Off-Design Operation and Cavitation Detection in Centrifugal Pumps Using Vibration and Motor Stator Current Analyses.

Centrifugal pumps are essential in many industrial processes. An accurate operation diagnosis of centrifugal pumps is crucial to ensure their reliable operation and extend their useful life. In real industry applications, many centrifugal pumps lack flowmeters and accurate pressure sensors, and therefore, it is not possible to determine whether the pump is operating near its best efficiency point (BEP). This paper investigates the detection of off-design operation and cavitation for centrifugal pumps with accelerometers and current sensors. To this end, a centrifugal pump was tested under off-design conditions and various levels of cavitation. A three-axis accelerometer and three Hall-effect current sensors were used to collect vibration and stator current signals simultaneously under each state. Both kinds of signals were evaluated for their effectiveness in operation diagnosis. Signal processing methods, including wavelet threshold function, variational mode decomposition (VMD), Park vector modulus transformation, and a marginal spectrum were introduced for feature extraction. Seven families of machine learning-based classification algorithms were evaluated for their performance when used for off-design and cavitation identification. The obtained results, using both types of signals, prove the effectiveness of both approaches and the advantages of combining them in achieving the most reliable operation diagnosis results for centrifugal pumps.

Resonance vibration interventions in the femur: Experimental-numerical modelling approaches.

MOTIVE: External vibration excitation might be key to many novel non-surgical interventions for pathologies in the musculoskeletal system and in other parts of the human organism. Lack of understanding about vibration patterns, their controllability, and reproducibility are three limitations of ongoing research. This study establishes a bovine vibration model and animal model replacements for future research. METHODS: We used biological samples (n=5) and one polyurethane sample of the bovine femur. Mechanical resonance was measured experimentally and analysed numerically by finite element method. MAIN RESULTS: The experiments obtained 5 distinct mode shapes for the biological sample set, with standard deviation < 7.5%. Finite element analysis of the biological samples can replicate experimental mode shape deflection. The use of polyurethane changes resonance character but results are also good approximations of the biological samples. CONCLUSIONS: A model of the bovine femur with consistent resonance behaviour is presented with alternatives (polyurethane and finite element analysis) that can serve in reducing the number of necessary biological samples. Future work will be to adapt results to human anatomy. Of clinical interest will be to influence bone pathologies such as post-surgical non-union, or bone functionality as part of haematopoiesis and endocrine secretion.

Implant resonance and the mechanostat theory: Applications of therapeutic ultrasound for porous metallic scaffolds.

The development of treatment strategies for improving secondary stability at the bone-implant interface is a challenge. Porous implants are one solution for improving long-term implant stability, but the osteoconduction process of implants into the bone can be slow. Strain-driven osteogenesis from the mechanostat theory offers insight into pathways for post-operative treatment but mechanisms to deliver strain to the bone-implant interface need refinement. In this work, the use of therapeutic ultrasound is simulated to induce resonance into a porous implant structure. Local strains through the scaffold are measured by varying systemic variables such as damping ratio, applied vibrational force, primary bone-implant stability, and input frequency. At the natural frequency of the system with applied forces of 0.5 N and a damping ratio of 0.5%, roughly half of the nodes in the simulated environment exceed the microstrain threshold of 1000 µÎµ required for new bone formation. A high degree of sensitivity was noted upon changing input frequency, with minor sensitivities arising from damping ratio and applied vibrational force. These findings suggest that the application of therapeutic resonance to improve osseointegration of the bone-implant interface may be viable for applications including dental implants or segmental bone defects.

Correction: On the quantification of local power densities in a new vibration bioreactor.

[This corrects the article DOI: 10.1371/journal.pone.0245768.].

On the quantification of local power densities in a new vibration bioreactor.

We investigate the power densities which are obtainable locally in a vibration bioreactor. These reactor systems are of great relevance for research about oncological or antibacterial therapies. Our focus lies on the local liquid pressure caused by resonance vibration in the fluid contained by the reactor's petri dish. We use for the excitation one piezoelectric patch which offer advantages concerning controllability and reproducibility, when compared to ultrasound. The experimental work is extended by finite element analyses of bioreactor details. The peaks of the vibration response for water, sodium chloride (0.1N Standard solution), and McCoy's 5A culture medium are in good alignment. Several natural frequencies can be observed. Local power density can reach multiple times the magnitude used in ultrasound studies. Based on the observed local power densities, we are planning future work for the exposure of cell cultures to mechanical vibration.

Response of Saos-2 osteoblast-like cells to kilohertz-resonance excitation in porous metallic scaffolds.

