Capstone: Diagnosing a Vibration Spectrum
The Scenario
Routine review of Bearing #2's vibration trend shows a gradual climb from 1.8 to 3.1 mils peak-to-peak over six weeks — still below alert level, but a clear, real trend rather than noise. This module walks through the full diagnostic process using every tool and test covered across this track, arriving at both a root cause and an appropriate response.
Step 1 — The Trend Itself Is the First Clue
A gradual six-week climb that hasn't crossed an alarm threshold is exactly the baseline shift pattern described in Module 4.6 — the kind of change only caught through trending, not a single reading or a discrete alarm. This shapes the whole investigation toward a developing mechanical condition rather than a sudden event.
Step 2 — Frequency Content Narrows the Field
The frequency spectrum shows vibration almost entirely at 1X, with minimal 2X and no subsynchronous content whatsoever. Per Module 4.1, this immediately rules out oil whirl/whip (Module 4.5), which would require subsynchronous frequency — that mechanism simply isn't present here. Minimal 2X content also argues against misalignment (Module 4.4) as the primary driver, leaving unbalance as the leading candidate.
Just as in the Track 1, 2, and 3 capstones, ruling out what the evidence doesn't support is as valuable as confirming what it does. Three of four major vibration mechanisms are eliminated using frequency content alone.
Step 3 — Confirming Unbalance With the Two Key Tests
Per Module 4.3, true mass unbalance requires confirmation beyond just 1X frequency: a stable phase angle over time, and amplitude tracking the speed-squared relationship. Both check out here — phase angle has remained essentially constant across the full six-week trend even as amplitude climbed, and amplitude measured across different load/speed conditions closely follows the predicted speed-squared curve. Both confirming tests pass.
Two additional checks close out the remaining possibilities. Low, stable axial vibration (Module 4.4) alongside the climbing radial vibration further argues against misalignment. A clean slow roll check from the most recent outage — minimal vibration at very low speed — rules out a bent shaft (Module 4.4), which would have shown elevated vibration even at slow roll regardless of centrifugal force.
Step 4 — Finding the Root Cause, Not Just the Mechanism
Confirming unbalance as the mechanism isn't the end of the diagnosis — per Module 4.3, real in-service unbalance usually has an identifiable physical cause. Reviewing recent operating history reveals a roughly four-week period of heavier-than-normal boiler carryover beginning right around week 2 of the vibration trend — closely matching when vibration began climbing. This strongly suggests blade deposits from that carryover period as the likely source of a developing heavy spot.
Heavy carryover period → blade deposits accumulating unevenly → developing heavy spot → rising 1X vibration with stable phase and speed²-tracking amplitude → confirmed mass unbalance from an identifiable, explainable cause. Every link traces back to a concept covered across this entire track.
Step 5 — What This Diagnosis Points Toward
Because the trend remains below alert level, the appropriate response is continued trending with increased monitoring frequency — not an immediate shutdown or field balance correction. The plan: watch whether the trend stabilizes, continues climbing toward alert level, or reverses as carryover conditions improve. If it continues climbing toward alert level, the diagnostic groundwork already laid here (confirmed unbalance, identified cause) directly supports planning a field balance correction (Module 4.3) at the next appropriate opportunity, rather than needing to restart the diagnosis from scratch.