60396 - A Model for Tilting Pad Thrust Bearings Operating With Reduced Flow Rate – Do Benefits Outweigh Risks? 

The literature on tilting pad thrust bearings (TPTB) calls for flow reduction as an effective means to reduce drag power losses as well as oil pumping costs. However, the highest level of flow reduction a bearing can undergo while maintaining reliable operation is a key question that demands comprehensive analysis. This abstract implements a model into an existing thermo-elasto-hydrodynamic (TEHD) computational analysis tool to deliver load performance predictions for TPTBs operating with reduced flow rates. For bearings operating with either a reduced flow or an over flow conditions, a sound model for the flow and thermal mixing in a feed groove determines the temperature of the lubricant entering a thrust pad. Under a starved flow condition, the analysis iteratively reduces the effective arc length of a wetted pad until matching the available flow. Predicted discharge flow temperature rise and pad subsurface temperature rise from the present model match measurements in archival literature for an eight-pad TPTB with OD=267 mm operating under a specific load per pad ranging from 0.7 MPa to 3.4 MPa and at 4 krpm and 10 krpm, producing laminar flow and turbulent flow regimes respectively. The bearing is supplied with 150% to 25% of the predicted nominal flow rate as well as with a flow rate recommended in the ancient literature (~170% to 420% nominal). A supply flow rate above the nominal rate increases the bearing drag torque but has little effect on the pad peak temperature rise or the pad minimum film thickness. Nonetheless, a reduced flow below the nominal rate produces areas denuded of lubricant at both the pads’ leading and trailing edges. Hence, a pad minimum film thickness substantially decreases while the film peak pressure largely increases. Also, the pad temperature rise substantially increases up to twice that with a nominal rate and results in a large pad thermal crowning. A lightly loaded (<1MPa/pad) bearing maintains a sufficiently large film thickness (> 8 μm = API critical limit) even with only ¼ predicted nominal rate. Under a heavy load of 3 MPa/pad, however, the minimum film thickness falls below 8 μm with a flow < 80% nominal rate. Compared to the bearing lubricated with a nominal rate, a starved flow TPTB shows a larger axial stiffness and a lesser damping coefficient. Despite a significant increase in the bearing drag torque, the recommended flow rate does not lead to either a sizeable increase in the minimum film thickness nor to a decrease in the maximum pad temperature rise. At a high rotor speed of 10 krpm, the example TPTB with a flooded configuration (with end-seals) operates with up to 11 μm lesser minimum film thickness, a 10 °C larger pad maximum temperature rise, and a 11% larger bearing axial stiffness compared to those in an identical bearing with evacuated ends. The bearing’s ends configuration, with seals or no seals, however, shows marginal influence on the bearing performance at a low speed of 4 krpm.