VIBRATION ALERT LEVELS IN SHIPBOARD MACHINERY CONDITION MONITORING AND DIAGNOSTICS

The vibration alert level plays a vital role in machinery condition monitoring and diagnostics for providing an early warning of fault development in the machinery. Approaches for setting an appropriate vibration alert level are presented for either the time or frequency domains, using the vibration signals generated by shipboard fire pumps. For the time domain, Cross Peak Analysis (CPA) is proposed to extract the dominant peak amplitude between the zero crossings. Statistical analysis of the measured data shows that the peak envelope distribution follows a lognormal distribution when using a linear velocity scale. However, it follows a Gaussian distribution when using a velocity dB(VdB) scale. The mean plus two standard deviations in VdB is calculated and suggested as a broadband vibration alert level with a 97.7% acceptance level. In the frequency domain approach, 1/1 full Octave Band Analysis (OBA) is proposed, which divides the frequency spectrum into constant percentage bands. The change in VdB level in a particular frequency band using OBA is more sensitive damage indicator than the change in broadband level.