Better blades

A new system that continuously monitors operating blades in gas turbines has surpassed 5,000 hours of operation, confirming that the technology is ready for commercialisation.

Developed by Siemens Power Generation, with funding from the US Department of Energy (DOE), the online monitor makes it possible for operators to replace turbine blades based on their actual condition, when the thermal barrier coating is worn or damaged. This capability optimises the life of the blades, avoids the cost of unscheduled replacement, and extends the time between preventative maintenance periods, increasing the availability of the plant.

The technological breakthrough could help keep electricity rates down by saving gas turbine utility operators an estimated $600 million per year.

The first full-scale high-temperature, full-pressure commercial system was installed in a Westinghouse 501FD gas turbine at Empire Stateline Electrical Company in Joplin, Missouri, during a scheduled outage last October to demonstrate its capabilities in capturing real-time infrared images of rotating blades. Siemens engineers are monitoring the performance of the first row of thermal-barrier-coated blades on both pressure (front) and suction (back) sides.

The demonstration will evaluate the mechanical design and integrity of the blades’ thermal barrier coating, camera performance, environmental enclosures, spectral filter, integration and development of the supervisory system, and the life model that predicts when the thermal barrier coating on a blade will fail.

Turbine blades operate in an exceptionally hostile environment; they rotate at more than 3,600 revolutions per minute, with a linear tip speed of 800 mph, under very high pressure (220 psi, nearly 15 times the force of gravity or about the same as under 450 feet of water) and extremely high temperature (in the range of 2,600 degrees F, about the melting point of steel).

To operate in these conditions, Siemens Westinghouse developed a cooled optical probe that is installed in the gas turbine reaching down to the moving blades. A near- and mid-wave infrared high-speed camera is also situated in a cooled housing and connected to the probe outside of the turbine.

Thermal radiation from the blades provides sufficient energy for the infrared camera to obtain images of the moving blades. And since such cameras have a short integration time, in the magnitude of one millionth of a second, the fast blade rotation does not blur the images. Control software can identify and record 85 percent of the blade surface and has the spatial resolution to capture very small design features. The images are evaluated automatically, and the entire system can be linked to remote diagnostic centres.

The ongoing success of this commercial demonstration is garnering keen interest by the energy industry and the military. Siemens has already ensured that this technology will be available on its next generation of gas turbines, and online blade monitor retrofits are currently being marketed for existing gas turbines.

Commercialisation of the technology is expected within 8 to 12 months for gas turbines, and the versatility of the technology may not yet be fully realised. By successfully operating in the most complex environment, the technology has proven it could monitor the performance of components in additional hostile areas of the turbine, such as combustor baskets and stationary vanes.

While DOE involvement in the demonstration is scheduled to end in September 2005, Empire Stateline will continue to use the blade monitoring technology at its Joplin plant. The site will serve as a test bed to expand commercialisation of the online monitor for gas, steam, and other types of turbines.