Piping Branch Junctions Research Articles

Effect of welding sequence on residual stress distribution in a multipass welded piping branch junction

This paper presents a validated three-dimensional thermomechanical model to investigate the effect of the welding sequence on weld induced residual stress distributions in a multipass welded piping branch junction. Three possible symmetrical welding sequences, i.e. one-welder, two-welder and four-welder sequence, which were expected to generate the least adverse stress distributions in actual welding circumstances, were proposed and evaluated. It is shown that the variations of the predicted maximum stresses along the fusion lines are not large for the three proposed welding sequences, although at both the branch and run cross section, the four-welder model predicts the highest peak residual stresses while the one-welder model gives the lowest values. As would be expected, the stress distributions predicted by the four-welder and two-welder models show more symmetrical features than those by the one-welder model. High residual stress is formed in the vicinity of the weld region irrespective of the sequence of the welding.

Plastic limit loads for piping branch junctions under out-of-plane bending

This paper presents approximate closed-form plastic limit load solutions for branch junctions under out-of-plane bending and under combined pressure and out-of-plane bending, based on three-dimensional finite element limit analyses for an elastic-perfectly plastic material. When bending is applied to the branch pipe, the plastic limit loads for out-of-plane bending are shown to be lower than those for in-plane bending. However, for bending to the run pipe, the opposite trend is found. For combined pressure and out-of-plane bending, either the circular interaction or the parabolic interaction rule can be used, depending on the bending location and the branch geometry. Comparison with published experimental plastic limit load data shows that the predictions agree relatively well with the test data.

Effect of local wall thinning on limit loads for piping branch junctions. Part 1: Internal pressure

This paper presents plastic limit pressures of piping branch junctions with local wall thinning, based on systematic three-dimensional finite element (FE) limit analyses using elastic—perfectly plastic materials. An ideal branch junction is considered, where no weld or reinforcement around the intersection is considered. Two locations of wall thinning are considered: one is in the run pipe (opposite to the intersection) and the other is in the branch pipe (next to the intersection). Based on the assumption of possible plastic collapse modes, approximate plastic limit pressure solutions for piping branch junctions with local wall thinning are proposed using two limit pressure solutions: plastic limit loads for straight pipes with local wall thinning and those for branch junctions without wall thinning. Comparison with FE results shows overall good agreement.

Effect of local wall thinning on limit loads for piping branch junctions. Part 2: In-plane bending

This paper presents plastic limit loads of piping branch junctions with local wall thinning under in-plane bending, based on systematic three-dimensional finite element (FE) limit analyses using elastic—perfectly plastic materials. As in Part 1, an ideal branch junction without weld or reinforcement around the intersection is considered with two locations of wall thinning; one in the run pipe (opposite to the intersection), and the other in the branch pipe (next to the intersection). Based on FE results, effects of thinning geometries on plastic limit moments are quantified and simple approximations of plastic limit loads for piping branch junctions with local wall thinning under in-plane bending are proposed.

Limit loads for piping branch junctions under internal pressure and in-plane bending—Extended solutions

The authors have previously proposed plastic limit load solutions for thin-walled branch junctions under internal pressure and in-plane bending, based on finite element (FE) limit loads resulting from three-dimensional (3-D) FE limit analyses using elastic–perfectly plastic materials [Kim YJ, Lee KH, Park CY. Limit loads for thin-walled piping branch junctions under internal pressure and in-plane bending. Int J Press Vessels Piping 2006;83:645–53]. The solutions are valid for ratios of the branch-to-run pipe radius and thickness from 0.4 to 1.0, and for the mean radius-to-thickness ratio of the run pipe from 10.0 to 20.0. Moreover, the solutions considered the case of in-plane bending only on the branch pipe. This paper extends the previous solutions in two aspects. Firstly, plastic limit load solutions are given also for in-plane bending on the run pipe. Secondly, the validity of the proposed solutions is extended to ratios of the branch-to-run pipe radius and thickness from 0.0 to 1.0, and the mean radius-to-thickness ratio of the run pipe from 5.0 to 20.0. Comparisons with FE results show good agreement.

