MSI conducted specialized vibration testing of a digester complex compressor piping in the US-Midwest. The main purpose of the testing was to find the root cause for the elevated noise levels.



MSI 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 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 drift in and out of phase with each other. While acoustic resonance was not contributing to the problem, the impact testing revealed a mechanical natural frequency in the piping system near the pressure pulsation frequency of 20 Hz that was exacerbating the problem.

  • Problem Statement (Challenge)
    • Digester Complex Compressor Piping
    • US-Midwest
    • The piping experienced loud chattering sounds at certain times
  • Work Performed
    • Condition Monitoring (Vibration and Pressure), Operating Deflection Shape (ODS), Modal Impact Testing, VibVue™ Motion Magnification Video of Piping
  • Results/ Solution
    • Resonant excitation of this natural frequency results in large deflections of a localized section of pipe during times when the compressors’ pulsations reinforce each other, as compared to reduced pressure excitation during times when the compressor’s pulsations cancel each other out. Addressing the motion of this particular section of pipe is key to eliminating the nuisance noise.
  • MSI recommended the following be done in order until the problem is resolved:
    •  Inspection and adjustment of piping hangers since they were loose or unloaded during testing
    • Add mass to shift the structural natural frequency
    • Addition of tuned vibration absorbers
    • Addition of VFDs

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.

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