Capstone: Diagnosing an Oil System Anomaly
The Scenario
Routine review of Bearing #3's oil supply data shows two abnormal trends over the past two weeks: rising oil supply temperature and rising water content. At first glance these might look like two unrelated problems requiring two separate investigations. This module walks through the full diagnostic process using every tool from this track to determine whether that's true.
Step 1 — Two Trends, Same Timeframe
Bearing oil supply temperature has climbed from 118°F to 128°F over two weeks, approaching the upper edge of the target band established in Modules 6.1 and 6.3. Since the automatic temperature control valve actively works to hold oil within that band, a sustained trend despite active control suggests the cooler itself isn't removing heat as effectively as it should — a capacity problem, not a control system problem.
Over the identical two-week window, water content (measured via Karl Fischer titration per Module 6.6) has more than doubled, from 180 ppm to 410 ppm. Per Module 6.4, water contamination is often linked specifically to cooler tube leaks. Two abnormal trends appearing over the exact same timeframe raises an important question: are these genuinely separate problems, or two symptoms of one root cause?
When multiple abnormal trends appear over the same timeframe, checking whether a single root cause could explain both is often more productive than investigating each parameter in isolation — exactly the approach this capstone applies.
Step 2 — Narrowing to the Cooler
A cooler heat transfer check, comparing current performance against baseline data, confirms meaningfully reduced efficiency — the cooler is removing less heat per unit of flow than it previously did. This directly explains the temperature trend and points toward a physical cooler problem rather than a control valve or cooling water supply issue.
A vibration cross-check at the affected bearing shows only a mild rise in 1X vibration, with no subsynchronous content and no elevated axial vibration. Per Track 4's diagnostic framework, this pattern is consistent with a secondary consequence of reduced oil film quality — from elevated temperature and water contamination degrading the hydrodynamic film — rather than a separate, independent mechanical problem like developing unbalance or misalignment. The vibration is a symptom, not a second root cause.
Step 3 — Ruling Out Broader Oil Degradation
TAN and particle count, the other two key parameters from Module 6.6, both remain stable and within normal range throughout this entire period. This helps rule out general oil chemical degradation or a filtration system failure as contributing factors — the anomaly is isolated specifically to water content and temperature, both consistent with one specific physical cause rather than a broad, system-wide oil quality decline.
Step 4 — Physical Confirmation and Root Cause
Physical inspection of the in-service cooler's water side reveals a leaking tube, allowing cooling water to migrate directly into the oil side. This single physical defect explains both trends simultaneously: the leak introduces water directly into the oil (explaining the rising water content), while the compromised tube reduces effective heat transfer surface area (explaining the rising temperature).
Rising temperature + rising water content, same timeframe → cooler heat transfer check confirms reduced efficiency → vibration cross-check rules out an independent mechanical cause → stable TAN/particle count rules out broader degradation → physical inspection confirms a single leaking tube explains both trends. What looked like two problems was one.
Step 5 — Correction Path
Per Module 6.3's dual-cooler design, the correction path is straightforward: transfer oil flow to the standby cooler using the no-interruption transfer valve, without ever disrupting oil supply to the bearings. The leaking cooler can then be repaired or retubed while cooling capacity continues uninterrupted from the standby unit. The oil itself should be treated through vacuum dehydration (Module 6.4) to remove the accumulated water before it causes further harm to bearing surfaces or promotes corrosion.