Limit loads for thin‐walled piping branch junctions under combined pressure and in‐plane bending

Closed‐form yield loci are proposed for branch junctions under combined pressure and in‐plane bending, via small‐strain three‐dimensional finite element (FE) limit load analyses using elastic—perfectly plastic materials. Two types of bending loading are considered: bending on the branch pipe and that on the run pipe. For bending on the run pipe, the effect of the bending direction is further considered. Comparison with extensive FE results shows that predicted limit loads using the proposed solutions are overall conservative and close to FE results. The proposed solutions are believed to be valid for the branch‐to‐run pipe ratios of radius and of thickness from 0.0 to 1.0, and the mean radius‐to‐thickness ratio of the run pipe from 5.0 to 20.0.

Effect of the interpass temperature on the predicted residual stress distributions in a multipass welded piping branch junction

A sequentially coupled three‐dimensional thermomechanical finite element model has been developed to predict residual stress distributions in a multipass welded piping branch junction. The residual stresses at the branch and run pipe cross‐sections, as well as along the circumferential weldlines on the outer surfaces of both the run and the branch pipes and on the inner surface of the branch pipe, are predicted. Three levels of interpass temperature have been selected to investigate their effect on the peak residual stresses. It is revealed that the interpass temperature has a significant effect on the residual stresses. As the interpass temperature is increased, both the peak hoop and the axial residual stresses at the run and branch cross‐sections decrease. The peak normal stresses along the circumferential weldline on the outer surface of the run pipes are also reduced. However, increasing the interpass temperature had a negligible effect on the peak tangential residual stresses along the circumferential weld line on the inner surface of the branch pipe. The results presented and the modelling technique described in the current study can be used towards formulating a recommendation to optimize residual stress profiles in multipass welded complex geometries through better interpass temperature control.

Finite element modelling of multi-pass fusion welding with application to complex geometries

The current paper presents recently completed work in the development of advanced multi-pass weld modelling procedures, with the ultimate objective of predicting weld residual stress distributions in thick-walled complex geometries. The modelling technique was first developed using simple three-dimensional geometries, for which experimental data was available for validation purposes. All the non-linearities associated with welding, including geometry, material, and boundary non-linearities, as well as heat source movement were taken into account. The element removal/reactivate technique was employed to simulate the deposition of filler material. Combined with a newly developed meshing technique, the model was then applied to predict residual stress distributions for a relatively thick stainless steel piping branch junction. Finally, a parametric study was conducted to assess the effects of various manufacture-related welding parameters on the final residual stress fields. The interpass temperature and cooling rate were found to be the two most sensitive parameters affecting resultant residual stresses. The residual stress profiles can be optimized relatively easily by adjusting these parameters. This research demonstrated that the developed modelling technique has potential in multi-pass welding process optimization and wide industrial applications including weld repairs.

Plastic Limit Loads for Piping Branch Junctions

The present work presents plastic limit load solutions for branch junctions under internal pressure and in-plane bending, based on detailed three-dimensional (3-D) FE limit analyses using elastic-perfectly plastic materials. The proposed solutions are valid for a wide range of branch junction geometries; ratios of the branch-to-run pipe radius and thickness from 0.0 to 1.0, and the mean radius-to-thickness ratio of the run pipe from 5.0 to 20.0.

Finite Element Prediction of Residual Stress Distributions in a Multipass Welded Piping Branch Junction

Piping branch junctions and nozzle attachments to main pressure vessels are common engineering components used in the power, oil and gas, and shipbuilding industries amongst others. These components are usually fabricated by multipass welding. The latter process is known to induce residual stresses at the fabrication stage, which can have severe adverse effects on the in-service behavior of such critical components. It is thus desirable if the distributions of residual stresses can be predicted well in advance of welding execution. This paper presents a comprehensive study of three dimensional residual stress distributions in a stainless steel tee branch junction during a multipass welding process. A full three dimensional thermomechanical finite element model has been developed for this purpose. A newly developed meshing technique has been used to model the complex intersection areas of the welded junction with all hexahedral elements. Element removal/reactivate technique has been employed to simulate the deposition of filler material. Material, geometry, and boundary nonlinearities associated with welding were all taken into account. The analysis results are presented in the form of stress distributions circumferentially along the weld line on both run and branch pipes as well as at the run and branch cross sections. In general, this computational model is capable of predicting three dimensional through-thickness welding residual stress, which can be valuable for structural integrity assessments of complex welded geometries.

