Lubricant Flow Variation in a Tilting Pad Journal Bearing: Quantification of the Effect on Operation Including Examination of the Response at Extreme Low Flows
Abstract
This thesis examines the change in performance of a tilt pad journal bearing due to variation in lubricant flow ranging from 150% to 25% (and below) of a theoretical value. The test bearing is a four-pad, 101.6mm (4 inch) diameter, center pivot TPJB, with a single orifice feed arranged in a flooded bearing housing. The results quantify the effects on pad metal temperatures, power loss, eccentricity, and the magnitude of the estimated dynamic coefficients (stiffness, damping and virtual mass) resulting from the variation in flow rate. The tests include two rotor surface speeds, 32 m/s and 64 m/s (105 and 210 ft/s), and three specific loads of 345 kPa to 2068 kPa (50 to 300 psi).
Experimentally measured bearing eccentricity decreases commensurate with an increase in shaft surface speed and increases with an increase in applied load (as expected). Eccentricity generally increases, modestly, with reducing flow. For flows reducing below 50% of the nominal flow, eccentricity increases 2 to 11 µm for operation at 6 krpm and 11 to 15 µm. While the magnitude of the total eccentricity is dominated by the displacement along the load direction, the results display a trend of slight increasing displacement perpendicular to the direction of the applied load as flowrate decreases.
Pad metal temperature rise over the inlet oil temperature (Tin=60°C) for the loaded pads increases for both increasing applied load and shaft speed. Pad metal temperature rise also increases nearly proportionally to decreases in flowrate for operation with flows between 150% and 50% of the nominal flowrate. However, for operation below 50% of the nominal flow, pad metal temperatures increase dramatically with further reducing flowrate, exceeding 64°C and 61°C for operation at 6 krpm and 12 krpm respectively. Thermocouples placed in the bearing housing oil supply annulus indicate that an asymmetrical temperature distribution develops within the annulus. The emergence of the uneven temperature distribution correlating with the dramatic increase in pad metal temperature rise. The temperature increase also correlates with a decreasing annulus supply pressure that eventually equalizes with the bearing exit pressure at very low flows.
Power loss decreases between 12% and 19% for a 50% reduction in nominal flowrate for operation at 6 krpm and applied specific loads between 345 kPa and 2068 kPa. Power savings of between 13% and 19% are realized for the same 50% reduction in flowrate from the 100% nominal flow for operation at 12 krpm. Power consumption for operation at 12 krpm is 3-4X the power consumption at 6 krpm. Power consumption increases roughly proportionally to load at all flowrates and for both operating shaft speeds.
Broadband subsynchronous vibration emerges at very low flows and, most prevalently, at lightly loaded operating conditions. The amplitude of the vibration, when it did emerge, is low relative to the synchronous vibration amplitude and in no cases resulted in unstable bearing vibration. The results demonstrate the ability of this flooded arrangement to attenuate SSV response, at least over the test conditions observed in this experiment.
The direct stiffnesses increase in magnitude with increasing applied load for operation at both shaft speeds. However, the direct stiffnesses are mostly invariant with respect to increasing shaft surface speed. The direct stiffnesses demonstrate orthotropy with Kyy > Kxx by up to 12% for operation at 6 krpm and up to 30% for operation at 12 krpm. Stiffness increases only modestly for operation with flowrates less than 25% of the nominal.
Direct damping decreases continuously, from 14% to 28%, with decreasing flowrate for operation at 6 krpm. The damping remains nearly constant between 150% and 50% of the nominal flow for operation at 12 krpm, then demonstrates a dramatic decline between 16% and 30% for flows below 50% of the nominal value. Damping for operation at 6 krpm exceeds that for operation at 12 krpm by up to 70% depending on applied load and flowrate. Damping, mostly invariant for loads above 1034 kPa, decreases by 14% to 34% for operation at 345 kPa, the lowest applied load.
The results are compared to model predictions for a direct lubricated, evacuated setup. The predictions for eccentricity, stiffness, and damping compare well with the experimental results for high flowrates (~50% to 100% and above of the nominal flow) with discrepancies between measured and predicted values increasing for reducing flowrate. The model generally underpredicts maximum pad temperature for flowrates greater than 100% of the nominal flow as well as bearing power consumption for all flowrates. The underprediction of pad temperature and power loss (as well as the differences in stiffness, damping, and eccentricity at low flows) likely stem from the difference in performance of an evacuated setup bearing compared to the experimental flooded arrangement over the range of flowrates observed.
Generally, the experimental results demonstrate the bearing’s ability to tolerate significant reductions in lubricant flowrate (50% or more) without suffering a catastrophic reduction of stiffness or damping and/or suffering from mechanical damage related to exceedance of allowable pad metal temperature limits. The comparison to the model results highlights the operating resiliency of this flooded bearing at exceedingly low flowrates, likely resulting from the retention of oil within the cavity afforded by the flooded arrangement end seals.
Subject
hydrodynamic bearingsCitation
Toner, Jonathan J (2021). Lubricant Flow Variation in a Tilting Pad Journal Bearing: Quantification of the Effect on Operation Including Examination of the Response at Extreme Low Flows. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /193132.