Vibration Behaviourof a Turbo-Generator Set

Wolfgang Hahn and Jyoti K. Sinha

Abstract In situ vibration measurements are carried out on a a typical steam turbo-generator (TG) unit at the West Burton Power Plant UK during the steady stateoperation. A typical phenomenon of appearance of low frequencies in band of7–12 Hz is observed mainly related to the vibration measurements on low pressure(LP) turbines bearing pedestals. This band of frequency observed to be modulatingwith vibration at the machine RPM (50 Hz) and its higher harmonics. The papersummaries the possible causes for such dynamic behaviour.

Keywords Steam turbine � LP blade cracking � Vibration measurement � Vibrationspectrum � Rotor stall

1 Introduction

West Burton Power Plant, UK owned by EDF Energy has 4 steam turbo-generator(TG) units for the power generation. Each TG unit consists of a high pressure (HP)turbine, an intermediate pressure (IP) turbine and three low pressure (LP) turbinestogether with a generator and an exciter [1, 2]. The 2 TG units have been sufferingfrom blade vibration problems in stages 3, 4 and 5 on the LP turbines and bladecracking in the last stage blades [1]. The cracking in the turbine blades have beenfound just above the blade root which is expected to be high stress location for thefirst bending mode for the blade. The last stage bladed disc is shown in Fig. 1.Typical blades with erosion and with crack are shown in Fig. 2. The blade erosionand then crack propagation is suspected as a result of blade excitation during the

W. HahnWest Burton Power Station, EDF Energy, Nottinghamshire DN22 9BL, UKe-mail: Wolfgang.hahn@edfenergy.com

J.K. Sinha (&)School of MACE, The University of Manchester, Manchester M13 9PL, UKe-mail: jyoti.sinha@manchester.ac.uk

© Springer International Publishing Switzerland 2015J.K. Sinha (ed.), Vibration Engineering and Technology of Machinery,Mechanisms and Machine Science 23, DOI 10.1007/978-3-319-09918-7_13

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normal and transient (run-up or run-down) operations [1]. The in situ blade tiptiming (BTT) measured data on the last stage blades (steam side) of the Unit 3 LP1turbine also confirm the high vibration amplitude of a few blades which could causethe blade cracking [2].

In situ vibration measurements are carried out on the Unit 3 during the steadystate operation at different power generation outputs to understand the machinevibration behaviour. A typical phenomenon of appearance of low frequencies in

Fig. 1 Typical LP1 last stage front bladed disc (new design blades) near IP turbine (steam end)

Fig. 2 Typical LP last stage blade with a crack

156 W. Hahn and J.K. Sinha

band of 7–12 Hz is observed mainly related to the vibration measurements on LPturbines. This band of frequency observed to be modulating with vibration at themachine RPM (50 Hz) and its higher harmonics. Such observations are also foundin literature related to rotating machines mainly compressors often related to thestall phenomena [3–9] which generally has high potential of damage. The papersummaries the possible causes for such dynamic behaviour of the TG set.

2 Vibration Measurements

A simple schematic of the TG set is shown in Fig. 3. A total of 14 numbers of fluidbearings in each TG unit are used in a TG unit to support the rotors of the steamturbines, generator and exciter. In situ vibration measurements were done on thebearing housings mainly in the vertical direction and also in the horizontal directionat the bearing B5 and B6 for Unit 3 during machine operation but at different poweroutputs.

3 Spectrum Analysis

The spectrum analysis of all the measured vibration data are carried out tounderstand the vibration behaviour of the TG set (Unit 3). It is observed that TGunit has in general strong peaks at 1× (1 times RPM) and 2× (2 times RPM) at allthe bearings. This indicates there could be some misalignment in the shafts of HP,IP, LP turbines and the generator rotors. Typical 3D waterfall diagram of themeasured vibration spectra at bearing B5 in the vertical direction are shown inFig. 4. Further inspection of spectra also indicates a presence of frequency peaks of7–12 Hz and this frequency band gets modulated with 50 Hz (1×) and higherharmonics. The averaged spectrum is also computed for all data. A few typicalaveraged spectra are also shown in Figs. 5 and 6. It is clear from the spectra that theexistence and the modulation of 7–12 Hz in several occasions during machineoperation. This effect is generally prominent in the LP turbines and often insig-nificant in the HP-IP turbines (see Fig. 7) and the generator.

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14HP IHP LP1 LP2 LP3 Gen Ex

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14HP IHP LP1 LP2 LP3 Gen Ex

Fig. 3 A simple schematic of TG unit

Vibration Behaviour of a Turbo-Generator Set 157

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Fig. 4 Typical waterfall diagram of the measured acceleration vibration spectra at the bearing B5in vertical direction

158 W. Hahn and J.K. Sinha

4 Further Investigation

The phenomenon of low frequency 7–12 Hz peak and its modulation with the 1×and higher harmonic components is not a usual feature for the steam turbine. It isalso important to note that it is not exiting continuously and not clearly observed inthe HP-IP turbines and the generator. The phenomenon is often intermittent andmay be depending upon several operating parameters and possibly related with theLP turbines.