Limit loads for thin-walled piping branch junctions under internal pressure and in-plane bending

The present work presents plastic limit load solutions for thin-walled branch junctions under internal pressure and in-plane bending, based on detailed three-dimensional (3-D) finite element (FE) limit analyses using elastic–perfectly plastic materials. To assure reliability of the FE limit loads, modelling issues are addressed first, such as the effect of kinematic boundary conditions and branch junction geometries on the FE limit loads. Then the FE limit loads for branch junctions under internal pressure and in-plane bending are compared with existing limit load solutions, and new limit load solutions, improving the accuracy, are proposed based on the FE results. The proposed solutions are valid for ratios of the branch-to-run pipe radius and thickness from 0.4 to 1.0, and the mean radius-to-thickness ratio of the run pipe from 10.0 to 20.0.

Limit load analysis for the piping branch junctions under in-plane moment

An approximate analysis approach for plastic limit load of piping branch junctions, by means of the relationship of the internal force between the main and branch pipes around intersection line, is presented in this work. The approach is built on the following process: based on the external force equilibrium condition, an equation between the limit load and internal force of the branch pipe around intersection is derived firstly. And then, taking this internal force as an external force acting on the intersection of the main pipe, the approximate solution of the internal force around the intersection on the main pipe is given as a function of the limit load. Finally, referring to the von-Mises yield criterion, the limit load of component with two intersecting cylindrical shells is then obtained. In use of the proposed approach, a closed form of limit load solution for piping branch junction under in-plane moment is developed. Finite element (FE) models of the idealized piping branch junction with various diameters and wall-thickness of the main and branch pipes were analyzed by using nonlinear FE software. The limit loads from FE analysis, from the proposed solution and six experimental data of real piping branch junctions are compared. Overall good agreement between the different limit loads was observed which provides confidence in the use of the proposed formulae in practice.

Limit Load Interaction of Welded Piping Branch Junctions Subjected to Combined Internal Pressure and Bending

Systematic detailed non-linear finite element (FE) analysis are described for limit load interaction of piping branch junctions subjected to internal pressure and bending. The results show that for the tees with a small diameter ratio, the limit load interaction closes to the linear expression; as diameter ratio d/D increasing, the interaction relationship tends to parabolic equation; for the piping branch junction with diameter ratio equaling to unit, the limit load combinations is approximately quadratic. Compared to the individual limit bending value, internal pressure slightly increases the bending capability as it is in the range of 0.2£P/PL£0.4, especially for the cases of the main pipe with thinner wall. A closed limit load solution is obtained from the FE results through accommodating the geometrical parameter influence, and validated by using experimental results.

Experimental Limit Load of Forged Piping Branch Junctions and Comparison with Existing Solutions

Limit load test results from fourteen forged piping branch junctions are reported. Eight were uncracked. Three had outside, non-through wall cracks running parallel to the run pipe centerline at the flank and the others had crotch corner cracks among the selected specimens. The collapse behaviors such as bulging deformation at the side flanks of junctions with and without crack were depicted. Based on the experimental data, the relationships of limit load with structural dimension and crack size were then summarized and the existing solutions were evaluated in use of the test results.

Finite Element Predictions of Temperature Distributions in a Multipass Welded Piping Branch Junction

This contribution deals with the complex temperature profiles that are generated by the welding process in the intersection region of thick walled, cylinder-cylinder junctions. These affect material microstructure, mechanical properties and residual stresses. Knowledge of the thermal history and temperature distributions are thus critical in developing control schemes for acceptable residual stress distributions to improve in-service component behavior. A comprehensive study of three-dimensional temperature distributions in a stainless steel tee branch junction during a multipass welding process is presented. A newly developed partitioning technique has been used to mesh the complex intersection areas of the welded junction. Various phenomena associated with welding, such as temperature dependent material properties, heat loss by convection and latent heat have been taken into consideration. The temperature distribution at various times after deposition of certain passes and the thermal cycles at various locations are reported. The results obtained in this study will be used for on-going and future analysis of residual stress distributions. The meshing technique and modeling method can also be applied to other curved, multipass welds in complex structures.

