Gas Turbine Inspection Checklist: Combustion, Hot Gas Path, and Major Overhaul
Gas Turbine Inspection Checklist
A full-scope technical framework for Combustion (CI), Hot Gas Path (HGPI), and Major Overhauls (MI). Protect turbine availability and eliminate multimillion-dollar catastrophic outages.
Combustion Inspection (CI)
Targets high-stress hardware inside combustion casing: fuel nozzles, liners, crossfire tubes, transition pieces, and end covers.
Hot Gas Path Inspection (HGPI)
Combines full CI scope plus deep inspection of turbine stage 1-3 buckets, stationary nozzles, shrouds, and ring segments.
Major Overhaul (MI)
Total unit disassembly: compressor rotor/stators, bearings, turbine discs, high-speed dynamic balancing, and full auxiliary overhaul.
What Drives Actual Inspection Intervals?
Calendar time is misleading. Operational severity consumes inspection life via Factored Fired Hours (FFH) and Starts multipliers.
Full Load Trip
Imposes severe thermal shocks, accelerating fatigue cracking in transition pieces and S1 nozzles faster than any other single event.
Cold Start Cycles
Fast startup from ambient temperatures subjects blades and combustion liners to heavy cyclic thermal fatigue stress.
Distillate / Heavy Fuel
Accelerates nozzle tip erosion, coke deposition, and hot corrosion on transition pieces compared to standard dry gas.
Continuous Natural Gas
The cleanest baseline operational profile with minimal transient thermal distress and standard OEM interval adherence.
Routine Operational Surveillance (Daily & Weekly)
Preventive BaselineDaily Shift: Performance & Combustion
- Compressor inlet air temp/pressure vs corrected output baseline
- Compressor discharge pressure (CDP) and temperature (CDT)
- Exhaust temperature spread across TCs (maximum spread < 30°C from mean)
- Combustion dynamics & acoustic pressure fluctuations within OEM limits
- Power output & heat rate degradation trends (>98% expected)
Daily Shift: Lube, Bearings & DCS
- Lube oil header pressure, sump level, and supply temperature
- Bearing metal temperatures (journals & thrust pads) below alarm limit
- Vibration tracking on all bearing pedestals (Shaft & Casing)
- Fuel gas supply pressure, temperature, and Wobbe Index
- DCS / Mark VIe active alarm audit (zero unacknowledged alarms)
Weekly: Compressor & Auxiliaries
- Inlet air filter differential pressure (ΔP) trending
- Inlet Guide Vane (IGV) angle tracking vs DCS feedback
- Online compressor wash effectiveness and power recovery checks
- Fuel gas filter differential pressure and automatic condensate drain trap
- Lube oil laboratory sampling (viscosity, particle count NAS/ISO, water content)
1. Combustion Inspection (CI) Checklist
Interval: 8,000–12,000 FFH / 900 StartsFuel Nozzles & End Covers
- Remove all fuel nozzles and execute flow-bench calibration (< 5% variation)
- Assess tip erosion, thermal distortion, and carbon buildup
- Inspect swirler vanes for cracking, fretting, or blockage
- 100% replacement of all elastomeric O-rings and metallic C-seals
- Verify torque sequence on end-cover bolts per OEM specifications
Liners & Transition Pieces
- Inspect combustion liners for oxidation, burnthrough, or hot streaks
- Verify clear passages across all film cooling hole patterns
- Ultrasonic thickness (UT) gauge inspection of transition piece side walls
- Inspect transition piece aft frame and floating seals for fatigue cracks
- Examine crossfire tubes, spring retainers, and flow sleeves
Return-to-Service Verification
- Compressed air leak check on all reinstalled fuel manifold lines
- Combustion tuning & dynamics verification across full load envelope
- Stack emission certification (NOx, CO, UHC) at base load
- Log refurbished serial numbers and update CMMS asset history
2. Hot Gas Path Inspection (HGPI) Checklist
Interval: ~24,000 FFHStage 1 Buckets (Turbine Blades)
- Disassemble, index, and log all Stage 1 buckets
- Fluorescent Penetrant Inspection (FPI) on leading and trailing edges
- Thermal Barrier Coating (TBC) spallation & oxidation evaluation
