Detection of erosive cavitation on hydraulic turbines through demodulation analysis (original) (raw)
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Methods for vibro-acoustic diagnostics of turbine cavitation
Journal of Hydraulic Research, 2003
Basic aspects of noise sampling, signal processing and analysis, and data processing, analysis, and interpretation in vibro-acoustic diagnostics of turbine cavitation are investigated in a series of prototype and model experiments. Several weak points of the practice are identified, and improvements and new techniques are developed. These techniques enable extraction of data on cavitation details and early detection of detrimental effects met in turbine exploitation. A brief review of weak points of the practice, developed improvements, and new techniques, as well as examples of application, are presented in the paper. RÉSUMÉ Des aspects de base de l'enregistrement du bruit, des traitements et analyses analogiques et numériques des signaux, avec leur interprétation dans le diagnostic vibro-acoustique de la cavitation de turbine, sont étudiés dans une série d'expériences de prototype et de modèle. Plusieurs points faibles de la pratique sont identifiés, et des améliorations et de nouvelles techniques sont développées. Ces techniques permettent l'extraction de données sur des détails de cavitation et la détection précoce des effets néfastes rencontrés dans l'exploitation de turbine. Un bref examen des points faibles de la pratique, des améliorations développées, et de nouvelles techniques, aussi bien que des exemples d'application, sont présentés dans le papier.
Multidimensional Diagnostics of Turbine Cavitation1
Journal of Fluids Engineering, 2002
A novel technique for vibro-acoustical diagnostics of turbine cavitation is introduced and its use demonstrated on a Francis turbine. The technique enables identification of different cavitation mechanisms functioning in a turbine and delivers detailed turbine cavitation characteristics, for each of the mechanisms or for the total cavitation. The characteristics specify the contribution of every critical turbine part to the cavitation intensity. Typical diagnostic results: (1) enable optimization of turbine operation with respect to cavitation erosion; (2) show how a turbine’s cavitation behavior can be improved; and (3) form the basis for setting up a high-sensitivity, reliable cavitation monitoring system.
Detection of cavitation in hydraulic turbines
Mechanical Systems and Signal Processing, 2006
An experimental investigation has been carried out in order to evaluate the detection of cavitation in actual hydraulic turbines. The methodology is based on the analysis of structural vibrations, acoustic emissions and hydrodynamic pressures measured in the machine. The proposed techniques have been checked in real prototypes suffering from different types of cavitation. In particular, one Kaplan, two Francis and one Pump-Turbine have been investigated in the field. Additionally, one Francis located in a laboratory has also been tested.
Cavitation in Hydraulic Turbines
International Journal of Heat and Technology, 2019
Hydroenergy is one of the richest and most useful renewable energy sources in the world. Hydropower is a vital source as it is the clean energy source, sustainable and last but not least it is also cost-effective. One of the most important parameters that affect the performance of the hydraulic machines is the cavitation phenomenon, which is defined as the formation of the vapor bubbles in the liquid through any hydraulic turbine. In this paper, hydraulic machines, cavitation, types of cavitation are briefly described. After theoretical studies, analytical and numerical researches about cavitation in hydraulic machinery are discussed extensively. With those studies which are summarized in this paper covers a lot of ground about cavitation on the other hand further studies are needed about cavitation in hydro turbines. Numerical methods provide sufficient predictions for cavitation. However, numerical results should be verified by experimental measurements and detection methods to decide what intensity and which shape of cavitation is hazardous and vital, where the local pressure is lower than the vapor pressure and at which static pressure cavities start to grow and collapse.
The identification of cavitation in Kaplan turbine runner
38TH MEETING OF DEPARTMENTS OF FLUID MECHANICS AND THERMODYNAMICS
The presented paper deals with monitoring of cavitation in a Kaplan turbine. The main goal of the experimental works was to verify the suitability of the chosen method for application in the technical diagnostics of water turbines. The experiment was carried out in three different ways, where a correlation was looked for between the results. The first method was the standard method used in water turbine test workbenches. A decrease in efficacy is a manifestation of cavitation. The efficiency drop is evaluated from the energy parameters and cavitation tests are necessary. Simultaneously with the mentioned standard measurement, the formation and development of cavitation in the turbine impeller was monitored by a visualization method and subsequent video analysis. A pulsed light source (stroboscope) with a parallel digital camera was used for visualization. The devices were synchronized via the Timing Hub. The third method used was based on sensing and analyses the acoustic emission in the ultrasonic band. To sense the signal, an ultrasonic probe was used. The probe was tuned in a way that it was not sensitive to vibrations and noise in the audible range. The signal was analysis only in the area of cavitation formation, which was verified by visualizing cavitation areas in the turbine wheel. The results from the experiments showed very good correlation for all three methods. One of the outputs of the experiments is the possibility to use experimental procedures for early diagnosis of cavitation formation. The indisputable advantage of determining the onset of cavitation by sensing acoustic emissions is that it is ab extra disassembly method and at the same time it is not necessary to measure the entire complex of energy parameters.
