This paper presents a case study in diagnosing an excessive pipe vibration due to the beating phenomenon. The outdoor process pipes in a sewage plant were found to vibrate viciously and resulted in emitting loud humming noise that affecting the surrounding community. The process pipes were connected to two identical blower units, each driven by a motor via belt and pulley system. Besides the loud noise, the excessive vibration had also posed a concern to the plant personnel that a possibility of premature machine failures may occur if the problem persists any longer. A comprehensive vibration investigation was conducted to map-out the vibrations of the entire machine train that includes pipes, blowers, motors, skid, plinth and floor slab of the blower house. Vibration investigation found that pipes vibration was most severe when the two units of blowers were operated simultaneously. It was found that the root cause of the excessive pipe vibration was caused by the beating phenomenon of which two adjacent machines operated under slightly different speeds. In this case, the two blowers were operated at 41.88Hz and 41.72Hz respectively. Beating is a phenomenon of constructive and destructive interference of two identical waveforms with slightly different frequencies. As such, the remedy measure undertaken was thus to fine-tune the operating speed of the two blowers. It was found that pipes vibration had subsided considerably when the two blowers’ speeds were adjusted to be 7.5 Hz apart. As a result, the loud humming noise emitted from the pipes was noted to be completely mitigated with the remedial action taken.

 

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Plymon Vibration Consultants, LLC 126 Ashford Circle Birmingham, AL 35242

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