- Clearance verification of internal cooling passages
- Radial tip clearance measurement against stationary shroud track
Stage 1 Nozzles & Shrouds
- Measure nozzle throat areas to track creep deflection
- Perform FPI on leading edge fillet radii and trailing edges
- Inspect shroud segments and abradable seals for rub depth
- Assess ring segment retaining hooks for thermal wear or distortion
Stages 2 & 3 Rotors & Casings
- Check S2/S3 bucket tip shroud interlocks and Z-notch contact surfaces
- Inspect turbine casing horizontal split-line for thermal distortion
- Verify labyrinth interstage brush seals and tooth condition
- Execute moment-weight balance sorting before re-blading
3. Major Inspection (MI) Checklist
Interval: ~48,000 FFHAxial Compressor & Rotor Train
- Full casing lift; unstack and lift complete GT rotor
- Inspect compressor rotor blades and stator vanes for FOD and erosion
- FPI inspection on all disc web faces, dovetail slots, and tie-bolts
- High-speed balance and total indicator runout (TIR) verification
Bearings, Seals & Alignment
- Inspect journal and thrust bearing babbitt surfaces and contact patterns
- Measure bearing clearances and record cold float readings
- Full replacement of oil deflector seals and labyrinth rings
- Comprehensive laser alignment of turbine, generator, and accessory gears
Auxiliary Systems & Protection
- Full lube oil reservoir flush and filtration to NAS 7 / ISO 16/13
- Calibration and functional proof-testing of electronic overspeed trips
- Hydraulic trip valve actuation timing and accumulator pre-charge test
- Exhaust expansion joint fabric bellows replacement
Inspection Tier Scope Matrix
At-a-glance comparison of major component actions across inspection levels.
| Component | Combustion Inspection (CI) | Hot Gas Path Inspection (HGPI) | Major Inspection (MI) |
|---|---|---|---|
| Fuel Nozzles | Remove, flow-test, replace seals | Remove & full refurbishment assessment | Complete replacement / OEM overhaul |
| Combustion Liners | Visual + UT thickness check; repair/replace | Refurbishment or complete replacement | 100% Replacement standard |
| Transition Pieces | UT wall thickness & aft frame crack NDT | Full shop qualification & repair | Replace or shop recertification |
| Stage 1 Turbine Blades | Borescope inspection only | Remove, FPI NDT, measure clearances | Replace or complete metallurgical re-life |
| Stage 1 Nozzle Segments | Borescope throat check | Remove, throat area mapping, FPI crack check | Replace or full shop overhaul |
| Compressor Blades | Operational monitoring | Borescope examination for FOD/rubbing | Remove, NDT disc slots, replace damaged rows |
| Journal & Thrust Bearings | Vibration & temperature logging | Clearance verification if accessible | Full disassembly, babbitt NDT, replacement |
| Rotor Assembly | Not accessed | In-situ visual on exposed discs | Full lift, TIR runout, high-speed dynamic balance |
Frequently Asked Questions
Key engineering considerations for gas turbine outage planning.
How often should a gas turbine combustion inspection be performed?
Typically every 8,000 to 12,000 fired hours or 900 starts (whichever occurs first). On peaking units, units burning liquid distillate, or turbines subjected to frequent trips, the starts or factored hours trigger will occur well ahead of standard calendar intervals.
What is the difference between Hot Gas Path Inspection and Major Inspection?
HGPI includes the full combustion scope plus complete removal and inspection of high-temperature turbine blades, nozzles, and shrouds (approx. 24,000 FFH). A Major Inspection is a complete flange-to-flange disassembly encompassing the compressor, turbine rotor, bearings, and all mechanical drive trains (approx. 48,000 FFH).
Can borescope inspections substitute for a scheduled combustion or HGPI outage?
No. While borescopes provide essential condition monitoring to detect early coating spallation or cracks, they cannot flow-test fuel nozzles, measure transition piece remaining wall thickness, or inspect concealed dovetail roots.
