Abstract
In order to increase efficiency and reliability of steam turbines, last stage blades are equipped with mid-span tie-boss and shroud. We can use Blade Tip-Timing method (BTT) to measure vibration of shrouded blades, but the measured level of vibration is dependent on position of sensor over blade shroud. Hence, the results of BTT measurement have to be compared with other measurement techniques, like strain gauges, and with FEM analysis, to interpret these results right. This article provides findings we made on measurement of shrouded blades in test rig with eddy-current sensors, optical sensors and strain gauges at different positions of sensors over blade shroud. Measurement is also compared to FEM analysis.
Highlights
Blade Tip Timing (BTT) measurement is in Doosan Skoda Power used mainly for two reasons: To prevent blade failure on some operational states, where can be increased level of vibration because of ventilation unpredictable asynchronous vibration can be excited by small aerodynamic forces
We have done two types of measurement: First, we investigated the influence of magnet and coil position to blade and secondly we excited blades on its natural frequencies to compare amplitudes from different sensors
The measurement brought a new look on measuring with eddy-current sensor and we found an optimal position of sensor over blade shroud for this type of blade
Summary
Blade Tip Timing (BTT) measurement is in Doosan Skoda Power used mainly for two reasons: To prevent blade failure on some operational states, where can be increased level of vibration because of ventilation unpredictable asynchronous vibration can be excited by small aerodynamic forces. For these cases, longterm monitoring with calculation of blade lifetime is used on power plants to plan shutdowns of turbine and measuring blade behaviour on wide range of operation points to optimalize operation of the turbine. The blade shroud has a great impact on signal shape from the sensor, in some positions (contact plane) it is complicated to receive a good data Another question is what point of shroud is exactly measured and it is necessary to interpret the amplitude of vibration for BTT right
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