Journal of Scientific & Industrial ResearchVol. 65, May 2006, pp. 402-409

Stress analysis on the major parts of a bulb turbine using FEA-A case studyN Kuppuswamy':",R Rudramoorthy' and K Mayilsamy'

'Maharaja Prithvi Engineering College, Avinashi, Coimbatore 6416542pSG College of Technology, Coimbatore 641 004

]Oepartment of Mechanical Engineering, Institute of Road and Transport Technology, Erode 638 316

Received 06 May 2005: accepted 15 February 2006

In the bulb turbine, main shaft and stay column play important role. Main shaft is positioned horizontally. More stressconcentration occurs near the junction of stay column with concrete at the top and bottom, since the entire load is beingtaken care of by stay columns at the top and bottom. A finite element analysis (FEA) has been made using ANSYS softwareto find out structural stability with stress distribution for bulb turbine and the reasons for high vibration and vertical cracksin the concrete structure of the powerhouse. Results from FEA analysis show that the bulb turbine operation is safe when thestay columns are fitted with pipe jacks. This method reduces stress concentration at the junction of stay column withconcrete.

Keywords: Bulb turbine, Main shaft, Simulation, Stability, Stay column, Vibration resonanceIPC Code: F03B13/06

IntroductionBulb turbine (Fig. 1) is similar to the tubular turbinewith a horizontal disposition of the shaft. The outersurface of the bulb is streamlined so as to form apassage for the flow of water from the head pond tothe turbine with a minimum loss of energy. Bulbcould be upstream or downstream of the runner but inmost cases it is on the upstream. The bulb turbines arealso quite suitable for tidal power plants. Bulbturbines are developed to utilize the smaller waterheads (up to 25 m) for power generation with higherspecific speeds (900-1100 rpm). The dischargesthrough these machines are 200-500 m3isec. Thewater flows all around the generator and the waterway is formed conical in shape by the turbine partssuch as stay cones and distributor cones. The innerstay cone houses the shaft and bearing assemblies.The distributor cone houses the guide vaneassemblies. A straight tapered water flow duct up tothe runner and after the runner brings about a verygood operating condition and improvements inhydraulic efficiency. The turbine runner is housedinside a cylindrical 'Discharge ring' and is boltedwith the distributor cone assembly at the upstreamside of the runner.

*Author for correspondenceTel: 04296-270066; Fax: 04296-270366E-mail: kuppuswamynks@yahoo.co.in

Vibration, occurred in the casing of the bulbturbine during the starting and stopping of machines,was observed due to sudden thrust of the water forceapplied to supporting steel and concrete structures.The rapid movement of the guide vane and the runnervane causes transient vibration. The steady statevibration is induced by the hydraulic thrust of flowingwater and due to the unbalanced rotating masses. Thevibration transmitted to the foundation of the bulbturbine is measured and found that the inducedvibration at the turbine is directly transmitted to thefoundation. The peak value of vibration at the guidebearing induced a peak vibration in the foundation butwith a reduced scale. This caused the entire civilstructure of the power station to vibrate in alldirections and attains peak amplitude of vibration atthe resonance condition. Hence the instabilityoccurred during the starting and shutdown of the

. machine and consequently vertical cracks appeared inthe concrete structure of the powerhouse building.

Literature SurveyHeavy vibration in the rotating shaft of the turbo

machinery was primarily responsible for the costlydown time, catastrophic failure and heavy IOSsl. Aheavy vibration in a rotating shaft had a very harmfuleffect on the machine performance.' Stability of theoperating system and chaotic response, which had animpact on the reliability and operation safety of highperformance rotating machines, was the most

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KUPPUSW AMY et at: STRESS A AL YSIS 0 BULB TURBL E USI G FEA 403

Oil Header Assembly

l3ulb NoseWater Flow ---11

Guide Vane

Stator

Fig. 1 - Cut section of bulb turbine

commonly seen failure modes attributed to thestiffness of the rotating machinery'. Turbine shaftinstability was caused by different parametric changesincluding gyroscopic effect, stiffness orthotropy ofrotor and support. partial rubbing conditions.clearance changes. etc '.

Wang & Lai.J used a combination of feed forwardand feedback control to minimize the structuralenergy during transient excitations. DeBedout et 0/

5

employed a combination of rough tuning open-loopcontrol and a gradient descent algorithm to tune anactive Helmholtze resonator, which is an acousticversion of the vibration neutralizer. This two-stageapproach is similar to that used by Long" who used alook-up table to roughly tune the neutralizer and thiswas followed by a fine tuning control loop using thephase between the velocity of the neutralizer massand the velocity of the host structure. Abc" used afuzzy rule base to alter the neutralizer stiffness as afunction of the transmissibility between theneutralizer and the host structure. Lai & Wang8 alsoadopted a fuzzy logic approach but used the structuralenergy as the criterion for their rule base. Li9 hasgiven the exact solutions and a new approach for theforced vibrations of single degree of freedom systems

with arbitrary variation of mass or stiffness with time.Yang et allo presented a novel method capable ofsimultaneous detection and characterisation ofnonlinear mechanical instability induced by atransverse crack in a rotary system.