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Often the fluid induced instability in the fluid bearing generates a frequency peakbelow sub-harmonic frequency (0.5×) [3], however the present observations aremuch different. The experimental study on a compressor by Bently et al. [4] sug-gests that this phenomenon could be related to the rotating stall which may oftenhave potential for any damage. This suggests the suspicion on the rotating stall inthe LP turbines. The rotating stall in the compressor is generally related to theinsufficient vacuum for the smooth air flow or fluid bearing related problem [4].

Another study by Wu and Qu [5] on the compressor also observed the modu-lation around the rotating speed due to inappropriate angle of attack of the bladeinlet causing the rotating stall. If the inlet steam relative velocity is not tangential tothe blade then it may result in the rotating stall. The inappropriate angle of attack forthe blade inlet seems to be the common problem for the rotating stall, back pressurepulse or flutter in the last stage LP turbine blades [6, 7]. Owczarek [8] gave anexcellent overview of the pressure pulse reflecting phenomenon between the bladesand rotating stall that can cause the blade failure.

Hence, considering the earlier studies on the rotordynamics and the presentobservations in the spectra of the TG unit, the possibility of rotating stall cannot beruled out. The presence of the low frequency 7–12 peaks and their modulation withthe machine 2× and higher harmonic components in vibration is definitely resultsinto the resonance of the machine at 4th critical speed (46.88 Hz [1]) and at theblade 1st bending natural frequency around 90 Hz [1, 2]. It is typically marked in ameasured vibration spectrum at the bearing B5 in the vertical direction shown inFig. 8 for the clarity. This phenomenon can have high potential for the failure.

5 Concluding Remarks

In situ vibration measurements were carried on the bearing housing on steamturbines and the generator of a typical TG set with a problem of the LP turbinesstage blades cracking. The spectrum analysis during different operating times and

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Fig. 8 Zoom view of Fig. 6a with comments to highlight the potential resonance at 4th machinecritical speed and at blade 1st bending mode during stall condition

160 W. Hahn and J.K. Sinha

powers indicate the presence of the frequency peaks in the band of 7–12 Hz andtheir modulation with the machine vibration at 1× and the higher harmonic com-ponents in a few occasion during the measurement process. The modulated fre-quencies with the 1× and 2× vibration components likely to trigger the resonancewith the machine 4th critical speed and the blade 1st bending mode during machinenormal operating speed which may have high potential of damage. The case studiesin the literature pointing the suspicion towards the possible stall in the TG set. Thestall is generally having sufficient potential for the damage. Plant is planning tocarry out in-depth study on the TG set by the online strain, blade tip timingmeasurements and possibly installation of camera to confirm the suspicion of thestall existence.

Acknowledgements Authors acknowledge Dr. Keri Elbhbah, the University of Manchester andMr. Joeff Tasker, West Burton EDF plant for their excellent help during the vibrationmeasurements.

References

1. Sinha JK, Hahn W, Elbhbah K, Tasker J, Ullah I (2012) Vibration investigation for lowpressure turbine last stage blade failure in steam turbines of a power plant. In: Proceedings ofthe ASME TURBO EXPO conference, Copenhagen, Denmark, 11–15 June 2012

2. Hahn W, Sinha JK (2013) Comparative study between in situ measured vibration data atbearing and BTT on a LP turbine last stage blades in a steam turbo-generator set. Paper numberGTINDIA2013-3614. In: Proceedings of the of the ASME gas turbine India conference,Bangalore, India, 5–6 Dec 2013

3. Muszyska A (1988) Stability of whirl and whip in rotor/bearing systems. J Sound Vib 127(1):49–64

4. Bently DE, Goldman P, Yuan J (2001) Rotor dynamics of centrifugal compressors in rotatingstall. Orbit, 2nd Quarter

5. Wu F, Qu L (2009) Diagnosis of subharmonic faults of large rotating machinery based onEMD. Mech Syst Signal Process 23:467–475

6. Rieger NF (2012) Progress with the solution of vibration problems in steam turbine blades.www.sti-tech.com/dl/vibnfr.pdf. Accessed 10 Jan 2012

7. Jonas O (2008) Steam turbine corrosion and deposits problems and solutions. In: Proceedings of37th turbomachinery symposium, Houston, Texas, 8–11 Sept 2008, pp 211–228

8. Owczarek JA (2011) On the phenomenon of pressure pulses reflecting between blades ofadjacent blade rows of turbomachines. ASME J Turbomach 133:021016-1-021016-11

9. Kita M, Iwamoto S, Kiuchi D, Kawashita R (2008) Prediction of subsynchronous rotorvibration amplitude caused by rotating stall. In: Proceedings of 37th turbomachinerysymposium, Houston, Texas, 8–11 Sept 2008, pp 97–102

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