Why the typical LTSA is inadequate for modern GTs

28 April 2020

The compressor technology employed in modern heavy duty gas turbines, with, for example, reduced number of stages, thin 3D airfoils, and super-polished coatings, achieves very high levels of performance but can also suffer from relatively fast degradation, with major consequences for downstream turbine components. It is therefore concerning that compressor maintenance scope is absent from the typical LTSA.

Above Image: Modern gas turbine compressor section

The idea for the LTSA (long term service agreement) can be considered to have originated in early 2000 when F-class gas turbine technology was first introduced. At the time of the rollout of the initial LTSA concept, compressor specific issues were not as relevant in terms of driving more rapid life consumption in downstream hardware as they are in the case of modern compressor technology.

Older compressor technology and load dispatch operation profiles were primarily baseload with no more than around 70% turn-down. In contrast, today’s technology must sustain wider residual swirl vibratory effects, and their consequent negative outcomes, such as enhanced compressor performance-degradation rate and reduced wear-and-tear integrity of new design features in the compressor.

Accelerated product development

The dash to claim market leadership in aero engine (AE) and heavy duty gas turbines (HDGT) for power plants has driven gas turbine technology development to focus on methods for achieving higher efficiency compressors that consume comparatively less turbine work for a given high pressure ratio, and several other advanced technologies for the combustor, turbine and sealing that are directed at achieving longer operating intervals before scheduled maintenance.

Also, manufacturers in both the aero engine and heavy duty gas turbine sectors have resorted to “platform strategies”, which speed- up market introduction by using a common core, sharing some common hardware across different engine ratings.

This method of product development is distinctly different from past approaches, which involved longer periods of technology maturation and validation, initially starting with military engines, followed by application in commercial aero engines, and subsequently in heavy duty gas turbines for power generation.

The older product development approach involved much longer design development lead times for transferring technology. But the outcomes from this type of incremental path benefited reliability because of the more disciplined structure of technology transfers from military engine, to commercial aero engine, to heavy duty gas turbine for power generation applications.

Modern gas turbine compressor section

Present day gas turbine development may be lacking the precursor vetting of advanced technology concepts in military engines, and many innovations are occurring comparatively more concurrently across aero engines and heavy duty power generation gas turbines.

The accelerated product development approach in both the AE and HDGT industries may be driving higher cost outcomes for insurers, lenders, owners/operators, and, in the case of power generation, the EPC contractor, due to more frequent disruptions in operation, and greater maintenance costs than were originally anticipated.

Faster compressor degradation

A side effect of modern compressor technology, is faster compressor degradation rates, and faster life consumption for various items of critical hardware such as turbine blades, vanes, ring segments (shrouds), seals, and rotor disc dovetails.

For example, upgraded F-class engines and other more advanced heavy duty gas turbines now have fewer compressor stages, and airfoils that are thin and 3D shaped (in contrast to older compressor technology). This combination exposes the compressor to faster erosion, abrasion and corrosion effects, leading to compressor degradation (ie, requiring more work from the turbine, and entailing greater fuel consumption), faster thinning of airfoils, and contamination of cooling-air bleed extractions with particle matter from abrasion, corrosion and erosion, thereby exposing the downstream combustor, turbine hot gas path, and rotor hardware to greater risks of cooling-hole clogging.

As a result, modern day compressor technology performance has more direct negative consequential effects on hardware endurance and engine reliability. Therefore, compressor performance related issues are actually driving higher overall maintenance costs.

Despite this link to advanced compressor technology, present day long term service agreements (also known as contractual services) between OEM and owner/operator specifically exclude compressor related maintenance scope.

Perplexing omission

The absence of compressor maintenance scope in typical LTSA contracts in the power industry, is rather perplexing, considering that there are several additional features that may require more systematic and specialised maintenance services.

An example of this is multiple variable guide vane technology, and ensuring the structural integrity of mechanical linkages.

Modern compressors also make extensive usage of coated surfaces (super-polished for performance), which pose greater risks of wear and abrasion, with entry of coating matter into the flow path, caused by residual swirl (mismatch between angle of attack and airfoil leading edge associated with wider load variability), and thinning of the highly loaded thin-section airfoils.

The industry must review and update LTSA maintenance to include modern heavy duty gas turbine compressor maintenance scope, rather than continuing with LTSA concepts grandfathered for older compressor technology. This would help in achieving heavy duty gas turbine reliability and life cycle cost objectives.

Author information: Vinod Kallianpur, Samsung C&T, Seoul, Korea.

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