Scan M. DeCamillo, Manager, Research and Development
Kingsbury, Inc., Philadelphia, Pennsylvania
Minhui He, Machinery Specialist
C. Hunter Cloud, President
James M. Byrne, Machinery Consultant
BRG Machinery Consulting, LLC, Charlottesville, Virginia 22903 USA
Peculiar, low-frequency, radial vibrations have been observed in various turbomachinery using tilt-pad journal bearings. Unlike discrete subsynchronous spikes that often indicate a serious problem, the vibrations are indiscrete and of low frequency and amplitude. The low level shaft indications have raised concern in witness tests of critical machinery, even in cases that comply with American Petroleum Institute (API) limits, owing to uncertainty regarding the cause and nature of the vibrations.
This paper presents shaft and pad vibration data from various tilt-pad journal bearing tests that were undertaken to investigate and better understand these subsynchronous indications. The vibration characteristics are defined and compared under the influence of speed, load, oil flow, and bearing orientation. Results are presented for conventional and direct lube tilt-pad bearing designs, along with discussions of parameters and methods that were successful in reducing and eliminating these low level vibrations.
The test results indicate that the low-frequency shaft indications are caused by pad vibration. Hypotheses and analyses are presented and discussed in relation to the test observations.
Many early technical papers report on the inherent stability of tilt-pad journal bearings in overcoming oil whirl and oil whip limitations of fixed geometry bearings. There are, however, other vibration phenomena associated with tilt-pad bearings that are topics of past and present research. In the late 70s, extensive babbitt fatigue cracking of upper, unloaded pads was a major problem in large, conventional tilt-pad journal bearings. These were flooded designs, which incorporate end seals to restrict oil outlet and flood the bearing cavity. A well-referenced study by Adams and Payandeh (1983) attributes the damage to self-excited, subsynchronous pad vibration.
Trends for larger and higher speed turbomachinery impose greater demands on bearings and rotor dynamics. Direct lube bearings have evolved to reduce the higher power losses, oil flow requirements, and pad temperatures associated with higher surface speeds. Direct lubrication is not a new concept as designs have been used in special applications with a long history of reliability. Use of direct lube journal bearings became more prevalent as machine size and speed increased, eventually spawning several papers in the early 90s investigating their steady-state performance. These include research by Booser (1990), Tanaka (1991), Harongozo, et al. (1991), Brockwell, et al. (1992), and Fillon, et al. (1993).
The references document pad temperature reductions derived from the efficient evacuation of hot oil from the bearing cavity. Direct lube bearings are therefore typically designed for evacuated operation, accomplished by opening up end seal and oil outlet restrictions. A direct application of oil to the journal surface prevents oil from bypassing the films, which can occur in conventional bearings when outlet restrictions are removed. Use of nozzles to spray oil on the journal surface between pads is one method of direct lubrication. An additional pad temperature advantage is gained by direct lubrication features.
Literature on vibration characteristics of direct lube journal bearings is not as prevalent. DeCamillo and Clayton (1997) present rotordynamic data for large, 18 inch (457 mm) generator bearings. The tests showed comparable vibration response for conventional and direct lube designs. Edney, et al. (1996), provide similar information for a small, high-speed, multistage steam turbine with 4 inch (102 mm) diameter journal bearings. Peculiar, low-frequency, radial vibrations were observed during these steam turbine tests, but levels were low and acceptable. However, similar vibrations in a high-speed compressor during acceptance tests in 1999 did encroach upon acceptable API limits, and significant time and resources were expended to address this issue (DeCamillo, 2006).
Personal experience and discussions among original equipment manufacturers (OEMs) and users over the past few years indicate that these low-frequency vibrations have been encountered in turbines, compressors, and gearboxes, using conventional and direct lube tilt-pad bearing designs. The signature has also been documented in separate research investigating stability (Cloud, 2007). Accurate stability prediction is a major topic of concern in the industry. Kocur, et al. (2007), highlight a large spread in predicted bearing stiffness and damping coefficients among computer codes, and associated ramifications regarding stability assessment of critical turbomachinery. At the same time, there are not many codes available that address direct lube performance and dynamic coefficients (He, 2003; He, et al., 2005).
Researchers are presently attempting to sort through some difficult questions. What is the source of the low-frequency vibrations? Are they attributable to direct lube bearings, pad flutter, starvation, high speeds, or low loads? Do they affect machine stability, safety, or reliability?
This paper presents a chronology of tests, investigations, and theoretical analyses aimed at providing answers to some of these questions. It is desired that the information be of value to researchers, OEMs, users, and other personnel involved with hydrodynamic bearings and vibration in turbomachinery.
Read full PDF