This paper presents a case study in diagnosing an excessive pipe vibration due to a beating phenomenon. The outdoor process pipes in a sewage plant were found to vibrate viciously and result in loud hamming noise 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 pipes vibration also posed a concern to the plant personnel that the possibility of premature machine failures may occur if the problem persists any longer. A comprehensive vibration investigation was conducted to map-out the vibration 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 operating simultaneously. It was found that the root cause of the excessive pipe vibration was caused by beating phenomenon of which two adjacent machines operating under a slightly different speeds, of which in this case, the two blowers were operating at 41.88Hz and 41.72Hz respectively. Beating is a phenomenon of constructive and destructive interference of two identical waveforms with slightly different frequency. 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 7.5 Hz apart, or fine-tuned to operate at the speed of 42.5Hz and 50Hz respectively. As a result, the loud humming noise emitted from the pipes was also noticed to have disappear altogether.
