Flow-Induced Vibration and Impact Fretting-Wear in Nuclear Steam Generators and Heat Exchangers

Abstract

Mechanical damage due to excessive vibration may limit the life of steam generators and heat exchangers and may cause costly station shutdowns. It is important to understand flow-induced vibration and damage mechanisms to prevent problems at the design stage and to assist station owners in predicting the life of heat exchange components. This paper outlines state-of-the-art technologies in the related areas of thermal-hydraulic analysis, flow-induced vibration and mechanical damage prediction. Vibration problems often occur locally in areas where excessive flow velocities exist, such as inlet regions and around sealing strips in heat exchangers. Detailed three-dimensional thermalhydraulic analyses are required to obtain flow velocity distribution in components. Recent development work to predict flow velocity distribution around sealing strips and in the U-bend tube region of steam generators is described. Vibration excitation mechanisms must be formulated to predict the vibration response of heat exchange components. Turbulence-induced excitation is important in axial-flow. In crossflow, fluid elastic instability is usually the cause of excessive vibration. While a reasonable amount of information is available in liquid and gas flow, very little work has been done in two-phase flow. The results of recent experimental work on vibration of tube bundles in two-phase cross-flow is presented. Damping, fluid elastic instability and turbulence-induced excitation are discussed. Design guidelines are suggested. Damage mechanisms must be understood to predict component life. Studies on fretting-wear damage of steam generator materials under simulated operating environments are discussed. Techniques to model the dynamic interaction between vibrating tubes and their supports are outlined. As an example, the effect of increased tube-to-support clearances due to chemical cleaning on fretting-wear damage will be discussed.