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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 ele-ment (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 pres-ent 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
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clearances (clearance at which the interfacial pressure starts to drastically drop) which are about 30% lower than 23311
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.
Keywords 3-D Finite element Tube-tubesheet joint
rolling Clearance effects Strain hardening
Overtolerances
Introduction
The frequent process of de-stubbing and re-tubing opera-
tions performed during the life of the heat exchangers in
petrochemical and energy producing plants results in the
over enlargement of the tubesheet hole to levels that may
exceed the levels prescribed by industrial standards
(Standard of the Tubular Exchanger Manufacturer Asso-
ciation ‘TEMA’ 1988). When new tubes are installed, they
are often expanded using the rolling process into the
enlarged tubesheet holes; this may result in a deterioration
in the strength of tube–tubesheet joint having large initial
clearance as well as in the strain hardening and thinning of
the expanded tubes and their surrounding ligaments.
Simulation of roller expanding of tubes into tubesheet by
the finite element method is associated with a number of
difficulties such as roller kinematics and the absence of
loading axisymmetry. In fact, themechanical rolling process
is a good example of periodic symmetry with respect to the
load application. A number of researchers have shown that
the complexity of the tube–tubesheet roller expansion can be
simplified by reducing it to an axisymmetric quasi-static
problem (Aufaure 1987; Merah et al. 2010, 2009; Updike
et al. 1992; Merah 2008; Al-Aboodi et al. 2009; Cizelj and Mavko 1995; Williams 1996, 2003). Displacements are
applied in steps up to the required expansion and then
decreased down to zero such that the tube–tubesheet contact
ensures the strength of the connection joint after transmitting
a large radial stress to tube’s outer surface. Andrieux and
Voldoire (1995) developed an axisymmetric FE model to
simulate roller expansion using five steps. Cizelj andMavko
(1995) used the axisymmetric model in ABAQUS code
to evaluate the residual stresses in one-step rolled tube to
tubesheet joint. Williams (1996), used three load steps to
complete the finalmechanical roll expansion. Because of the
absence of loading symmetry other researchers, however