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Cav03-OS-6-007 Fifth International Symposium on Cavitation (cav2003)
Osaka, Japan, November 1-4, 2003
VIBRATION CAVITATION DETECTION USING ONBOARD MEASUREMENTS
Xavier Escaler
Universitat Politècnica de Catalunya [email protected]
Eduard Egusquiza Universitat Politècnica de Catalunya
Mohamed Farhat Ecole Polytechnique Fédérale de Lausanne
François Avellan Ecole Polytechnique Fédérale de Lausanne
ABSTRACT Experimental tests have been carried out in a Francis
turbine suffering from inlet edge cavitation erosion on the runner blades. The tests have comprised an onboard accelerometer mounted on the shaft in radial direction just above the turbine guide bearing. Simultaneously, vibrations in fixed parts have also been monitored such as turbine guide bearing pedestal and guide vane. Besides, an acoustic emission sensor has been located next to the fixed accelerometer in the bearing pedestal. The machine has been operated at various output loads and the corresponding signals have been recorded and analyzed following traditional cavitation detection techniques such as amplitude demodulation of high frequency bands. This permits to investigate the erosion process from a location which is directly connected to the blades. The advantage of such a situation is that the influence of the fluid film bearing and of the rotating frame of reference are avoided compared with measurements from a fixed position. As a consequence, the validity of external measurements to detect cavitation dynamic behavior can be checked. In the paper, the suitability of the telemetry system used for the tests is analyzed and then the results are presented and discussed.
NOMENCLATURE Rotating speed N Number of runner blades Zb Number of guide vanes Zv Fundamental frequency ff= N/60 [Hz] Blade passing frequency fb= Zb⋅ff [Hz] Guide vane passing frequency fv=Zv⋅ff [Hz] GVO Guide Vane Opening [%]
INTRODUCTION Cavitation erosion in hydro turbines is a problem that has
not been solved yet and 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 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. A brief description of various techniques and results can be found in Escaler et al. [1-2] and Kaye and Farhat [3]. However, such experiments lack information from measurements taken on the rotating components where cavitation is in fact taking place. Therefore, in this paper it is proposed to carry out traditional cavitation monitoring tests on a Francis prototype suffering from erosion on the runner blades combined with an onboard sensor fixed to the rotating shaft. Similar measurements had already been done by Bourdon et al. [4] which are explained in detail in his PhD thesis (Bourdon [5]). Unfortunately, they could not be completely analyzed due to noise appeared on the signals. In spite of that, they suggested the equivalence between measurements on the fixed parts of the machine and measurements taken with the onboard transducer. Later on, again Bourdon et al. [6] and Farhat et al. [7] tried to validate the vibrations induced at the turbine guide bearing by comparison with vibrations measured directly on a single cavitating blade, but no measurements were carried out at this time on the shaft.
The aim of this paper is to find out if measurements taken on the shaft in a position as close as possible to the runner can improve the detection techniques. The comparison of such measurements with the ones taken from the traditional positions on bearing pedestal and guide vane should permit to confirm the validity of the present monitoring techniques.
EXPERIMENTAL SET-UP AND MEASUREMENTS
Prototype A vertical Francis turbine has been chosen for the
experimental investigation. This unit is the second of four identical units belonging to the same hydropower plant. These
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turbines can operate up to a maximum output power of about 11 MW per unit. The nominal flow rate per unit is 28 m3/s, the net head is around 51 m, N is 250 rpm, Zb is 15 and Zv is 24. Previous results concerning cavitation erosion detection on this particular hydropower plant are available in Escaler et al. [1-2] and Vizmanos et al. [8]. A particular aspect of these machines is that their turbine bearing is made of rubber and works without oil film.
For those machines, periodic inspections have proved that cavitation erosion is well localized next to the leading edge in the suction side of the blades In fact, after several years of operation advanced stages of mass loss occur due to inlet cavitation.
