Comparison of Residual Stresses in the Mechanical Roll Expansion of HX Tubes Into TEMA Grooves

Abstract

The mechanical roll expansion of heat exchanger tubes into tubesheets containing TEMA grooves, which are thought to aid in the mechanical integrity of the tube-to-tubesheet (TTT) joint, has for many years provided an acceptable means of completing a TTT joint. Inherent with the intentional roll expansion of the tube is the creation of a tensile residual stress field within the tube that is greatest in the transition region between the expanded and unexpanded zones of the tube. An additional complicating factor in the tube-to-tubesheet joint design is the choice of utilizing a seal weld or a “full strength” weld at the tube end in conjunction with a level of roll expansion quantified by the degree of tube wall reduction. This paper presents the results of an initial study of the mechanical roll expansion of 1in. diameter tubes into a typical TEMA-R designed tubesheet, utilizing two grooves in the tubesheet hole. Two combinations of tube and tubesheet materials are studied that include duplex stainless steel tubes and tubesheet, while the second combination includes type 321 tubes roll expanded into a 2‐1∕4 Cr-1 Mo tubesheet, clad with 321 SS overlay. The residual stress fields are calculated by the finite element method and employ a simplified two-dimensional nonlinear axisymmetric model.