An approximative solution for limit load of piping branch junctions with circumferential crack and finite element validation

This paper is concerned with the prediction of limit load of the piping branch junctions with circumferential crack under internal pressure. Recently, we have developed a new approach for predicting the limit load of two-cylinder intersection structures with diameter ratio larger than 0.5, which has been successfully applied to defect free cases under various loading conditions. In the present work, we consider the extension of the approach to cover cracked piping branch junctions. On the basis of stress analysis in the vicinity of intersection line, a closed form of limit load solution for piping branch junctions with circumferential crack was developed. Then, 36 finite element (FE) models of piping branch junction with various dimensions of structure and crack were analyzed by using nonlinear finite element software. The limit loads from FE analysis and the proposed solution are compared with each other. Overall good agreement between the estimated solutions and the FE results provides confidence in the use of the proposed formulae for defect assessment of piping branch junctions in practice.

Finite element-based limit load of piping branch junctions under combined loadings

The limit load is an important input parameter in engineering defect-assessment procedures and strength design. In the present work, a total of 100 different piping branch junction models for the limit load calculation were performed under combined internal pressure and moments in use of non-linear finite element (FE) method. Three different existing accumulation rules for limit load, i.e., linear equation, parabolic equation and quadratic equation were discussed on the basis of FE results. A novel limit load solution was developed based on detailed three-dimensional FE limit analyses which accommodated the geometrical parameter influence, together with analytical solutions based on equilibrium stress fields. Finally, six experimental results were provided to justify the presented equation. According to the FE limit analysis, limit load interaction of the piping tees under combined pressure and moments has a relationship with the geometrical parameters, especially with the diameter ratio d/ D. The predicted limit loads from the presented formula are very close to the experimental data. The resulting limit load solution is given in a closed form, and thus can be easily used in practice.

Cracked piping branch junctions under pressure and in-plane bending: Comparison of experimental plastic loads and finite element limit loads

A series of piping branch junctions has been manufactured by computer numeric control (CNC) machining, fully instrumented and subjected to in-plane bending or internal pressure tests. For some of the junctions, crack-like defects have been introduced at strategic locations. All data recorded during the loading of the branch junctions have been used in an attempt to establish an acceptable plastic load for each junction using the twice-elastic slope method. Measured plastic loads and maximum experimental loads achieved are compared with finite element (FE) predictions of limit loads.

Evaluation of plastic limit load of piping branch junctions subjected to out-of-plane moment loadings

Under out-of-plane moment loadings, the piping branch junctions (also called tees in engineering) exhibit three kinds of failure mode, namely collapse failure of the branch pipe, global collapse of the intersection due to plastic hinges forming along the intersection line and local instability of the main pipe at the flank. In this work, the common piping branch junctions utilized in petrochemical and power industries with a failure mode of global collapse were investigated, and a new approximate formula for an out-of-plane plastic limit moment was presented. The formula was built on the following process: firstly, an equation between the out-of-plane limit moment and internal force of the branch pipe along the intersection is set up on the basis of the force equilibrium condition. Regarding this internal force as an external load for the main pipe shell, the internal force and moment along the intersection of the main pipe, under the plastic limit state, are then obtained. Finally, referring to the von Mises yield criterion, the approximate plastic limit load of the piping branch junctions subjected to the out-of-plane moment is derived. The accuracy of the new formula is validated by comparison with finite element analysis and experimental results.

Plastic limit load of welded piping branch junctions under internal pressure

In this work, a simplified estimation formula for plastic limit load of defect-free welded piping branch junctions subjected to the internal pressure is established. The relationship takes into account the effect of the internal forces between the run and branch pipe around the intersecting line. The formula is built on the following process: First, an equation between the limit load and internal force of branch pipe surrounding the intersection is derived on the basis of the force equilibrium condition. Secondly, regarding this internal force as an external force acting on the intersection of run pipe, the approximate solutions of internal force around the intersection on run pipe, under the plastic limit load, are given. Finally, referring to the von-Mises yield criterion, the plastic limit load of two intersecting cylindrical shells is then obtained. The formula’s suitability and feasibility applied in engineering practice is also validated with finite element analysis and experimental results in the paper.