The gas turbine industry must focus on several key factors that will make

its future power generation technology successful in the electric power

generation market sector. These factors are as follows:

• competitive economic performance (i.e. higher efficiency and

optimised life-cycle cost);

• reliable operation under a cycle duty (repeated gas turbine startups

and shutdowns);

• increased dependability of current and future plants (reliability,

availability, maintenance and durability, or RAM–D);

• the ability to meet regulatory emissions levels and achieve high

thermal efficiencies; and

• reliable fuel-switching capability and fuel flexibility.

Gas turbines will be one of the most important horizontal technologies

and will play an essential role in meeting these requirements. It is

considered a horizontal technology due to its capacity to be integrated

into multiple power plant configurations, while running with different

fuels (coal gas, natural gas, hydrogen, liquid fuels, etc.).

The Spanish electricity utility Endesa is participating in the promotion of

initiatives to improve gas turbine technology, in particular the European

Turbine Network, which is dedicated to the application of highly efficient

and environmentally friendly technologies. Following the 3rd International

Conference on The Future of Gas Turbine Technology in Brussels, the

following conclusion can be made: “Gas turbine technology is one of the

best available options today and in the years to come for power

generation, however, they will continue to be affected or influenced by

their user´s technology and development needs pending resolution.”

This statement is reinforced by the fact that, in a deregulated and

increasingly competitive power generation market, power producers are

continually asking themselves, “How can we get the edge over our

competitors? How can we improve our decision-making processes?

How can we continually operate our plants in the most efficient and

cost-effective way? How can we limit damage and improve availability?

How can we reduce maintenance costs and extend service life? How can

we know fixed asset remaining value throughout power plant life?”

The key issues are the development of gas turbine assets and

performance management; these are the ways to achieve competitive

advantages that will enable companies to get the edge over their

competitors. The focus is therefore on gas turbine technology. The ‘hot

gas path’ of a gas turbine is the core of the engine, which includes the

combustion chamber, the transition piece and the turbine section. The

main drivers for improving hot gas path behaviour are:

• gas turbine performance – this is highly dependant on the turbine

entry temperature, which results in a greater need for the hot gas

path components to achieve high thermal efficiencies with low

nitrogen oxide (NOx) emissions; and

• gas turbine life-cycle cost – this is strongly affected by hot gas path

cost and maintenance, which gives rise to maintenance practices

and inspection techniques that in turn allow the improvement of

gas turbine dependability, i.e. its RAM–D.

Background

The blades and vanes in the turbine section will to a large extent

determine the ultimate efficiency of the gas turbine. These parts have to

work under extreme conditions, operating in high temperatures in an

oxidising environment while being subjected to large thermal and

mechanical stresses. In order to increase the durability of the blades and

vanes in these extreme conditions, special metal superalloys have been

developed. The high-quality technologies used in the manufacture of the

turbine blades make them the most expensive parts of the gas turbine.

In order to achieve higher thermal efficiencies, higher combustion

temperatures are needed; however, higher combustion temperatures –

from around 1540ºC (2,800ºF) – exacerbate NOx emissions. To combat

excessive NOx emissions, oxygen is limited during the combustion

process, but this can lead to unacceptably high levels of carbon

monoxide and unburned hydrocarbon emissions. Further adding to these

technological limitations, extremely high operating temperatures –

greater than 1,290ºC (2,350ºF) – are beyond the material tolerances of

the turbine blades and vanes.

Therefore, the goal of achieving 60% efficiency while staying below

10ppm NOx emissions is constrained by the thermal, emission

reduction and material limits of the gas turbine system. There are four

main innovations that are critical in meeting this need for high

efficiency and low emissions: closed-loop steam cooling; single-crystal

superalloy casting; thermal barrier coating; and lean pre-mix dry low-

NOx combustors. In order to optimise the life-cycle cost of gas turbines,

special attention must be paid to the hot gas path components:

typically, around 70% of the total maintenance cost corresponds to

schedule, maintenance, parts and materials. This will lead to the

establishment of mechanisms for risk mitigation, such as long-term

service agreements (LTSAs), business interruption insurance, extended

guarantees and part-cost guarantees. Apart from the above

Towards the Future of Gas Turbine Asset and Performance Management

© T O U C H B R I E F I N G S 2 0 0 7

Turb

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Dr Tomás Alvarez Tejedor is Head of the EndesaCombined Cycle Technology and Maintenance

Department. He has been working in the Spanishelectrical market for more than 15 years, covering R&Dprojects on advanced power generation systems, asset

management and combined cycle power generation andgas turbine technology. Dr Alvarez obtained his BSc,

PhD and MBA at the Polytechnic University of Madrid,his MSc in the Gas Turbine Engineering Group at

Cranfield University, and his postgraduate specialisationon the Spanish electrical sector at Carlos III University.

a report by

Tomás Alvarez Tejedor

Head, Combined Cycle Technology and Maintenance Department, Endesa Generación

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Towards the Future of Gas Turbine Asset and Performance Management

considerations, it is also necessary to take into account current

operational conditions in a deregulated electricity market. These

conditions require more flexible operations with high efficiency and

low emissions for the whole power range, high operational reliability

and better maintainability.

