Sensor Technology - Trends

For most people, the word "turbine" probably brings to mind images of jet engines. Yet the propulsion unit of a jumbo jet is tiny in comparison to the turbines used to generate electricity in a power plant. In a gas turbine, for example, the rotor alone can weigh anything up to 75 t—as much as a diesel locomotive. Mounted onto the rotor is an arrangement of rims with increasingly smaller blades that suck in and compress air before forcing it into the combustion chamber, where the energy of the fuel is converted into heat. As it expands in the downstream turbine, the hot exhaust gas drives the rotor, which is also connected up to the generator used to produce electricity.

Powered by a stream of exhaust gas heated to a temperature of up to 1,500 °C, the turbine blades rotate at a speed of around 3,600 r.p.m. Only high-tensile alloys which are cooled and covered with a protective ceramic coating are able to handle such stresses. Power-plant turbines are therefore expensive acquisitions. In fact, a single turbine blade costs as much as a family car. For the power plant manager, monitoring of these parts is enormously important—and difficult. On the one hand, the gap between the blades and housing must be as small as possible in order to achieve a high energy yield; on the other, any contact must be avoided as this reduces efficiency and damages the blades.

Siemens Power Generation (PG) in Berlin has now developed a sensor system that measures the so-called radial gap—the distance between the blade tips and the turbine wall. A sensor mounted directly in the turbine housing determines the distance to the rotating turbine blades on the basis of changes in electrical capacitance between itself and the blade tips speeding past. A hydraulic system is then used to move the rotor into an optimal position. "We’re talking here about rotors of between two and three meters in diameter," says Olaf König, Manager of the Berlin Test Center, "and a radial gap of only a few millimeters." The sensor must therefore be able to determine the position of the huge rotor to a 10th of a millimeter—and do so at extremely high temperatures.

Online diagnosis via camera. The extreme temperatures within a turbine are one of the greatest impediments to the use of complex measurement technology. In a project led by Dr. Hans-Gerd Brummel, development engineers at Siemens Westinghouse Power Corporation in Orlando, Florida, have equipped their sensor unit—a high-speed infrared camera plus a flange-mounted optical probe—with a refined cooling system. This is because the camera is directly exposed to the immense heat of the turbine blades, the heat-protective coating of which can reach temperatures of 1,200 °C. The aim is to continuously monitor the condition of the blades during turbine operation, as the ceramic coating has a tendency to flake with use, which naturally shortens their service life. In order to preempt blade failure and the enormous damage potential that this entails, power-plant operators generally replace blades after a certain number of operating hours—a procedure associated with high costs and damaging downtimes. By contrast, the new system opens up a whole new dimension of online diagnosis. Previously, temperature and pressure measurements were the only means of drawing imprecise conclusions as to events inside the turbine, since the speed of the blade tips (approximately 1,400 km/h) plus the extreme temperature and high pressure prevented any direct access.

Tim Schröder