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

ABSTRACT

This paper discusses a newly found high speed, light load hydrodynamic tilting-pad thrust bearing phenomenon. The phenomenon was first witnessed on centrifugal compressors operating above 10,000 rpm. Testing results have shown that in high speed, light load applications, thrust bearing babbitt temperatures are much higher than expected at low load conditions, and then drop to expected levels as the thrust load is further increased. This behavior has been observed in flooded and directed lube type bearings as well as center-and offset-pivot designs, occurring at sliding velocities above 300 feet per second (fps) at the mean pad diameter and at thrust unit loads between zero and 100 pounds per square inch (psi).

Additionally, the authors discuss the initial anomalies encountered at an original equipment manufacturer's testing facility; extensive testing at the bearing manufacturer's facility to first reproduce the bearing behavior and then to introduce geometry modification to address the phenomenon. Finally, successful low load testing as well as high load testing is presented with the enhanced thrust bearing design.

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.

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.

[NOTE: Download the full PDF to read the testing overview, procedures and results]

CONCLUSIONS

A testing program was conducted in order to determine the cause of the higher than normal temperatures and reduction of oil flow rate on leading edge groove (LEG) thrust bearings operating at high speed and light loads. Using the bearing manufacturer's test rig, the problem was duplicated and the reason for the behavior was discovered. Modifications to the bearings led to a solution that successfully eliminates the undesirable effects of this newly documented phenomenon. The following conclusions have been formulated through the testing program:

• The effect of this phenomenon yields higher than expected pad temperatures at light loads. As the load increases, the pad temperatures return to expected values. Additionally, with LEG type bearings, the oil pressure required to supply the recommended flow rate becomes very high.
• The phenomenon is observed in all types of hydrodynamic tilting-pad thrust bearings including flooded, directed lube, center-and offset-pivot, steel, and chrome copper backed bearings.
• The phenomenon occurs at mean sliding velocities above 300 fps and at light thrust loads. The actual load at which it occurs depends on the speed, but typically is in the range of zero to 100 psi unit load.
• The phenomenon is believed to occur when the moment due to viscous drag on the shoe surface exceeds the moment from the hydrodynamic oil film force. Viscous drag increases with speed and its moment can exceed the film force moment under light load conditions.
• A taper modification has proven to effectively address the phenomenon in directed lube and flooded designs by exerting additional hydrodynamic force at the leading edge. This additional force maintains the proper tilt of the shoe and keeps the leading edge gap open. Tests show that pad temperatures are reduced by as much as 80°F and oil supply pressures lowered by as much as 25 psi.
• The taper modification has been successfully applied at light loads in the OEM facility and at high loads in the field. The OEM reported that a handful of high speed centrifugal compressors, incorporating the modification, is running well at full load and that "no problems have been reported regarding any of the respective thrust bearings."

Read the full paper:
http://www.kingsbury.com/pdf/High Speed Light Load in Thrust Bearings.pdf