Challenges in Circumferential Entry Blades for Structural Integrity of Closing Blade in Steam Turbines

Abstract

The bladed disc of an LP last stage rotor assembly in a steam turbine often employs circumferential entry blades. The closing blade is fastened with the aid of a pin type locking mechanism, which locks the closing blade onto the disc. Radial entry blade root design demands for a groove in the disc through which blades are inserted and guided to their respective final positions which alters the stiffness in the disc. Closing blade is the most highly stressed weak link zone in the circumferential entry rotor disc achieving mechanical integrity for in-service condition is a challenge. Location of pin and its insertion is the key to achieve the desired in-service life with good material model. The present research work focus on emphasis of pin positioning to lock the T-root blade at operating range. Various material model configurations and pin positions are considered in both blade and disc to improve the reliability in design. Customized methodology is developed to address the elasto-plastic kinematic hardening in pin, disc and blade grove during in-service life. Finite element approach along with classical design equations followed by API 616 standards is employed to achieve the mechanical integrity in bladed disc assembly.