How to Detect Bearing Wear in Sealless Pumps
What are methods for detecting bearing wear in sealless pumps?
Sealless pumps (canned motor pumps or magnetic driven) generally have sleeve bearings that are cooled and lubricated by the process fluid. As the name suggests, sealless pumps do not have dynamic shaft seals and are designed with the rotor within a containment shell.
Table 1. Failure modes for pumps with sleeve bearings (Courtesy of Hydraulic Institute)Excessive wear or failure of the bearings due to insufficient lubrication, abrasives in the lubricant, or cavitation could cause contact between the rotor and the containment device. This could result in equipment failure and leakage of the process liquid. Therefore, providing some means to monitor the condition of these bearings is desirable. Table1 provides some common failure methods for sleeve-type (journal) bearings, their causes and indicators.
The detection method to monitor wear of the bearing depends on the material of the bearings. For instance, silicon carbide bearings are more prone to sudden failure, so a progressive wear monitoring device might not be suitable.
Carbon bearings, on the other hand, may exhibit progressive wear, and a progressive wear monitoring device may be beneficial.
With progressive wear monitoring, proximity sensing devices can be used to monitor the position of the rotor within the containment device.
Positional changes to the rotor are used to determine the direction and amount of bearing wear. This method allows wear to be detected prior to contact between the rotor and the containment device. The sensing devices should be configured to detect axial and radial changes in the rotor position.
Since bearing wear will cause contact between the rotor and the containment device due to positional changes in the rotor, use of a contact detection device may be useful.
Contact can be detected using an acoustic detection device, power monitor, vibration sensor, containment shell temperature probe, continuity probe or contact switch.