Dynamic loads in Francis runners and their impact on fatigue life

Seidel, U.; Mende, C; Hübner, Björn; Weber, W.; Otto, Andrew W.

doi:10.1088/1755-1315/22/3/032054

Cited by 82 publications

(77 citation statements)

References 1 publication

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“…In the present case, the stochastic part of the flow is not relevant for our discussion as the frequency range is very low. At BEP only the RSI is observed with a higher amplitude, which corresponds to the typical behavior in Francis turbines [35]. No vortex rope is observed at deep part load or BEP, and also not detected in the spectrum of the pressure fluctuation, although this rope may appear for some operating condition in between.…”

Section: Hydraulic Excitations From the Cfdmentioning

confidence: 54%

“…Energies 2018, 11, x 8 of 16 amplitude, which corresponds to the typical behavior in Francis turbines [35]. No vortex rope is observed at deep part load or BEP, and also not detected in the spectrum of the pressure fluctuation, although this rope may appear for some operating condition in between.…”

Section: Hydraulic Excitations From the Cfdmentioning

confidence: 62%

“…Figure 3 shows that at deep part load, and BEP this frequency is clearly observed in the simulation results as predicted with the analytical Energies 2018, 11, 2320 9 of 16 equation of the RSI (Table 2). For the BEP condition the relative amplitude of this component is higher, as the load is larger [35]. It has been also checked, comparing phase differences between homologous point in the other blades, that the dominating excitation shape in this frequency is a k = 4 mode, rotating in the same direction than the runner.…”

Section: Natural Frequencies and Mode Shapes Of The Runner Without Anmentioning

confidence: 99%

“…Usually the excited frequencies are in the low frequency range from 0 Hz to few times the rotating speed (≈5 Ω) [28,33]. Under these conditions the associated dynamic stresses on the runner can be high and have to be considered for the calculation of long term Energies 2018, 11, 2320 5 of 16 fatigue [34][35][36]. Nevertheless, due to the fact that the excited frequencies are in the low range and are stochastic rather than periodic, it is believed that these vortices itself cannot explain a structural resonance of the runner.…”

Section: Deep Part Load Flow and Inter Blade Vorticesmentioning

confidence: 99%

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Cavitation Effects on the Structural Resonance of Hydraulic Turbines: Failure Analysis in a Real Francis Turbine Runner

et al. 2018

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When discussing potential resonances in hydraulic turbine runners, cavitation effects are usually neglected. Nevertheless, recent studies have experimentally proved, that large cavitation volumes in the proximity of flexible simple structures, such as hydrofoils, greatly modify their natural frequencies.In this paper, we analyze a resonance case in a Francis runner that leads to multiple fractures on the trailing edge of the blades, after just one day of operation at deep part load. If simple acoustic Fluid-Structure-Interaction (FSI) simulations are used, where the runner's surrounding fluid is considered as a homogenous acoustic medium (water), the risk of structural resonances seems to be limited as the predicted natural frequencies are far enough from the excited frequencies by the flow. It is shown that the only hydraulic phenomenon which could have produced such fractures in the present case is the Rotor Stator Interaction (RSI). In order to analyze possible cavitation effects on the natural frequencies of the turbine runner, CFD simulations of the deep part load conditions have been performed, which predict large inter-blade vortex cavities. These cavities have been then introduced in the acoustical FSI model showing that under such conditions, natural frequencies of the runner increase approaching to some of the RSI excited frequencies. In particular, a possible resonance of the four-nodal diameter (4ND) mode has been found which would explain the fast behavior of the crack propagation. Furthermore, the shape and the position of the real fracture found agree with the local maximum stress spots at the junction between the trailing edges and the crown.Energies 2018, 11, 2320 2 of 16 almost one century ago. More recently, the researches of Amabili et al. [4,5] and Kwak [6] revise and improve the analytical work performed by Lamb. Furthermore, many studies performed in the last decade show that acoustical FSI simulations are perfectly capable to predict natural frequencies of submerged structures [7][8][9]. In all the aforementioned studies simple thin plates have been considered.In the particular case of reaction hydraulic Turbines, which are completely submerged, added mass effects play an important role and have to be considered for the calculation of the natural frequencies as well. In this case the added mass phenomenon is even more complicated, as the gaps turbine-casing (usually considered as rigid boundaries) are extremely small (less than 1% of the turbine diameter), the turbine is rotating, and the water couples the turbine with the casing. Some effects such as rotation [10], influence of non-rigid surfaces [11] and acoustic modes [12] have been recently discussed using numerical FSI simulations and contrasting experimental results. Nevertheless, most of these effects have not been yet determined in real turbines. A review of actual research works shows that for turbine runners in still water, natural frequencies and mode shapes obtained with finite element simulations have a good agreement with exp...

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Section: Hydraulic Excitations From the Cfdmentioning

confidence: 54%

Section: Hydraulic Excitations From the Cfdmentioning

confidence: 62%

Section: Natural Frequencies and Mode Shapes Of The Runner Without Anmentioning

confidence: 99%

Section: Deep Part Load Flow and Inter Blade Vorticesmentioning

confidence: 99%

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Cavitation Effects on the Structural Resonance of Hydraulic Turbines: Failure Analysis in a Real Francis Turbine Runner

et al. 2018

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“…In theory, HPPs should only perform a normal shutdown during planned maintenance and for reservoir water level control. The continuous operation of large HPPs near the best efficiency point (BEP) demands low intermittency, which in turn depends on the reservoir gross head constancy, regular rainfall periods, and rigorous maintenance plans [1]. The national system operator (NSO) forces all Brazilian HPPs to start, shut down, or change their generated power to attend the grid's instantaneous requirements.…”

Section: Introductionmentioning

confidence: 99%

Bearing Diagnostics of Hydro Power Plants Using Wavelet Packet Transform and a Hidden Markov Model with Orbit Curves

Pino

Ribas

Guimarães

2018

Shock and Vibration

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The contribution of a medium-sized hydro power plant to the power grid can be either at base load or at peak load. When the latter is the most common operation mode, it increases the start and stop frequency, intensifying the hydro turbine components' degradation, such as the guide bearings. This happens due to more frequent operation in transient states, which means being outside the service point of the machines' nominal condition, consisting of speed, flow, and gross head. Such transient state operation increases the runner bearings' mechanical vibration. The readings are acquired during the runner start-ups and filtered by a DC component mean value and a wavelet packet transform. The filtered series are used to estimate the relationship between the maximum orbit curve displacement and the accumulated operating hours. The estimated equation associated with the ISO 7919-5 vibration standards establishes the sojourn times of the degradation states, sufficient to obtain the transition probability distribution. Thereafter, a triangular probability function is used to determine the observation probability distribution in each state. Both matrices are inputs required by a hidden Markov model aiming to simulate the equipment deterioration process, given a sequence of maximum orbit curve displacements.

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Introduction

Yang¹

2019

Hydropower Plants and Power Systems

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Dynamic loads in Francis runners and their impact on fatigue life

Cited by 82 publications

References 1 publication

Cavitation Effects on the Structural Resonance of Hydraulic Turbines: Failure Analysis in a Real Francis Turbine Runner

Cavitation Effects on the Structural Resonance of Hydraulic Turbines: Failure Analysis in a Real Francis Turbine Runner

Bearing Diagnostics of Hydro Power Plants Using Wavelet Packet Transform and a Hidden Markov Model with Orbit Curves

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