Post-operative therapy for joint replacement is often performed to optimize bone volume and bone-implant contact. Methods, such as pulsed therapeutic ultrasound, have been shown to be a valuable addition to regular physiotherapy to increase bone regeneration. To evaluate the efficacy of kilohertz-frequency (kHz) resonant stimuli to additively manufactured implant analogues, Saos-2 cells were seeded onto porous stainless steel scaffolds and flat substrates. Resonant frequency modes were mapped in the low kHz range, and cells were subjected to daily stimulus for 10 min at a frequency of 1.278 kHz. kHz-frequency excitation was found to increase normalized alkaline phosphatase production by almost twofold on metallic substrates relative to non-vibrated control scaffolds, while peak velocity influenced alkaline phosphatase production on porous scaffolds but not flat substrates. Total cell proliferation was downregulated by excitation, and all excited samples displayed larger variability. This work indicates that vibration within the range of 0.16-0.48 mm/s may reduce cell proliferation, but favour osteogenic gene expression. This study highlights the potential of using kHz-resonance therapy to mitigate early-onset pore occlusion to achieve uniform osseointegration through porous metallic scaffolds.

Detection of Hydraulic Phenomena in Francis Turbines with Different Sensors.

Nowadays, hydropower is demanded to provide flexibility and fast response into the electrical grid in order to compensate the non-constant electricity generation of other renewable sources. Hydraulic turbines are therefore demanded to work under off-design conditions more frequently, where different complex hydraulic phenomena appear, affecting the machine stability as well as reducing the useful life of its components. Hence, it is desirable to detect in real-time these hydraulic phenomena to assess the operation of the machine. In this paper, a large medium-head Francis turbine was selected for this purpose. This prototype is instrumented with several sensors such as accelerometers, proximity probes, strain gauges, pressure sensors and a microphone. Results presented in this paper permit knowing which hydraulic phenomenon is detected with every sensor and which signal analysis technique is necessary to use. With this information, monitoring systems can be optimized with the most convenient sensors, locations and signal analysis techniques.

Experimental and Numerical Design and Evaluation of a Vibration Bioreactor using Piezoelectric Patches.

In this present study, we propose a method for exposing biological cells to mechanical vibration. The motive for our research was to design a bioreactor prototype in which in-depth in vitro studies about the influence of vibration on cells and their metabolism can be performed. The therapy of cancer or antibacterial measures are applications of interest. In addition, questions about the reaction of neurons to vibration are still largely unanswered. In our methodology, we used a piezoelectric patch (PZTp) for inducing mechanical vibration to the structure. To control the vibration amplitude, the structure could be excited at different frequency ranges, including resonance and non-resonance conditions. Experimental results show the vibration amplitudes expected for every frequency range tested, as well as the vibration pattern of those excitations. These are essential parameters to quantify the effect of vibration on cell behavior. Furthermore, a numerical model was validated with the experimental results presenting accurate results for the prediction of those parameters. With the calibrated numerical model, we will study in greater depth the effects of different vibration patterns for the abovementioned cell types.

A Review of PZT Patches Applications in Submerged Systems.

Submerged systems are found in many engineering, biological, and medicinal applications. For such systems, due to the particular environmental conditions and working medium, the research on the mechanical and structural properties at every scale (from macroscopic to nanoscopic), and the control of the system dynamics and induced effects become very difficult tasks. For such purposes in submerged systems, piezoelectric patches (PZTp), which are light, small and economic, have been proved to be a very good solution. PZTp have been recently used as sensors/actuators for applications such as modal analysis, active sound and vibration control, energy harvesting and atomic force microscopes in submerged systems. As a consequence, in these applications, newly developed transducers based on PZTp have become the most used ones, which has improved the state of the art and methods used in these fields. This review paper carefully analyzes and summarizes these applications particularized to submerged structures and shows the most relevant results and findings, which have been obtained thanks to the use of PZTp.

Sensor-Based Optimized Control of the Full Load Instability in Large Hydraulic Turbines.

Hydropower plants are of paramount importance for the integration of intermittent renewable energy sources in the power grid. In order to match the energy generated and consumed, Large hydraulic turbines have to work under off-design conditions, which may lead to dangerous unstable operating points involving the hydraulic, mechanical and electrical system. Under these conditions, the stability of the grid and the safety of the power plant itself can be compromised. For many Francis Turbines one of these critical points, that usually limits the maximum output power, is the full load instability. Therefore, these machines usually work far away from this unstable point, reducing the effective operating range of the unit. In order to extend the operating range of the machine, working closer to this point with a reasonable safety margin, it is of paramount importance to monitor and to control relevant parameters of the unit, which have to be obtained with an accurate sensor acquisition strategy. Within the framework of a large EU project, field tests in a large Francis Turbine located in Canada (rated power of 444 MW) have been performed. Many different sensors were used to monitor several working parameters of the unit for all its operating range. Particularly for these tests, more than 80 signals, including ten type of different sensors and several operating signals that define the operating point of the unit, were simultaneously acquired. The present study, focuses on the optimization of the acquisition strategy, which includes type, number, location, acquisition frequency of the sensors and corresponding signal analysis to detect the full load instability and to prevent the unit from reaching this point. A systematic approach to determine this strategy has been followed. It has been found that some indicators obtained with different types of sensors are linearly correlated with the oscillating power. The optimized strategy has been determined based on the correlation characteristics (linearity, sensitivity and reactivity), the simplicity of the installation and the acquisition frequency necessary. Finally, an economic and easy implementable protection system based on the resulting optimized acquisition strategy is proposed. This system, which can be used in a generic Francis turbine with a similar full load instability, permits one to extend the operating range of the unit by working close to the instability with a reasonable safety margin.