Onboard Instrumentation A mini telemetry system has been used to measure
vibrations on the rotating shaft of the water turbine. The system is composed of very small and light modules which are mounted around the shaft next to the turbine guide bearing. The signal acquisition module for ICP sensors includes signal condition and anti-aliasing low pass filter. It has a 12 bit resolution and is controlled by an Encoder module. This Encoder is a multiplexer that generates a PCM output signal that is in turn connected to a radio frequency transmitter module. The powering of the modules is 5V DC generated by a battery. The receiver station includes a receiving antenna and a 4 channel decoder. Four analog output signals are available via BNC connectors. All channels are simultaneously sampled and the cut-off frequency for a two channel configuration is 6000 Hz per channel. The full system accuracy is about ± 0.5% without the sensors.
The telemetry system has been equipped with a high frequency accelerometer having a mounted resonance frequency of about 52 kHz. This sensor has been fixed to the shaft with a cementing stud.
Fixed Instrumentation The same type of sensors than the onboard one have been
used to monitor vibrations on the fixed parts of the machine. A total of 3 accelerometers have been mounted with cementing studs. They have been conditioned and amplified prior to their recording. The instrumentation set-up has also comprised an acoustic emission sensor having a resonance frequency of about 200kHz and a photoelectric tachometer probe for contact-free detection of the rotating shaft.
The analogue output signals have been simultaneously digitized and recorded at a sampling frequency of 48000 Hz.
Measurements The accelerometer mounted on the shaft, which is indicated
by Ashaft, has been glued close to the top end of the turbine guide bearing as it can be seen on Figure 1. The rest of the accelerometers have been located on fixed parts of the machine. In particular, two of them have been placed on the upper part of the turbine guide bearing wall in radial directions at 90° one from the other. They are named by A13 and A14 and their location relative to the penstock is shown in Figure 1. The last accelerometer, D13, has been positioned on the top of one wicket gate in axial direction. Finally, position AE13 indicates
the acoustic emission sensor mounted in radial direction next to A13.
A13D13
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ASHAFT
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A13 AE13AE13
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Figure 1: Outline and photographs of the measured positions.
For the tests, the turbine was operated at various conditions. Previous investigations carried out in unit 1 by Vizmanos et al. [8] had shown that maximum cavitation erosion was occurring at around full load. Therefore for unit 2, it was decided to concentrate the measurements in the operating range from 70% to 100% guide vane opening. Specifically, the machine was run at 70, 75, 80, 85, 95 and 100% guide vane opening during the coast up. Each load was maintained during 15 minutes to make the measurements.
RESULTS Auto power spectra have been computed from time domain
signals measured in fixed and onboard sensors. In Figure 2 some results are presented corresponding to the onboard accelerometer Ashaft, the fixed accelerometer A13 and the fixed acoustic emission sensor AE13. As it can be observed, the signal from the onboard accelerometer has a limited bandwidth due to the telemetry system cut-off frequency at 6 kHz. For the rest of the signals the frequency content reaches 20 kHz. In these graphs, it is possible to compare the evolution of the signals as the operating condition of the machine is changed by increasing the guide vane angle. In this case, 100% guide vane opening corresponds to full load. In all the measured positions the spectra begin to change with load at frequencies above 3 kHz. In the lower frequency range no clear distinction between the various machine conditions is observed. On the other hand, for higher frequency bands the spectral lines are quite parallel and show different amplitudes depending on the GVO. However, if
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the spectra corresponding to different positions are compared it can be confirmed that each of them has a distinctive shape especially at high frequencies.
UNIT2 ASHAFT
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Figure 2: Auto power spectra of onboard (Ashaft – 10 kHz baseband) and fixed vibrations (A13 – 20 kHz baseband) and of fixed acoustic emissions (AE13 – 20 kHz baseband), for different guide vane openings.
