Case – Modal analysis
A client in the energy sector has 4 frequency-controlled vertical pumps. During a periodic measurement the company noticed high vibrations on 1x rpm. Vibration values up to 22 mm/s may significantly decrease the life span of the components and the structure.
Bumptest on running machine:
This could indicate a resonance problem, so a bump test was conducted. By means of ‘negative averaging’ this excitation measurement could be conducted on a running machine. It led to the following spectrum:
We notice a (first) resonance area, at 1306 cpm, right next to the number of revolutions.
The bump test confirms the suspicion that this is a resonance problem.
To gain more insight in the resonance area, a modal analysis was conducted. The idle machine was bumped and measured on 48 points and in 3 different directions, according to the following overview:
These measurements resulted in an arithmetic model. This model shows resonance at 1300 cpm, with a motion according to the following figure or movie clip:
The desired modulation range is between 1050 and 1500 rpm. The data from the modal analysis were used to create several possible scenarios.
The best results were achieved by using a rear plate (purple in the overview) that was 6 mm thicker.
This will shift the natural frequency up to +/- 1805 cpm.
This adaptation will make sure that the structural resonance will cause no further hindrance.
The adaptation can be effected without removing the pump/engine, thus minimizing the down time.
By means of a modal analysis we have been able to demonstrate the cause of the high vibrations. Additionally, we have managed to make a proposal for a structural adaptation, only causing a minimal increase in production time.
This measurement and analysis results in a maximally deployable machine over the entire modulation range, without excessive vibrations.