Figure 1. Disassembled industrial gas turbine with casing removed. Source: photosoup/Adobe StockFigure 1. Disassembled industrial gas turbine with casing removed. Source: photosoup/Adobe StockOne of the key factors behind maintaining industry-best, fleet-wide reliability for turbomachinery equipment like turbines is developing and executing a planned and robust turbine maintenance program. These maintenance focus areas for turbines and their auxiliary components can be divided into three overarching categories.

  • Preventive maintenance (PM): A planned, reliability-centered and maintenance-based program. Failure modes are identified and risk mitigation steps are implemented through proactive replacements and inspection.
  • Deficient maintenance: A failure avoidance-based program. Repairs need, or deficiencies, are identified and remedies planned at the next opportunity, such as in a planned outage.
  • Corrective maintenanc­e: A reactive, failure-based program and approach. Component failure with collateral component damage, often leading to a forced outage (system functional failure) and subsequent repair work.

For revenue generating components like turbines, it is critical for the implemented maintenance program to be able to predict failures through inspections and assessments, as implemented under a preventive maintenance program, and prioritize and proactively replace defective critical and aging components in order to mitigate outage risk. The end goal is minimizing the size of the corrective maintenance program on turbomachinery.

Maintenance inspections

A comprehensive scope of inspections, assessments, and diagnostics allows for a collaborative and data-driven analysis of gas turbine equipment and their auxiliary equipment. The performance of a facility’s turbomachinery directly impacts the organization’s bottom line, and from a financial as well as asset management perspective, businesses must be aware of the equipment health parameters and know where it is underperforming and what the deficiencies are. The ultimate goal is to address those deficiencies based on inspection results with a view of minimizing potential downtime in the form of a potential forced system outage.

Combustion section inspection

A key controlling factor that determines the service life of a gas turbine engine is the state of the hot combustion section. This includes the inspection and cleaning of the hot combustion section, which comprises of the combustion liners and cans, end caps, fuel nozzle assemblies, crossfire tubes, and transition pieces. For example, the external combustion casing should be inspected for evidence of exhaust leaks, hotspots, and distortions before the casing is disassembled. After the combustion casing has been opened, the combustion chambers should be inspected for evidence of localized overheating and erosion, excessive wear, or cracks. The inspection generally includes that of the combustion chamber outlet ducts, covers and turbine fuel nozzle for blockages, cracks, pits and corrosion in addition to the inspection of the first stage turbine blades and nozzle guide vanes for warping, cracks, or foreign object damage.

Additionally, in applications in areas of high salt water or other chemicals, a thorough turbine rinse is also generally recommended. To take a step further, cleaning with chlorinated solvent method ensures the parts are left absolutely dry.

If two cracks are observed progressing from a free edge with imminent signs of meeting in the middle over time, this warrants for a replacement or repair of the combustion chamber.

Hot gas path inspection

Some of the most surveillance and condition monitoring intensive components of a gas turbine engine are those that are exposed to the system’s high temperature gases discharged from the hot combustion section. Generally, the full scope of a hot gas path inspection includes the combustion system components and a detailed inspection of the turbine nozzles, shrouds, blades and buckets, bearings, rotor, cross-fire tubes, fuel nozzle sets, and the turbine casing.

Borescope inspection

Borescope inspection offers an economical method of visually inspecting the innermost parts of a turbine such as the gas path, rotor, or the impeller blades. Such areas would otherwise prove to be virtually impossible to reach and physically view without requiring a resource intensive and intrusive equipment disassembly, which in most cases end up requiring an extensive system or plant outage, due to the need to remove the entire turbine casing.

Borescopes can have both rigid as well as flexible cable options, a lighting and optical system, and an eyepiece or a digital screen to view the inspection pathway and can be available in lengths as long as 30 feet, which provides a long enough coverage for such inspections. Using a flexible and articulating video borescope to conduct the required inspections, eliminates significant resource impact including labor, rigging, and maintenance costs which it would otherwise require to disassemble equipment of such size.

A good chunk of scope of the hot gas path as well the combustion section inspection can be conducted with a borescope inspection.