So, it is reasonable to use the frequency band from 3 to 6 kHz for a detailed investigation of cavitation induced vibrations. First of all, the RMS values of the time signals in this narrow band have been computed. They are presented in Figure 3 as a function of GVO. At first glance the tendencies are similar for accelerometers A13, A14 and acoustic emission sensor AE13, all of them located on the turbine guide bearing. Their values always increase with GVO with the exception of a sudden decrease at 90%. This behavior is not observed with the onboard accelerometer on the shaft. In this case, the RMS value has a steady increasing tendency with GVO. Finally, position D13 in
the guide vane differs from all the rest but also has its maximum at 100% like them.
UNIT2 [3k-6k Hz]
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ASHAFT A13 A14 D13 AE13
Figure 3: RMS values of time signals filtered between 3 and 6 kHz as a function of guide vane opening.
The next analysis has consisted in determining the predominant modulating frequencies by filtering the time domain signals in various frequency bands. Figure 4 shows the averaged auto power spectra of demodulated signals in the band from 3 to 6 kHz for the onboard accelerometer and the fixed accelerometer on the bearing A13 at 70, 85 and 100% GVO. In this case, the time envelopes have been computed digitally using the Hilbert transform.
The spectra corresponding to 70% GVO do not show any clear frequency peak but at 85% GVO the blade passing frequency fb (62,5 Hz) appears and it is the highest peak in both positions. Its second harmonic 2fb (125 Hz) can also be observed as well as the guide vane passing frequency fv (100 Hz) that has a very low amplitude. Other hydrodynamic frequencies are also observed such as the fundamental frequency ff (4,16 Hz) and its second harmonic 2ff (8,33 Hz). At full load (100% GVO) it is remarkable how fv, strongly modulated at ff, reaches a similar amplitude level to fb which is still an important peak in the whole spectrum. For both positions, the 2fv peak also appears but on Ashaft the 2fb disappears. And finally, on A13 the ff peak and many of its harmonics dominate the low frequency band of the spectrum. In summary, the main hydrodynamic frequencies that modulate the signals are quite the same in Ashaft and A13 for any operating condition, although on A13 some additional peaks appear at 100% GVO.
In Figure 5 (top), the comparison of amplitude demodulation results is plotted. The frequencies show a coincidence between the onboard vibration in the frequency range from 3 to 6 kHz and the acoustic emission on the bearing in the frequency range from 17 to 20 kHz. In both cases the frequencies fb and fv are the most important ones, as well as 2fv with an intermediate amplitude. On the contrary, there is a only a slight similitude between Ashaft and the guide vane accelerometer D13 as it can be seen on the same Figure 5 (bottom). For instance, in the guide vane the fb peak is among the lowest ones although 2fb and 3fb are the highest ones, which does not happen in Ashaft.
The output signal from the tachometer has permitted to average synchronous data in order to try to improve the results.
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Concerning the frequency content of the demodulated signals no significant differences have been observed between results obtained from averaged spectra and from time averaged envelopes as the ones shown in the top of Figure 6. The time averaged envelopes serve to enhance the periodic amplitude increase of the vibration and acoustic emission during one shaft revolution that corresponds to 0,24 seconds. So, these signals have been used to estimate the averaged coherence between different measuring positions. For instance, the plot at the bottom of Figure 6 presents this function computed between averaged envelope measurements from Ashaft and A13 in the frequency range from 3 to 6 kHz at 100% GVO. A coherence value above 0.8 after 10 averages is found for ff, 2ff, 5ff, fb, fb+2ff, fv, fv+ff and 2fv. Among these peaks the highest coherence level is found for fv. These results demonstrate the equivalence of the amplitude demodulations calculated from onboard and fixed measurements.
UNIT2 70% GVO [3k-6k Hz]
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Figure 4: Auto power spectra comparison of demodulated filtered signals in the band from 3 to 6 kHz between onboard
accelerometer (Ashaft - Y axis on the left) and fixed accelerometer on bearing (A13 - Y axis on the right) at 70, 85 and 100% guide vane opening.
