Yagoub N. Al-Nassar

FEA of the Effects of Initial Tube-Tubesheet Clearance, Wall Reduction and Material Strain Hardening on Rolled Joint Strength

The tube-to-tubesheet joint strength is measured in terms of residual contact pressure between the tube’s outer surface and tubesheet hole surfaces. The joint integrity is affected by several design parameters, including the type of tube and-tubesheet materials, level of expansion and the initial radial clearance between the tube and tubesheet. In the present work, an axisymmetric finite element model based on the sleeve diameter and rigid roller concepts is developed. The model has been used to evaluate the combined effects of clearance, wall reduction level and the-strain hardening of tube and tubesheet materials on the interfacial pressure between tube and tubesheet. The FE results show that the initial clearance effect is dependent on the strain hardening capability of the tube material. For low strain hardening tube material the interfacial pressure remains almost constant well above the TEMA (Tubular Exchanger Manufacturing Association) maximum radial over tolerance of 0.0254 mm. These results are validated by the experimental data developed during the research program. As expected, a drastic reduction in joint strength is observed at high values of radial clearances. The cut-off clearance (clearance at which the interfacial pressure starts to drop) is found to vary linearly with the level tube material hardening level. The residual pressure is found to increase slightly for moderate strain hardening tube materials but shows lower cut-off clearances. Wall reductions ranging from 1% to 10% were utilized in calculating the contact pressure as a function of radial clearance. The results show that for low strain hardening materials the optimum value of residual contact stress is obtained for the industry recommended value of 5%. Finally, because of the absence plastic deformation in the ligament, the level of tubesheet material strain hardening does not have any noticeable effect to the joint strength.Copyright

Development of Residual Stress during Manufacturing of Spiral Welded Pipes

Spiral welded pipes (SWPs) are produced from forming and welding of plate or strip material with seam running its entire length in a spiral form. The relative structural weakness of these pipes is due to high residual stress developed during welding, which depends on various parameters and their interaction. Finite element method (FEM) was used to predict the temperature and stress fields in a 56-inch spirally welded steel pipe. Temperature-dependent thermal and mechanical properties of steel were incorporated in the model. Arc welding was modelled as a three-dimensional (3D) volumetric moving heat source. The residual stresses produced after the completion of the welding process were investigated. For validation, the numerically predicted residual stress was compared with measured values using split-ring and hole-drilling methods. It was found that von Mises stress attained high values in the cooling cycle after the solidification of the molten zones. Moreover, the effect of welding speed on the level of residual stress was studied. Increasing the welding speed with constant power resulted in reduction of the width of the high stress zone.

3-D finite element analysis of roller-expanded heat exchanger tubes in over-enlarged tubesheet holes

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 element (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 present 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 clearances (clearance at which the interfacial pressure starts to drastically drop) which are about 30% lower than 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.

Convective Heating of Solid Surface: Entropy Generation Due to Temperature Field and Thermal Displacement

Convective heating and cooling of the surfaces find application in process industry. During the heating or cooling cycle of the process, thermodynamic irreversibility which can be associated with the process parameters occurs. Moreover, thermodynamic irreversibility associated with the heating cycle can be quantified through entropy analysis. In the present study, convective heating of the solid surface is considered. A mathematical formulation of the temperature rise and thermal stress development during the transient heating process is presented. Entropy generation due to temperature field and thermal displacement is also formulated. The simulation for temperature rise, thermal displacement, and entropy generation are carried out for steel substrate. It is found that thermal displacement does not exactly follow the temperature distribution inside the substrate material. The magnitude of entropy generation due to temperature field is considerably higher than that corresponding to the thermal displacement.

Estimation of Residual Stress in Spiral Welded Pipe: Regression and Numerical Model

Double submerged spiral-welded pipe (SWP) is used extensively throughout the world for large-diameter pipelines. Fabrication-induced residual stresses in spiral welded pipe have received increasing attention in gas, oil and petrochemical industry. Several studies reported in the literature verify the critical role of residual stresses in the failure of these pipes. Therefore, it is important that such stresses are accounted for in safety assessment procedures such as the British R6 and BS7910. This can be done only when detailed information on the residual stress distribution in the component is known. In industry, residual stresses in spiral welded pipe are measured experimentally by means of destructive techniques known as Ring Splitting Test. In this study, statistical analysis and linear-regression modeling were used to study the effect of several structural, material and welding parameters on ring splitting test opening for spiral welded pipes. The experimental results were employed to develop an appropriate regression equation, and to predict the residual stress on the spiral welded pipes. It was found that the developed regression equation explains 36.48% of the variability in the ring opening. In the second part, a 3-D finite element model is presented to perform coupled-field analysis of the welding of spiral pipe. Using this model, temperature as well as stress fields in the region of the weld edges is predicted.Copyright

Optimization of Pipe Repair Sleeve Design

For the repair of pipeline defects, repair sleeves are the most widely used method in petro-chemical industry. The objective of this work was to optimize the thickness of non-pressure containing repair sleeve, by refining the existing design practice. Laboratory studies involving instrumentation of small-scaled repair sleeve system coupled with finite element analysis were carried out to refine the design procedure and optimize the thickness of the sleeve. Using a unified approach for finite element modeling, including failure pressure estimation and simulation of sleeve installation pressure, twenty-four cases ranging from 6–60 inches nominal diameter were investigated. In this paper, the details of the optimization approach used in this investigation have been presented.Copyright

Effects of Initial Tube-Tubesheet Clearance and Grooves on Axial and Hoop Residual Stresses of Roller Expanded Joints

An axisymmetric finite element model is used to evaluate the effect of large initial tube-tubesheet radial clearance and the effect of grooves machined in the tubesheet hole on the distributions and magnitudes of the radial deformation of expanded tube walls, residual contact stress, residual hoop stress, and residual axial stress along the expanded length of the tube and in the transition zone. The results have revealed the presence of residual axial stresses and residual hoop stresses in the inside and outside surfaces of the tubes with various degrees of severity. These tensile residual stresses may cause crack initiation and propagation and may lead stress corrosion cracking in corrosive environments.Copyright

Integrity of Roller Expanded Tube-to-Tubesheet Joints With Over-Enlarged Tubesheet Holes

This paper presents the results of an experimental investigation conducted to evaluate the effect of initial tube-tubesheet diametral clearance on the integrity of roller expanded tube-to-tubesheet joints in over enlarged tubesheet holes. The criteria of evaluating joint integrity include the pull out force and hydraulic tightness of the joints in which tubes were expanded to 5% wall reduction after metal to metal contact of the tube with the tubesheet hole. The levels of clearance investigated were selected to cover a range that exceeded by ten times that prescribed by the Tubular Exchangers Manufacturers Association (TEMA) standards [1]. The pull out test results showed that, within the investigated range of clearance, the joint strength has not declined below the strength of joints with clearance levels within the TEMA range. In fact the pull out force exhibited a slight linear increasing trend with the increase in initial clearance, which is attributed to the effect of tube strain hardening. The hydro test results indicated that all joints within the investigated clearance levels were hydraulically tight when they were tested at 1.5 times the design pressure of the heat exchanger for duration of 30 minutes. A relationship between rolling specific energy and initial joint clearance has been developed.Copyright