Olkiluoto, the world’s largest and most modern nuclear power station, with an output of 1.600 Mwatts, relies on German technology. During the final assembly of turbine manufacturing at Siemens, blades are installed to the rotor and measured in the housing. This is a balancing act between optimal efficiency and necessary safety margins between blades and housings involving measurements of just tenths of a millimeter.
The steam turbine, which is more than 12 meters long and has a diameter of 6.7 meters, is currently waiting for the final cross section before distribution to the Olkiluoto 3 nuclear power station in Finland. The freestanding blades of the low-pressure turbines are aligned to tenths of a millimeter, despite their mass and weight. “The deciding factor for these blades is their distance to the housing,” says Stefan Hoeyng, plant engineer for final assembly of steam turbines at Siemens AG Power Generation. The gap between turbines and housings must be kept as small as possible, as the larger the distance, the more steam will pass through and this lowers the efficiency. Turbine blades are not allowed under any circumstances to come in to contact with the housing as this causes damage.
Better efficiency with the employment of laser trackers
During the final assembly, large disks are shrunk and inserted into the blade’s groove on the rotor. Olkiluoto´s low-pressure turbine has a blade weight of about 320 kilograms. When the rotor is bladed, it is loaded in the force shelter with excess rotation speed to obtain an appearance of compression. Subsequently, the freestanding blades are measured. “The more exact a measurement system is, the smaller the tolerances for the distance between the blades, which improves efficiency,” explains Hoeyng. An accuracy of at least 1/10 millimeters was therefore the requirement for the new measurement system. After some tests, Siemens decided on the API Omnitrac, thanks to its ease of use and lightweight portability of just 8.5 kilograms. If the housings are to be measured in the final assembly, the laser tracker must reach heights of 3 to 5 meters, and a lightweight system is a lot easier to handle in such tasks. It is planned that the laser tracker will be additionally used on construction sites, for example when measurement revisions in the power station housings are required.
Custom-made measurement software
Additionally, Siemens has a specialized and easy-to-use software to measure turbines. The laser tracker can also be used by untrained employees. “If I want to measure distances and geometry with the 3-D coordinate system, then I need to perfectly understand the system and be able to work with it every day, otherwise I will have too many errors,” states Hoeyng. A strict program sequence leads the user step-by-step through the measurement task. Metrolog, an open-measurement software also supplied by API, delivers the basics. API has developed a measurement routine in close cooperation with Siemens in order to check measurements.
Collaboration with system suppliers
As Hoeyng explains, Siemens have already provided internal measurement procedures, however not to this extent and in such detail. Therefore, API’s offer to deliver the hardware, software, and programming contributed to Siemens’ final decision to purchase. “In principle I needed something to practice with in order to be able to use the laser tracker to recapture the laser beam, but this is not difficult with the software operation,” adds Steffen Linnemann, API account manager. The measurement project was released internally for this measurement. The measurement system is suitable for the task if different operators work with the system and want to reproduce different blades or acceptable values. The laser tracker completely fulfilled these requirements and is officially used in combination with the custom-made software for blade measurement.
Better results in less time
Documentation and indication of the single blades takes place automatically. This was one of the main requirement specifications for the custom-designed software. The laser tracker has been in employment at Siemens since February 2006 and measured values have been reproduced with measurement analyses.
Wide range of employment
Laser trackers are not only used for the measurement of blades. They are a great tool if you need to measure axial distances or diameters in a housing. With a height of 36 centimeters, it is small enough to position exactly where you want it, and with its rotating ranges of +/-320° horizontally and +80° to –60° vertically, the laser tracker is also suitable for measuring large objects at long range. The Omnitrac is certified with a range of 60 meters and at Siemens it measures with an accuracy of 10 ppm to NIST—a distance of 40 meters. As Hoeyng explains, the laser tracker has no boundaries. Siemens is very happy with its laser tracker and is already thinking about purchasing the API Intelliprobe V2 handheld sensor in the near future. Siemens will continue to remain in close contact with API, even more so with the continuous growing presence of API in Europe and the opening of the German office in Mannheim in 2007.
About Automated Precision Inc. (API)
Automated Precision Inc., founded in 1987, offers modern measurement and sensor systems such as laser trackers, 2-D and 3-D systems, and interferometers for coordinate measuring machines. API is the inventor of the laser interferometer, which laser tracker technology is based on. API’s headquarters is based in Rockville, Maryland, and the company’s European branch is based in Heerenveen, the Netherlands, with marketing and sales based in Mannheim, Germany.
API products are used in the aerospace, automotive and machine tool industry and by the manufacturers of coordinate measurement machines. For further information, visit www.apisensor.com.
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