Detection of cavitation in hydraulic turbines

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. First, a brief description of the general features of cavitation phenomenon is given as well as of the main types of cavitation occurring in hydraulic turbines. The work presented here is focused on the most important ones which are the leading edge cavitation due to its erosive power, the bubble cavitation because it affects the machine performance and the draft tube swirl that limits the operation stability. Cavitation detection is based on the previous understanding of the cavity dynamics and its location inside the machine. This knowledge has been gained from flow visualisations and measurements in laboratory devices such as a high-speed cavitation tunnel and a reduced scale turbine test rig. The main techniques are the study of the high frequency spectral content of the signals and of their amplitude demodulation for a given frequency band. Moreover, low frequency spectral content can also be used in certain cases. The results obtained for the various types of cavitation found in the selected machines are presented and discussed in detail in the paper. Conclusions are drawn about the best sensor, measuring location, signal processing and analysis for each type of cavitation, which serve to validate and to improve the detection techniques.

Détection vibratoire de l'érosion de cavitation des turbines Francis

Cavitation erosion of Francis turbines continues to impose costly repairs and loss of revenues. This erosion is primarily caused by leading edge cavitation. A revue of the literature reveals the non-existence of an absolutely validated detection technique of this cavitation in operating Francis turbines. Considering the impulsive character of the implosion of a cavitation structure, the assimilation of cavitation erosion to a process of mechanical fatigue of materials and the vibratory response at high frequencies of a linear structure to multiple random excitations whose responses add on a power basis, we have proposed that the vibratory response of a Francis turbine at high frequencies is tightly related to its cavitation erosion rate. The absolute measurement of cavitation aggressiveness on the blades can be made by the normalization of lower guide bearing vibration measurements by an average Transmissibility function measured by reciprocity with the runner underwater. The proposed method has been validated and it is now possible to state that the MSV of inferred forces at the cavitation impact rate of hydrodynamic origin is proportional to the erosion rate. The approach has been validated in excellent manner in laboratory set-ups with little or no flows by comparison to absolute cavitation aggressiveness assessment methods such as the DECER electrochemical technique and the pit counting technique on polished material samples. The validation was then pursued with laboratory homologous flows on a NACA 009 profile in the LMH high-speed cavitation tunnel with blade mounted DECER sensors as reference. Once again, impact frequencies of hydrodynamic origin, following here a Strouhal law, were observed and the MSV of the profile acceleration at high frequencies was proportional to the average erosion rate on the profile. Notwithstanding the difficulty of the measurements, the results were excellent and fared well with the erosive power of the flow in V3L2 (V= flow velocity, L= length of the attached vapour cavity). Preliminary measurements on a Francis model with blade mounted DECER sensors agreed well with the MSV of lower guide bearing acceleration at high frequencies. Later measurements on the Rapide Blanc model confirmed the damage model observed on the NACA profile. Hydrodynamic impact frequencies were however not governed by the tunnel Strouhal law but were found to be determined by the blade passage frequency. Stationary measurements at the lower guide bearing were equivalent to runner on board measurements. Reciprocity, typical to linear structures, was verfied. As on the profile, maximum aggressiveness on the runner blade occurs in the leading edge cavity closure area. This was confirmed by the application of "Prescale" pressure sensitive film on the blades for operating conditions generating maximum vibratory response at the cavitation hydrodynamic frequencies. High-speed video recordings shot at 2000 frames/s allowed detailed observation of the dynamics of the cavitation structures at the blade passage frequency. Visual observations of cavitation developments along with implosion frequencies allowed observing that the MSV of vibratory response at high frequencies at the cavitation impact occurrence rate was coherent with the calculated value of the flow erosive power inasmuch as the implosions occurred on the blades and not beyond the blade trailing edges. These results on the Francis model therefore supported the proposed vibratory detection approach. The application of the developed techniques, including the measurements of Transmissibility with the runners underwater, allowed assessing the difference in absolute cavitation aggressiveness on an existing and proposed replacement model runner for the Beauharnois power plant. The applicability of the proposed measurement and calibration methods was confirmed on prototype N°6 of the Rapide Blanc power plant. Measurements on fixed and rotating elements of the machine are equivalent and reciprocity also applies. The recurrence frequency of the cavitation impacts was however different from that of the model. The difference appears to be due to the semi-spiral case utilized on the model. It was not fully homologous to the full spiral case of the prototype. It would thus appear that on the model, the wake of the semi-spiral case tongue plays a determinant role in the generation and implosion process of cavitating structures while on the prototype the wakes of the wicket gates fill that function. The ultimate validation of the vibratory approach to the detection of erosive cavitation on Francis prototypes occurred in 1995 on unit 44 of the Manic 5 PA power plant. This was done using on board sensors (pressure, DECER and accelerometer) on severely eroded blade 4 and polished 316L stainless steel disks mounted in this same blade after removal of the pressure sensor housings. Paint removal and pit counting tests had previously correlated well in 1993 with the MSV of global modulation of high frequency acceleration at the lower guide bearing at the cavitation hydrodynamic impact occurrence rate which was for this prototype the wicket gate passing frequency for a rotating observer. The 1995 correlation of the MSV of the blade 4 high frequency acceleration modulation at the wicket gate passage hydrodynamic frequency with the volume pitting rate of the 316L stainless steel disks and the DECER erosion current was excellent. The existence in the flow of the 60Hz (rotation speed, 180 r.p.m., 20 wicket gates) hydrodynamic frequency was confirmed by dynamic pressure measurements on blade 4 and in the draft tube just below the runner. The per blade inferred forces were calculated for each set of test conditions with the average Transmissibility measured by reciprocity on five blades with the runner underwater. The MSV of high frequency inferred forces agrees better with the real aggressiveness than does the wideband acceleration but not as well as the MSV of the acceleration modulation at the hydrodynamic frequencies of the cavitation impacts. Improvements on this subject have been incorporated in the cavitation erosion monitoring system now in operation at Hydro-Québec. Tu summarize, the MSV of the vibratory response at the lower guide bearing modulated at the cavitation hydrodynamic frequencies describes well its relative aggressiveness with varying operating conditions and the use of an averaged Transmissibility measured with the runner underwater allows to estimate it on an absolute basis.

