Steel Elbow Research Articles

Damage analysis in welded steel pipe elbow under strong cyclic loading

According to cyclic loading modes, highly pressurized tubular elements, such as those in elbows, differ from other straight tubular structures in their modes of stress and damage. This work aims to perform a numerical prediction of a tubular structure. It consists of an X52 steel elbow welded to X65 grade ends by straight tubular sections. This pressurized structure is subjected to strong cyclic loading in bending moment applied until damage occurs. The bending moment is combined with different orientations between the closing or opening moment and the out-of-plane moment, presenting a possible but rarely analyzed situation. This cyclic loading mode is taken as an evaluative parameter in its various amplitudes, orientations, and patterns. The cyclic hardening of the elbow and straight tubular sections, including those of the welded joints, are all formulated by the combined isotropic and kinematic model introduced in the ABAQUS calculation code with parameters calibrated according to the Voce model and experimental results, using plasticity by the Von Mises equivalent stress flow theory. The appropriate XFEM (Extended Finite Element Method) technique is used in conjunction with the hardening model to overcome numerical calculation difficulties and predict damage in traction laws, separation by crack initiation and propagation. The results, in the form of hysteresis curves in cyclic bending moment and angular displacement, have shown a strong dependency that conditions the structure’s response as well as the level of its damage.

Effects of fluid dynamics parameters on flow-accelerated corrosion at elbow of carbon steel pipeline

Flow-accelerated corrosion (FAC) has always posed a significant threat to the safe operation of the secondary circuit in nuclear power units. In this study, we investigated typical carbon steel elbow pipe sections susceptible to FAC failure using fluid dynamics software to analyze the hydrodynamic characteristics at varying inlet velocities (2 m s−1, 4 m s−1, and 6 m s−1). The distribution of the FAC rate was monitored in real time using an array electrode. The results revealed that the outermost side of the elbow pipe section was the most susceptible location to FAC. By comparing different fluid dynamic parameters with the FAC rate, we identified radial velocity as an effective parameter for characterizing the FAC rate. Additionally, we established an empirical formula for predicting flow-accelerated corrosion in elbow pipe sections using the least squares method. The implications of this research are pertinent to the design and operation of pipelines in nuclear power plants.

Very low-cycle fatigue failure behaviours of pipe elbows under displacement-controlled cyclic loading

In this study, quasi-static very low-cycle fatigue tests were conducted on pipe elbows at room temperature to investigate the effects of the pipe thickness, internal pressure, and material on the failure behaviours under large-amplitude cyclic loads. Three types of elbow specimens comprising two different materials and two different pipe thicknesses were used in the tests. Elbow specimens were subjected to displacement-controlled, fully reversed, in-plane mode cyclic bending with and without internal pressure. All specimens failed due to through-wall cracking of which the location and orientation depend only on the pipe thickness regardless of the internal pressure and elbow material. For thin-walled elbows of Schedule 40, the axial crack at flank was developed, whereas for thick-walled elbows of Schedule 160, the circumferential crack at intrados. The number of cycles to failure decreased with increasing internal pressure in thin-walled elbows. However, the effect of internal pressure on the number of cycles to failure was negligible in thick-walled elbows. Regardless of internal pressure level, stainless steel elbows were more resistant to failure, compared with carbon steel elbows of identical dimensions. Finally, the strain-based evaluation model considering the void growth and shrinkage effect was applied to predict fatigue failure location and cycles. Two parameters in the model, the multi-axial fracture strain locus and void shrinkage ratio, were determined from monotonic tensile test data and the fatigue life curve of the material. The predicted crack initiation site and orientation were consistent with experimental finding. For all cases, the predicted failure cycles were less but overall close to experimental data.

Erosion impact on mild steel elbow pipeline for different orientations under liquid-gas-sand annular flow

In this paper, the quantitative results of erosion of a horizontal-vertical (H-V) downward long radius 90° elbow and horizontal-horizontal (H-H) elbow for abrasive annular flow conditions is evaluated using experimental and Computational Fluid Dynamics. Experimental results indicate that the critical erosion eventuates when the angle reaches approximately 45°, which is perceived in both H-V and H-H orientation. The excessive pitting and microcutting are the main causes of erosive wear for both oriented elbows after the abrasive particle collision. Meanwhile, the 1018 steel H-H exhibited the lowest thickness loss, surface roughness, hardness, and mass loss in comparison with H-V oriented elbow. The corresponding erosion rate of the H-V elbow was comparatively 21% higher than 90° H-H elbows for identical annular flow conditions.

