Phosphors light way to longer life

3 June 2003

By embedding phosphors in gas turbine thermal barrier coatings they can be made "smart", able to indicate their temperature and condition by phosphorescence under UV light. This opens up new, and much needed, possibilities for NDE and condition monitoring.

"There is no current way to assess wear of components within the hot gas path in real time without shutting down the turbine" and consequently "maintenance and component replacements are scheduled on conservative design practices," said a 2000 press release from the US Department of Energy's National Energy Technology Laboratory.

But all this could change in the not too distant future if technology under development by Southside Thermal Sciences (STS) Ltd a start-up company spun out of Imperial College, London can deliver what its inventors promise.

The relentless increase in firing temperatures and the effects of market liberalisation have placed a new emphasis on the need for monitoring of gas turbine hot gas path components, says STS.

In particular, in order to use thermal barrier coatings (TBCs) to their maximum thermal insulation capacity and provide flexible and reliable gas turbines for operators, a new approach, not only to condition monitoring but also life prediction for hot gas path components, is required, the company believes. Existing condition monitoring systems are simply not up to it, they say.

The basic concept of the STS Sensor Coating approach (invented by Dr Jörg Feist during PhD research at Imperial College under the supervision of co-inventor Dr Andrew Heyes) is to embed small amounts of ceramic phosphor into standard thermal barrier coating materials creating a "smart TBC". It so happens that thermographic phosphors (as used on TV screens, for example) consist of a ceramic host doped with a rare earth ion, while a gas turbine thermal barrier coating typically consists of yttrium-stabilised-zirconia (YSZ) doped with the same rare earth ions, eg, YSZ:Eu, YSZ:Dy.

When illuminated with UV light the Sensor Coating phosphoresces, which can be used to provide an indication of:

• temperature within the coating;

• erosion of the coating;

• degradation of the coating.

Temperature measurement (which has been successfully done at 1100°C) can be based on the rate of decay of the phosphorescence, which is temperature dependent (increasing with temperature, see plots opposite). Erosion is detected when dopant layers are found to be missing. Degradation, due to the phase change from t'-phase to monoclinic when a typical YSZ thermal barrier coating is exposed to high temperatures, can be detected by using the fact that the phosphor activators can respond, albeit weakly, to changes in the atomic environment and thus could pick up phase changes.

STS's initial objective is to "develop a prototype Sensor Coating that satisfies the coating engineers as well as the monitoring people." Development of the associated instrumentation will be limited to the modification of already existing equipment. This could be done by using pyrometer systems for on-line temperature monitoring or modified boroscopes for inspections of coating quality during minor maintenance intervals.

With the potential capability for parallel measurement of erosion, degradation and temperature, in both off-line and on-line mode, the Sensor Coating technology of Southside Thermal Sciences "can provide turbine OEMs and operators with a tool to meet tomorrow's requirements in condition monitoring", say its developers. Its use off-line will also provide a valuable addition to the NDE armoury. In addition the Sensor Coating is ideally suited to providing reliable information inputs for analytical tools such as the Hot Section Life Management Platform of EPRI or the Logistic Support Analysis Record (LSAR) system of Rolls-Royce.

By measuring temperatures and collecting life prediction data during operation, the Sensor Coating will enable turbines to run at optimum/higher temperatures, with consequent efficiency gains, while minimising the risk of exceeding temperature limits and avoiding failure of protective coatings. As STS says, "currently, there is no coating on the market which can indicate its own remaining lifetime!"

Failure of the coating can be catastrophic and very costly. The price of a single blade can easily reach $10 000, replacing a row can cost $2-$3 million and if a blade breaks off during operation and causes further damage downstream, the cost can be in the tens of millions of dollars. Overall, STS estimates the total power plant gas turbine services market at over $10 billion per year.

Competing technologies

Several methods are already in use for inspecting and monitoring hot section components but they can measure only one or two parameters simultaneously.

The most widely used commercial temperature monitoring devices are pyrometric systems and thermocouples. Pyrometry is an optical technique that can be used on running engines to measure blade temperatures, while thermocouples are intrusive and not well suited for temperature measurements on blades.

Thermographic phosphors have been used for temperature detection, but generally in research rather than commercially. They are usually applied as paints and thus have a limited lifetime of a few hundred hours or less.

Looking at other monitoring methods, boroscope inspection is the most common technique for analysing hot section components, but it can only be used when the turbine is not in operation. Similarly, eddy currents are used to measure coating thickness, but only off-line.

For stress and structural changes, piezo spectroscopy can be used but the light intensities are weak and on-line monitoring may be impossible.

Other metallurgraphic techniques are usually of a destructive nature and components cannot be reused after testing.

Coating degradation due to phase concentration changes can be detected today by employing X-ray diffraction techniques. But a phosphorescence-based approach promises a cheaper visual inspection tool (perhaps using modified boroscopes during outages), which is easier to deploy than X-ray equipment (although no specific development programme is yet underway on this).

The US Department of Energy (DoE) has identified development of "smart" materials for on-line, non-destructive evaluation of ceramic coatings as one of its key priorities for gas turbine research, with work currently underway at Oak Ridge and Argonne. On the OEM side, the DoE currently sponsors research at General Electric and Siemens Westinghouse on monitoring of hot gas path components using combustor flame sensors and pyrometric technology.

However, there seems to be no direct competitor with a product on the market.

The core business of STS, which started operations in October 2002 and owns the intellectual property on which the Sensor Coating is based, will be built on licensing the tehnology to major gas turbine manufacturers. As well as O&M applications, the Sensor Coating could also have uses in manufacturing and design, its developers believe.

While currently the main focus is applications in gas turbines, further potential markets are envisaged. STS says its "eventual goal is the use of the knowledge-based, multifunction coating as a tool for condition monitoring and life prediction which will optimise the life-cycle of industrial systems."

The company is led by Jörg Feist and Udo Dengel. Two experienced industry professionals were recently appointed as non-executive directors. In November 2002 the company received the European Technology Innovation Award from The Wall Street Journal Europe.

For more information contact [email protected] or [email protected]

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