External vibration – case with HPS unit el. motor and Main Engine HPS

Vibration analysis in marine industry is more complicated than offshore. One of the complexities is that external vibration can be transferred from other rotating machinery through vessel’s hull. Many times I have seen that overall vibration level exceeded ISO limits but maintenance wasn’t necessary. It is due to fact that high vibrations are coming from another machine and aren’t caused by any misfunction of tested machinery. It is a big mistake when machinery for maintenance is chosen based only on overall velocity values without analysis of FFT spectrum. A good example of such situation we recently observed on HPS unit el. motor (Hydraulic power supply) – location of measurement points and its construction is shown below:

Fig. 1. Measurements points of HPS unit el. motor.

The observed high vibration values that exceed ISO limits were as below:

Fig.2. Overall measurements results: Velocity in RMS (mm/s) and Envelope bearing in Peak (m/s2).

First column is measurement points location described in figure 1. Second column is velocity in RMS (mm/s) according to ISO 10816. Red colour is class “D” and Yellow colour is class “C” according to ISO. Third column is bearing envelope in Peak (m/s2) which give us early information about bearing condition.

The FFT spectrum shows that the highest peak was at 9Hz and it is not correlated to machinery speed, which is 1778 RPM (30 Hz).

Fig.3. FFT spectrum of velocity signal from point HH1.

During analysis it was noticed that 9Hz can comes from Main Engine which have 6 cylinders. It was checked in “measurement card” (received from the Crew) that during test Main Engine was running on 90 RPM (1,5Hz). Additionally, the fact that highest vibrations were observed on Non Drive End suggested that there is a problem with stiffness or external vibration. Taking all above information into account, the Crew was asked to perform additional measurement with Main Engine stopped.

The next measurement obtained from the Crew confirmed our diagnosis. The overall vibration signals were significantly decreased and the FFT spectrum shown no sign of additional peaks, except the peaks from tested machinery (30Hz).

Fig.4.Second overall measurements results (Main Engine stopped).

Fig.5. FFT spectrum of velocity signal from point HH1, second measurements (Main Engine stopped).