This' paper analyzes the structural stability of thebulb turbine using ANSYS software.

Materials and MethodslIarmonic Response Analysis (liRA)

Any sustained cyclic load produces a sustainedcyclic response (a harmonic response) in a structuralsystem. HRA enables to predict the sustaineddynamic behavior structures, thus enabling to verifywhether or not designs wi 11 successfu lly overcomeresonance, fatigue. and other harmful effects of forcedvibrations. Hence this technique is used to determinethe steady state response of the structure at variousloads that vary with time. The idea is to calculate thestructure's response at several frequencies and obtaina graph of some response quantity (usuallydisplacements) versus frequency. "Peak" responsesare then identified on the graph and stresses reviewedat those peak frequencies.

404 J SCI IND RES VOL 65 MA Y 2006

Location

Table I - Vibration amplitude level in main shaft

Experimental reading displacement, urn ANSYS software prediction displacement, urn

Horizontal Axial Vertical Horizontal Vertical

On turbine guide bearing 50 36

On the thurst bearing 42 40 22 36.8 20.9

Axial

46 45 33 44.2

37.4

Load Bearing support

Fig. 2 - Main shaft with loadings

Fig. 3 - Blade with element and reaction forces

Simulation on Main ShaftThe element used for main shaft analysis is a 3D

beam element for flexural vibration and torsionalvibration (Fig. 2). Assumptions for main shaftanalysis are: i) Bearings are assumed to support theshaft which is restricted to the motion in x and ydirection; ii) Self-weight of the shaft is applied assurface load; and iii) Runner and rotor load areapplied at both the extreme ends of the shaft.

Simulation on Turbine BladeThe element used for the turbine blade for model

analysis is a 3-D 4 NODE tetrahedral structural solidwith rotations (Fig. 3). One end of the blade isassumed to be fixed (all degree of freedom is zero).

Bearing support Load

Simulation on Stay ColumnAssumptions for stay column analysis are: (i) In a

fixed structure turbine, boundary conditions are fixed(all degree of freedom is zet'o);(ii) Bearing contactsare at the part, which is attached to the main shaftthrough which loads are transmitted; (iii) Axial thrustis given to the blade along the line; (iv) Runnerweight is given at one end of the main shaft;(v) Similarly rotor weight is acting at another end, forpictorial representation it is shown as a rectangularcross section; (vi) Bulb weight is acting along thebearing itself; (vii) Self-weight of the shaft is actingalong the main shaft; (viii) Water pressure is acting atthe front end of the bulb turbine along the line;(ix) Upper and lower stay column in bulb turbine isfixed in the concrete, so it is assumed for fixedboundary conditions and stay column assembly(Fig. 4); and (x) Bulb turbine cross section along they-axis is taken for the analysis. Loading in the bulbturbine is as follows: rotor weight, 105; stator weight,75; shaft weight, 40; and turbine runner and hubassembly, 72 ton; water pressure, 1.6 kg/ern"; watervelocity at the exit of turbine, 9.37 rnIsec; water flowin bulb turbine, 270 m3/sec; and rotational speed,75 rpm.

Results and Discussion;.

Analysis of Main ShaftStress induced in shaft is well within the limit and

fundamental frequency calculated using flexuralvibration and torsional vibration does not coincidewith operating frequency. This shows that the systemis in safe condition. Values from ANSYS softwarematch with experimental readings (Table 1).

KUPPUSWAMY et al: STRESS ANALYSIS ON BULB TURBINE USING FEA

Top stay columnembeded in concrete

~~h

Bulb nosel

Water flow ----.-

Water flow

405

Water thrust

Bottom stay columnembedded in concrete

Fig. 4 - Simplified 2D section of a bulb turbine with stay columns (meshed)

Deflection of blade during modal analysis007

0.06

E 0.05E.~ 0.04

~ 0.03

'"Q 002

0.01

9-2·_······· .•..•.. -_ .•..

0.03368

2 3 4 5

Mode

Fig. 5 - Deflection Vs mode for the turbine blade

Analysis on Turbine BladeThe modal analysis of turbine blade predicts

fundamental frequency as follows: Mode 1, 1.157;Mode 2, 2.3621; Mode 3, 3.1703; Mode 4,4.1327;and Mode 5, 4.6820 Hz. Fundamental frequency is faraway from the operating frequency. Hence thevibration produced by the blade is well within thelimit and one can see the behaviors of blade duringvibrations. Modal analysis shows that deflection ofthe blade increases with frequency (Fig. 5). Hence itis necessary to operate the turbine within the ratedspeed. Deflection of the runner blade for the firstmode frequency (1.116 Hz) is shown in Fig. 6.

