Evaluation of True Frequencies of the Blade Vibrations from Blade Tip Timing Measurement

The world of electricity production is currently increasingly affected by the variability of production from renewable sources. This increases need for the flexibility of standard sources, especially coal-fired power plants. The turbines of these power plants are often operated in non-design conditions, and one of the critical components that can be damaged as a result are the impeller blades, especially the last stage blades of low-pressure turbines. In order to ensure their reliable operation, it is necessary to measure the vibrations of the blades online and monitor their condition. A popular method of non-contact vibration measurement of rotating blades is the blade tip timing (BTT) method. The principle of BTT is measuring the passage times of individual blades under a sensor mounted in the turbine stator body. The two most commonly used methods for frequency analysis of blades from BTT measurements are the single-blade spectrum, i.e. the spectrum of the sequence of vibrations of individual blades, and then the all-blade spectrum, i.e. the spectrum of the sequence of vibrations of all the blades as they pass under the sensor. Both methods have the negative property that in the given spectra, blade vibrations are not displayed at the actual excited frequencies, but at apparent frequencies given either by an insufficient sampling frequency or by modulation caused by the very principle of BTT measurement. In this article, we will introduce the so-called true-spectrum calculation method, based on the transformation of the all-blade spectrum, in which the blade vibrations are already displayed at real frequencies. The presented method uses the knowledge of nodal diameters corresponding to individual natural frequencies of the bladed disc. The results of this method can be used not only for the correct evaluation of the blade vibrations, but also subsequently for the calculation of the residual life of the blades.