ASME Trans . , Vol. 63, No. 1 , 1941, pp.

29-40.11.

12.

13.

14.

15.

16.

17.

18.

19.

Gris t , E ., "Net t Pos i t ive Suct ion HeadRequirements for Avoidance ofUnacceptable cavi ta t ion-Eros ion inCen t r i fuga l Pumps; c a v i t a t i o n , FluidMach inery Group , H e r i o t - W a t tUnivers i ty , Edinburgh, Scot land , Sept.3-S, 1974, I m e c h ~ ConferencePubl icat ion CP13-1974, London 1974,Paper Cl63-74.Dernedde, R ., and Stech , P.R. , "Designof Feed Pump Hydraulic Components",Symposium Proc: Power Plan t Feed Pumps- Th e Sta te of the Art , EPRI CS-3158,Cherry Hi l l , New Je r sey , June 2-4,1982, Session 2, pp. 11-23.Flor janc ic , D ., "Development and DesignRequirements fo r Modern Boi ler FeedPumps", Symposium Proc : Power PlantFeed Pumps - Th e Sta te of the Art , EPRICS-3158 Cherry H i l l , New Je r sey , June2-4 , 1982, Sess ion 4, pp. 74-99.Palgrave, R . , and Cooper , P . , "VisualStudies o f Cav i t a t ion in PumpingMachinery", Proc. of the 3rdI n t e r n a t i o n a l Pump Symposium,Turbomachinery Laborato ry , Departmentof Mechanical Engineer ing , Texas A & MUnivers i ty , College Sta t ion , Texas, May1986, pp. 61-88.Sch iave l lo , B. , "Visual Study ofCavi tat ion -A n Engineer ing Tool toImprove Pump Re l i a b i l i t y " , Proc. ofEPRI 1 s t I n t e r na t i ona l Conference on:Improved Coal -Fi red Power Plants , PaloAlto, Cal i forn ia , Nov. 19-21, 1986.McNulty, P . J . , and Pearsa l l , I .S . ,"Cavi tat ion Incept ion in Pumps", Trans.ASME, Journa l of Fluids Engineering,Vo l 104, March 1982, pp. 99-104.Doolin, H.J . , "Judgecavi ta t ion P e r i l with AidEight Fac to r s" , Power, 130,77-80 (October 1986).

Relat iveof These( 10 ) 1 PP

Vlaming, D.J . , "A Method fo r Est imat ingth e Net Pos i t ive Suct ion Head Requiredby Centr i fugal Pumps", ASME Paper No.81 - WA/FE-32.Cooper, P., an d Antunes, F.F . ,"Cavi tat ion in Boi ler Feed Pumps,Symposium Proc. Power Plan t Feed Pumps- The Sta te o f the Art , EPRI CS-3158,Cherry H i l l , New Je r sey , June 2-4,1982, Session 2, pp. 24-29.


20/27

20 . Schiavel lo, B., " 'Cr i t ical Flow of the 5th In te rna t iona l Pump Use::sSymposium Panel Session onSpecifying Minimum Flow, TurbomachineryLaboratory, Dept. of MechanicalEngeineer ing, Texas A&M Universi ty ,College Sta t ion , Texas, May 1988, pp .186-192.21.

22.

23 .

24.

25.

26.

27.

Engendering Appearance of Recircula t iona t Centr i fugal Pump Impeller In le t :Determinant Phenomena, DetectionMethods, Forecast ing Cr i ter ia" LaHouille Blanche, No. 2/3 July 1982 pp .139-158 ( in French).Gulich, J . , and Pace, s . , "Quanti ta t ivePredict ion of Cavita t ion Erosion inCentr i fugal Pumps", Proc . of th e 13thIAHR Symposium on Progress inTechnology. Montreal , Canada,September 1986, Paper No. 42.Okamura, T ., an d Miyashiro H."Cavi ta t ion in cen t r i fugal ' P u m p ~Operat.ing a t Low Capac i t i e s" , PolyphaseFlow l.n Turbomachinery, ASME WinterAnnual Meeting, San FranciscoCal i fornia , Dec. 10-15, 1978 pp. 243:251. ISchiavel lo, B ., Arisawa, T ., andMarenco, G ., "Flow Visual iza t ion - ADesign Tool to Improve Pump Cavita t ionPerformance", Proc . of the secondIn te rna t iona l Symposium on Transpor tPhenomena, Dynamics and Design ofRotating Machinery, Honolulu, HawaiiApri l 1988. 'Schiavel lo, B., Marenco, G., andArisawa, T ., "Improvement of Cavita t ionPerformance an d Impel le r Life in HighEnergy Boi le r Feed Pumps", Proc. of_14th IAHR Symposium on "Progress

