Troubleshooting Bearing and Lube Oil System Problems

TROUBLESHOOTING BEARING AND LUBE OIL SYSTEM PROBLEMS by Thomas H. McCloskey Manager, Turbomachinery, Generation Group Electric Power Research Institute Palo Alto, California The electric utility industry, in conjunction with users, original Thomas H. (Tom) McCloskey, is the Man equipment manufacturers and bearing vendors, has diagnosed ager, Turbomachinery, in the Generation various modes of thrust and journal bearing failures, linked the Group at the Electric Power Research Insti failure modes to potential causes and, for each failure-inducing tute ( EPRl) in Palo Alto, California. Before root cause, developed a guideline manual for remedial actions joining EPRI in 1980, Mr. McCloskey was a and bearing refurbishment [1]. The manual classified 16 sepa project/design engineer fo r steam turbines rate modes of bearing failure that are likely to occur in hydrody at Westinghouse Electric in Philadelphia. namic bearings on power plant rotating equipment (Table 1). Mr. McCloskey has over 25 years experience In the following six sections, symptoms, mechanisms and in the design, operation, maintenance and characteristics of six fairly common modes of failure-abrasion, troubleshooting of steam turbines, pumps, corrosion, electrical pitting, fatigue, overheating, and wiping and related auxiliaries, and holds six patents are described and ways of visually identifying and diagnosing in mechanical engineering and steam turbine design. bearing failures in each of the six modes are treated. Then the Mr. McCloskey is a Fellow member of the ASME, past Chairman root-cause analysis and remedy of an actual power plant bearing ofthe ASMESteam Turbine Committee, and Member ofthe Advisory failure is outlined. Committee fo r the International Pump Users Symposium and a corresponding member of the Turbomachinery Symposium. He is Table 1. Modes of Bearing Failure. also a Charter Member of the Technical AdvisoryBoard fo r Pumps and Systems Magazine. Mode Other Names Mr. McCloskey was the recipient of the ASME George Westing house Gold Medal in 1995, as well as the Edison Electric Institute Abrasion Gouging; scoring;scratching Prime Movers Award in 1984. He is an author of over 70 technical Bond failure Spalling papers on power plant equipment and, in particular, on steam Cavitation erosion Cavitation turbine generator and pump availability, lifeassessment, and ther Corrosion Chemical attack mal performance improvements. Electrical pitting Frosting Erosion Worm tracks Fatigue Fretting Fretting corrosion ABSTRACT High chromium damage Wire-wool; black scab Information and an approach intended to help engineering and Non-homogeneity Blistering; porosity other plant personnel troubleshoot bearing problems and diag Overheating Mottling; anisotropy; ratcheting; nose the mode of bearing failures are presented. Six leading sweating modes of bearing failure (abrasion, corrosion, electrical pitting, Seizure fatigue, overheating, and wiping) are covered in some detail. Each mode is defined and mechanisms of occurrence are de Structural damage scribed. Also, for each mode, the visual appearance of failed Surface wear Black scale bearings is discussed and possible causes of failure are re Tin oxide damage Wear viewed. An illustrative example is provided: symptoms on a Wiping Smearing; polishing bearing surface are described and the mode of failure is identi fied, as is the root cause of failure and possible remedial actions. Because lubrication system problems are a leading cause of Familiarity with and recognition of visual and laboratory bearing failures, effective means of monitoring oil condition are symptoms that may be considered representative of a particular also discussed. mode of failure can be instrumental in identifying the failure mode when a bearing is damaged and a number of failure modes INTRODUCTION are possible. Therefore, the following sections include several Turbine generator bearing failures are a leading cause of photographs of damaged bearing surfaces; these and similar power plant unavailability and can cause serious damage not visual aids can be important tools for persons attempting to only to bearing systems but also to rotors, stators, and nearby diagnose the mode of bearing failure involved in a particular equipment. In addition to failures in turbine generators, bearing incident. failures in other rotating equipment, including pumps, fans, and A number of possible underlying causes are listed for each of auxiliary gas turbines and motors, can also lead to plant outages. the six modes of failure under consideration. One should keep in Given the serious consequences of such breakdowns, determina mind the distinction between direct and indirect causes. Thus, tion of the causes of bearing failure and methods of