Alignment and measurements

Accurate and precise measurements allow for the development of targeted data-driven repair strategies or simple component alignments, while also retaining the data for longer term trending. Some of the common measurement and alignment scope includes:

  • Internal alignment measurements and realignment: measurement of the internal bore to ensure that the bearings, diaphragms and stationary turbine components are correctly aligned in order to limit high vibrations
  • Shaft and coupling alignment, concentricity and runouts
  • Shell casing levelness and flatness
  • Turbine geometry dimensional analysis, horizontal and vertical joint surface mapping
  • Turbine nozzle axial clearance, blade tip, and bearing clearances, and more
  • Tolerance checks on critical dimensions such as journal bearings

Non-destructive testing/examination (NDT/NDE)

Advanced NDT/NDE technologies can allow the detection material flaws and defects sooner and with a high Probability of Detection, or POD. Current technologies are able to detect the smallest of flaws in turbine componentry that could potentially limit equipment operation and risk further asset damage. The testing methods are designed to detect some of the most common failure modes for gas turbines, including corrosion, cracking, fatigue, weld defects, and leaks, allowing businesses to make crucial asset management decisions with a view of optimizing maintenance resources.

Some of the common NDT/NDE techniques include:

  • Eddy current inspection: Used for crack detection for example in blade and rotor dovetail joints
  • Phased array ultrasound (PAUT): Similar to eddy current inspections, PAUT inspections are better suited for more complex geometry as well with a higher accuracy of predicting crack length
  • Magnetic particle inspection: Best suited for visually detecting surface cracks
  • Liquid/dye penetrant weld inspection: Used for checking weld integrity for new installs as well as checking aging component integrity for life extension initiatives

Component repair

Figure 2. Gas turbine impeller blade inspection. Source: Christian Kuhna/CC BY-SA 3.0Figure 2. Gas turbine impeller blade inspection. Source: Christian Kuhna/CC BY-SA 3.0Gas turbine components can be refurbished through several metallurgical processes that restore their metallurgical and dimensional properties to make them more reliable and extend equipment life. The methods involve various options, ranging from an isolated repair of, for example, a damaged shaft to applying preventative coatings on the entire body of the main rotor.

Coating

Coating technologies provide a thermal insulation barrier between the process as the internal turbine components. Several advanced coating technologies are available to extend lifespan of existing components as well increase the service life of newly installed ones:

  • Air plasma spraying
  • Electric arc wire spraying
  • High-velocity oxygen fuel spraying (HVOF) – liquid fuel and gas fuel

Repairs and replacements

Shafts can be repaired with welding methods which involve pre- and post-weld heat treatments to ensure proper metallurgical/mechanical properties. Repairs are often prescribed following NDT/NDE assessments as aforementioned. In most cases, a proactive repair result in a significant reduction in costs and overall repair times by mitigating the risk of future unforeseen equipment failures. Some additional repair techniques and scope of work include

  • Body panel replacement
  • CNC machining/tip welding restoration
  • Hardware replacement

Additional processes

After the main repair, reconditioning and coating, a number of additional operations are also often carried out, as required

  • Shot peening: This extends operating life of components that endure cyclic loading, or are fatigue-prone, by pre-stressing (compressive) their surfaces.
  • Blade balancing: Balanced by moment-weight calculation and sorting. It is highly recommended to stay as close as possible to the OEM sequence
  • Guide vane repair: They must be repaired to OEM recommended throat openings

Bottom line

There are multiple approaches and even more techniques business can apply towards gas turbine maintenance strategy to stay competitive in terms of facility reliability, depending on the industry, system age and operating finances. But there a few aspects best practices common to all in terms of creating a maintenance program.

  • Knowing the status of assets: This involves understanding the health of the equipment with a data driven approach
  • Knowing the operating procedures: The conditions and practices of operating the equipment is the building block to creating a maintenance plan
  • Efficient work management and planning: Maintenance scope of work should be properly defined and completed within designated maintenance windows
  • Expert knowledge: The importance of subject matter expertise cannot be overemphasized
  • Reliable supply chain: Parts quality and having a supply chain you can trust is the final piece of the puzzle