M alik Spahic

Abstract

Over the last few years, cracks appeared at an early stage of operation on several steam turbine rotors due to low cycle fatigue induced by thermal transient events. These cracks led to expensive maintenance and unavailability. An extensive fatigue lifetime assessment program was set up to avoid similar issues in other units. Although numerical and finite element (FE) models can offer accurate results in this matter, regular 3D and 2D FE models are too time-consuming when simulating the rotor’s entire operational history. Instead, a more time-efficient numerical method, using the rotor’s measured physical data as input, was developed.

This method follows two steps.

In a first step, a 2D axisymetric thermomechanical finite element model is used to define  accurate stress levels on a reference period of a few days.

In a second step, the simulation parameters of a simplified time-efficient thermomechanical finite differences model of the rotor are calibrated to give the same results as the 2D model for the reference period. It is then processed over the rotor’s entire operational history to determine the stress history at the most critical location of the rotor.

In combination with theoretical fatigue models, this method was able to reliably reproduce the crack initiation in several cracked rotor cases. The results were then used to extend the operational period of different steam turbine units by modifying their operational parameters or by doing skim cuts to remove damaged metal surfaces.

Session