Root cause analysis for cracked pipe in auxiliary system in nuclear power plant

Penetrated cracks were found in succession in 316LN stainless steel elbows in auxiliary system in a Chinese nuclear power plant after about 19 years’ service. In order to clarify the root cause of the cracking, the cracked elbow with nominal diameter of 12 in. was cut and its chemical composition, hardness, microstructure and residual strain were investigated. The results show that this elbow was heavily cold worked during push-bending and the degree of cold work was within 25∼40%. The manufacturer did not obey RCC-M M3317 to solution treat this elbow before delivery. The heavily cold-worked elbow suffered stress corrosion cracking in occluded primary water.

Ratcheting-fatigue behavior and life prediction of Z2CN18.10 austenitic stainless steel elbow

The ratcheting fatigue behavior of Z2CN18.10 austenitic stainless steel elbow under internal pressure and cyclic loading was studied. The evolution of axial and hoop ratcheting strains at four different positions, respectively at intrados (0°), 45°, crown (90°) and extrados (180°) on the outer surface of the elbow was discussed. It is shown that the hoop ratcheting strain of the elbow is always greater than the axial ratcheting strain. Under cyclic test with force control, the order of ultimate hoop ratcheting strain is intrados (0°)>crown (90°)> 45°> extrados (180°). Under cyclic test with displacement control, the order of ultimate hoop ratcheting strain is 45°> intrados (0°)> crown (90°)> extrados (180°). Under cyclic tests with both force and displacement control, the crown (90°) of the elbow leaked first, which is the critical point to evaluate ratcheting fatigue life. Afterward, the ratcheting fatigue life is predicted based on ASME code, and it is found that the prediction results tend to be non-conservative. Then, a modified Manson-Coffin equation considering ratcheting strain is proposed to predict fatigue life of elbow well.

Experimental and numerical investigation of ratcheting behavior in seamless carbon steel 90° elbow pipe with small dimensions under constant internal pressure and in-plane cyclic bending

The purpose of this study is to identify the effects of mean fatigue load, amplitude fatigue load, internal pressure and loading history on ratcheting behavior of seamless A234 WPB steel elbow pipe. So, in this paper the ratcheting behavior of seamless 90° elbow pipe of A234 WPB steel under constant internal pressure and in-plane cyclic bending is investigated experimentally and numerically. At first, in order to the confirm of elbow pipe material, a series of experimental tests were performed on elbow specimen, including: chemical composition analysis, metallography testing, hardness testing and uniaxial tensile testing. In the next step, the fatigue tests were performed on elbow pipe specimens at ambient temperature (25 °C) under asymmetric cyclic loading with load-control conditions. According to the experimental fatigue data, concluded that the increasing of mean load, amplitude load and internal pressure, increases the accumulation of ratcheting strain in axial direction at intrados and extrados locations, and also in hoop direction at crown location. So that the increasing of mean load or amplitude load has the greatest effect on the ratcheting strain at intrados and extrados locations. Also, it was found that the loading history affected to ratcheting behavior and premature failure of pressurized elbow pipes. Finally, the finite element simulation was carried out by ANSYS software based on the nonlinear kinematic/isotropic hardening model, and the FE simulation results were compared with experimental fatigue data and it was found that the numerical results are in good agreement with experimental fatigue data.

Novel assessment of in-operations mapped stainless steel elbow pipe internal wall for detection of corrosion and scale deposition

Novel assessment of in-operations mapped stainless steel elbow pipe internal wall for detection of corrosion and scale deposition

Experimental investigation of a method for diagnosing wall thinning in an artificially thinned carbon steel elbow based on changes in modal characteristics

Curved cylindrical structures such as elbows have a non-uniform thickness distribution due to their fabrication process, and as a result have a number of complex mode shapes, including circumferential and axial nodal patterns. In nuclear power plants, material degradation is induced in pipes by flow accelerated erosion and corrosion, causing the wall thickness of carbon steel elbows to gradually thin. The corresponding frequencies of each mode shape vary according to the wall thinning state. Therefore, the thinning state can be estimated by monitoring the varying modal characteristics of the elbow. This study investigated the varying modal characteristics of artificially thinned carbon steel elbows for each thinning state using numerical simulation and experimental methods (MRIT, Multiple Reference Impact Test). The natural frequencies of specified mode shapes were extracted, and results confirmed they linearly decreased with increasing thinning. In addition, by comparing single FRF (Frequency Response Function) data with the results of MRIT, a concise and cost effective thinning estimation method was suggested.

