Estimation of Hydraulic Pressure for Expansion of Heat Exchanger Tubes

Abstract

The paper describes the development of a calculation procedure for determining the required hydraulic forming pressure for tube-to-tubesheet joints. The procedure has been incorporated into software, which is now being used in a fabrication shop environment. The bases for the computations, which are internal to the software, are principles of structural mechanics, easily measured dimensions, and material properties taken from mill test reports. The user of the software need not be well-versed in structural analysis. The basic idea behind the procedure is the separate treatment of the structural behavior of the expanded portion of the tube, the transition zone, and an axisymmetric sleeve model of the tubesheet. Precalculated solutions of the tubesheet are stored in nondimensional form in a data base. When these results are needed, they are retrieved from the data base, interpolated, and dimensionalized. Displacements and forces are then matched for equilibrium and compatibility during the hydraulic pressure cycle on the expanded section of the tube. The software, which produces results in a matter of seconds, calculates the peak hydraulic pressure, final dimensions, final contact pressure, and transition zone residual stresses corresponding to several degrees of expansion. After examining the trends, the user can select a desired degree of expansion and record the hydraulic pressure to produce it. This avoids the risk of overexpanding the ligaments in exchange for a very small increase in final contact pressure or holding force. The paper also compares measured final dimensions of some successfully formed joints with those predicted by the analysis.