A field vibration survey generates a large amount of data — velocity measurements, acceleration measurements, frequency spectra, and phase relationships. Knowing what that data means and how to use it to diagnose the root cause of a problem is the difference between a useful investigation and an expensive exercise in data collection.
The Basic Measurements
Field vibration surveys typically measure velocity in mm/s RMS or peak, sometimes expressed in inches per second. Velocity is the most useful measure for assessing fatigue risk because it is directly related to dynamic stress in the piping through the pipe wall stiffness and geometry.
Acceleration measurements are useful for high-frequency phenomena and for assessing impact loads. Displacement measurements are useful for assessing slow, large-amplitude motion.
Frequency Analysis
Raw vibration amplitude tells you how much something is vibrating. Frequency analysis tells you why. A Fast Fourier Transform of the time domain vibration signal produces a frequency spectrum showing the vibration amplitude at each frequency. The peaks in this spectrum identify the excitation sources.
In a reciprocating compressor system, the dominant peaks in the vibration spectrum appear at the running speed (1×) and its integer multiples (2×, 3×, 4×, etc.). A peak at exactly 1× running speed indicates that the primary excitation is at fundamental frequency. A peak at 2× indicates second-harmonic dominance, which often points to specific acoustic resonance conditions.
A peak that does not correspond to any running speed harmonic indicates a structural resonance being excited by a broadband source — or a separate mechanical issue such as looseness or bearing defect.
Phase Measurements
Phase measurements between two simultaneous measurement points tell you about the mode shape of the vibration. Two points moving in phase are vibrating together; two points moving out of phase are the endpoints of a vibrating span. Phase analysis is essential for identifying which structural mode is active and where support or stiffening would be most effective.