Analysis on Stay ColumnThe analysis shows that more stress has been

generated near the junction between the stay columnand the concrete foundation (Figs 7 & 8). It can bereduced by providing the crossbar arrangement in thestay column (Figs 9-12). Fig. 9 shows some deflectionnear the outer stay column with single cross bararrangement; hence it is necessary to add anothercross bar in upper stay column. In case 2, deflectionof the stay column is reduced by including twocrossbars in the top stay column and one set of crossbars in the bottom stay column. It also reveals thatstress concentration near the stay column and concretefoundation is reduced. Similarly, stress concentration(Fig. 12) is reduced by providing a solid bar (diam, 40mm) and an angle ofinclination of about 45°.

ConclusionsBased on the finite element simulation, modified

final design has been found to be better than theprevious design in terms of deflection and stress value.The stress induced in the main shaft is well within

the allowable stress. The flexural vibration analysisshows that the main shaft is in safe condition.Similarly, torsional vibration analysis shows that it isin safe condition. Experimental values match wellwith the results obtained from ANSYS software.

406 J SCI IND RES VOL 65 MA Y 2006

Fig. 6 - Deflection of runner blade for 151 mode (1.116 Hz)

NODAL SOLUTIONSTEP=lSUB =1FREQ=1.116USUM (AVG)RSYS=OPowerGraphicsEFACET=lAVRES=MatDMX =.033683SMX =.033683

- ~003743.007485.011228.01497.018713.022455,.026198.02994.033683

Fig. 7 - Deflection of stay column for the existing condition in x direction

10:45:24NODAL SOLUTIONSTEP=1SUB =1TIME=1USUM (AVG)RSYS=OPowerGraphic.9EFACET=1AVRES=MatDMX =.332E-04SMX =.332E-04

o.369E-05.738E-05.111E-04.148E-04.185E-04.222E-04.258E-04.295E-04.332E-04

- ----- - ---------------------',

KUPPUSWAMY et al: STRESS ANALYSIS ON BULB TURBINE USING FEA

Fig. 8 - Stress distribution for present conditions

Fig. 9 - Deflection of stay column in x direction for case 1

407

JAN 18 200310:47:17NODAL SOLUTIOr.STEP=1SUB =1TIME=1SEQV (AVG)

PowerGraphic::JEFACET=1AVRES=MstDMX =.332E-04SMX =.131E+07

o14546029092043637958183'9727299872759.102E+07.116E+07.131E+07

JAN 18 200312: 10: 44NODAL SOLUTIONSTEP=1SUB =1TIME=lUSUM (AVG)RSYS=OPowerGraphic3EFACET=1AVRES=MatDHX =, 112E-03SHX =. 112E-03

o.12'1E-04.248E-04.373E-04.'197E-04.621E-04.745E-04.870E-04.994E-04.112E-03

408 J scr IND RES VOL 65 MAY 2006

Fig. 10 - Stress distribution in x direction for case I

Fig. II - Deflection in x direction for case 2

JAN 18 200312:13:06NODAL SOLUTIONSTEP=lSUB =1TIME=lSX (AVG)RSYS=OPowerGraphicsEFACET=lAVRES=MatDMX =.112E-03SMN =-.445E+07SMX =.3':11E+07_ -.445E+07_ -.357E+07_ -.270E+07_ -.183E+07

-955592-83001789591.166E+07.253E+07.341E+07

JAN.<- 200312:55:35IWDAL SOLUTIO:STEP=lSUB =1TIME=lUSUM (AVGRSYS=OPowerGraphics'l!FACl!T=lAVRl!S=MatDMX =.996l!-06SMX =.996E-06

• ~111E-06• .221E-06• .332E-06• .443E-06• .553E-06o .664E-06

.775E-06• .886l!-06

.996l!-06

KUPPUSWAMY et al: STRESS ANALYSIS ON BULB TURBINE USING FEA

Fig. 12 - Stress distribution along x direction for case 2

Model analysis shows how the turbine blade willdeflect for various speeds, which then shows thefundamental frequency. The first fundamentalfrequency does not match the operating frequencyfrom the analysis. Changing the material can increasestiffness of the blade, but it should withstand thecavitations damage. It is better to operate the turbinebelow the frequency as obtained from the modalanalysis to avoid resonance condition.Stay column takes the entire load from bulb

turbine. More stress concentration occurs near theconcrete foundation for the stay columns. Thisproblem can be avoided or reduced by providing crossbar inside the stay column. Due to this, stress will bedistributed evenly and deflection of the stay columnwill also be reduced. The cross bars are solid shaft ofmild steel (diam, 40 mm).ReferencesI Suh C S & Chan A K, Wavelet-based technique for

detection of mechanical chaos, SPIE's Proc, Wavelet ApplVII, 4056 (2000) 267-274.

409

JAM 18 200313 :01:41NODAL SOLUTIO]STEP=lSUB =1TIME=lSX (AVG:RSYS=OPowerGraphic3EFACET=lAVRES=MatDMX=.996E-06SMI~=-44072SMX =160310

-44072

-. -21363134624055467656947492183114892137601160310

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