Dirt analysis case study: Motor bearing

Location: Nuclear power plant
Description of sample: Oil from motor inboard journal bearing

Background: This motor is a critical asset that is sampled and analyzed monthly for metals, particle count and physical properties. The particle count for this motor bearing oil had been consistently higher than other motors of the same make and model (ISO Code 4406 = 21/18/14 versus 16/14/10). Emission spectroscopy revealed no indication of silicon or wear metals. The sample was subjected to SEM/AFA analysis because the root cause investigation needed to determine what these particles were and their sizes.

SEM Observing conditions: The specimen was observed under vacuum at 15 keV beam energy to perform AFA on a nonconductive substrate. Working distance was approximately 16 mm to facilitate X-ray spectroscopy.

General observations: Approximately 9.5% of the surface area of the filter was analyzed with the automated process, which identified and classified 5,618 particles by elemental composition and size (Table 4). This extrapolates to 1,182,740 particles per 100 ml, which is approximately 6.67% lower than indicated by HL-1185 particle count. The difference is attributable to the way the two processes account for particles smaller than 4 micron.

Most of the debris in the sample is tin and iron. Tin, the dominant material, ranges from nearly pure tin to various brass-like alloys (copper, zinc and lead). The iron-dominant particles are mainly a low-chromium alloy and oxides, however, about 5% of the iron particles contain more than 5% chromium. The particles were classified by chemical composition to sort out the most frequently appearing types.

Summary of results: This data from the nuclear power plant shows the journal bearing was wearing, producing large Babbitt and iron wear particles. The data suggests the bearing was wiped from too thin an oil film.

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