Detection of erosive cavitation on hydraulic turbines through demodulation analysis

Alexandre Presas,Zhengwei Wang,Eduard Egusquiza,Yongyao Luo,Mònica Egusquiza,Carme Valero,David Valentin

doi:10.1088/1755-1315/240/6/062048

Abstract

Erosive cavitation in hydraulic turbines affects severely the runner structure, increasing maintenance costs and reducing the remaining useful life of the component. Actual vibro-acoustic techniques used to detect this kind of cavitation in hydraulic turbines are based on analysing high-frequency vibrations in different parts of the unit. Particularly, the demodulation of high frequency bands has proven to give relevant results regarding the erosive cavitation behaviour. However, demodulation is not absolutely conclusive since the excitation and the transfer function from the excitation to the measuring point, which depend on every particular prototype, are partially unknown. In this paper, the demodulation method to detect erosive cavitation in hydraulic turbines is reviewed and analysed in detail. To do so, first, an experimental study in a test rig in laboratory has been carried out. This test rig is based on a rotating disk instrumented with a piezoelectric patch and with different sensors, such as accelerometers and acoustic emission sensors, in the rotating and stationary parts. Different excitation patterns simulating erosive cavitation have been applied to the rotating disk. These patterns include pseudo-random excitations of different frequency bands modulated by one low carrier frequency, which model the erosive cavitation characteristics. In this way, it is possible to understand how mechanical vibrations similar to those produced by erosive cavitation are transmitted in such complex systems involving fluid and solid mediums. The knowledge obtained in the test rig helps to interpret the results obtained with demodulation analysis in prototypes. Particularly, in this paper these conclusions have been used to analyse erosive cavitation in a real Francis turbine.

Highlights

Erosive cavitation is a well-known phenomenon that erodes hydraulic turbine components such as turbine blades
In order to have a better understanding of what is expected to detect when using demodulation techniques for erosive cavitation detection in hydraulic turbines, in this paper, an experimentation performed in a simplified model test rig with the purpose of determining the transmission mechanism of erosive cavitation has been performed
Concluding remarks This paper makes an experimental approach on the complex topic of differentiate erosive and not erosive cavitation with vibro-acoustic methods and focused on the demodulation technique of the envelope of the high-frequency signals

Summary

Introduction

Erosive cavitation is a well-known phenomenon that erodes hydraulic turbine components such as turbine blades. This particular behavior of unstable cavities can be theoretically detected with demodulation techniques[1, 6], not all researches have confirmed it [5]. In order to have a better understanding of what is expected to detect when using demodulation techniques for erosive cavitation detection in hydraulic turbines, in this paper, an experimentation performed in a simplified model test rig with the purpose of determining the transmission mechanism of erosive cavitation has been performed For this purpose, several excitation types emulating cavitation have been applied in a rotating disk test rig. This effect has to be considered as it can produce some modulation frequencies that could be confused with those one produced by erosive cavitation, as it will be discussed

Test rotating disk test rig

Sensors and location

Findings

Assessment in prototype