Fluids
Korto’s multidimensional method for vibro-acoustical diagnostics and monitoring of turbine cavitation is based on a high number of spatially distributed sensors and the signal and data processing that systematically utilises three data dimensions: spatial, temporal, and operational. The method delivers unbiased data on cavitation intensity and rich diagnostical data on cavitation mechanisms. It is applicable on Kaplan, Francis, bulb, and reversible pump turbines, as well as pumps. In this paper, the theory of the method is introduced, and its application is illustrated on a prototype and three models of a Kaplan turbine. In the considered case, two distinct cavitation mechanisms responsible for the two erosion patches found in an overhaul are vibro-acoustically identified, quantified, and analysed. The cavitation quality of the models is compared. Cavitation as a source of vibration is discussed.
Journal of Fluids Engineering, 2007
The goal of the study was to explain the relationship between different acoustic signals and visual appearance of cavitation. Measurements of acoustic emission, vibration, and noise were performed on a Kaplan turbine model, with only two blades, in a cavitating condition. Since a model with only two blades was used, most of the side effects were eliminated, and it was concluded that the cavitation itself is the source of the recorded signal. Results showed an interesting relationship between the extent of the cavitation and the recorded data from sensors. At a decreasing cavitation number, the recorded amplitudes from all measurements first rose, experienced a local maximum, then fell to a local minimum, and finally rose again. The cavitation was also visually observed. It was concluded from the measurements that there are distinct correlations between acoustic emission, vibration, and noise on one side and the topology, extent, and type of cavitation structures on the other side. A physical explanation for the phenomenon was introduced and included in a semi-empirical model that links the visual appearance of cavitation on the blade of the turbine to the generated noise and vibration.
Detection of Cavitation in Kaplan Water Turbines
Cavitation causes excessive pressure pulsations, which damage the surfaces of the runner and channels of a turbine. As a result, the overall operating efficiency of the water turbine decreases and repair costs increase. Traditionally, there have been efforts to detect cavitation using vibration, pressure and acoustic emission measurements. For instance, extensively used vibration velocity measurements are not effective enough to detect all cavitation areas so more sensitive and accurate signal processing methods are still demanded. This study concentrates on vibration measurements in the real operating environment of a Kaplan turbine. Altogether 29 measurement periods were carried out at different power levels from 1.5 to 59.4 MW. The vibration analysis was based on the use of traditional velocity and acceleration signals and novel higher order derivatives: x(3) and x(4). The features used were rms, peak, kurtosis and crest factor. Normal accelerometers could be used and the upper cutoff frequency did not have to be high in order to detect all cavitation areas reliably. The sample length has to be over 30 seconds in order to detect all cavitation areas accurately. The rms value works sufficiently well at high powers, whereas kurtosis and crest factor are effective at low powers only. The feature working well throughout the whole power range is peak value.
Turbine Cavitation Diagnostics and Monitoring Multidimensional and Simple Techniques
2010
The consequences of cavitation erosion are best assessed directly, during an overhaul. However, in order to find out from which operating points they stem and clarify the role various turbine parts play in cavitation, one must apply vibro-acoustic measurements or monitoring. Based on the example of the large Francis turbines at the Grand Coulee Dam in the USA, the multidimensional vibro-acoustic technique for cavitation diagnostics and monitoring is presented and compared to simple techniques.
INTRODUCTION TO CAVITATION IN HYDRAULIC MACHINERY
Design, operation and refurbishment of hydraulic turbines, pumps or pump-turbine are strongly related to cavitation flow phenomena, which may occur in either the rotating runner-impeller or the stationary parts of the machine. The paper presents the cavitation phenomena featured by fluid machinery including type of cavity development related to the specific speed of machines in both pump and turbine mode, the influence of the operating conditions, such as load, head and submergence. Therefore, for each type of cavitation illustrated by flow visualization made at the EPFL testing facilities, the influence of cavitation development on machine efficiency, operation and integrity are discussed.