The Role of the Combustion System in Gas Turbines
The combustion system is the heart of every gas turbine, converting the chemical energy in fuel into the high-temperature, high-pressure gas that drives the turbine stages. It is also the section that demands the most frequent maintenance attention, with combustion hardware typically requiring inspection or replacement at intervals of 8,000 to 25,000 equivalent operating hours depending on the turbine model and operating conditions.
This guide provides a comprehensive overview of the major combustion system components, their functions, common failure modes, and practical guidance for procurement and maintenance planning. Whether you operate GE aeroderivative turbines (LM2500, LM6000) or heavy-duty frame turbines (Frame 5, Frame 6B, Frame 7), understanding these components is essential for controlling maintenance costs and ensuring reliable operation.
Combustion System Architectures
Gas turbines use two main combustion system designs, each with distinct component requirements:
| Architecture | Description | Turbine Models |
|---|---|---|
| Single Annular Combustor (SAC) | One continuous ring-shaped combustion chamber surrounding the engine axis | GE LM2500, LM6000, LMS100 |
| Can-Annular | Multiple individual combustion cans (typically 6, 10, or 14) arranged in a circle | GE Frame 5, Frame 6B, Frame 7, Frame 9 |
The SAC design offers advantages in terms of combustion uniformity and lower pressure drop, while the can-annular design allows individual combustion cans to be removed and serviced without a complete engine teardown. Both designs use similar fundamental components — fuel nozzles, liners, and ignition systems — but the specific hardware differs significantly between architectures.
Fuel Nozzles: The Most Critical Combustion Component
Fuel nozzles atomize and distribute fuel into the combustion chamber in a precisely controlled spray pattern. The quality of fuel atomization directly affects combustion efficiency, emissions levels, flame stability, and the thermal life of downstream components. Poor fuel nozzle performance is one of the leading causes of uneven exhaust gas temperature (EGT) spread, which can accelerate wear on first-stage nozzles and buckets.
Types of Fuel Nozzles
| Type | Fuel | Design Features | Application |
|---|---|---|---|
| Simplex pressure nozzle | Liquid fuel | Single fuel circuit, pressure atomization | Older designs, backup fuel systems |
| Duplex pressure nozzle | Liquid fuel | Primary and secondary fuel circuits for better turndown | Most liquid-fuel turbines |
| Air-blast nozzle | Liquid fuel | Uses compressor air to assist atomization | Modern aeroderivative turbines |
| Gas fuel nozzle | Natural gas | Multiple gas injection ports for mixing | Gas-fired turbines |
| Dual-fuel nozzle | Gas and liquid | Combined gas and liquid fuel passages | Dual-fuel installations |
| DLE premixer | Natural gas | Pre-mixed lean combustion for low NOx | LM6000 PD/PF/PG, Frame 7FA DLN |
Fuel Nozzle Failure Modes
Fuel nozzles degrade over time due to several mechanisms:
Carbon buildup (coking) occurs when fuel residues carbonize on the nozzle tip, partially blocking fuel passages and distorting the spray pattern. This is more common with liquid fuels, especially heavier distillates, and is accelerated by high nozzle tip temperatures during shutdown.
Erosion of the fuel passages occurs from abrasive particles in the fuel or from the high-velocity fuel flow itself. Erosion changes the flow characteristics of the nozzle, leading to uneven fuel distribution between combustion chambers.
Thermal distortion results from repeated thermal cycling and can cause the nozzle tip to warp, changing the spray angle and pattern. This is particularly problematic in peaking turbines that undergo frequent starts and stops.
Internal leakage between fuel circuits (in duplex or dual-fuel nozzles) can cause fuel to enter the wrong circuit, leading to poor atomization and potential flame instability.
Fuel Nozzle Maintenance
Fuel nozzle maintenance typically involves removal, cleaning, flow testing, and either refurbishment or replacement. Flow testing is critical because it verifies that each nozzle delivers the correct fuel flow at a given pressure, ensuring uniform fuel distribution across all combustion chambers. Nozzles that are outside flow tolerance must be replaced or sent for repair.
For the LM2500, which uses 30 fuel nozzles in its single annular combustor, a complete set of flow-matched nozzles is essential for achieving the tight EGT spread required for optimal performance and component life.
Combustion Liners
Combustion liners are the structural shells that contain the flame and direct the combustion gases toward the turbine. They must withstand temperatures exceeding 1,200°C on the flame side while maintaining structural integrity and acceptable metal temperatures through various cooling techniques.
