Alarming Sounds From Wastewater Digester Complex Compressor: Pipe Vibration
MSI troubleshooting engineers conducted specialized vibration testing of digester complex compressor piping in the US-Midwest. The main purpose of the testing was to find the root cause for elevated noise levels that made a loud chattering sound. 
MSI troubleshooting consultants performed specialized vibration testing that included experimental modal analysis (EMA), continuous monitoring, operating deflection shape (ODS) characterization, and collected motion magnified video data using VibVue®. This testing indicated that a beating phenomena was occurring because of slight variations in the compressor speeds, causing the pressure excitation to rise and fall in severity as the pulsations from each compressor drifted in and out of phase with each other. While acoustic resonance was not contributing to the problem, impact testing revealed a mechanical natural frequency in the piping system near the pressure pulsation frequency of 20 Hz that was exacerbating the problem.
Resonant excitation of this natural frequency resulted in large deflections of a localized section of pipe during times when the compressors’ pulsations reinforced each other, as compared to reduced pressure excitation during times when the compressors' pulsations canceled each other out. Addressing the motion of this particular section of pipe was key to eliminating the nuisance noise.
MSI recommended the following be done in order until the compressor piping problem was resolved:
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- Inspect and adjust piping hangers, since they were loose or unloaded during testing
- Add mass to shift the structural natural frequency
- Add tuned vibration absorbers
- Add Variable Frequency Drives (VFDs) to positively control compressor speeds
Locations of the twelve triaxial accelerometers used
Vibratory response of the pipes at the indicated measurement location and direction
Frequency response function plot of a modal impact test point
Pipe hangers of particular importance to contain the motion exhibited during noisy periods
