Inspections of gas turbine components

High-efficiency gas turbines (GTs) for power generation employ critical components of state-of-the-art design and advanced metallurgy. Replacement and refurbishment of these capital parts represents a significant portion of the maintenance costs incurred over the life cycle of these machines, even in older B/D/E-class designs.

(Frame 7FA combustion cap – full disassembly for condition assessment)

Maximizing component life, and thus minimizing procurement of expensive capital spares, can substantially enhance plant profitability. A sound parts management strategy must include a detailed condition assessment and inspection plan for each component once removed from service, which invariably takes place in the power plant’s repair shop of choice.

But how well are these plans defined in practice? Are the right inspections employed and in the right sequence? Are these assessments properly documented in order to accurately track each component’s condition through a number of refurbishment cycles?

It is only recently that most plant owners have specified a shop “Inspect and Advise (I&A)” step prior to release of repair processing. However, the I&A scope of work used by shops can vary dramatically and is designed primarily to define repair costs, rather than as a tool to manage parts life.

It is incumbent upon GT plant owners and operators, therefore, to devise and specify I&A work scopes to their repair vendors that generate the necessary information to allow management of component life. Once devised, monitoring the work in the shop is crucial in order to ensure proper execution of the plan on a part-by-part basis.

Engineering of these I&A processes requires a thorough understanding of the component design, critical features of that design, prevalent service damage mechanisms and the modern inspection techniques available to characterize damage and residual life. Execution of the plan and the reports generated must then answer the question — Can this part be repaired, perform to design intent and run safely for the next planned service interval?

Parts management considerations

Establishment of inspection plans for condition assessment as part of an overall parts management strategy should take into account a variety of factors. For example, adopting the OEMs fleet-wide component life recommendations often results in retirement of parts with significant residual life.

These recommendations are typically quantified by number of service intervals, such as a particular component being a “2 hot gas path interval (HGPI)” part. Service damage can vary based on site-specific operating history; and using the I&A inspection results to independently evaluate component life can result in safe life extension for many parts beyond OEM recommendations.

OEMs do not always design new parts for optimum reparability. For example, some use designs with component assemblies joined by brazing, where the disassembly necessary to conduct proper inspection and repair processes necessitates the destruction of one or more parts. In these instances, repair costs are higher due to the requirement for having new parts available for refurbishment.

In fact, careful disassembly of these often complex component assemblies not only represents a major portion of the I&A work, but ensures maximum re-use of expensive detail parts.

More in the May/June 2014 issue of Turbomachinery International