Gas Turbine Vibration Analysis: Fault Detection & Monitoring
By Willam Jerry on October 23, 2026
A heavy-duty gas turbine spinning at 3,600 rpm is a rotating mass that tolerates misalignment in thousandths of an inch and bearing wear in microns. The engine does not fail suddenly — it tells its story in the vibration spectrum weeks before the trip, if the plant is listening. ISO 10816-4 defines the severity bands, API 670 defines the instrumentation, and the FFT spectrum carries the fault signatures a trained analyst reads the way an ECG is read. This guide walks the sensor placement, the ISO 10816-4 bands, the six fault signatures, the shaft-centerline view, the trend-over-startup curve and the live monitoring board. OXMAINT AI is the AI-powered power plant maintenance software that ingests the vibration monitoring stream, tags the signature, raises the WO before the band crosses and keeps the trend as the lineage.
Gas Turbine Vibration Analysis: Fault Detection & Monitoring
Six fault signatures, four ISO 10816-4 bands, one API 670 instrumentation stack. OXMAINT AI reads the live spectrum, tags the signature and raises the WO before the band crosses — the trip never happens.
Turbine & Sensor Anatomy — Where API 670 Puts The Probes
A heavy-duty gas turbine carries proximity probes at every bearing, velocity sensors on the casing, key-phasor for 1× reference and discharge temperature probes throughout the hot section. The diagram below is the standard placement; OXMAINT AI stores each probe as a sub-asset of the turbine — load the probe register in the power plant maintenance software.
ISO 10816-4 Severity Bands — Gas Turbine Specific
ISO 10816 is the general machinery vibration standard; -4 is the gas-turbine-specific part. The bands differ from general machinery — a gas turbine runs at higher baselines and reaches Band D sooner. OXMAINT AI tags every reading against the right band.
A
Newly commissioned
0 - 3.8 mm/s RMS
New or recently overhauled gas turbine; vibration at or below the OEM baseline. Normal operating state.
B
Acceptable long-term
3.8 - 7.5 mm/s RMS
Normal running wear; no action required beyond trending. Most mid-life turbines sit in Band B.
C
Limited run · schedule repair
7.5 - 11.8 mm/s RMS
Vibration past the long-term acceptable; schedule inspection or repair at next planned outage. Watch rate of change.
D
Shutdown trigger
> 11.8 mm/s RMS
Mechanical damage possible on continued operation. Shutdown trigger per the OEM trip card.
The Six Spectrum Signatures — What A Gas-Turbine FFT Reveals
Six fault types cover almost every heavy-duty gas-turbine vibration event. Each shows up at a specific frequency relative to running speed; OXMAINT AI pattern-matches the signature and tags the WO at the moment the match crosses confidence.
1×
Rotor unbalance
Peak at running speed; smooth, radial, in-phase across the shaft. Causes: blade erosion, FOD, cooling-hole fouling, blade loss.
Fix: trim-balance / blade inspection
2×
Shaft misalignment
Peak at 2× running speed, strong axial component, 180° phase across the coupling. Causes: coupling misalignment, thermal growth differential.
Fix: hot-align after soak-out
BPFI / BPFO
Rolling-element bearing fault
Non-synchronous peaks at computed bearing frequencies with sidebands; progression days to weeks before failure.
Fix: scheduled bearing swap
BLADE ×N
Blade-pass excitation
Peak at (blade count × running speed). Rising amplitude: blade-tip rub, stator wake issue, loose shroud.
Fix: borescope blade inspection
0.4-0.5×
Rotor rub / oil whirl
Sub-synchronous peak at ~0.42-0.48× running speed. Rotor-stator rub or bearing instability. Progression hours, not weeks.
Fix: shutdown + inspect
COMB
Combustion dynamics
Pressure pulsation picked up by casing accelerometers at combustor acoustic frequencies (~50-500 Hz band).
Fix: fuel-split / pilot tuning
The Shaft Centerline / Orbit View — What The Proximity Probes Show
A spectrum is only half the picture. Proximity-probe pairs at each bearing render a shaft-centerline plot (gap voltages, cold vs operating) and an orbit plot (X vs Y vibration shape). The orbit is where a trained analyst reads rub, imbalance, misalignment and bearing instability.
Clean · circular small orbit
Low amplitude, nearly circular. No fault. The healthy baseline.
Imbalance · large circular
Enlarged circular. 1× peak drives the shape. Trim-balance returns it to clean.
Misalignment · banana
Flattened / banana-shaped. 2× peak dominates. Coupling alignment returns it to circular.
Rub · clipped / inside loop
Clipped or inside-loop shape. Sub-synchronous content present. SHUTDOWN & inspect.
Trend Over Startup → Steady State → Shutdown
Vibration tells different stories at different operating points. Startup transients, steady-state baseline, trip-related events and shutdown coast-down each have their own characteristic trace. OXMAINT AI stores the trend per start.
