BACKGROUND: This is a case of a pump exciting a resonance in the discharge piping.
This pump had recently been overhauled. The unit was showing "high vibrations". REDLINE was called in to identify and suggest a remedy.
FINDINGS: During our initial information gathering process we found that the unit had always had a vibration issue.
2. The base under the pump was found to be loosened from the grout and therefore the pump is showing signs of looseness. The amplitude of this looseness was only at an alert stage and did not warrant replacement at this time.
However, The discharge pipe was showing signs of resonance.
The unit could not be shut down, so we could not do some of the different resonance testing, such as Coast-downs and bump test. However, we did conduct a Mode Shape Test and the results show that the discharge pipe was indeed in resonance.
The image below is a representation of the discharge pipe. The red lines represent the pipe hangers. The dotted blue and pink lines represent the pipes vertical movement during resonance.
The graph below is also a representation of the discharge pipe. Again the red lines represent the hangers. The blue line represents the bend in the pipe. This chart shows the actual Mills P/P that the pipe is moving at each of the 16 points at which data was collected.
SOLUTION: We suggested installing a rubber flex-type coupling between the pump and the discharge piping.
This eliminated the resonance by doing two things:
- Dampened, by absorbing the forcing function so that it can not travel through to the discharge pipe.
- Changed Mass, by virtually disconnecting the pump from the discharge pipe they changed its mass.
