Dynamics of loosely supported heat exchanger tubes

Tube failure due to excessive flow-induced vibrations is a major concern with regards to the operation of heat exchangers in nuclear power and chemical process plants. Typical consequences include unplanned and expensive plant shutdowns to plug the failed tubes. Fretting wear resulting from tube/support impact is considered as a major contributor to tube failure. Impact forces, which occur when tube vibration amplitudes exceed the local support clearance, play a vital role in determining tube wear. Turbulence is one of the possible excitation mechanisms which can drive tube vibrations and is of particular concern in heat exchangers. Unlike other excitation mechanisms, turbulence has a persistent effect and thus determines the long term reliability of the heat exchangers.

To address these issues, a point contact algorithm describing the tube/support interaction was implemented in an in-house finite element program and validated by several published examples. Pseudo-forces were utilized in conjunction with modal superposition in solving the nonlinear equations of tube motion. The equilibrium equations were solved iteratively to calculate the contact forces required to oppose any tube/support overlaps. The impact model was further modified to consider more realistic tube/support contact configurations. The new model considers a finite support width. The contact forces arising from the tube/support overlap are due to two different contact situations that may be encountered (point and segment contact). A distributed stiffness along the tube/support interface was utilized to model the segment contact. The resulting contact pressure was calculated using the displacement profile along the contact segment. The general point contact model considers any tube/support overlap that may occur between the principal contact node and the neighbouring node.

Time-domain simulations of the nonlinear response of the tube are presented to determine the effect of various tube/support parameters on the system's vibratory characteristics. Special attention was paid to the effect of clearance enlargement due to fretting wear on the response of tubes in lattice-bar supports. The tube response, the impact force, and the contact ratio (ratio of the contact time to the total time) were analysed and presented in a dimensionless form. The dimensionless parameters utilized proved to be effective at collapsing all the data pertaining to different flow velocities over a single curve. This aids in identifying the role of support variables in influencing tube dynamics. Moreover, these parameters may also be used to scale the results in order to account for differences in geometrical and material properties. In addition, simulations were conducted to investigate the effect of the support type, the flow orientation, and the lattice-bar offset on the tube dynamics. The study indicates that some flow orientations, support types, and support offsets provide a favourable support geometry for higher normal work rates. This, in turn, increases susceptibility to fretting wear damage. These results provide new insights and a better understanding of the underlying phenomena of nonlinear tube behaviour in loose lattice-bar supports.