Ultrasonic examination of LP steam turbine shrunk-on discs: method (designed through modelling), process and tools, data capture, analysis and post processing)

Abstract

The different manufacturing technologies of LP steam turbines include solid turbines (monoblock), welded turbines, and shrunk–on disc turbines. For shrunk–on disc LP turbines, the turbine rotor is constructed by thermally expanding the discs and mounting each disc onto the spindle allowing it to 'shrink fit'. The discs are aligned and locked in their circumferential positions by way of a pin and drive pin hole arrangement. This type of assembly is a source of risk of cracking due to the inherent stress. Additional factors contribute to the risk stress corrosion, fretting action, and fatigue. The areas of concern are the shrink–fit area between shaft and disc, drive pin holes on the lateral face of the discs. Without any dismantling the only means of inspecting the suspect area is to use an ultrasonic test method. This paper presents through modelling (CIVA) the process followed from the stage of receiving the specification to the inspection.