High-Speed Thermal Imaging Captures Stills of Turbines
Brent D. Johnson
Siemens Westinghouse makes land-based steam and combustion turbine engines for plant applications such as the production of nuclear power, conventional coal power and natural gas power that burns gas directly. For combustion turbines, very high pressure and high-velocity combustion gases rotate the turbine engines at 3600 rpm, creating blade speeds in excess of 1000 mph and generating as much as 230 MW for a single turbine generator.
The Phoenix camera from Indigo Systems captures medium-wavelength IR images, such as these turbine blades in operation to help monitor the status of the thermal barrier coating that protects the blades from combustion gases.
The turbine blades are nickel-based superalloys with a thermal barrier coating that protects them from combustion gases at temperatures as high as 2900 °F, which is the melting point of steel. The coating also protects the blades with a layer of ceramic so that the metal temperature does not exceed 1700 °F. However, in this extreme environment, failure of the coating can cause the turbine to age very quickly. Something that's designed for a two-year maintenance sequence may require repair much sooner.
To avert potential problems, the entire system must be shut down for eight to 10 hours at a stretch, and a flexible borescope that inspects the blades for damage is lowered through an access port in the turbine. The most common defects are disbonds, or separation of the ceramic from the base metal. Because this approach significantly restricts power production, Siemens engineers wanted an inspection system that would allow them to test the turbine while in operation. This meant imaging the blade at a shutter speed of 1 µs to capture delamination of the coating.
The company hired consultants from Wayne State University in Detroit to choose the best camera. After evaluating a number of systems, the Manufacturing Research Institute at the university gave the nod to Indigo Systems Corp.'s Phoenix Mid camera for its sensitivity at extremely high speeds.
"We knew Indigo Systems made state-of-the-art cameras for high-speed events like this in the near- or mid-IR," said Siemens engineering manager Paul Zombo.
The company is building one of the first production systems that will integrate the camera with one of its own industrial gas turbine engines. It has been working with Indigo, of Goleta, Calif., for about two years and has tested a full-size prototype using the Phoenix Mid.
The camera can thermally image in the 3- to 5-µm waveband at shutter speeds as fast as 1 µm, and it uses indium-antimonide detectors in a 320 x 256-pixel focal plane array. The array uses an ultralow-noise, high-gain readout integrated circuit to allow imaging with very high sensitivity. The camera head is connected to a digital acquisition system that records data at rates of up to 40 megapixels per second.
Zombo said that the initial images captured are surprisingly good and that the system has generated very clear thermal images. "We always think we are going to see smear, but this is better than our mental models," he said.
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