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Mitigating high vibrations of a new hydro turbine dewatering pump

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Mitigating high vibrations of a new hydro turbine dewatering pump

By Bernard F. Boueri and Joanna Rice-McVicars

At an Ontario Power Generation hydroelectric project, in Canada, the motor for the new dewatering pump for a newly installed hydro turbine was experiencing high vibrations. Site personnel engaged the Machine Dynamics (MD) Group at OPG to measure and assess the severity of the vibration. This article details the approach taken to identify the structural natural frequency, the mode shapes associated with it and the solution implemented to reduce the vibrations to acceptable levels.

Introduction

During vibration diagnostics of a newly installed hydropower turbine, site personnel could feel vibration when touching the motor for the new dewatering pump. Due to this, they suspected the motor was experiencing high vibrations. Site personnel requested that scientists with the MD Group measure vibrations and assess the severity of the levels.

The motor is rated at 90 kW with a synchronous speed of 1,200 rpm. Preliminary vibration measurements at the top of the motor indicated casing vibration levels close to 0.7 inches per second (ips) root mean square of the velocity (RMS). The range for acceptable vibration is below 0.07 ips RMS, so this level was considered excessive and was well beyond any ISO or API standard limits. Thus, operating the pump at these vibration levels may cause severe damage to the motor bearings as well as the pedestal.

MD personnel suspected the cause of the elevated vibration levels was most likely a structural natural frequency close to the running speed of the motor. They needed to conduct additional measurements and impact tests to confirm the cause of the high vibrations and develop a mitigation plan to reduce the vibrations to acceptable levels.

Results

Measurements were taken at the top of the motor using a velometer in two radial directions, East-West (EW) and North-South (NS) (see Figure 1).

Figure 1: New Dewatering Pump Motor

Figure 2 shows the time waveforms and spectra of the measured locations. Both time waveforms and spectra show that the dominant vibration component is the 1x rpm (20 Hz) of the motor.

Figure 2: Time Waveform and Spectrum at Top of Motor

The spectra show that the vibration levels in the EW and NS directions were about 0.65 ips RMS and 0.3 ips RMS, respectively. The difference in the vibration levels is most likely because the support structure is stiffer in the NS direction than in the EW direction.

ISO 10816-3 states that for newly installed pumps, acceptable vibration levels should not exceed 0.07 ips RMS. Thus, the measured values are well above the recommended value. Looking at the time waveforms and spectra, the authors suspected that a structural natural frequency exists close to 20 Hz and was being excited by the speed of the motor.

An impact test at the top of the motor was conducted to identify any structural natural frequencies close to any forcing frequencies. Impact tests are performed by a hammer with an instrumented load cell and a velometer/accelerometer mounted at the structure being impacted, and the response is recorded on an analyzer. Figure 2 shows the response at the top of the motor in the EW direction. The Frequency Response Function (FRF) shows a frequency at 20 Hz with a phase shift of about 180degrees and a coherence of 92%. This confirms that the natural frequency of the motor/pump structure is being excited by the speed of the motor. To reduce the vibration levels, personnel would need to shift the natural frequency, either by increasing the stiffness of the structure or by installing a dynamic absorber.

Figure 3: Impact Response at Top of Motor in East-West Direction

Having confirmed that the cause of the high vibrations is because of the structural natural frequency, the next step was to perform a modal analysis to identify the mode shape associated with the 20 Hz natural frequency. Modal analysis is done by impacting the motor at the same location (usually at the top of the motor with the highest deflection) and roving the accelerometers to map the motor/pump structure (see Figure 4).

Figure 4: Modal Testing of Motor and Pedestal

After completing the modal testing, the FRFs are entered into a model and animated to identify the mode shape associated with the 20 Hz (see Figure 5). Figure 6 indicates that the 20 Hz is the first cantilever mode of the motor-pedestal. The decision was made to stiffen the structure to shift the 20 Hz natural frequency by at least 10% from the rotational speed of the motor.

Figure 5: Motor-Pedestal 20 Hz Mode Shape

To investigate how to stiffen the structure, a finite element analysis (FEA) model was created (see Figure 6). Simulation of the FEA model yielded a structural natural frequency of 19.7 Hz, which is very close to the 20 Hz measured in the field. The ideal place to stiffen the structure will be at the top of the motor. However, this is not practical as there is no provision on site to anchor the stiffeners to a wall nearby.

Figure 6: FEA Model and Simulation

Instead, the decision was made to modify the pedestal by adding stiffeners, and the FEA model was updated to include these stiffeners (see Figure 7). The simulated model identified that the first natural frequency would be at about 27 Hz.

Figure 7: FEA Model of Modified Pedestal with Stiffeners

Based on these results, the decision was made to manufacture and install the new pedestal (see Figure 8). The work too two to three weeks to complete, depending on the availability of personnel.

Figure 8: Dewatering Pump with New Pedestal Installed

An impact test on the modified pump-motor structure identified the first natural frequency at 29 Hz and the second natural frequency at 36 Hz (see Figure 9). Both these frequencies are well-removed from the forcing frequency of 20 Hz. The actual first natural frequency was very close to the FEA predicted one of 27 Hz.

Figure 9: Impact Response at Top of Modified Motor in East-West Direction

The vibration spectrum at the top of the motor shows that the 1x rpm (20 Hz) vibration component level is about 0.05 ips (see Figure 10). This is a significant improvement from the 0.7 ips recorded prior to the modification. The overall levels are now below those recommended by ISO 10816-3.

Figure 10: Spectrum Measurement at Top of Modified Motor

Conclusion

Vertical pumps are inherent of their first structural natural frequency being close to the running speed of the motor. It is highly recommended that vibration testing be done of the motor/pump set at the factory and in situ and impact tests be performed to confirm that structural natural frequencies are at least 15% removed from any forcing frequencies. The previous statement should also be included in the pump specifications. If this problem is not tackled early, it will lead to complication at site, resulting in major design changes as noted in this case study.

The work took about three months to complete, at an additional cost of about $200,000, including engineering hours.

Bernard F. Boueri is a vibration specialist and Joanna Rice-McVicars is a senior technical officer/engineer with the Machine Dynamics Group at Ontario Power Generation.

Bernard F. Boueri has 25 years of experience in the field of vibrations and rotordynamics and their application in the diagnostics of rotating equipment. He is a graduate of the University of Florida in mechanical engineering. In the Machine Dynamics Group at Ontario Power Generation, he has been responsible for vibration diagnostics across the OPG fleet including steam turbine, hydro units and all rotating auxiliary equipment. He is a Category IV certified vibration analyst by the Vibration Institute.

Joanna Rice McVicars is a senior technical officer/engineer in the Machine Dynamics and Component Integrity department at Ontario Power Generation. She has extensive experience creating CAD models and performing FEA to support vibration issues and fitness-for-service evaluations at OPG’s hydroelectric, thermal and nuclear stations. She achieved her Vibration Analysis Cat. II certification in 2018. Her work is primarily focused on reverse engineering, which includes 3D laser scanning, for a variety of projects.