QMT Features: January 2008
Tracking goes nuclear
Laser trackers ensure optimal distance between massive turbine blades and their housing for the world's largest and most modern nuclear power station in Finland.

Olkiluoto, in Finland,is the world’s largest and most modern nuclear power station, with an output of 1600 MWatts. It relies on German technology from Siemens Power Generation AG to achieve its impressive output. 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 with clearances of just tenths of a millimetre.

The steam turbine, which is more than 12 m long and has a diameter of 6.7 meters, is currently waiting for the final cross section before distribution to Olkiluoto 3 nuclear power station.  The free standing blades of the low pressure turbines are aligned to tenths of a millimetre, despite their mass and weight. “The deciding factor for these blades is their distance to the housing”, describes Stefan Hoeyng, plant engineer for final assembly of steam turbines at Siemens 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 laser trackers
During the final assembly large disks are shrunk and inserted into the blade´s groove on the rotar. Olkiluoto´s low-pressure turbine has a blade weight of about 320 kg. When the rotor is bladed, it is loaded in the force shelter with excess rotation speed to obtain an appearance of compression. Subsequently, the free standing blades are measured. “The more exact a measurement system is, the smaller the tolerances for the distance between the blades are - which improves efficiency”, explains Stefan Hoeyng. An accuracy of at least 1/10 mm 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 kg. If the housings are to be measured in the final assembly, the Laser Tracker must reach heights of 3-5 meters and a lightweight system is a lot easier to handle in such tasks.

The laser tracker, employed at Siemens since February 2006, is used in combination with the custom-made software for blade measurement. To-date, five employees at Siemens are trained on the laser tracker. With the help of the special measurement software, developed by API in close cooperation with Siemens,  the untrained employees can now measure after a short fundamental briefing on measuring with the system. Metrolog, an open measurement software, also supplied by API, delivers the basics. “If I want to measure distances and geometry with the 3D 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”, explains Stefan Hoeyng. A strict program sequence leads the user step by step through the measurement task.

The  system allows different operators to work with the system and measure different blades with acceptable values.
Documentation of the individual blades takes place automatically - which was one of Siemens  main requirement for the custom designed software.  As Stefan Höyng expl ains, Siemens have  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 their final decision to purchase.

Wide range of employment
Laser trackers are not only used for the measurement of blades. Trackers can also be used to measure axial distances or diameters in a housing. With a height of 36 cm it is small enough to position exactly where it is needed. With rotating ranges of +/- 320 degrees horizontally and +80 to -60 degrees 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 Stefan Hoeyng explains, “The laser tracker has no boundaries. Siemens is very happy with their laser tracker and is  thinking about purchasing the API Intelliprobe V2 hand-held sensor in the near future.” In addition, it is planned that the laser tracker will also be used on construction sites, for example, when measurement revisions in the power station housings. 


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