VIBRATION TREND · TYPICAL START-TO-STEADY · BRG 3
Crank & purge
1.2 mm/s
Ignition & warm-up
3.6 mm/s
Critical-speed traverse
6.4 mm/s
Full-speed no-load
2.9 mm/s
Loaded steady state
3.1 mm/s
Shutdown coast-down
5.8 mm/s
Critical-speed traverse peak is the single most informative point on the start — rising peak start-over-start is the leading indicator of unbalance growth
The Spectrum Said So Days Ago. The Trip Was The Confirmation.
A disciplined vibration-monitoring program reads the signature at the moment the band shifts, not when the trip fires. OXMAINT AI runs the ingest, holds the signature library and raises the WO on the pattern match — days of warning become planned work.
Live Monitoring Board — What A Reliability Engineer Sees
The live board shows every bearing of every turbine against ISO 10816-4 bands, the current signature match and any open WO. OXMAINT AI renders it live from the vibration-monitoring stream.
GT-02 BRG 51× peak rising 11 d · unbalance growthBand C · trim-balance planned
GT-07 BRG 2BPFO sidebands emerging · LOI stableINVESTIGATE · WO raised
GT-01 all bearingsBand A/B steady · post-overhaul 240 OHOK
GT-05 all bearingsBand B steady · 11,400 OH on cycleOK
GT-04 coast-down 15:30 · borescope team mobilized · rotor-stator rub inspection scope on the WO
API 670 Instrumentation Reference — What Goes Where
API Standard 670 defines the machinery-protection system for turbines, compressors and other critical rotating equipment. The six-sensor set below is the heavy-duty gas turbine baseline; OXMAINT AI holds each as a tagged sub-asset.
01
Radial proximity probes (X-Y)
Two probes 90° apart at each bearing. Reads shaft orbit, shaft centerline position, 1× 2× vibration vectors.
02
Axial thrust probes
Dual axial probes at the thrust bearing. Reads shaft axial position for thrust-bearing wear.
03
Key-phasor
One probe reading a notch or key on the shaft at BRG 1. The 1× phase reference the whole system is referenced to.
04
Casing velocity sensors
Velocity sensors on bearing casings or compressor casing. Picks up casing-side vibration proximity probes miss.
05
Casing accelerometers (hi-freq)
High-frequency accelerometers for bearing-fault BPFI/BPFO envelope analysis and blade-pass excitation.
06
Combustion-dynamics probes
Dynamic pressure transducers in the combustor section. Reads combustion-driven acoustic pulsation.
How OXMAINT AI Runs The Vibration-Analysis Program
The probe register, ISO 10816-4 band ingest, six-signature library, orbit-and-centerline view, startup-trend history and the live board all live natively in the OXMAINT AI power plant maintenance software — wire the vibration monitoring system into OXMAINT AI.
Probe Register
Every proximity probe, velocity sensor, accelerometer and combustion-dynamics transducer tracked as a sub-asset per bearing.
Band Tagging
Every live reading tagged ISO 10816-4 Band A/B/C/D; Band C auto-raises an inspection WO, Band D the trip card.
Signature Library
The six classic signatures held as pattern templates; a match raises the WO with the suspected cause pre-populated.
Orbit View
Live orbit per bearing with reference shapes (clean / imbalance / banana / rub) for operator reading.
Start-Over-Start Trend
Vibration at the critical-speed traverse captured per start; rising peak start-over-start raises an unbalance watch WO.
Outage Scope Export
End-of-run vibration summary exported into the next outage HGPI / MI scope so the inspection path starts with evidence.
Frequently Asked Questions
What's the first signature to look for after a hot restart?
2× misalignment. Thermal growth differential between hot and cold alignment is the single most common restart signature; it decays as the machine soaks out. A persistent 2× past 4-6 hours is a real coupling-alignment finding, not a thermal transient.
Why do we also need casing accelerometers if we have proximity probes?
Proximity probes read shaft motion inside the bearing clearance; they miss casing-side faults and high-frequency bearing-fault signatures. Accelerometers at the bearing housing catch BPFI/BPFO sidebands and blade-pass excitation that proximity probes do not resolve.
How often should we re-baseline a turbine?
After every HGPI (hot gas path inspection), every MI (major inspection), after a trim-balance and after a bearing swap. The baseline is per-asset per overhaul cycle; comparing today to a pre-overhaul baseline misleads the trend. OXMAINT AI stores the baseline against the overhaul cycle so the comparison is always like-for-like.
What's the typical lead-time on a bearing-fault signature?
10-14 days for a typical BPFI progression on a heavy-duty gas-turbine bearing — enough to plan a controlled coast-down and bearing swap during a scheduled outage window. Faster progression (3-5 days) usually points to lubrication breakdown rather than classic wear.
Read The Spectrum · Watch The Orbit · Trend The Start · Trip Never Fires.
Wire the vibration monitoring into the OXMAINT AI power plant maintenance software — API 670 probe register, ISO 10816-4 band tagging, six-signature library, orbit view, start-over-start trend and the live board. The spectrum reads days before the trip fires; the WO lands days before the outage would have.
