DC electrical resistance techniques (potential drop methods) are well-established methods for detecting both corrosion and crack growth during laboratory-based experiments. However, application of the techniques to large industrial plant has, to date, been somewhat limited. Rowan Technologies has, over the past nine years, been developing and refining its scanner technology to monitor corrosion/erosion, thermal parameters and more recently crack initiation and propagation on an EPRI-sponsored project. Crack monitoring is achieved by welding matrices of electrodes to the external (air-side) metal surfaces, from which a number of high-precision measurements are made to detect and quantify changes in electrical resistance associated with crack growth. Extensive laboratory-based work preceded a full-scale trial of the scanner on an 830MW supercritical boiler in the USA, where circumferential cracking of weld overlaid boiler tubes is of major concern. This paper discusses the background to the technique, reports on the laboratory-based results of simulated crack growth and also gives initial results from the boiler trial.

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A research and development project has recently been carried out to develop ceramic thermocouple probes (CTPs) capable of measuring temperatures up to 2000°C and rugged enough to withstand extended service in high-temperature gas turbine environments. Existing metallic thermocouple technology cannot withstand such conditions for sustainable periods of time. Following initial laboratory studies, CTP trials were carried out in power generation boilers (Farrell and Higginbottom, 1995). Prototype CTPs were subsequently developed for evaluation in gas turbine (GT) combustors (at atmospheric and elevated pressures) and in a Spey engine (Patent, 1996). The CTPs performed well under the harsh conditions imposed, demonstrating their mechanical integrity and consistency/sustainability of signal output. Initial studies have also been carried out with a view to applying ‘thin-layer’ ceramic thermocouples directly onto thermal barrier coatings to give surface temperatures on stator or other hot gas surfaces, and are briefly mentioned. Rowan Technologies and TÜV Energy Services are currently looking for companies interested in exploiting this new ceramic thermocouple technology.

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