Evaluation of a High Speed, Light Load Phenomenon in Tilting-Pad Thrust Bearings

Joseph J. Wilkes, Chief Engineer and Scan M. DeCamillo, Manager, Research and Development
Kingsbury, Inc., Philadelphia, Pennsylvania

Mark J. Kuzdzal, Manager, Development Engineering and James D. Mordell, Bearing and Seal Engineer
Dresser-Rand Olean Operations, Olean, New York

Introduction

Common sense, "rules of thumb," and generalizations are useful in understanding bearing performance until you must explain why tilting-pad thrust temperatures would decrease with increasing thrust load.

As in any industry, there is a continual push to improve products. In the world of turbomachinery, directed lubrication has many advantages over traditional flooded bearing designs. As a result, the end users often ask the turbomachinery original equipment manufacturer (OEM) to include a directed lube thrust bearing as a product offering. It has been the goal of bearing designers to decrease oil flowrate and horsepower (hp) consumption without increasing pad temperatures within the bearing. Bearing manufacturers conduct numerous tests and present data to show the industry the relative improvements of a directed lubrication bearing with respect to a flooded bearing. The tests are conducted at high loads and speeds because that is normally where high temperatures limit the application.

Typically, a centrifugal compressor must pass a low-pressure API mechanical spin test before it is shipped to the site. This test looks for, among other things, bearing performance as measured in oil flowrate and temperatures. Typically, a low-pressure test produces light loads on the thrust bearing. These light loads are expected to result in low pad temperatures. In recent test experiences, higher than expected acceptance test temperatures were encountered with directed lube thrust bearings, which has caused delays.

The problem was first identified on a centrifugal compressor on the OEMs test stand in March 1998. The compressor unit's thrust bearing configuration was a 10.5 inch leading edge groove (LEG), and was operating at approximately 400 fps mean sliding velocity with light axial load. The test failed due to pad temperatures in excess of 240°F and a drain temperature that exceeded 200°F.

Immediately following the OEMs March test failure, at least a dozen high risk contracts were identified. These contracts were approaching the test phase in the OEMs facility. Each consisted of compressors containing thrust bearings, which would run at speeds where the phenomenon was experienced. The importance to find a solution became critical.

The behavior has been observed in center- and offset-pivot bearings, occurring at sliding velocity above 300 fps at the mean pad diameter and thrust loads between zero and 100 psi. However, at these higher speeds, directed lube bearings are almost exclusively used.

Another observation has been that the bearing, which is orificed on the supply side, passed the prescribed amount of oil at slow rotational speeds. But as the rotational speed increases, the flowrate to the bearing is not constant. As a matter of fact, the faster the bearing runs the less flow the bearing would accept. Hence, at full speed and light load, the bearing performance is unsatisfactory with respect to the low-pressure API mechanical spin test temperature criteria.

Regardless of the measures taken, the flow versus speed dependency could not be overcome. The result was failed tests and loss of client confidence. The bearing manufacturer has tested his design at much higher loads with acceptable results while low load compressor testing yielded unacceptable results. This fact was perplexing and spawned an extensive low load thrust bearing testing program at the bearing manufacturer's facility.

This paper discusses a hydrodynamic tilting-pad thrust bearing temperature phenomenon that occurs at high speeds and at low thrust loads. Further, it discusses a testing program held at the bearing manufacturer's site, which was successful at reproducing the behavior and solving the problem. Finally, successful low load testing as well as high load testing is presented with the enhanced thrust bearing design. To the best that the authors could determine, this problem has not been reported in prior literature.

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