W l . ~ h l . ~ Large an d High-Specific EnergyUnl.ts , Trondheim, Norway, June 1988,Paper J3 .Gulich, J . , and Rosch, A., "Cavita t ion

E r o s i ~ n in Centr i fugal Pumps", SulzerTechnl.cal Review 1/1988, pp. 28-32.Schiavel lo, B ., and Presco t t , M. ,"Fie ld Cases Due to Various Cavita t ionDamage Mechanisms: Analysis an dSolut ions", Proc. of EPRI symposiumon: Power Plant Pumps, Tampa, Flor idaJune 26-28, 1991.Schiavel lo, B ., "The Impact of FluidDynamics Design on Minimum Flow", Proc.

lA5lE l RAW SEWAGE PUMP f iElD OBSERVATIONSres PUMP Ps Pc Pv H 0 NPSHRiPSIG! (PSIGI . IPSIG) (F'!) (GPM! (F'!)

LO 36 .36 (3.1)": 86 650 - 12

-()5 J7 J7 134)" 87 sac - 12

28.

29.

30.

Hallam, J . L . ,Which SuctionAcceptable?" ,Apri l 1992.

"Centr i fugal Pumps:sp ec i f i c ~ p e e d s Ar eHydrocarbon Processing,Janigro, A ., and Fer r in i , F.,"Inducers: Optimizing Pump Performancein Indus t r ia l Applicat ions", PumpWorld, 1976, Vol. 2, No. 1.Grohrnann, M., "Extended PumpApplication with Inducers" , HydrocarbonProcessing , Dec. 1979.

31. Palmateer L. 1 an d Grout 1 P. 1 "PumpSuct ion Specif ic Speeds Above 20,000",Hydrocarbon Processing , Aug. 1984.

32 . Boccazzi, A., Schiavel lo, B ., e t a l . ,"Cavita t ion Behavior and In te rna l FlowMeasurements by Laser DopplerAnemometry in a Low Sol id i ty Inducer",Proc. of the XXth Hydraul ics Open Days- "Hydraul ic Machinery", Lyon, France,Apri l 4-6, 1989.

ACKNOWLEDGEMENTSThe author wish to acknowledge theManagement of Dresser Pump Division for thepermission to present t h i s paper. Also 1thanks are due to T. Arisawal U. Capponi;W. Droop, c . Grossi an d several othercol leagues of the engineer ing departmentsof the Dresser Pump Division worldwide, whoact ively col laborated in solving the abovef ie ld problems. Final ly , the author wishesto thank a l l the plan t opera tor s who openlyan d act ively contr ibuted to th e successfu lso lu t ion of the above f ie ld pump t r oublesby providing a l l the requested andpert inent input .

NPSHA NPSHA . tJ>v ! REMARKS(FT)3 6 - ~

35.6

NPSHR' (PS9 ; i CAVITATION NOiSE FROM 3.07 [- 0 (3)>; PUMP - SUGHT INCREASE 1I WITH CLOSING CHECK VALVE

2.97 - 0 (3)*1 AS FOR POINT 1 (ASOVE)CAVITATION NOISE (PUMP)

JA 75C - 12 35.2 2.93 - 0 CHECK VAlVE OPEN

J8 560 -1 2 347 2!!9

02 - .. . 102 (150) - 12

3C -oe o - 40 94 oo - 120 S SUCTION PRESSUREPc DISCHARGE PRESSUREP.. P R E S S ~ ~ E AF>ER CHECK VAlVE4 >. ?HESSVRE OROF ACROSS iHE CHECK VA:..VE

, :MEC" V A ~ \ ;::;ART!A\.:.." C ~ t . O S E : : ; :

CAVTTA T(ON NOISE- 0 INCREASES.

