Structural Integrity Or Large Steam Turbine Rotors.

Abstract

The disks and shafts of large steam turbines are subject to cracking from a variety of causes that may limit the service life of the rotor. These mechanisms include thermal-mechanical (low-cycle) fatigue resulting from startup cycles, creep-rupture from long-term service at elevated temperature, and stresscorrosion cracking in regions of condensation. Cracks may initiate in stress concentration areas such as bores and diaphragm grooves, but in many cases they initiate from preexisting flaws. Degradation of material, especially temper embrittlement, can combine with crack growth to reduce the remaining service lifetime. Nondestructive inspection methods and lifetime prediction computer programs have been developed for turbine rotors. The prediction of lifetime requires detailed analysis of potential flaw linkup in regions of high ultrasonic indication density. Transient stress and temperature analysis for the specific turbine duty cycle and rotor geometry, and the calculation of crack growth rates for several possible fracture mechanisms are discussed. The time for a crack to reach critical size is determined by linear elastic fracture mechanics. Illustrations of these analysis procedures and comparisons with actual field service case studies are presented.