Combined effects of grooves, tube projection and clearance on the integrity of rolled tube-tubesheet joints

Abstract

The integrity of tube-tubesheet joints plays a critical rule in heat exchanger durability and life cycle cost. The integrity of the joint is affected by many parameters including joint configurations, expansion process and materials properties. A considerable work in the literature has studied the effect of these parameters on the joint integrity. However most of this work has focused on the joints when the clearance level is within the value specified by Tubular Exchanger Manufacturers Association (TEMA) standard. Clearance increases over time because of repeated maintenance processes and re-tubing of the joint. A need exist to study and investigate the effect of grooves and projection for the overenlarged joints having initial radial clearance greater than the clearance value specified in TEMA standard. In this thesis the combined effects of the joint parameters that include tubesheet grooves, tube projection and radial clearance on the integrity of rolled tube-tubesheet joints has been investigated with special attention to the joints with overenlarged tubesheet holes. A nonlinear axisymmetric finite element (FE) model has been developed to study the effect of the above mentioned parameters on the residual contact pressure and residual stresses on the expanded tube surfaces with special focus on the residual stresses in the tube transition zone. In addition the FE model has been used to investigate the effect of friction coefficient on the joint contact pressure and on the critical radial clearance after which the contact pressure starts to decline. An experiment similar to the one reported in Shuaib et al [1], has been designed to investigate the effects of the above mentioned parameters on the integrity of the joint and used to verify the results of the finite element model. The results of the FE analysis and the experiment have shown that a radial clearance up to three times TEMA standard clearance provides a reliable joint in terms of the contact pressure, joint strength and tightness. Also the model and the experiment results indicated that grooves tend to enhance the tube-tubesheet joint strength by up to 15%. Both The finite element model and the experimental results have shown that within the investigated range of tube clearance (from 0 to 0.38 mm) and tube projection (0, 1.5 and 3.2 mm), tube projection does not seem to have significant effect on pull out force, or joint strength. The FE model results indicated that very high tensile residual stresses that reaches around 80% of the yield strength of the materials exists at the transition zone of the tube internal surface, however clearances higher than TEMA standard clearance will not add extra penalty to the maximum residual stress level for both grooved and ungrooved joints. On the other hand residual stresses in the secondary transition zone at the tube projection are lower than those at the primary transition zone.