Vibration analysis in the marine industry is more complicated than offshore. One of the complexities is that external vibration can be transferred from other rotating machinery through the 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 the FFT spectrum. A good example of such a situation we recently observed on HPS unit el. motor (Hydraulic power supply) – location of measurement points and its construction is shown below:
The observed high vibration values that exceed ISO limits were as below:
The first column is the measurement points location described in Figure 1.
The second column is the velocity in RMS (mm/s) according to ISO 10816. The red colour is class “D” and the yellow colour is class “C” according to ISO.
The third column is the bearing envelope in Peak (m/s2) which gives us early information about the 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).
During analysis, it was noticed that 9Hz can come from the Main Engine which have 6 cylinders. It was checked in the “measurement card” (received from the Crew) that during the test Main Engine was running at 90 RPM (1,5Hz). Additionally, the fact that the highest vibrations were observed on the 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 measurements with the Main Engine stopped.
The next measurement obtained from the Crew confirmed our diagnosis. The overall vibration signals were significantly decreased and the FFT spectrum showed no sign of additional peaks, except the peaks from tested machinery (30Hz).
