Combined Effects of Tube Projection, Initial Tube-Tubesheet Clearance, and Tube Material Strain Hardening on Rolled Joint Strength

Abstract

The tube-to-tubesheet joint strength is measured in terms of interfacial pressure between the tube’s outer surface and tubesheet bore. The strength of a rolled joint is influenced by several design parameters, including the type of tube and tubesheet materials, initial tube projection, and the initial radial clearance between the tube and tubesheet, among other factors. This paper uses finite element analysis (FEA) to evaluate the effect of several parameters on the strength of rolled joints having large overtolerances, a situation that applies to used equipment. An axisymmetric finite element model based on the sleeve diameter and rigid tube expanding roller concepts was used to analyze the effects of tube projection, initial tube-tubesheet clearance, and tube material strain-hardening property on the deformation behavior of the rolled tube and on the strength of the tube-tubesheet joint. The FEA results show that for zero tube projection (flush) the initial clearance effect is dependent on the strain-hardening capability of the tube material. For low strain-hardening tube material the interfacial pressure remains constant well above the Tubular Exchanger Manufacturer’s Association maximum overtolerance. A drastic reduction in joint strength is observed at high values of radial clearances. The cut-off clearance (clearance at which the interfacial pressure starts to drop) is found to vary linearly with the tube material hardening level, and the contact stress increases slightly for moderate strain-hardening tube materials but shows lower cut-off clearance levels. Furthermore, with flush tubes the maximum contact pressure occurs close to the secondary face (at the end of rolling) while for joints with initial tube projection the contact pressure shows two maxima occurring near the primary and the secondary faces. This is attributed to the presence of two elbows in tube deformation near the primary and secondary faces. The average interfacial pressure increased with increasing projection length for all clearances. Tube material strain hardening enhances the interfacial pressure in a similar fashion for all initial tube projection lengths considered in the analysis.