Feasibility of Detecting Natural Frequencies of Hydraulic Turbines While in Operation, Using Strain Gauges.

Nowadays, hydropower plays an essential role in the energy market. Due to their fast response and regulation capacity, hydraulic turbines operate at off-design conditions with a high number of starts and stops. In this situation, dynamic loads and stresses over the structure are high, registering some failures over time, especially in the runner. Therefore, it is important to know the dynamic response of the runner while in operation, i.e., the natural frequencies, damping and mode shapes, in order to avoid resonance and fatigue problems. Detecting the natural frequencies of hydraulic turbine runners while in operation is challenging, because they are inaccessible structures strongly affected by their confinement in water. Strain gauges are used to measure the stresses of hydraulic turbine runners in operation during commissioning. However, in this paper, the feasibility of using them to detect the natural frequencies of hydraulic turbines runners while in operation is studied. For this purpose, a large Francis turbine runner (444 MW) was instrumented with several strain gauges at different positions. First, a complete experimental strain modal testing (SMT) of the runner in air was performed using the strain gauges and accelerometers. Then, the natural frequencies of the runner were estimated during operation by means of analyzing accurately transient events or rough operating conditions.

Accurate Determination of the Frequency Response Function of Submerged and Confined Structures by Using PZT-Patches†.

To accurately determine the dynamic response of a structure is of relevant interest in many engineering applications. Particularly, it is of paramount importance to determine the Frequency Response Function (FRF) for structures subjected to dynamic loads in order to avoid resonance and fatigue problems that can drastically reduce their useful life. One challenging case is the experimental determination of the FRF of submerged and confined structures, such as hydraulic turbines, which are greatly affected by dynamic problems as reported in many cases in the past. The utilization of classical and calibrated exciters such as instrumented hammers or shakers to determine the FRF in such structures can be very complex due to the confinement of the structure and because their use can disturb the boundary conditions affecting the experimental results. For such cases, Piezoelectric Patches (PZTs), which are very light, thin and small, could be a very good option. Nevertheless, the main drawback of these exciters is that the calibration as dynamic force transducers (relationship voltage/force) has not been successfully obtained in the past. Therefore, in this paper, a method to accurately determine the FRF of submerged and confined structures by using PZTs is developed and validated. The method consists of experimentally determining some characteristic parameters that define the FRF, with an uncalibrated PZT exciting the structure. These parameters, which have been experimentally determined, are then introduced in a validated numerical model of the tested structure. In this way, the FRF of the structure can be estimated with good accuracy. With respect to previous studies, where only the natural frequencies and mode shapes were considered, this paper discuss and experimentally proves the best excitation characteristic to obtain also the damping ratios and proposes a procedure to fully determine the FRF. The method proposed here has been validated for the structure vibrating in air comparing the FRF experimentally obtained with a calibrated exciter (impact Hammer) and the FRF obtained with the described method. Finally, the same methodology has been applied for the structure submerged and close to a rigid wall, where it is extremely important to not modify the boundary conditions for an accurate determination of the FRF. As experimentally shown in this paper, in such cases, the use of PZTs combined with the proposed methodology gives much more accurate estimations of the FRF than other calibrated exciters typically used for the same purpose. Therefore, the validated methodology proposed in this paper can be used to obtain the FRF of a generic submerged and confined structure, without a previous calibration of the PZT.

Feasibility of using PZT actuators to study the dynamic behavior of a rotating disk due to rotor-stator interaction.

In this paper, PZT actuators are used to study the dynamic behavior of a rotating disk structure due to rotor-stator interaction excitation. The disk is studied with two different surrounding fluids-air and water. The study has been performed analytically and validated experimentally. For the theoretical analysis, the natural frequencies and the associated mode shapes of the rotating disk in air and water are obtained with the Kirchhoff-Love thin plate theory coupled with the interaction with the surrounding fluid. A model for the Rotor Stator Interaction that occurs in many rotating disk-like parts of turbomachinery such as compressors, hydraulic runners or alternators is presented. The dynamic behavior of the rotating disk due to this excitation is deduced. For the experimental analysis a test rig has been developed. It consists of a stainless steel disk (r = 198 mm and h = 8 mm) connected to a variable speed motor. Excitation and response are measured from the rotating system. For the rotating excitation four piezoelectric patches have been used. Calibrating the piezoelectric patches in amplitude and phase, different rotating excitation patterns are applied on the rotating disk in air and in water. Results show the feasibility of using PZT to control the response of the disk due to a rotor-stator interaction.