Finite element analysis of roller expanded tube -tubesheet joints with over enlarged tubesheet holes

Abstract

The integrity of tube-tubesheet joints in heat exchangers are influenced by several parameters including the initial clearance, tube/tubesheet materials properties, type of expansion, and tube wall reduction, among other factors. Used heat exchangers tend to have over-enlarged tubesheet holes as a result of repeated de-tubing and re-tubing during the life cycle of the equipment. The technical literature is short of studies that address the effect of over-enlarged tubesheet holes on the integrity of rolled tube-tubesheet joints. This research work has investigated the effect of initial tube-tubesheet clearance, tube tubesheet material properties, tube wall reduction, and joint geometry on the integrity of roller expanded tube-tubesheet joints having over enlarged tubesheet holes using the axisymmetric, two dimensional (2-D) planar, and three dimensional (3-D) finite element (FE) models. The results of each of the three FE models (Axisymmetric, 2-D planar, 3-D) have shown that: at a given combination of tube and tubesheet material properties and tube wall reduction the level of the rolled joint strength, represented by the residual contact stress, versus clearance remains at the same level of the strength that corresponds to the clearance prescribed by the tubular heat exchanger manufacturer association (TEMA) until a critical clearance level, where the strength experiences a sudden dropping trend. The residual contact stresses of the joints with clearances lower than the critical clearance are independent of the type of the FE model used but they are directly proportional to the tube material strain hardening and to the tube wall reduction. The level of critical clearance at which the joint strength starts to deteriorate increases with the increase of tube wall reduction but it decreases with the increase in tube material strain hardening. An empirical relation has been developed to predict the critical clearance and corresponding contact stress as a function of wall reduction and material strain hardening. The FE results were found to agree with numerical, experimental, and analytical approaches of the other researches for the same joint geometry and material combination. The critical clearance at which the contact stress starts to decay below the contact stress corresponding to the TEMA clearance obtained from the 3-D FE model is similar to that of the axisymmetric model while it is higher for the 2-D model. This leads to the conclusion that the relatively simpler and computation efficient axisymmetric model is capable of predicting allowable clearance limits for over enlarged tubesheet holes more conservatively. The FE analysis results also show that, for the investigated joint geometry and tube and tubesheet materials, tube projection does not have a significant effect on the residual contact stress of the roller expanded joint. The results also indicate that the effect of increasing groove width is to increase the interfacial residual contact stress in hydraulically expanded joints while it has the effect of decreasing the pressure in roller expanded joints. On the other hand, other groove parameters have no significant effect on the contact stress for both hydraulic and roller tube expansion joints.