UNIT2 100% GVO
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ASHAFT [3k-6k Hz]AE13 [17k-20k Hz]
Figure 5: Auto power spectra comparison of demodulated signals between onboard accelerometer (Ashaft – Y axis on left) in the frequency band from 3 to 6 kHz and: (top) fixed acoustic emission sensor on bearing (AE13 – Y axis on right); (bottom) fixed accelerometer on guide vane (D13 – Y axis on right), filtered from 17 to 20 kHz at 70, 85 and 100% guide vane opening.
Finally, the signal energy distribution in the joint time-frequency domain has been computed using the Short-Time Fourier Transform with a Hanning sliding window. Some results are presented on Figure 7 corresponding to onboard vibrations at 70 and 100% GVO. Significant differences are observed between these two operating conditions in the upper frequency range that confirm the results previously presented.
Other results have consisted in the study of the measured response to impacts applied on the machine still with an instrumented hammer. They have served to confirm the weak transmissibility from the shaft next to the onboard sensor location and the guide vane sensor.
DISCUSSION The evolution of the auto power spectra of raw signals
above 3 kHz seems to indicate the presence of cavitation activity. In the case of the fixed sensors, the high frequency energy is observed up to 20 kHz which is the maximum frequency of the measuring system. For the onboard sensor the bandwidth is restricted to 6 kHz due to the telemetry system limitation.
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The RMS values of the time signals filtered in the frequency band from 3 to 6 kHz permits to have an initial information about the most aggressive cavitation conditions, although this must be confirmed with amplitude demodulation results. The energy content in this band as a function of guide vane opening shows two tendencies. For the fixed sensors located on the turbine guide bearing it increases with GVO but when passing from 85 to 90% there is a sudden decrease. Such behavior is analogous in any frequency band up to 20 kHz. On the contrary, for the onboard sensor the tendency is to increase continuously. There is a doubt about what would be observed for higher frequencies. In the case of the guide vane sensor a similar growing behavior is observed for higher bands but is not evident in the frequency band from 3 to 6 kHz.
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Figure 6: (Top) Time averaged envelopes for onboard (Ashaft – Y axis on left) and fixed (A13 – Y axis on right) vibrations in the frequency band from 3 to 6 kHz at 100% guide vane opening. (Bottom) Coherence function between envelopes.
At this point the analysis is dedicated to detect hydrodynamic frequencies modulating the shedding process of transient cavities that collapse on the blades and cause erosion. Their appearance in the spectra of the envelopes of filtered signals indicates cavitation and their amplitude is related to its relative aggressiveness. Concerning the actual measurements, similar results are obtained in all the measuring positions with the exception of the guide vane. At 70, 75 and 80% GVO no significant trace of any particular frequency is observed. Then, at 85% GVO the blade passing frequency fb appears and dominates the baseband. This peak is still visible for the rest of the operating conditions up to full load. Finally, the guide vane
passing frequency fv comes out at 95% and grows at 100%. In fact, the strongest modulation in terms of amplitude and number of frequencies is found at full load in all the sensors including the guide vane.
Hz sec
m/s2 RMS
Hz sec
m/s2 RMS
ASHAFT 100% GVO
ASHAFT 70% GVO
Hz sec
m/s2 RMS
Hz sec
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ASHAFT 70% GVO
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m/s2 RMS
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m/s2 RMS
Hz sec
m/s2 RMS
Hz sec
m/s2 RMS
ASHAFT 100% GVO
ASHAFT 70% GVO
Figure 7: Short Time Fourier Transform spectrograms for onboard vibrations (Ashaft) at 100% (top) and 70% (bottom) guide vane opening.
Looking in detail the demodulation results from the onboard accelerometer it can be confirmed that they are analogous to the ones from any fixed accelerometer on the turbine guide bearing. This is reinforced by the good coherence levels obtained above 0,8 for the most important peaks when comparing these positions. Only small differences are found mainly in the low frequency range below fb and for some harmonics or modulations. Besides, the onboard results are also very similar to the results from the acoustic emission sensor in a higher frequency band which is known to be free from other sources of noise not related to cavitation. Therefore this supports the idea that the onboard signals are suitable to detect
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cavitation in a frequency band that does not necessarily needs to be in the tenths of kHz.
