What unwanted factors can curtail the lifetime of a pump? Sometimes, pumps are not operated under the conditions they were designed for. For instance, the process may change during the lifetime of the pump. Perhaps the pump was incorrectly specified. Or maybe the operator is using a standard pump that does not exactly fit the requested operating conditions. All these factors may cause a pump not to run at its best efficiency point.
Investigation of hydrodynamic phenomena
Increased vibration, noise levels, and reduced pump efficiency can be an indication for the off-design operation of pumps. In the case of pumps with high power concentration — above 1 MW per stage — these phenomena can become serious enough to affect the mechanical integrity of the pump components. The Sulzer R&D team decided to execute a thorough investigation of hydrodynamic phenomena and their interaction with pump components.
This detailed research project was conducted largely in collaboration with the Laboratory for Hydraulic Machines at the Swiss Federal Institute of Technology Lausanne (EPFL). The EPFL is one of the leading research centers for hydraulic machines in the world. Thanks to the cooperation with this renowned Swiss University, Sulzer was able to use the most sophisticated and advanced measurement methods and tools.
Based on a real customer case study, the researchers analyzed the root cause of the pump failure. The root-cause-failure analysis resulted in four hypothetical causes of vibration.
- Acoustic resonance in stationary volumes of the pumps
- Impeller excitation at its natural frequency and eigenmode
- Non-uniform pressure distribution in stationary domain
- Fluid-structure coupling with impeller side room flow
Sulzer assessed all four hypothetical causes by numerical and experimental means (Fig. 1).
|Step 1||Step 2||Step 3||Step 4|
|Investigated subject||Acoustic resonance in stationary volumes of the pumps||Impeller excitation at its natural frequency and eigenmode||Non-uniform pressure distribution in stationary domain||Fluid-structure coupling with impeller side room flow|
Frequency measurements at the shaft
|Pressure sensors inside the pump||Video with tuft movements|
What happens inside a pump?
During the operation of a pump, the pressure-velocity field of the impeller interacts with the pressure field surrounding the diffuser vanes. This interaction is the main source of hydraulically induced vibrations. The vibrations were measured and analyzed during the project at EPFL. The hydrodynamic interaction between the rotating impeller blades and the stationary diffuser vanes can be decomposed into series of cosine functions (Fig. 2). This method is called Fourier transformation. It describes the summation of different pressure modes at different frequencies.
If these pressure modes coincide with structural or acoustic eigenfrequencies, large excitations can occur and lead to unacceptably high vibration levels. At off-design conditions, these periodic pressure fluctuations can be superposed by other influences. Nonperiodic pressure irregularities are usually triggered by flow separations and can add mechanical loadings in stationary and rotating pump components. The variation in the measured pressures shows the following: hydrodynamic interaction between the impeller and the stationary diffuser leads to changing forces inducing vibrations in the system. The video shows the periodically changing pressure fluctuation at the impeller outlet (Fig. 3, Video).
Investigation methods in use
Simulations and experimental investigations deliver results to improve the development of the pump itself. They also help the R&D team to enhance future simulations with real, measured values. For these experiments, Sulzer used over 90 sensors installed inside the pump model.
Computational fluid dynamics simulations
Computational fluid dynamics (CFD) uses numerical analysis and algorithms to solve and analyze problems that involve fluid flows. R&D engineers execute these calculations to simulate and predict the hydraulic performance of our pumps. Sulzer engineers performed comprehensive transient CFD simulations for the pump stages under investigation. They processed computer simulations of the pump for two states: for the designed conditions of the pump and for off-design conditions. The results of these simulations are used to define the experimental, hydroacoustic, and mechanical investigations. They point out the critical spots and influence the selection of measurement locations.
Together with the Swiss Federal Institute of Technology Lausanne (EPFL), Sulzer created a one-dimensional model of the last stage of the pump being investigated for a hydroacoustic simulation. The results of the CFD simulation were entered into a special software package of the EPFL. Through the simulation, the hydroacoustic behavior of the stationary flow passages can be analyzed separately for different areas. To optimize development, it is an advantage to know the acoustic behavior for each hydraulic element, like the diffuser, the exit chamber, and the discharge nozzle, individually. With experimental methods, it is almost impossible to analyze the behavior separately.
Sulzer’s R&D department created two different single-stage model pumps for the investigations. One pump represented the last stage of a high-energy pump under real operating conditions (Fig. 4). For this part of the investigation, Sulzer designed a test loop capable of withstanding pressures up to 100 bar. The pump had a maximum rotational speed of 5 600 rpm and a maximum power use of 1.2 MW. The rotating and stationary components were instrumented with numerous sensors. The impeller was equipped with 17 dynamic pressure sensors, 8 strain gauges, and 3 miniature accelerometers. The stationary part of the pump was instrumented with 64 dynamic pressure sensors (Fig. 5). Additionally, 2 proximity probes measured shaft vibrations at the drive end and non-drive end. The engineers at the Laboratory for Hydraulic Machines at EPFL developed a brand-new 32-channel data acquisition system. The impeller signals were acquired, conditioned, and digitized using this data acquisition system, rotating together with the pump shaft. During the experiment, the engineers acquired stationary and rotating domain data synchronously.
A more conventional test loop and setup was used in a second phase of the investigation. Sulzer built a special pump (Fig. 6) with optical access to the diffuser channels that allowed engineers to view the part-load flow phenomena in the pump diffuser. The experiments focused on the stationary domain pressure fluctuations, hydraulic forces, and flow patterns. The video shows that specially implemented tufts were moving back and forth with the change in the flow direction within the different diffuser channels (Fig. 7, Video). The tuft movement allowed the engineers to judge visually that this was one cause of the pump vibration.
Optimizing future simulations
The Sulzer engineers responsible for finite element analysis (FEA) transformed all test results into a dimensionless form. The values are used as boundary conditions for FEA and high-cycle-fatigue evaluations. Furthermore, the CFD simulation results were directly coupled to an FEA simulation. These simulations confirmed the measurement results. Simulations, which are validated by real measured data, improve the future pump developments of Sulzer.
Awarded research and findings
This fundamental research on high-energy pumps, which was carried out in collaboration with a leading university, improved the specific knowledge of Sulzer’s R&D engineers. The results advance product development activities, e.g., in the form of updated design and calculation procedures. Thus, the reliability of Sulzer products is further enhanced. Furthermore, the design has been improved to allow the pumps to run in a wider application range without disturbing vibrations and to allow them to consume less energy. The scientific results of the project have been documented in the form of a PhD thesis. Sulzer delivered several conference contributions on this project. The IMechE (Institution of Mechanical Engineers, UK) awarded one conference paper with the Donald Julius Groen Price for the scientific findings.
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