EPFL2000

Evaluation sur modèle réduit et prédiction de la stabilité de fonctionnement des turbines Francis

Stability of operation of Francis turbines Hydraulic disturbances often come with the operation of Francis turbines outside of design head and flow. In some cases, the excessive amplitude of the dynamic system response to these disturbances leads to restriction in the normal operation of the plant. Disturbances largely depend upon operating conditions and the particular turbine design. The response amplitude is under a strong influence from the feed pipe dynamics. Technical and economic consequences of a faulty stability of operation are such that the prediction of stability from model tests is one of the major challenges in present research on hydraulic machinery. Acceptance tests performed on a scale model in a hydraulic laboratory may provide the necessary elements for a full description of characteristics significant for the stability of operation of the full-size turbine in its piping system. This thesis is discussed in the report. 2. Dynamic characterization on the scale model The diagnosis for the dynamic behaviour associated with a machine design and given operating conditions is based on the observation of pressure fluctuations in various positions on the model and test circuit. Auxiliary measurements and surveys of cavitation, noise and vibrations support this data. This study presents a set of evaluation criteria for the elaboration of a diagnosis. The method is illustrated using practical examples from published works and from the numerous experiments performed by the author. A discussion of similitude shows that a diagnosis set using this method provides a good characterisation of the dynamic behaviour associated with the particular turbine design. Adapting acoustic power methods to Francis turbine tests provides a quantitative evaluation of disturbance sources. This tool makes it possible to look forward to an actual prediction of oscillation amplitudes associated with the full-size turbine operation. 3. Elements for the prediction The test laboratory provides a description of the dynamic behaviour associated with a particular turbine design in a guaranteed range of operation. This description is based on three elements of evaluation: discussion of the various dynamic phenomena according to operating conditions: relative frequency and amplitude of the part load precession, of 80 % load oscillation, relative frequency of draft tube column free oscillations, vortex-free region, organisation of the full load pulsation; survey of the draft tube cavitation compliance, lumped at the runner outlet, according to operating conditions; relative emission of the acoustic power toward the piping system, according to operating conditions. These three elements allow an adequate representation of the Francis turbine for a computation of stability of operation involving the piping system dynamics. However, this is the limit of what the test laboratory can say; the prototype piping system layout is liable to change after the turbine model acceptance tests. Once the turbine is adequately described the prediction of stability is performed using known methods and is done under responsibility of the technical consultant. 4. Conclusions The proposed method complies with technical and economic requirements of a laboratory acceptance test on a scale model. Following a simple experimental procedure, if yields a good description of the dynamic behaviour associated with a particular Francis turbine design and its guaranteed range of operation. It provides the elements for computations of stability of operation. Systematic use of the method and comparisons with field observations are the logical sequel to this research. This necessary phase of development will make the stability computations finer, for instance in handling damping in the connecting pipes. It will also provide the basis for a definition of scale effects to be used for the correction of similitudes.

