Multimode Tip-Timing Analysis of Steam Turbine Rotor Blades

This article presents a multimode tip-timing analysis of steam turbine rotor blades using the least-squares technique. The rotor blade velocity obtained from the measured time of rotor blade arrival is used in the functional of the least-squares technique, which is a novelty. The approach requires only two sensors in the casing for the algorithm to find blade multimode vibration components. For comparison, three and four sensors have also been used. An additional technique has been devised to determine the frequencies of blade vibration coupled modes, which is also a novelty. Verification of the algorithm is based on experimental and numerical analyses of steam turbine low-pressure last-stage rotor blades vibrating during rundown in a vacuum spin chamber, for synchronous vibrations, and during nominal work at 3000 r/min in a real working turbine for asynchronous vibrations. A multimode analysis is carried out for blade vibration at a nominal rotation speed of 3000 r/min. The considered steam turbine rotor blade is found to vibrate with two-mode shapes. This article considers the use of this method in blade tip-timing (BTT) measurement uncertainties in the form of noise uniform distribution and noise normal distribution.