Digital Replicas of Low Pressure Steam Turbine Moving Blades - Assessment of Geometrical and Mechanical Quality Requirements Using Roboter-Assisted Optical Scanning
Authors: Popig Frederik 
Published: October 2022

Steam turbines are primarily used as direct drives for generator applications to produce electrical power for regional grids. To ensure the safe operation of the steam turbine during the entire lifetime it is important to minimize the unbalance of the rotor and to avoid blade vibrations at operational speed. One measure to minimize the unbalance is the specification of an assembling order of the last stage moving blade row. The required mass and center of gravity of each blade are experimentally determined during the final inspection process. The avoidance of last stage blade vibrations at fixed rotational operating speeds is achieved by designing the freestanding last stages moving blades to be free of resonances in a specific speed range. Nevertheless, the disadvantages of freestanding blades are the increased sensitivity to both the occurrence of flutter and forced response limitation in terms of mode localization and amplitude magnification of single blades due to mistuning effects. Those disadvantages as well as the requirement that no resonances shall occur in a specific speed range lead to quality requirements that define the blades’ eigenfrequencies and the blade row’s mistuning pattern at standstill. The blade eigenfrequencies are experimentally determined during the final inspection process via modal testing with an impact hammer in a test bench facility.

Besides the mechanical requirements the manufactured blades have to fulfill geometrical requirements. For this purpose, geometrical features and parameters of the blades as manufactured are inspected and have to be within the defined geometrical tolerances. Commonly geometrical features are inspected at local inspection sections regarding the blade’s root and airfoil using a coordinate measurement machine. An alternative approach is an optical measurement that enables the digitalization of manufactured 3D parts and the inspection of geometrical features based on digital replicas. Furthermore, the digital replicas can be used to calculate the blades’ eigenfrequencies as well as mass and center of gravity.

The presented paper addresses the final inspection of geometrical features and parameters as well as mechanical properties of last stage moving blades based on roboter-assisted optical scanning. The measurements are performed using an automated roboter-assisted scanning approach with a newly developed modular reference frame and a high-speed optical scanning system. On the one hand the blade airfoil geometrical inspection based on the obtained geometrical replica is compared to the corresponding CMM-measurements. On the other hand, the optical measurements are used to determine the blades’ eigen-frequencies, mass and center of gravity numerically via Finite Element Analyses. Therefore, an approach is presented that uses a mesh-morphing algorithm to adapt the FE-mesh of the dis-cretized nominal CAD geometry by means of deviations between the optical scan data and nominal designed CAD geometry.

Finally, the numerically predicted results are compared to experimentally obtained eigenfrequencies and static moments.