Bearing Envelope Technique – Case With FW Ejector Pump

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

                          Fig. 1. Measurement points in FW ejector pumps.

 

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

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

 

The first column is the measurement point’s location described in figure 1.

The second column is the velocity in  RMS (mm/s) according to ISO 10816. Yellow colour is class “C”, Gray colour is class “A” or “B” according to ISO.

The third column is the bearing envelope in Peak (m/s2) which gives us early information about the bearing condition.

The rise of the “bearing envelope” on the Drive End bearing was noticed in January  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

 

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

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

The next measurement was done in April and the maximum envelope increased during those 3 months.

Fig.4 Measurements results: Velocity in RMS (mm/s) and Envelope bearing in Peak (m/s2) – April 
                                                                                                                                         Fig.5 Trend of maximum overall value from bearing envelope signal.

 

We recommended replacing the Drive End bearing. The crew performed our recommendation and sent us pictures which confirmed our diagnosis.

                 Fig.6 Damaged rolling element in bearing
                       Fig.7 Damaged rolling element in bearing

 

The first signs of wear of this element were noticed 3 months earlier. The Crew was informed about the possibility of replacing this bearing in the future and had enough time to order a new one. Monitoring of bearing condition based only on basic vibration signals like Velocity doesn’t give enough time to plan maintenance. It gives only time to shut down the machine. Based on this fact, knowing the bearing envelope technique is so crucial in the marine market.