Hot Gas Path Management

From the condition monitoring, instrumentation and control

standpoint, many improvements can be made to hot gas path

management. The ultimate goal is to manage the hot gas path section

by knowing its condition (known as condition-based maintenance, or

CBM) and its performance (known as performance monitoring). In

order to get in-depth knowledge of the condition of the hot gas path

components, it is necessary to combine both online and offline

techniques: together, these will show the real status of the main

components of the hot gas path section. In order to look at these

subjects in more depth, we need to divide the process into different

levels that allow us to set the needs assessment findings for each.

Level 1: Sensor Level

Typically, gas turbine units are equipped with minimal instrumentation.

On the one hand, this means the supplied sensors are adequate to ensure

safe operation and to monitor performance and emission requirements,

but on the other hand these units are not equipped with any

instrumentation that would provide the ability to: optimise performance;

define the risk of extending operating periods; monitor component

degradation; provide early warning of faults in the system; and monitor

hot section environment and failure mechanisms. In terms of needs

assessment findings, the following diagnostic instrumentation is required:

• combustion pressure pulsation;

• flame temperature sensors (semiconductor photodiode);

• fuel low-heating value (LHV) measurement (LHV sensor);

• turbine blade surface temperature (optical pyrometers);

• turbine blade vibration (optical probes);

• turbine blade tip deflection (blade tip clearance sensor);

• air inlet mass flow (ultrasonic sensor);

• turbine circumferential inlet temperature distribution (optical

fibre thermometers, high-temperature research and technology

development); and

• coating life degradation sensor (odometer, infrared sensor).

Level 2: Control and Supervision

Once the output from the instrumentation has been acquired, it can be

input into the local unit’s control station, which consists of a PC

equipped with Original Equipment Manufacture (OEM) software. This

provides the operator with a series of windows-based viewing screens

that present measured and calculated data. Factored starts and hours

are determined by empirical formulae provided by OEM. The power

industry is not completely comfortable with this approach due to higher

than desirable maintenance costs for gas turbines, and generally does

not have confidence in OEM’s stated hot gas path component life and

replacement interval. The industry would prefer to have a more

machine-specific, condition-based approach to determining the timing

of maintenance. As a result, control systems play a critical role in data

collection, conditioning and analysis within the automated CBM

infrastructure. Advanced process control systems include analysis

algorithms that enable them to diagnose and report malfunctions to the

CBM system. This ultimately supports system-wide asset management.

In terms of needs assessment findings, the following information

technologies are required for data manipulation:

• Online data acquisition – the data acquisition module provides

system access to digitised sensor or transducer data. It may

represent a specialised data acquisition module that receives

analogue feeds from sensors, or it may collect and consolidate

sensor signals from a data bus.

• Data processing and validation:

• signal-processing approach:

• signal correlation;

• high pass filtering; and

• correlation matrix and response statistics.

• physics-based approach:

• correlation matrix and response statistics;

• statistical neural network; and

• fuzzy logic rule based.

• Data-fusion techniques – a formal framework used to express

convergence data from different sources and the means and tools

for the alliance of these data.

• Data-mining tools – these provide new insights into wear and

failure mechanisms in engine components:

• neural nets;

• statistical analysis; and

• generic algorithms.

• Advanced control algorithms:

• predictive control algorithms for combustion instability;

• adaptative controller;

• closed-loop steam cooling control;

• active control technologies for enhanced performance,

enhanced reliablity and reduced emissions;

• fault-tolerant engine control (smart sensors and actuators); and

• closed-loop optimisation.

Level 3: Condition Monitoring

The primary function of the condition monitor is to compare features

against expected values or operational limits and output enumerated

condition indicators (e.g. level low, level normal, level high, etc.). The

condition monitor may also generate alerts based on defined operational

limits and, when appropriate data are available, may generate

assessments of operational context (current operational state or

operational environment). Ultimately, this would lead to automatic

assessment of the condition of the hot gas path section, thereby

reducing human inspection tasks and the unnecessary maintenance that

occurs in a traditional periodic maintenance scheme. System assessment

would also provide valuable realtime decision support data for

operational planning. In terms of needs assessment findings, one of the

unmet needs at this level is component life monitoring, either through

direct or indirect monitoring of component properties:

• Online monitoring of component life would allow some

assessment of when the next shutdown might occur.