Liner Cooling Technologies
| Cooling Method | Description | Effectiveness |
|---|---|---|
| Film cooling | Rows of small holes inject a layer of cool air along the liner surface | Good — widely used in older designs |
| Effusion cooling | Thousands of laser-drilled angled holes provide dense cooling coverage | Excellent — used in modern designs |
| Impingement cooling | Cool air jets directed at the liner from an outer sleeve | Very good — used in combination with film cooling |
| Thermal barrier coating (TBC) | Ceramic coating on the hot side reduces heat transfer to the metal | Supplementary — extends liner life by 200-400°C |
Liner Inspection and Replacement
During combustion inspections, liners are examined for cracking (especially around dilution holes and cooling hole rows), oxidation and scaling, distortion, and coating degradation. Common repair techniques include weld repair of cracks, re-coating with TBC, and dimensional restoration. However, liners that have exceeded their repair limits must be replaced with new components.
Key combustion liner part numbers in our inventory include:
| Part Number | Description | Engine |
|---|---|---|
| 1703M32G03 | Liner, Assembly — Outlet Guide | LM6000 |
| 1703M58G01 | Liner Assembly, Stage 13 Vane | LM6000 |
| 2083M50G01 | Liner, Turbine Mid Frame (Field) | LM2500 |
| 1808M63P03 | Tube, Combustion Cup | LM2500 |
| 3042M10P01 | Sleeve, Combustion Chamber Dome | LM2500 |
Transition Pieces
Transition pieces are unique to can-annular combustion systems and serve as the duct between the circular combustion can outlet and the annular first-stage nozzle inlet. They are among the most geometrically complex components in a gas turbine, with compound curves that must maintain precise tolerances to ensure proper gas flow distribution.
Transition pieces experience severe thermal gradients — the leading edge (closest to the combustion can) operates at much higher temperatures than the trailing edge (at the nozzle interface). This thermal gradient causes differential expansion and can lead to cracking, distortion, and seal wear at the downstream interface.
Modern transition pieces for Frame 6B and Frame 7 turbines feature advanced cooling schemes and thermal barrier coatings that extend service life. However, they remain one of the most frequently replaced components during combustion inspections, with typical service lives of 24,000 to 32,000 equivalent operating hours.
Crossfire Tubes and Retainers
Crossfire tubes connect adjacent combustion chambers in can-annular systems, serving two critical functions: flame propagation during startup (allowing the flame from the two igniter-equipped chambers to spread to all other chambers) and pressure equalization during normal operation.
Crossfire tubes are relatively simple and inexpensive components, but their failure can have significant consequences. A cracked or broken crossfire tube can prevent flame propagation during startup, cause uneven combustion, or allow hot gases to leak into the combustion casing area.
| Part Number | Description | Engine |
|---|---|---|
| 3041M99P01 | Retainer, Igniter — Combustion Chamber Dome | LM2500 |
Flow Sleeves
Flow sleeves surround the combustion liners in can-annular systems and direct compressor discharge air around the liner for cooling before the air enters the combustion zone through the liner's dilution and cooling holes. The flow sleeve also provides structural support for the liner and helps maintain the correct air distribution between cooling and combustion air.
Ignition System Components
Gas turbine ignition systems use high-energy spark igniters (similar in principle to aircraft engine igniters) to initiate combustion during startup. Typically, only two of the combustion chambers are equipped with igniters — the flame propagates to the remaining chambers through the crossfire tubes.
Igniter components include the igniter plug (which produces the spark), the igniter exciter (which generates the high-voltage pulse), and the igniter lead (which connects the exciter to the plug). Igniters are consumable items with limited service lives and should be replaced at regular intervals or whenever ignition performance degrades.
Combustion Inspection Planning
A well-planned combustion inspection minimizes downtime and ensures that all necessary parts are available when needed. The typical planning timeline is:
| Timeline | Action |
|---|---|
| 12 months before | Conduct borescope inspection to assess current condition; develop preliminary parts list |
| 9 months before | Obtain quotes from multiple suppliers; order long-lead-time items (liners, transition pieces) |
| 6 months before | Confirm parts delivery schedule; arrange for repair services if needed |
| 3 months before | Receive and inspect all parts; prepare tooling and consumables |
| Outage start | Execute inspection; order any additional parts identified during teardown |
Sourcing Combustion System Parts
BDB Turbine Parts maintains a comprehensive inventory of combustion system components for GE aeroderivative and frame turbines. Our catalog includes fuel nozzles, combustion liners, transition pieces, crossfire tubes, igniters, seals, and all associated hardware. With over 19,000 part numbers in our database and a global network of suppliers, we can help you find the right parts at competitive prices.
Browse our parts catalog or submit a quote request for fast, personalized pricing on combustion system components.
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