<title>Integrated nondestructive evaluation, materials, and fracture mechanics approach to life extension of steam turbine rotors</title>

Abstract

ABSTRACFThe ageing hollow forged steel rotors ofsteam turbine and generator units typical to utility power piants are a major area of concern for the future safety and integrity of the overall power plant. These components are also critical in determining future inspection intervals for the turbine unit and its remaining life. Over the past five years a number of lifetime extension projects have been conducted using an integrateJ nondestructive evaluation (NDE), materials and fracture mechanics approach to detennining inspection intervals and remaining life. These projects have resulted in extending inspection intervals, predicting remaining life, and restoring original equipment manufacturer (OEM) retired rotors to operation. Key aspects ofthe integrated program will be covered using the results ofa recently completed project.' INTRODUCHONThe importance ofcontrolling the rate at which steam turbines are started up, loaded, unloaded, and shut down became evident during the early 1950's when shell cracking from thermal transients became prevalent. After shell design modifications were made to reduce the effects ofthennal cracking, it wa-clear that turbine rotors had become the most susceptible component to thermal cracking.2 Therefore, during the late 1960's, new operating procedures were developed to control the rates oftemperature change experienced by the turbine rotors.3 These new procedures were intended to reduce thermal cycling on rotors in order to accomplish two main objectives:1 .Prevent bore surface cracking, which could lead to rotor bursting. 2.Eliminate thermal cracking on rotor surfaces.In order to prevent bore surface cracking, the procedures were aimed at limiting the combined centrifugal and thermal stresses at the bore surface to 9O% ofthe yield strength ofthe rotor material. When a material is stressed above its yield strength, it no longer behaves elastically and undergoes permanent plastic deformation. A simple example of this involves squeezing an aluminum soda can; after lightly squeezing a can it returns to is original shape, but heavily squeezing the can will result in permanent (or plastic) deformation. The same principle applies to a turbine rotor as it expands and contracts from centrifugal and thermal stresses. Overstressing a rotor with a bore surface crack of critical size could cause catastrophic rotor bursting. Modem principles of fracture mechanics allow engineers to determine the critical crack size for bore surfaces ofvarious turbines. In addition, fracture mechanics also allows the calculation ofthe number ofcycles required to propagate a small bore surface flaw to its critical size.Controlling the thermal cycles ofa rotor is most important during a cold start. A cold start is typically defmed by a situation when a rotor has been offline long enough to cool below its Fracture Appearance Transition Temperature (FATT). The FATT defmes a temperature below which the rotor material under impact loading will fail in a brittle manner and above which the rotor material will be tough a...