Effective asset management of gas turbines in Australia has been prevented due to a lack of understanding of hot gas path component degradation processes, life prediction methods and effective repair/refurbishment strategies. Without recourse to such knowledge, gas turbine users have had to rely on original equipment manufacturer (OEM) recommendations for component longevity and performance. Consequently, gas turbine users often dispose of hot gas path components based on elapsed time rather than on their condition.

Following feedback received at the annual IBC Australian Gas Turbine Users conference Quest Reliability (formerly MPT Solutions) and the Australian Nuclear Science and Technology Organisation (ANSTO), as participants in the Intelligent Diagnostics Program of the Cooperative Research Centre for Integrated Engineering Asset Management (CIEAM) devised a three-year research project to understand:

- How hot gas path components degrade in service - How life prediction methods can be employed to extend component life - How effective repair/refurbishment strategies can be employed to reduce the cost of replacing components.

Industrial Gas Turbines

In recent years the use of industrial gas turbines has grown substantially as they produce a large amount of energy relative to their size and weight. Gas turbines also offer the flexibility of being able to run on multiple fuels such as natural gas, diesel fuel, naphtha, vaporised fuel oils, biomass and syngas.

In the last ten years the world-wide industry has chosen to adopt combined cycle power plants, placing gas turbine technology at the heart of power plant operation. Over 150 gigawatts of power plant have been installed replacing the old steam turbine plants which were previously the backbone of the fossil fuel power industry. This trend is set to continue.

The largest power turbines in use in Australia are the ‘Frame Type’ gas turbines. These industrial units range from 3 to 480 megawatts with efficiencies approaching 50 per cent, and up to 60 per cent in combined cycle mode. Considerable research into new materials and coatings and improvements in cooling design and cooling systems has enabled these efficiencies to be achieved.

Gas turbine construction consists of a compressor section where filtered air is compressed and fed into a combustion zone where a fuel and air mixture is combusted. Combustion is accompanied by a rapid expansion of the hot gas produced, which enters the turbine stages and produces rotational motion.

The zone from the combustor to the turbine exhaust is termed the ‘hot gas path’. The components within the hot gas path have to withstand the most severe combination of temperature, stress and environmental conditions. Hot gas path components, such as combustors, transition pieces, liners, seals, nozzles (vanes), turbine blades (buckets) and discs generally represent the highest risk of failure and are the major life-limiting components. Turbine blades, having the most severe combination of stresses and temperatures, are often the limiting component when it comes to setting operational parameters and determining maintenance and overhaul schedules.

Although there are significant maintenance cost savings to be made by the rejuvenation or repair of components, even further cost reductions may be made using condition and life assessment data to safely extend component life.

Interaction with gas turbine users

Interaction with the gas turbine community was essential in prioritising the research tasks and also resulted in placing emphasis on the need for gas turbine disc life management methodologies to be researched. Several major power and oil companies assisted by supplying ex-service components and detailed operating histories. The components supplied included nozzles, buckets/blades and disc sample materials.

The project was brought to a successful conclusion in 2007. It has delivered outcomes in several areas:

- Life management strategies for hot gas path components - Effective condition assessment methods for determining thermal exposure in relevant Ni-base superalloy hot gas path components thereby enabling remaining life to be determined - Initial retirement for cause assessment methodology for steel turbine discs including proof of concept by case study. - An understanding of the economic benefits to be gained from utilising the appropriate repair of hot gas path parts for specific industrial units to extend useful life.

These outcomes were achieved by conducting applied and fundamental research that involved studying creep properties employing materials modelling and developing an understanding of grain boundary engineering. This work was undertaken with support from the New Zealand Government FRST program. The mechanical properties of all components are fundamentally determined by the microstructure of the materials and coatings used. Gaining an in-depth knowledge of this microstructure at the nano-level was key to understanding both the time and stress dependent degradation processes experienced by hot gas path components.

Benefits for gas turbine users

The research work performed in this CIEAM project successfully established a technology platform to enable Australian gas turbine users to understand how hot gas path components degrade in service, how life prediction methods can be employed to extend component life and how effective repair/refurbishment strategies can be employed to reduce the cost of replacement parts.

Hot gas path components comprise the major maintenance and overhaul costs associated with operating gas turbines. Knowledge of remnant life can be utilised in the risk assessment process for high value parts including turbine discs, nozzles, blades, combustion liners and transition pieces. Having the ability to be able to use repaired or refurbished hot gas path components which can be less than 25 per cent of the cost of purchasing new or being able to run existing components for an extended period provides significant cost saving benefits in the order of millions of dollars.

The outcomes of this research have been, and continue to be, promulgated to the Australian gas turbine community through a series of presentations and workshops. The benefits of adopting a life management approach as part of an effective gas turbine asset management strategy have been successfully demonstrated from feedback received from Australian gas turbine users.