Ensuring proper journal bearing selection and design is crucial to the overall reliability of pumps – especially when it comes to journal bearings in boiler feed service. These bearings incorporate a “pressure dam” that provides a dampening force to insure stable operation of the rotor. This dam has a unique taper geometry whose proper characteristics and design are difficult to assess with the naked eye. That’s what one power gen operation learned over the course of a routine pump element overhaul.
During the facility’s spring 2002 outage, the #51 boiler feed pump underwent a rotor change-out. Upon start-up, the pump’s vibration readings had improved, but they still weren’t acceptable. A review of the unit’s vibration spectrum showed vibration amplitude in excess of 0.42 ips (prior to repair the reading was in excess of 0.9 ips) and bearing temperatures at or above 185° F. Large vibration amplitude at running speed can be induced by many factors; imbalance, misalignment, component looseness, internal rubbing, loose bearings, and/or improper bearing design, to name a few. Even after new bearings and gaskets were assembled into the feed pump element, the unit still operated unsatisfactorily.
Further research into the situation by HydroAire Engineering, a division of Hydro, Inc., concluded that the root cause of the destructive vibration was improper bearing design. The problems with the design were: 1) excessive taper length across the bore; 2) improperly machined oil relief chamfers at the bearing split line to circulate and remove the lubricating oil.
HydroAire re-designed the bearing and after re-installation, the pump’s vibration amplitudes were at the desired safe levels.
Radial Bearing Design Specifics
It is essential that radial bearings be designed to provide a large centering force. The minimal design to provide acceptable performance incorporates a taper in the bearing bore that is 0.015” deep and 120° in length.
Radial bearings in boiler feed pumps are lightly loaded as a result of two factors:
Another crucial factor in bearing design is the capability to provide continuous lubricity and heat transfer via force-feed lubrication. This requires the proper flow avenues to effectively introduce and remove oil from the bearing cavity. When these design elements are missing, rotor instability along with high bearing temperatures can lead to accelerated pump degradation.
This bearing design upgrade increased the unit’s meantime between repairs (MTBR) from 4 to 7 years. In conjunction with HydroAire’s best-in-class repair processes and stringent acceptance standards, this will ensure that internal clearances are maintained for extended periods of time. The net impact is significant energy savings stemming from lower internal leakage and vibration levels for the element’s operating life. When compared to historical efficiency losses experienced through “normal” repair practices, the annual operating savings for this application are calculated to be $44,500, based on a $0.6/hour average retail energy rate.
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