Combustion vs Hot Gas Path vs Major Inspection: What Each Covers and When It's Due

A combustion inspection (CI) opens only the combustion system: fuel nozzles, liners and transition pieces. A hot gas path inspection (HGPI) adds the first turbine stages: nozzles, buckets and shrouds. A major inspection (MI) opens the whole unit; compressor and rotor included. Each fall due on factored fired hours or factored starts, whichever limit comes first. For GE 6B.03 and 7E.03 units, the HGPI baseline is 24,000 factored fired hours or 1,200 factored starts. 

This guide covers what each level includes, how intervals are set, how aeroderivative units such as the LM2500, LM6000 and TM2500 differ, and how to plan the outage. Prismecs supports both fleet types through its gas turbine inspection and overhaul services, independent of the original equipment manufacturer (OEM). 

Quick Answer: The Three Inspection Levels at a Glance 

Each level includes the scope of the one below it and adds more of the machine. 

The HGPI baseline comes from GE Vernova's GER-3620P, Heavy-Duty Gas Turbine Operating and Maintenance Considerations (February 2021). Intervals are typical only and vary by OEM, model, fuel and duty. 

Why Inspection Timing Matters More over the Next Decade 

Gas turbines will carry more load, and more cycling, in the years ahead: 

Frequent starts to use inspection intervals faster than running hours do, and tighter outage windows leave less room for a missed scope item. 

Combustion Inspection: Scope and Parts 

On a gas turbine, the combustion inspection is the shortest and most frequent disassembly inspection. It removes the parts that hold the flame without opening the turbine casing. 

Parts Covered 

Inspection Methods 

Crews use visual and dimensional checks and fluorescent penetrant inspection (FPI), which uses a dye that glows under ultraviolet light to reveal cracks. A borescope inspection (BSI), using a camera probe through casing ports, checks the turbine stages without opening the casing. Dry low NOx (DLN) units usually need retuning after combustion work. 

Borescope Inspection: The Check Between Outages 

A borescope inspection on a gas turbine is the check that runs between disassembly inspections. A camera probe goes through casing ports to show the compressor, combustor, and turbine stages without taking the unit apart. BSIs are done between outages, before planned outages to confirm the scope, and after events such as trips. On aeroderivative, the BSI is the core Level 1 check: Baker Hughes notes that a Level 1 inspection on PGT25 machines is typically done semi-annually or every 4,000 fired hours. A BSI catches TBC loss, cracks, burned nozzle edges, tip rubs and foreign object damage (FOD). Tracked over time, BSI results show how fast parts are wearing, which is the evidence condition-based decisions rely on. 

Hot Gas Path Inspection: Scope, Parts and What Drives Replacement 

A hot gas path inspection covers everything in a CI, then removes the turbine casing upper half. The parts it reaches see the highest gas temperatures in the machine. 

HGPI Scope: Parts Inspected 

Repair, Replace or Return to Service 

Each part is judged against OEM limits on crack length, coating condition, wall thinning and repair history. Inspectors use FPI, eddy current testing (an electromagnetic check for cracks under coatings), dimensional checks, and coating evaluation. 

What Happens during a Hot Gas Path Inspection, Step by Step 

• Shut down, cool down, and apply lockout/tagout (LOTO).
• Remove the enclosure roof, combustion cans, and crossfire tubes.
• Remove the turbine casing upper half.
• Take as-found clearance and concentricity measurements.
• Remove, clean, and inspect the stage 1 and 2 buckets and nozzles.
• Repair, replace, or refurbish parts based on the findings.
• Reassemble, take as-left measurements, and run return-to-service tests.
• Deliver the inspection report. 

Planning an inspection in the next 12 months? Get an independent scope review from Prismecs. 

Major Inspection: When the Whole Unit Opens 

A gas turbine major inspection opens the unit flange to flange. It adds the compressor, rotor, bearings, seals, and casings to the HGPI scope. In the compressor, crews check blades and vanes for erosion, FOD, tip rubs, and coating conditions. The rotor is checked against its life limits: GER-3620 calls for rotor inspections at 5,000 factored starts or 200,000 factored hours unless otherwise specified, while the 7FA rotor limit is 144,000 factored hours or 5,000 factored starts. 

Why It Is a Decision Point 

The MI is the largest scheduled event in the cycle. Owners often decide on rotor life extension, upgrades and OEM versus independent support at this point. 

How Gas Turbine Inspection Intervals Are Set: Fired Hours vs Starts 

Every interval has an hour limit and starts a limit. The inspection is due at whichever limit the unit reaches first. 

Factored Hours, Factored Starts and EOH 

How Starts and Trips Change Inspection Intervals 

Starts cause low-cycle fatigue (LCF), cracking from repeated heating and cooling. Hours cause creep and oxidation. Trips, peak firing, liquid fuel, and water injection all shorten intervals. 

Which Limit Hits First? 

This illustrative example uses a 24,000-hour / 1,200-start baseline and a maintenance factor of 1. 

A peaker can owe an HGPI within 4 years despite low hours. Always check the GER-3620 revision and any OEM Technical Information Letters (TILs) that apply to your unit. 

How Aeroderivatives Differ: Hot Section Inspection, Borescope Levels and Engine Swaps 

Aeroderivative units such as the LM2500, LM2500XPRESS, LM6000 and TM2500 are built in modules, so whole sections can be swapped instead of repaired in place. 

