Maintenance and Repair of Gas Turbine Nozzle Assemblies
Nozzle assembly components don’t last long in the challenging environment inside a gas turbine engine. Thermal cycling causes fatigue, high temperatures cause creep, and corrosive gasses erode precisely machined surfaces. If not monitored closely and inspected regularly parts can fail catastrophically, taking the whole engine with them.
The only way of avoiding such costly in-service failures is through rigorous inspection, maintenance and when necessary, component repair or replacement. As a leader in the manufacture of nozzle assemblies for gas turbines, Impro has the knowledge, experience and resources to carry out this work.
Function, Environment and Design
Located immediately after the combustion stage, the nozzle components direct hot gasses onto rotating blades. These parts are typically produced from creep-resistant nickel-based superalloys. They have good strength at high temperatures but their melting point is only around the temperature of the combustion gasses.
To ensure their survival, many of these nozzle parts incorporate small cooling channels that distribute lower temperature air over their outer surfaces. Some also use thermal barrier coatings (TBCs) to increase their heat resistance.
Nozzle components are produced to exacting tolerances that maintain tight clearances inside rotating assemblies that need precise balancing. In addition, high loads demand great structural integrity, so cracks in cast and machined parts must be avoided.
Wear and Failure Modes
In continuous duty turbine engines, nozzle assembly components suffer mainly from thermal creep and oxidation, corrosion and erosion caused by the combustion gasses. This alters dimensions, potentially reducing clearances, and can impair operation of the cooling channels.
Turbines used in quick response or peaking applications suffer more from thermally-induced mechanical fatigue. This can lead to cracking in highly loaded regions of nozzle assemblies.
For both use cases, foreign object damage (FOD), where the engine ingests debris, is another potential cause of component damage and failure.
Gas turbine manufacturers set out strict recommendations for inspection intervals. Accordingly, users are expected to log operating hours and schedule downtime for periodic inspection. These inspections take the form of:
- Visual checks for damage (by borescope where necessary)
- Checks of TBC thickness
- Dimensional inspection (after removal from the engine)
- Nondestructive evaluation (NDE) – crack detection, sometimes also x-ray analysis
- Flow testing (to verify rates are within the manufacturers limits)
- Destructive evaluation – to assess the progression of wear and find cracking in regions that cannot otherwise be observed
When damage, excessive wear, distortion or other defects are found a decision must be made: repair or replace?
Options for Nozzle Assembly Repair
If it is determined a component can be brought back to OEM specifications, the processes used are:
- Hard facing (applying a layer of harder metal through welding)
- Welding with an appropriate filler (electron beam welding (EBW) for higher precision and smaller heat affected zones)
- Machining (mostly grinding but possibly also turning, milling and electro-discharge machining (EDM))
In the case of TBCs that are below minimum thickness, these will be stripped and reapplied.
On passing inspection, repaired components are reassembled. For rotating assemblies concentricity, runout and straightness are checked and the assembly is balanced.
Rely on Impro’s Experience
Maintenance and repair of gas turbine nozzle assemblies requires specialized equipment and high levels of skill. As a leader in production of such components, Impro has the materials, components and process knowledge needed. What’s more, we use advanced manufacturing equipment like EDM and EBW, along with sophisticated inspection tools, to perform and verify the repair work undertaken. Contact us for more information.