[3] Kaye, M., Farhat, M., 2002, “Classification of Cavitation in Hydraulic Machines Using Vibration Analysis”, Proceedings of the Hydraulic Machinery and Systems 21st IAHR Symposium, September 9-12, Lausanne.
On the other hand, for this type of machine configuration, the guide vane does not seem to be a good position for erosive cavitation detection occurring on the runner blades. First of all the transmissibility from the shaft to this sensor is not good and secondly the results from the analysis of the induced vibrations in this position do not agree with what is found in the rest. This is also confirmed by the lower coherence values between modulating frequencies on the onboard signals and on the guide vane ones, except for fv.
[4] Bourdon, P., Simoneau, R., Avellan, F., 1993, “Erosion Vibratory Fingerprint of Leading Edge Cavitation of NACA Profile, and of a Francis Model and Prototype Hydroturbine”, Proceedings of the ASME Symposium on Bubble Noise and Cavitation Erosion in Fluid Systems, ASME Winter Annual Meeting, November 28-December 3, New Orleans, Louisiana.
Finally, it has also been proved that the time-frequency analysis is a tool that can be used successfully with the onboard signals to visualize the cavitation activity and to see the cavitation impacts.
[5] Bourdon, P., 2000, “La Détection Vibratoire de l’Érosion de Cavitation des Turbines Francis”, PhD Thesis, EPFL.
[6] Bourdon, P., Farhat, M., Simoneau, R., Pereira, F., Dupont, Ph., Avellan, F., Dorey, J.M., 1996, “Cavitation Erosion Prediction on Francis Turbines Part 1: Measurements on the Prototype”, Proceedings of the 18th IAHR Symposium, Valencia.
CONCLUSIONS A telemetry system comprising an accelerometer mounted
on the rotating shaft above the turbine guide bearing has been successfully installed and used to monitor cavitation erosion induced vibrations.
[7] Farhat, M., Bourdon, P., Lavigne, P., Simoneau, R., 1997, “The Hydrodynamic Aggressiveness of Cavitating Flows in Hydro Turbines”, ASME Fluids Engineering Division Summer Meeting, June 22-26. The onboard signal has been analyzed in the frequency
band from 3 to 6 kHz using traditional detection techniques usually applied to fixed sensors on the turbine guide bearing and guide vanes.
[8] Vizmanos, C., Egusquiza, E., Jou, E., 1996, “Cavitation Detection in a Francis Turbine”, Conference Monitoring for Hydro Powerplants II, July 8-11, Lausanne. The similarity of modulating frequencies demonstrates the
equivalence between measurements taken on fixed parts of the prototype, in particular the turbine guide bearing, and measurements on the rotating shaft.
Furthermore, the vibration measurements in a relatively low frequency range taken on the shaft are analogous to the acoustic emission measurements in a higher frequency range taken on the turbine guide bearing.
In this prototype, measurements taken on a guide vane do not agree with the onboard signals.
So, it seems that onboard measurements are more reliable than measurements on fixed parts. A detailed study of these measurements must be carried out to investigate the cavitation erosion process from a location which is directly connected to the blades.
ACKNOWLEDGMENTS The authors would like to acknowledge the help received
from Jesús Cabrera during the on-site measuring campaign. ENDESA GENERACIÓN and the CICYT (DPI2000-0702) are also thanked for their financial support and collaboration.
REFERENCES [1] Escaler, X., Egusquiza, E., Mebarki, T., Avellan, F., Farhat,
M., 2002, “Cavitation Detection and Erosion Prediction in Hydro Turbines”, Proceedings of the 9th of International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, February 10-14, Honolulu, Hawaii.
[2] Escaler, X., Egusquiza, E., Mebarki, T., Avellan, F., Farhat, M., 2002, “Field Assessment of Cavitation Detection Methods in Hydropower Plants”, Proceedings of the Hydraulic Machinery and Systems 21st IAHR Symposium, September 9-12, Lausanne.
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