effective while wiping is classified as a distinct mode, it is often a repair are of paramount importance. consequence of other mechanisms such as fatigue or overheat- 147 148 PROCEEDINGS OF THE TWENTY-FOURTH TURBOMACHINERY SYMPOSIUM ing; the root cause, therefore, would not be a condition likely to Contaminants cause wiping, but one leading to either fatigue or overheating. Many different types of particulate, varying in size and prop The troubleshooter's ultimate goal is to determine from a num erties and in potential for causing damage, may enter the lubri ber of possible mechanisms the one distinct cause of the prob lem. This is often specific to local situations and involves the cation system. The most important characteristics of these particles are size, hardness, and shape. Additional properties of particular power plant, its history, and practices. Whenever importance are compressive strength and crushability; a materi pertinent, therefore, the troubleshooter will consider these ele al that is hard but brittle may be less harmful than a softer ments too, to facilitate identification of the specific condition material. Particles below micrometers mil) in diameter that led to a bearing failure. 10 (0.4 can usually be considered too small to cause grooving in bear Finally, because one third of turbine bearing failures, includ ings for large power plant rotating machinery. ing most failures due to abrasion, involve contaminated lubri The more common particulates found in lubrication systems cants or malfunctioning lubricant supply system components, and their possible origins are as follows: some aspects of maintaining a turbine lubrication system are also discussed. • Quartz or Sand. Sand is a very hard material that will scratch the hardest steel as well as chromium. ABRASION • Grit Blasting Substances. Grit is not only damaging when it Abrasion is a mode of bearing failure due to the erosive action enters the clearance space but it often lodges behind the bearing of a large number of solid particles that are harder than the shell causing high spots in the bearing. This is particularly bearing surface. Under certain conditions both the bearing and hazardous in thin shell bearings. the shaft may be damaged by the abrasive action of the particles. • Metal Chips. These are usually leftovers from manufactur ing. Although they are too soft to scratch hard shafts, they often Mechanisms undergo undesirable changes. In the presence of water they will rust and may form hematite (Fe 0 ), which is a very strong When the lubricant carries few particles, they eventually 2 3 become embedded harmlessly in the bearing surface. When abrasive. there is a large number of particles, they recirculate through the • Weld Spatter. These, too, are leftovers from the construc clearance, causing wear and scoring (Figure 1). The particles tion period and are usually egg shaped. may be metallic or nonmetallic, large or small. When they are • FlyAs h. These are relatively small combusted and noncom large they may become partially embedded in the babbitt. They busted coal particles, ranging from 25 micrometers mil) will then protrude against the journal and function as a cutting (1.0 down to less than micrometer mil) in diameter. tool. 1.0 (0.04 • Silicon Carbide. These are synthetic abrasives close to 25 micrometers (1.0 mil) in size with many sharp jagged edges. • Cast Iron Chips. These may come from the bearing housing, which on occasion is made of cast iron. The appearance of typical oil contaminants as seen under a microscope is shown in Figure 2. Appearance of Abraded Be_arings The main visual characteristic of abrasion is the presence of parallel, circumferential, tracks running over most of the loaded part of the bearing. Small particles may bounce between shaft and bearing, causing intermittent scratches. Frequently, the pits have smooth bottoms; but ordinarily they do not have the melted appearance of electrical pitting. Often a series of pits of identical shape will be spaced at regular intervals circumferentially over a portion of the bearing; this is unmistakable evidence of pitting due to foreign particles. Naturally the most frequent location of scoring is near the minimum film thickness, h . (Figure 3). As �t Figure 1. Abrasion of a 12 in 30 em) Turbine Journal Bearing. illustrated in Figure 4, scoring at low speed or artup produces ( a more irregular pattern than scoring lit high speed. The embed ment of particles is, of course, indicated by the termination of a particular groove, as documented in Figure 5, for the case of a tin Occasionally, embedded particles are lifted out of their seats babbitt matrix magnified 15 times. and deposited elsewhere, leaving gouged out indentations, as While scratches and tracks

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