EPFL1993

Etude de l'influence des instationnarités des écoulements sur le développement de la cavitation

Jean-François Caron

Flows in hydraulic machinery show interactions between guiding and rotating components. The wakes of the fixed components will be part responsible for the pressure fluctuations sensed on the blades. Those fluctuations become more important when the distance between the fixed and rotating components decreases. Global instabilities induced by the rope in the draft tube will also be sensed all over the installation. Cavitation on the blades will react in some ways to those fluctuations. This work is devoted to the influence of flow unsteadiness on the cavitation phenomenon. Two different flow configurations are tested, both subject to macroscopic fluctuations of the flow variables. The first part of this work involves the use of a fixed and oscillating NACA hydrofoil in the test section of the High Speed Cavitation Tunnel. Different leading edge configurations corresponding to hydrodynamically smooth and rough surfaces are tested. Observations of cavitation figures are done in parallel with pressure measurements on the hydrofoil suction side. It is observed that the pressure field leads the hydrofoil motion. This positive phase lag increases with the oscillation reduced frequency. For a particular reduced frequency, the phase lag between the movement and the pressure signals increases from the leading to the trailing edge of the hydrofoil. The transition location of the boundary layer on the suction side is modified by the movement. This transition follows the hydrofoil movement if compared with the fixed incidence case. Leading edge cavitation on the hydrofoil suction side interacts with the fluctuating pressure field. Cavitation appearance becomes intermittent when the leading edge is smooth. It disappears over all oscillation cycles if the oscillation frequency is further increased. This phenomenon is not observed when the leading edge is rough. Cavitation is also observed on the smooth parts of the leading edge when a rough patch is put at mid span. Intermittence is therefore not related to the modification of the pressure field, but to the state of the laminar boundary layer in the low pressure region. The positive phase lag of the pressure field also act to lower cavitation aggressiveness when the leading edge is rough on the oscillating hydrofoil. Those observations are validated by the measured acceleration. The second part of this work is devoted to the study of cavitating flow field in Francis turbines. A model turbine is used for pressure measurements on the blades without cavitation. The operating conditions correspond to the best efficiency point, highest power output at highest head, part load at the lowest head and part load at the highest head. It is observed that pressure fluctuations measured on the blades have the highest amplitude at part load. Those pressure fluctuations are related to the swirl of the flow in the draft tube. Other pressure fluctuations sources are the distribution of velocity at runner inlet and the wakes of the guide vanes. Experiments with cavitation on the blades show that the distribution of the flow at the runner inlet has a strong influence on the pressure pulses measured. Those pulses are detected in the pressure signals before cavitation becomes visible on the blades. An increase in the Reynolds number of the flow acts to higher the incipient cavitation parameter. This observation suggests that cavitation tests should be done at the highest Reynolds number reachable with the model turbine.

EPFL2002