Calibration of blade tip-timing sensor for shrouded blades

Significance 

Power generation has been of great significance in the production of electric energy for numerous household and industrial use. However, the rapid increase in the demand for electric power has compelled researchers to improve the already existing power generations techniques as well as develop alternative sources to supplement the deficiency. Consequently, the increasing global call for sustainable development as a way of reducing pollution through limiting dependency on fossils fuels have also contributed greatly to such developments. For instance, steam turbines which are highly used in today’s power generation stations have undergone numerous improvements to enhance their efficiency. Unfortunately, it is difficult to realize the smooth operation of turbines, especially for large power generation.

Generally, turbines are made of blades which are susceptible to numerous faults due to the natural frequency, misalignment, unbalancing and vibration. Asynchronous vibrations, for example, are more prevalent, especially for high loaded blades. On the other hand, steam turbine blades commonly undergo synchronous excitation while aerodynamic excitation is caused by resonances with nonsynchronous excitation. To this end, several methods have been devised to prevent unpredictable excitation, dynamic stresses and to improve the lifetime of blades. Blade vibrations have been measured in power stations during wide range operation because mechanical stress damages the blades thus reducing their lifetime. Unfortunately, it is expensive and difficult to use these methods to monitoring more blades due to the short sensor life, especially in corrosive environments. Therefore, researchers have been looking for alternative methods and have identified blade tip timing as a promising solution.

Recently, Zdenek Kubín at the University of West Bohemia in Pilsen in collaboration with Dr. T. Mísek, J. Hlous, T. Dadaková and Dr. J. Kellner at Doosan Škoda Power and Dr. T. Bachorec at SVS FEM investigated the use of eddy current and optical sensors in blade tip timing measurements and calibration in steam power stations. They performed measurements in various conditions and presented the detailed calibration procedure. Eventually, the obtained results were compared to the theoretical ones to validate the feasibility of the model. They purposed to improve the operation efficiency of the steam turbines, reduce damages and improve their lifespan for large power output generation. Their work is published in the journal, Mechanical Systems and Signal Processing.

From the modeled magnetic interference between the blade shroud and sensor, the authors observed that it was capable of predicting the voltage amplitude and shape of signals emanating from the sensors. Consequently, it efficiently identified the axial position responsible for bad signals production. The measurement results correlated well with the existing theoretical results. This included the calibration curve and tip deflection. For instance, best axial positions for the sensors were selected based on the calculated vibration limits and safety factors which were determined based on the measurements and calibration uncertainty. Furthermore, possible causes of such uncertainties included gage position, axial shifts, blade untwists among others.

The authors successfully developed a detailed calibration procedure for blade tip timing sensor that will help advance the efficiency and operation of steam turbines in power generation. It emphasizes on choosing the sensor size and position which was successfully validated by the developed model. The study also provides a groundwork for future studies which will promote power generation to meet the increasing demand for power.

Calibration of blade tip-timing sensor for shrouded blades - Advances in Engineering
Fig. 1 – Comparison of measurement and FE calculation of induced voltage during shroud passes the sensor

About the author

Zdenek studied Cybernetics and Artificial Intelligence at University of West Bohemia. He got master degree in 2007. After the studies he become programmer for machine monitoring systems at Profess LTD which provides on-line machine protection and monitoring systems for rotating components. In 2009 he started to work for Doosan Skoda Power as signal processing engineer. In 2010 he established the blade vibration monitoring system in Doosan Skoda Power and he became the head of component development department which provides blade vibration measurement, seal development and hydrodynamic bearing testing that contains identification of dynamic stiffness and damping.

Nowadays his team has measured the blade vibration on over the 20 steam turbines from 35 to 1100MW. The most valuable work for today is the calibration of blade tip-timing measurement in terms of correct blade tip angle determination, which is the key to identify dynamic stress load on the blades from the tip-timing measurement for shrouded blades. This work is the main part of his PhD. thesis at University of West Bohemia.

Reference

Kubín, Z., Mísek, T., Hlous, J., Dadaková, T., Kellner, J., & Bachorec, T. (2018). Calibration of blade tip-timing sensor for shrouded 40″ last stage blade. Mechanical Systems and Signal Processing, 108, 88-98.

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