Blade Damping Mechanisms and Some Recent Failures

Abstract

The rotating blades of a steam turbine perform the important task of converting the thermal energy of the steam into rotating mechanical work or torque. Hence, these blades have to be very robust and they need to be installed properly in the discs of the turbine. Damping mechanisms are used to improve the stiffness of the blades and to minimize the vibrations of the blades. Different types of blade damping mechanisms are used in steam turbines. Shroud bands, lashing or damping wires, pins, tie-bolts and lashing stubs/ snubbers are the most common types of damping mechanisms used in steam turbines. Several factors such as stress distribution, additional centrifugal stresses due to the added weight of the damping mechanisms, aerodynamic effects, resulting harmful mode shapes and critical vibration frequencies etc., need to be considered in the selection and the design of the damping mechanisms. Due to the various types of operating conditions and stresses, the damping mechanisms can experience failure in service. Metallurgical evaluations of some of the recent failures of damping mechanisms are discussed here. Failures of shroud bands, blade tenons, tie-bolts and damping wires are the particular examples presented. SCC (stress corrosion cracking), fatigue and overload were found to be the main modes of failures in these examples. The purpose of discussing these examples is to make the designers, the manufacturers and the users aware of these potential issues so that corrective actions can be taken.