There are two commonly used testing methods to determine a vertical pump’s natural frequency. The first method is called a startup or coast down. In order to perform this method a tach signal is required for the speed to be tracked. The pump is started and the amplitude and phase are recorded during start-up and coast down, however when a pump is started across the line (connected directly to a power source without a drive or soft-start circuit) it is very difficult to use this method. The problem is that when a pump is started across the line it goes from zero rpm to full speed so quickly that there is not enough time to obtain valid data. The coast down method is not normally successful in these cases. When the stop is initiated the pump comes to a complete stop in a very short period of time as the liquid inside the pump column falls back to the wet well acting as a brake. However start-up and coast down testing can be performed successfully if a pump is being operated using a VFD (Variable Frequency Drive) as the rate of speed can be controlled.
The other method of determining structural natural frequencies on a vertical pump is to conduct an impact test. This test is more commonly known as a bump test. This test requires that the pump be stopped and impacted using a block of wood or a large hammer that has a soft tip (modal hammer). The bump test provides a response curve that will identify the natural frequency and/or frequencies of the pump. It is recommended that the testing be performed in two separate directions. One direction would be in-line with the pumps discharge pipe and the other direction should be 90 degrees from the discharge pipe. The two different directions will usually result in two different natural frequencies as the pumps discharge pipe tends to stiffen the structure. This vibration data can be shown as a higher natural frequency from that direction. The other direction which is 90 degrees from the pump discharge will usually have a lower natural frequency. This is due to the fact that the pump manufacturers typically cutout part of the structure. This allows access to the coupling or seal which also dampens the structure in that direction.
Both of the mentioned methods can assist with discovering the natural frequencies of a pump. Once the frequencies have been identified on the pump; the proper corrections can be made to make certain that the pump is not operating on a resonance frequency.
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by Dave Leach CRL CMRT CMRP
Smith Pump modeled the discharge head in Solidworks. Since the measured natural frequency was 1500 cpm, and the pump operating range was 1500 to 1800 rpm, we wanted to lower the natural frequency below 1500 cpm. We knew by removing stiffeners from the discharge head we would lower the unit’s natural frequency. We removed stiffeners and ran a finite element analysis to determine how much we would lower the natural frequency. Our model study showed that by removing all the external stiffeners and half of all the internal stiffeners we would lower the natural frequency by 30%. Pump #2 was removed and its discharge head modified by removing stiffeners.
Pump #2 was put back into service with its modified discharge head and vibration testing was performed. The new bump test data gave a measurement of 86 cpm in line with discharge. The vibration measured at 50 HZ at the top of the motor was 0.05 in/sec rms. The vibration dropped from 0.47 to 0.05 in/sec rms.
In conclusion, determining a unit’s natural frequency is very important when designing a vertical turbine pump. Every fabricated steel discharge head that Smith Pump makes is modeled in Solidworks and goes through a finite element analysis to make sure the unit’s natural frequency (mainly discharge head and vertical motor) is 25% away from any running speeds. In this example, the discharge head (built by others) was too stiff and had a natural frequency at the pump operating speed causing high vibration. Since Pump #2 was so successful, we are currently modifying Pumps #1 and #3 the same way. Common sense tells us that the stiffer and stronger the discharge head the better, but this case study clearly shows us that is not the case!
Special thanks to our customer Josh Jurgensen, service engineer at Smith Pump Company for sharing this case study with us!
by Yolanda Lopez