• Online indication of component degradation could alert operators

to failures that could propagate through the unit. For example,

online monitoring of the combustor status or blade coating

integrity would alleviate downstream consequences.

• Offline non-destructive evaluation (NDE) of component life would

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Towards the Future of Gas Turbine Asset and Performance Management

help determine if component replacements are needed before the

next scheduled shutdown:

• eddy current metallic coating life (aluminium depletion versus

conductivity); and

• time-base corrector (TBC) integrity (NDE methods).

• NDE measurements and supporting analysis to determine:

• physical condition of components referenced to baseline;

• cyclic fatigue status of components; and

• component coating wear and integrity status.

• Online assessment of the risks of extending the outage schedule

would also be useful to determine whether it is possible to operate

for extended periods.

• Sensors that map the blades and vanes for integrity. For example,

a temperature profile of the blades and vanes could indicate

blocked cooling passages or coating failures.

• Online monitoring of exhaust gases for metal particles.

• In situ repair technologies (TBC repair).

Level 4: Performance and Health Assessment

The primary function of the performance and health assessment level

is to determine whether the health of a monitored system, subsystem

or piece of equipment is degraded in terms of its thermodynamic and

mechanical condition. If its health is degraded, this assessment level

may generate a diagnostic record that proposes one or more possible

fault conditions with an associated confidence. This level should take

into account trends in the health history, operational status and loading

and maintenance history of the system, subsystem or piece of equipment.

The needs assessment findings are as follows:

• Combustion process diagnostic module – automated assessment

of exhaust gas temperature (EGT) spread, fuel flow manifold and

supply pressure, vibration/dynamic pressure, emission data, etc.

• Hot gas path analysis – determination of hot gas path condition

based on the thermodynamic relationships that exist between the

engine components and various gas path performance parameters.

• Hot section damage assessment – automated trending and fault

pattern classification and fusion.

• Aero-thermal performance-based module.

Level 5: Prognostics

The primary function of the prognostics level is to project the current

health and performance state of equipment into the future, taking into

account estimates of future usage profiles. The prognostics level may

report health and performance status at a future time or may estimate

the remaining useful life of an asset given its projected usage profile.

Assessments of future health or remaining useful life may also include a

diagnosis of the projected fault condition. Prognostics therefore allow us

to predict the onset of hot gas path component failure to match its use

or to enhance maintenance support. Prognostic capabilities expand

support options and allow for cost-effective planning and management.

The needs assessment findings are as follows:

• Life consumption tracking module of hot gas path components:

• assessment model for coating degradation;

• assessment model for creep fatigue damage; and

• assessment model for thermal mechanic fatigue.

• Hot gas path components life-cycle prognostics.

• ‘What if’ analysis for the performance and health of the hot gas

path components.

Level 6: Decision Support

The primary function of the decision support module is to provide

recommended actions and alternatives and to advise on the implications

of each recommendation. Recommendations include maintenance action

schedules, modifications to the operational configuration of equipment in

order to accomplish mission objectives or modifications to mission profiles

to allow mission completion. The decision support module needs to take

into account operational history (including usage and maintenance),

current and future mission profiles, high-level unit objectives and resource

constraints. The needs assessment findings are as follows:

• Computer-based gas turbine condition- and health-monitoring

predictive systems might offer the potential for providing decision

support for the following items:

• reduced nuisance shutdowns and unplanned outages;

• optimum engine operation;

• continuous realtime maintenance scheduling;

• extended time between overhauls based upon determination

of remaining component life;

• protection against catastrophic failure via realtime fault

assessment; and

• estimating operations and maintenance (O&M) cost based on

condition monitoring.

• Automated logistics for advanced scheduling and co-ordination of

maintenance actions. Advanced triggering of logistics support

improves system availability and utilisation of resources.

Level 7: Human Interface

Typically, high-level status reports (health assessments, prognostic

assessments or decision support recommendations) and alerts would

be displayed at this level, with the ability to access information from

lower levels when anomalies are reported. In many cases, the human

interface level will include multiple layers of access depending on the

information needs of the user. The needs assessment findings are:

• More user-friendly human interfaces are required to supply

actionable information to the operator and maintenance staff:

• actionable information provides users with the necessary

details for effective decision-making; and

• actionable information should be in the form of ‘what

happened, where, when, how bad is it and what should be

done about it?’.

Conclusion

Future advanced thermal power plants will need to be highly complex

in order to fulfil the requirements of a global society that is increasingly

sensitive to environmental issues. Gas turbine technology will play a key

role as the core technology for different plant configurations. Gas

turbine asset and performance management will allow the industry to

develop the competitive skills required for the new power generation

arena, with the aim of optimising life-cycle cost by improving both gas

turbine dependability and performance. ■

A longer version of this article, containing graphics, can be found

in the Reference Section on the website supporting this briefing

(www.touchbriefings.com).

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