3D stress/deflection analysis of tubesheet and U-tubes

Abstract

Service records of many shell-and-tube type heat-exchangers demonstrate the vulnerability of tube penetration joints to structural failures, especially due to the combined action of corrosion and thermomechanical stresses. For a reasonably complete assessment of such failures and related issues (e.g., remedial measures) it is often essential to know the stresses and relative importance of various factors causing these stresses. The main objective of this paper (and its companion) is to present and discuss the results of stress analyses for a specific U-tube type heat-exchanger typical of a pressurized water reactor (PWR) design. First, the deformations imposed by the overall structure - consisting of the thick tubesheet and its shell attachments - on the tube-to-tubesheet junctions are estimated for normal operating conditions of pressure and temperature; the restraining influence of divider plate (between the hot and cold legs) on the tubesheet deformation is explicitly included. The thermal-structural analysis was done by means of a 3-dimensional finite element model using eight-noded isoparametric brick elements under elasto-static conditions. The significant results are presented as deformation patterns for the in-plane and out-of-plane displacements of the tubesheet; also, the maximum rotation of the tubesheet and the tube-end displacements relative to the first support (baffle) plate holes are estimated. The above deformations produce axial loads and sectional bending moments on the tube-to-tubesheet junctions; these loads and moments are also impacted by the thermal differential expansion of the hot and cold legs of a typical U-tube. Therefore, using the results of above model to define the end conditions, the load-deflection response of three typical sizes of U-tubes was investigated; the full length tubes were modeled by planar beam elements with nodal temperatures defined by the heat transfer analysis. Relatively high stiffness of the tubesheet and flexibility of the U-tubes are confirmed by the analysis. These results are discussed in relation to the expected stress influence of the various factors investigated in this work.