Different Names for Similar Work 

Baker Hughes cites typical aeroderivative intervals of 8,000, 25,000 and 50,000 hours, extended to 10,000, 30,000 and 60,000 hours on the LM6000PF+. 

Maintenance Levels 1 to 4 

Aeroderivative work is commonly grouped into levels. Lower levels cover on-site inspections, including routine BSIs, and on-site module work such as a hot section exchange. Higher levels are shop repairs, up to a full depot overhaul. 

Engine and Hot Section Swaps 

A rotable hot section or lease engine can go in while the original goes to the shop, limiting downtime to the swap itself. The trade-off is the cost of the spare and the planning lead time. 

Prismecs crews work on these units in the field, including the 260 MW TM2500 dual-fuel project at Birr and the 110 MW TM2500 mobile gas turbine project. 

Planning Timeline: 12 Months, 6 Months and 30 Days Out 

12 Months Out 

Pulls factored hours, starts and trips, review the last report and borescope trends, and flag long-lead parts. 

6 Months Out 

Order parts or book exchange modules, reserve crews and tooling, and agree on the outage window. 

30 Days Out 

• Run a pre-outage borescope inspection, stage parts on site, and finalize the work plan. 

• Placing parts of orders at the 12-month mark is the simplest way to protect the outage schedule. 

Inspection Findings That Change Scope and Budget 

Common Findings That Expand the Work 

Budgeting for Scope Growth 

A finding during a CI can force an unplanned HGPI. Build in a contingency and line up exchange parts early, so a finding does not turn into a long wait for parts. The stakes are high: EPRI puts the cost of a 7EA hot gas path inspection at $750,000 to $1 million, and reports that planned maintenance accounts for more than 70% of gas turbine unavailability, largely because of reassembly and workmanship errors. 

When to Stick to the OEM Interval and When Condition-Based Maintenance Makes Sense 

Interval-based maintenance follows the OEM's fixed hour and starts limits. Condition-based maintenance times inspections on measured condition instead. Condition-based timing suits some units, but not all. 

Condition-Based Timing Can Make Sense When 

Stick to the OEM Interval When 

Extending an interval without data only moves the risk to a later outage. 

Frequently Asked Questions 

What Is the Difference Between a Combustion Inspection and a Hot Gas Path Inspection? 

A combustion inspection opens only to the combustion system: liners, end caps, fuel nozzles, crossfire tubes and transition pieces. A hot gas path inspection includes all of that, then removes the turbine casing upper half to inspect the stage 1 and 2 nozzles, buckets and shrouds. 

What Parts Are Inspected or Replaced during a Hot Gas Path Inspection? 

An HGPI covers the combustion parts plus the first-stage nozzle, turbine buckets, shrouds and stage 2 hot parts. Each part is judged against OEM limits on crack length, coating condition, wall thinning and repair history, then repaired, replaced or returned to service. 

How Often Is a Hot Gas Path Inspection Required? 

It depends on the model, fuel, and duty. For GE Frame 6B (6B.03) and Frame 7EA (7E.03) units, GER-3620P sets a baseline of 24,000 factored fired hours or 1,200 factored starts, whichever comes first. A maintenance factor above 1 shortens the interval. 

How Do Starts and Trips Change Inspection Intervals? 

Every start heats and cools the hot section, causing low-cycle fatigue, so starts count toward their own limit alongside hours. Trips count more: GER-3620 treats a trip from full load as equal to about eight normal starts. Two full-load trips in one week would add 16 factored starts and bring the next HGPI forward. 

How Much Does a Hot Gas Path Inspection Cost? 

Cost depends on the model, parts strategy and findings. EPRI cites $750,000 to $1 million for a 7EA hot gas path inspection. Parts are usually the biggest driver, followed by outage days and scope of growth from findings, so pre-staged parts and a budget contingency matter as much as the quoted price. 

How Long Does a Hot Gas Path Inspection Outage Take? 

It depends on the model, crew size, parts of readiness, and findings. In one example reported by Power Magazine, MD&A completed a 7FA HGPI with a generator inspection in 19 days. Staging parts before the outage is the biggest factor you control. 

Do Aeroderivative Turbines (LM2500, LM6000, TM2500) Have Hot Gas Path Inspections? 

Yes, but they are usually called hot section inspections. Because aeroderivative units are modular, the hot section is often swapped for a rotable module instead of being repaired in place, which keeps downtime short. Intervals vary by model and duty. 

Can Hot Gas Path Inspection Intervals Be Extended? 

Sometimes, if borescope trends are stable, operating data is reliable and the unit runs steady baseload on clean gas, with an engineer's sign-off. Units under an LTSA or warranty, on liquid fuel or with recent trips should keep the OEM interval. 

Plan Your Next Inspection with an Independent Review 

Planning a combustion, hot gas path or major inspection in the next 12 to 18 months? Prismecs reviews inspection scope for heavy-duty and aeroderivative units, independent of the OEM. Share your unit model, fired hours, starts, and planned outage date. Our field engineers will check the scope line by line, flag long-lead parts, and help you set up a realistic outage window. 

Prefer to talk it through? Call our team at 1 (888) 774 7632 24/7, or email us at sales@prismecs.com.  

Request an inspection scope review from Prismecs today.