Vibration measurement pump curve
A client in the energy industry has 4 frequency controlled vertical pumps. Most of the time 3 are running at the same time. Sometimes a strange throbbing sound was observed on vertical water pump number 3. The client asked for a measurement to explain this noise.
Observation during measurement
All pumps were measured in normal operating condition. We detected no problems on the machines. After this initial measurement P2, 3 and 4 were started. This was the situation in which the noise was detected previously. When all three have a modulation of +/- 43Hz, there is no noticeable problem.
The problem only shows as from 42Hz on P3. From this point onwards, there is a stochastic rumbling sound on the lid of the basement. In lower modulation, there is a crackling sound near the bottom of the pump. The following graph shows the P4V spectrum:
The blue spectrum represents the measurement at 47Hz, the brown one at 40Hz. Where normally there are operating peaks, forests have appeared. This indicates turbulent behaviour. When we look at the time signal of the peak vue measurement, we get the following picture:
The audible impacts are clearly visible. They occur irregularly in amplitude and frequency. We tested if the same occurred on P4. The pump doesn’t show this phenomenon at this point.
We are dealing with a cavitation/turbulence phenomenon. As from 42Hz there is noticeable turbulence. As soon as we go even lower, there is a throbbing sound. From this point onwards we get the formation of cavitation. The exact location of this tipping point can vary slightly, depending on production circumstances.
We advise avoiding this range. Scada is set to a minimal modulation of 41.5Hz. Pump 3 should stay above 42Hz. By modulating all 3 or 2 pumps in a similar way all desired ranges can be reached. This way of working avoids the problem range.
This case teaches us that ‘identical’ pumps always differ slightly in practice, due to construction differences or differences in duct systems. By performing a measurement we can determine if the pump is running in a stable place. This has a major effect on the life span of the pump.