Random Thermal Stress Oscillations and Fatigue Life Estimation for Steam Generator Tubes

A one-dimensional computation model was used to determine the temperature and stress fields under conditions corresponding to those found in once-through steam generator tubes. The random oscillatory nature of the water/steam convective boundary conditions in the transition boiling zone was simulated by postulating random variations in heat transfer coefficients or flow temperatures. Statistical parameters describing the tube temperatures and thermal stress oscillatory history are determined allowing appropriate fatigue life estimations to be made. The results indicate that the tube wall temperature oscillation parameter provides a strong influence on thermal stress and the predicted fatigue life. In contrast, Biot number variations provide only a relatively small effect. An improved method of determining the allowable thermal stress limitations due to certain wall temperature oscillations is proposed.