Calibration of blade tip-timing sensor for shrouded 40″ last stage blade

Today's power generation market requests steam turbines of high efficiency, reliability and wide operation range. The last stage moving blades are designed with integral shroud, mid-span, and fir-tree dovetail. The blades are continuously coupled by the blade untwist due to the centrifugal force, and thus the vibration control and increased structural damping are provided. Unfortunately this shroud complicates the blade vibration measurement using contactless methods. This paper describes in detail the Blade Tip-Timing measurement and calibration provided in test rig and in steam power station where the new eddy-current and optical sensors were used together to measure blade vibration in various conditions. Detail calibration procedure for Blade Tip-Timing sensors will be presented as well as the results of the measurements and comparison to FEM Maxwell calculation and to FEM stress analysis.