Bearing envelope technique – based on case with FW ejector pump

Monitoring of bearing condition can be made by various techniques. One of them is “bearing envelope” that can indicate early symptoms of wear down of components. Recently during our analysis of Fresh Water ejector pump el. motor we noticed high increase of amplitude. The locations of measurement points are shown below:

Fig. 1. Measurements points in FW ejector pumps.

The measurements with normal level of amplitude, taken in September 2019 were as below:

Fig.2 Measurements results: Velocity in RMS (mm/s) and bearing Envelope in Peak (m/s2) – September 2019.

First column is measurements points location described in figure 1. Second column is velocity in RMS (mm/s) according to ISO 10816. Yellow colour is class “C”, Gray colour is class “A” or “B” according to ISO. Third column is bearing envelope in Peak (m/s2) which gives us early information about bearing condition.

The rise of “bearing envelope” on Drive End bearing was noticed in January 2020 and was almost 10 times higher than previous measurement:

Fig.3 Measurements results: Velocity in RMS (mm/s) and Envelope bearing in Peak (m/s2) – January 2020.

The FFT spectrum of bearing envelope shows many nonsynchronous peaks. One peak in spectrum was defined as ball spin frequency. 

Fig.4 FFT spectrum of bearing envelope from point H2.

According to fact that such high envelope signal appeared so fast, we decided to ask the Crew to make additional measurements to confirm trend of the signal.

Next measurement was done in April 2020 and maximum envelope increased during those 3 months.

Fig.5 Measurements results Velocity in RMS (mm/s) and Envelope bearing in Peak (m/s2) – April 2020.

Fig.6 Trend of maximum overall value from bearing envelope signal.

We decided to replace the Drive End bearing. Crew performed our recommendation and sent us pictures which confirm our diagnosis.

Fig.7 Damaged rolling element in bearing.

Fig.7 Damaged rolling element in bearing.

The first wear of this element was noticed 3 months earlier. The Crew was informed about possibility to replace this bearing in future and have enough time to order new bearing. Monitoring of bearing condition based only on basic vibration signal like Velocity don’t give enough time to plan maintenance. It gives only time to shut down machine and based on this fact, knowing bearing envelope technique is so crucial in marine market.