- 0 l (ISOLATED VAlVE CLOSED) iNO NOISEMAX lEVEl OF- 0 CAVITATION NOISE


21/27

SPSHI INC IP IENTSPSHd DAMAGE {AFFECTISG IMPELLER L l FE)SPSHR : 3:1; HEAD DROPPS : BLADE PRESSt 'RE SIDESS BLADE SUCTION SIDE

STALL ----oojPOSITIVE

Os1I

INCIDENCE 0 '

~ R U N O l J TCAPACITY

..

eCHOCK ISGNEGAT IVE

Figure 1. NPSH Pecul iar Curves DefiningVarious Cavi tat ion Hodes ....

a+ooti;~ 3 0 0

50PERCENTAGE

SPEED CONSTA." 'TNPSH CONSTANT

SHOCKLESS150

FLOWRATE

Figure 2 . Variat ion o f Erosion Ratewith Capacity (11) .

F IXEDCAVITATIONVORTEX I

CAVITATION

Figure 3 . Al ternat ingSheet cavi tat ion withVortex Cavi tat ion(22) .

REVERSEFLOW

Figure 4 . Suct ion Recirculat ion (a-b) asSource o f th e Vortex Cavitat ion (C-d)(22) .


22/27

UNIT Lc,ed Lc,el EDmax A 8 c D () b.%1 2.05 2.36 1.18 0.16 0.39 0.59 o .o1

1.97 2.28 0.98 0.08 0.20 0.39 0.01 in 104

. --l-i-IFigure 5. Field Cavi ta t ion Erosion Pat tern ( Impel ler A) .

~ 5 0..!:).... 40l,oJ::!:; :30

~ 2 0....~ 1 0l,oJ0...0 0

0

UNIT 1

(a )

1 2 3 4 5 6UNIT POWER

7 0LOAD

UNIT 4

(b)

1 2 3 4P (100MW]

5 6 7

Figure 6. Operating Hours ( \ Total) vs . Power Loads .

"; 1.4 . -------. . . . --. . --. . . . ---.--. .---,;.;


23/27

1-...0 10s0 8(....:X:(. ) 6:;:(z 40_,< 21-0

03::::ecc

I I .I2

SUCTIONI RECIRC.r-oRIGINALf - - -NEW DES

3 4 5 6 7CAPACITY (1000 GPM)

-;:: :4 800

;:;3- 60....w 2 40..


24/27

Figure 12. Impe l l e r Li f e Predic t i on .

J

( a )

Figure 13. Comparison between the or ig ina l Suct ion CaseGeometry (a) and the Modified One (b ) .


25/27

f Patm)WATEH LEVEL.

OOIJSL.E SUCTIO:"i -OOUSL.E VOLUTE'A ' I MPEL.L.ER 3550 RP M

0 8 12 16 20 24CAPACITY X 100 GP M

REGUL.AR ELBOWX*+D

D " Dl l \ OF PIPE ( 10 ' /

Figure 14. Scrubber Recycle ~ u m p :Performance an d Operating P o ~ n t .Scrubbe r Recycle Pump: Suct ionFigure 15.Pip ing .

SHEET CAVITATION

l - t v ~ \

( b )

Figure l6 . Cavi tat ion Bubble Visual izat ion with High IncidenceAngle at the Hub Case a: free f low at the impe l l e r i n l e t ; (b):th ro t t l ing cone at the impel ler hub.

GRIND THE V IS IBLE SIDEOF EACH Vl\ l ' iE I Q!.lTBOARO EYE

''

VA:"E SECTION

VISIBLEID E

l....GRI!'-10 HERE

GROUNDSURFACE

Figure 17. Impel ler Modi f icat ions as Temporary Fix .


26/27

1. 5

1 .0"';c."::::t' - .0 .5

:X

ORIGINALIMPELLERNs =3380

( a )0.58 o. 70 0.85

:..IE ~

:;;:,

(i;'O . O L - - - - - ~ - - - - ~ - - - - ~ - - - L ~ ~ ~ - - ~ ~ - - ~0 .0 0.2 0.+ 0.6 0- 8 1. 0 1.2

..

1. 0 J.+

Q/Qbep-des

0.68 0.82 1. 0Q/Qbep -dd

Q/Qbep -des

( c )

2 .0 3.0NPSH / NPS .HRdu t yB

1.34

-z0 . 20 c0.16:::

..0.12 ;.:zco.os i:


27/27

3 0 ~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~

00Q 25->

cavitation n recirculation field problems

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