1. Cavitation Erosion Workshop, May 27-28, Bassin d’Essais des Carenes, Val de Reuil, 2004
On the Vibratory Approach For Cavitation Erosion Monitoring in Hydraulic Turbines
X. Escaler, E. Egusquiza
Technical University of Catalonia
M. Farhat, F. Avellan
Swiss Federal Institute of Technology
Thank you very much Mr. Chairman.
Good morning Ladies and Gentleman,
I would like to present to you this work about …

2. OUTLINE Introduction Leading edge cavitation Detection techniques
Instrumentation
Test cases: results & discussion
Conclusions & Ongoing work
The outline of my presentation is the following one:
I will begin with a brief introduction about the motivation of the work. Then I will describe the prototype under investigation and the instrumentation. And after explaining the measuring campaign I will expose the results to finish with the conclusions.

3. INTRODUCTION
Cavitation erosion in hydro turbines is increasing due to:
Power concentration ( velocities   )
Operation far from b.e.p. (off-design)
Detection during operation is of prime importance
Predictive maintenance
Extending intervals between erosion repairs
Avoidance of erosive regimes
Optimization of turbine operation
Validation of turbine rehabilitation
Acceptance tests
Models + Prototypes
Cavitation erosion in hydro turbines is a problem that is probably going to increase in a near future. This is due to the fact that current tendencies are to design new units with higher power concentration and to operate the existing turbines far from their best efficiency point. Therefore, the detection of erosive cavitation during prototype operation is of prime importance to be able to control its negative consequences.
The traditional approach for erosive cavitation detection is based on the analysis of external vibrations measured on fixed parts of the machine. In that case, the best position to monitor cavitation induced vibrations in the prototype is found to be the turbine guide bearing pedestal. This is the case when dealing with inlet edge sheet cavitation on the runner blades (is the most erosive form of cavitation).
However, this type of cavitation takes place on the rotating blades.

4. LEADING EDGE CAVITATION
Attached cavitation on a 2D hydrofoil
Cloud cavitation: unstable and highly erosive regime
Shedding frequency follows a Strouhal like law

5. LEADING EDGE CAVITATION
Attached cavitation on the blades of an hydraulic runner
Results in strong erosion on the suction side
Shedding frequency forced by hydrodynamic phenomena !

6. LEADING EDGE CAVITATION
Unsteady incoming flow
Guide vane wakes
Von Karman vortex shedding
Unsteady W and 
Sheet fluctuation
Pressure fluctuations
Induced vibrations
High frequency noise
Strong erosion

7. LEADING EDGE CAVITATION
Typical fixed measuring positions:
Turbine guide bearing pedestal
Wicket gate arm
Draft-tube wall
Signal is attenuated
Relative quantification
For absolute quantification transmissibility functions are needed
Dynamic calibration
The inconvenient of measuring vibrations on fixed parts such as bearing and guide vane is that the influence of the shaft rotation and of the effects of the fluid film and the water in the transmission path from the excitation location to the measuring positions are not well known.
So, it is believed that a sensor located on the shaft should be advantageous since it has a mechanical link to the blades which permit to avoids these aspects.
Therefore a measuring campaign on a water turbine prototype has been proposed to investigate this possibility.

8. DETECTION TECHNIQUES
Monitoring cavitation-induced signals next to the runner
Vibrations
Acoustic emissions
Detection techniques
High-frequency content
Amplitude demodulation of band-pass filtered signals
Phase or synchronous averaging of envelopes
Quantification techniques
Dynamic calibration of transmission paths
Inferring absolute intensities from impulsive vibrations induced by cavity collapses
Comparison of estimates with lab results on a given material

9. DETECTION TECHNIQUES
Amplitude demodulation of band-pass filtered signals
Signal acquisition at a high sampling rate
Band pass filtering above several kHz
Envelope extraction by the use of the Hilbert transform
Low-pass filtering and decimation
Spectral analysis and phase averaging of resulting signals

10. INSTRUMENTATION Sensors Instrumented impact hammers
Miniature high-frequency accelerometers
Acoustic emission sensors
Dynamic pressure sensors
Instrumented impact hammers
Conditioning units and filters
Recording devices
Digital tape recorders
A/D converters
Spectrum analyser
Signal processing software

11. KAPLAN TURBINE 1 Max. output power = 73 MW
# blades = 6, # guide vanes = 24
Cavitation erosion observed on blades
Radial measurements in turbine guide bearing:
Broadband vibration > 3 kHz
Vibration amplitude is maximum at full output load

12. KAPLAN TURBINE 1 Signals band-pass filtered from 5 to 10 kHz.
Envelope spectra indicates that wicket gate passing frequency predominates at maximum load
Amplitude of significant frequencies are much higher in bearing closer to runner

13. KAPLAN TURBINE 2 Max. output power = 100 MW
# blades = 5, # guide vanes = 20
Aerated draft tube
High frequency accelerometers
Guide bearing
Wicket gates
Tachometer

