Effect of Reduced Oil Flow Rate on the Performance of a Load on Pad Journal Bearing: Flooded Versus Evacuated Conditions

Means to decrease the energy consumption of tilting pad journal bearings (TPJBs) without affecting their performance and structural integrity are mandatory in a cost efficient operation. Reducing the lubricant flow supplied to a bearing is a distinct method to diminish drag power losses along with savings in oil storage and pump equipment. However, a too low flow remains questionable in industrial practice. Starved flow conditions produce hot pad surfaces that could lead to Babbitt failure; and under certain loads, generate subsynchronous vibrations (SSV). This paper aims to resolve some of the issues above via measurements of the load performance conducted with a four-pad TPJB configured as load-on-pad (LOP) and having its ends sealed or open to make flooded and evacuated conditions. Prior measurements were conducted with the same bearing under load-between-pads (LBP); see Refs. [1] and [2]. The nominal supplied flow (Q) of ISO VG 46 oil at 60 degrees C is proportional to shaft speed (max. 12 krpm: 62.8 m/s surface speed). In the tests, the flow Qs ranges from 1.5 Q to just 1/4 Q, and the applied units load reaches 2.07 MPa. Compared to the flooded bearing, the evacuated bearing produces a slightly larger eccentricity across the range of flow rates. For a unit load = 2.07 MPa and shaft speed of 6 or 12 krpm, the highest pad subsurface temperature reaches similar to 130 degrees C for Qs below 50% nominal. For both bearings, flooded or evacuated, drag power losses decrease to similar to 30% as the oil flow drops from 100% to 50% Q. As the oil flow increases to 1.5 Q, the drag power increases similar to 10% for both bearing types at 6 krpm and for the flooded one at 12 krpm, while the evacuated bearing shows a reduction of similar to 7%. The drag power grows as the static load increases; the evacuated bearing producing up to similar to 40% lesser power loss than the flooded bearing. Both bearings produce similar size direct stiffnesses though largely orthotropic, Kyy >> Kxx. Direct damping coefficients Cxx similar to Cyy increase with shaft speed and unit load but dramatically decrease as Qs drops, in particular for the evacuated bearing. The current measurements and those in Refs. [1] and [2] demonstrate that LOP and LBP bearings can safely operate with 50% of nominal flow thus saving drag power, and without too large pad, temperature rises. Alas, too low Qs produces a significant reduction in the damping coefficient orthogonal to the applied load direction. The effect is most evident in the LOP evacuated bearing, which is most prone to show SSV Hash.