3-D finite element analysis of roller-expanded heat exchanger tubes in over-enlarged tubesheet holes

Abstract

Repeated de-stubbing and re-tubing of heat exchangers in petrochemical plants during their useful lifetime may result in over-enlarged tubesheet holes with overtolerances that exceed those prescribed by Tubular Exchanger Manufacturing Association (TEMA) standards (1988). Roller expansion of tubes in these over-enlarged holes may result in tube thinning and weakening of the joint due to a decrease of interfacial pressure between the tube and tubesheet. In the present work, a 3-D finite element (FE) model of a tube-tubesheet joint was used to determine displacement and stress distributions along the axial direction of roller expanded tube-tubesheet joint and to evaluate the combined effects of large initial clearance and strain hardening of tube material on interfacial pressure and tube deformation. The results obtained from the present model are compared to those of axisymmetric FE analysis and to the experimental results. Both axisymmetric and 3-D models are found to yield comparable trends showing that for elastic perfectly plastic tube material the residual contact pressure remains constant well above the prescribed TEMA maximum overtolerance values. In addition, both models show that for strain hardening tube materials the interfacial pressure increases with increasing clearance. An appreciable difference is observed at high overtolerances where the 3-D model predicts cut-off clearances (clearance at which the interfacial pressure starts to drastically drop) which are about 30% lower than those predicted by the axisymmetric models. The tube inner surface deformation and pull out forces estimated from 3-D results compares very well with those obtained from the experimental tests.