14. Upstream & Downstream levels
KAPLAN TURBINE 2
Test conditions:
Test Id
a [%]
P [MW]
Upstream & Downstream levels
H1 & H2 [m]
Aeration T0
H1=31.02, H2=14.13
Open T1 29
12.5 T2 37 20
H1=30.96, H2=14.13
Closed T3
H1=30.93, H2=14.13 T4 49
36.5
H1=30.96, H2=13.92 T5 60
50.7
H1=30.92, H2=13.93 T6 70
55.6
H1=30.89, H2=13.96 T7 75
63.6
H1=30.88, H2=13.98 T8 80
71.3
H1=30.86, H2=14.00 T9 90
89.3
H1=30.85, H2=14.02
T10
100
H1=30.83, H2=14.08

15. KAPLAN TURBINE 2
Spectra of guide bearing and wicket gates envelopes

16. KAPLAN TURBINE 2
Phase averaging of the guide bearing acceleration envelope
Zone I : a < 45 %
Random Acceleration
No modulation  NO CAVITATION
Zone II : 45 < a < 80 %
Strong & Periodic Impacts at the blade passing frequency
Phase shift analysis  Mechanical contact of the blades  NO CAVITATION

17. KAPLAN TURBINE 2 Zone III : a > 80 %
Strong modulation at the wicket gate passing frequency
 ATTACHED CAVITATION
Model tests showed attached cavitation for wicket gates opening > 85 %

18. FRANCIS TURBINE 1 2 Vertical Francis turbines
Hydropower Plant A, Units 1 and 2
65 MW per unit, 250 rpm
# blades = 15, # guide vanes = 24
High freq. Accelerometers
Guide bearing
Wicket gates
Draft-tube
Acoustic emission sensor
Sampling freq. 100 kHz

19. FRANCIS TURBINE 1 Averaged RMS values in the band [1-49k] Hz
Different behaviour in draft tube
Similar trends in bearing and wicket gates
Levels in wicket gates higher than in bearing
Maximum levels at 30 and 60 MW
Acoustic emission and acceleration qualitatively analogous

20. FRANCIS TURBINE 1
Auto-power Spectra of vibrations in Turbine Guide Bearing
UNIT 1
UNIT 2
High frequency region is very sensitive to operating condition
Maximum amplitudes and shapes of spectra depend on measuring position, sensor and unit

21. FRANCIS TURBINE 1
Auto-power Spectra of Envelope in the band [30k-40k] Hz
UNIT 1
Wicket Gate
Passing Freq.
UNIT 2
Outlet Swirl Freq.
Peaks appear at hydrodynamic frequencies
At low loads, draft tube swirl frequency
At high loads, wicket gate or blade passing frequency

22. FRANCIS TURBINE 2 Pitting Tests: Onboard Vibration:
4 polished stainless steel samples flush mounted (45 mm diameter)
Onboard Vibration:
4 kHz accelerometer
Onboard conditioning
Signal transmission:
Hollow turbine shaft
Slip ring. Freq. coding
Guide bearing vibration
50 kHz accelerometers

23. FRANCIS TURBINE 2 Dynamic Calibration :
Instrumented hammer
Multiple impacts on the runner blade
Synchronous measurements of onboard and guide bearing vibrations
Impact force & Response (in air)
Transfer function and coherence

24. Downstream level, Hd [m]
FRANCIS TURBINE 2
Test conditions:
40 min
205.9
206 72 M5
15 min
205.5
201 70
M4,4
219 75
M4,3
16 min
205.6
234 80
M4,2
17 min
205.7
247 85
M4,1
207.5
253 90 M3
41 min
207.4
223 78 M2
206.2
227 M1
Exposure Time
Downstream level, Hd [m]
P [MW]
a [%]
Test Id

25. FRANCIS TURBINE 2 Pitting tests:
Attached cavitation with maximum erosion located in the cavity closure
The cavity length depends on the downstream level
Pits size increases with the cavity length
Pits density decreases with the cavity length

26. FRANCIS TURBINE 2 Onboard vibration:
No possible to derive the contribution of cavitation induced vibration from raw acceleration signals with valuable accuracy

27. FRANCIS TURBINE 2 Validation of the guide bearing location
E0: Averaged power spectrum of onboard vibration envelope
E1: Averaged power spectrum of guide bearing vibration envelope
E0,p (E1,p): Averaged coherent power spectrum of onboard (guide bearing) vibration envelope referred to pressure fluctuation in the draft tube
Validation of the guide bearing location
Modulation at wicket gate passing frequency

28. FRANCIS TURBINE 2
Cavitation aggressiveness as a function of the guide vane opening
Maximum erosion risk : 80% of guide opening

29. CONCLUSIONS
Turbine guide bearing pedestal is the best fixed measuring point for detection of erosive leading edge cavitation on blades
Guide bearing measurements correlate with onboard measurements on the blade
High frequency accelerometers and acoustic emission sensors serve for detection
Amplitude demodulation techniques are the best method to identify hydrodynamic frequencies governing cavity fluctuations

30. CONCLUSIONS
Maximum erosive cavitation as a function of machine load is indicated by the amplitude of wicket gate passing frequency fv.
Forces on the blades are well transmitted to the turbine guide bearing through the shaft
Amplitude of fv is not sufficient to quantify in absolute terms erosive forces if transmission properties are not considered
Transmissibility functions can be obtained with an instrumented hammer with good coherence up to 15 kHz

31. ONGOING WORK
Detailed study of onboard measurements in various prototypes
Improve and apply calibration techniques for cavitation aggressiveness quantification.
Include the material resistance from laboratory measurements.