Experimental and numerical analysis of ratcheting behavior of A234 WPB steel elbow joints including corrosion defects

In this paper, ratcheting behavior of intact and corroded 90o elbow pipe specimens of A234 WPB steel is investigated at intrados, crown, and extrados positions. A special fixture was designed and constructed to carry out the fatigue experiments. Then, the effects of fatigue loading parameters (mean and amplitude) and defect depth were examined on ratcheting responses of intact and corroded specimens at intrados, crown, and extrados positions. To simulate the ratcheting behavior of specimens, a finite element analysis was performed based on the combined nonlinear isotropic/kinematic hardening model. As a reasonable agreement was found between the finite element (FE) simulation results and experimental data, the influence of constant internal pressure during the cyclic loading was examined on ratcheting responses through FE simulation. Results indicated that decreasing the load amplitude was more effective on decreasing the ratcheting strain than that of the mean fatigue load. Moreover, the most critical ratcheting response was associated with the intrados location of the pipe. For defective specimens, decreasing fatigue loading parameters resulted in a shakedown behavior for a defect placed at extrados, while, for the defects at intrados and crown, an increasing behavior was observed for the ratcheting strain. It was revealed that, at extrados position, the ratcheting behavior was arrested up to 1 mm defect depth. The ratcheting strains were increased at intrados and crown by increasing the internal pressure, while they were decreased at extrados.

Neural network method for the modeling of SS 316L elbow corrosion based on electric field mapping

Abstract Stainless steel is known for its superior corrosion resistance in industrial applications. In this work, corrosion modeling of stainless steel 316L is presented using artificial neural networks. The experimental setup consists of a loop containing stainless steel elbow with simulated seawater of known concentration continuously flowing at a specific flow rate, thus allowing to study the effect of flow dynamics and salt concentration on corrosion. Electric field mapping setup is used to collect the voltage and current information along with the temperature of the elbow section. In addition to modeling, characteristics of the observed scale deposits are also studied in-depth and briefly reported in this work.

Investigation of Erosion-corrosion of Carbon Steel Elbow in 3.5% Sodium Chloride Containing Quartz

Investigation of Erosion-corrosion of Carbon Steel Elbow in 3.5% Sodium Chloride Containing Quartz

MODELING OF STEEL ELBOW MANUFACTURING PROCESSES VIA FINITE ELEMENT METHOD AND OPTIMIZATION OF PRODUCTION PARAMETERS

With the advancement of today's technologies, parts of pipe and elbow elements, natural gas pipelines, petrochemicals, machinery manufacturing industry, water distribution and flow systems, sewage systems, foundation piles, shipbuilding industry, nurse containers, aviation, space and defense industry, pipe networks, refineries , steel structures, tube and pipe engineering, etc. widely used in areas. In this study, extrusion method, which is one of the cold forming methods of elbow pipes, has been investigated. When the literature is examined, the production conditions of steel pipes and the problems that occur in production have been examined by different people and suggestions have been made. In this study, the production process of AISI 316L (EN 1.4404) quality stainless steel elbow pipes has been investigated. The mold design model created with the Solidworks program was transferred to the non-linear finite element analysis software ANSYS program for simulations. The process was modeled and simulated in the transient structural analysis section of the ANSYS program. In the process of transforming the pipe into an elbow product, we have learned about the stress distributions and obtaining the total deformation changes on the elbow. The parameters of the production process have been optimized by comparing the work done in the simulation and experimental data. In this way, cost-effective production is aimed by minimizing production costs. The data was extracted by simulating the production environment and conditions.

Atmospheric Corrosion Analysis on Low Carbon Steel Plate Profile and Elbow in Medan Belawan District

The objective of this study is to carry out an atmospheric corrosion analysis on low carbon steel in the form of plates and elbows in the port area of Belawan. The exposure of the test specimen was carried out at five locations for 12 months. The location points are notated as A, B, C, D, and E. The process of specimen exposure and data collection followed the ASTM G-1 and ASTM G-50 standards. The measurement results indicated a slight difference in the corrosion rate at each location. Location C and D have the highest corrosion rate and location A has the lowest corrosion rate. Meanwhile, for locations B and E have a corrosion rate value that is not much different from location A. This difference in value is probably a consequence of the distance from the exposure location to the seaside. Locations C and D are adjacent to the seaside while the other locations are not. The steel plate has an average value of corrosion rate 0.2214 mpy and steel elbow has a rate of 0.2412 mpy. From the measurement results obtained, the two specimens exposed at each location are still in the "outstanding" category.

Internal cathodic protection to study the erosion‐corrosion of AISI 1018 carbon steel

AbstractSteel elbows used in pipelines carrying potash slurry may undergo material loss due to mechanical erosion and/or electrochemical corrosion. Cathodic protection was used to study the effects of flow velocity and slurry concentration on the protection current density required to maintain AISI 1018 carbon steel elbows at a set electric potential. A flow loop with a peristaltic pump was used to pump a slurry consisting of silica sand in a saturated potash brine through the steel elbows. The protection current density measurements were performed under varying slurry flow velocities of 2.5, 3.0, 3.5, and 4.0 m/s and varying slurry concentrations ranging from 0‐35 wt%. The results show maximum values of protection current density were required for mid‐range slurry concentrations. It is concluded that, within the parameters of study, high flow velocities require higher protection current density than low flow velocities. Furthermore, high flow velocities result in a local maximum protection current density being reached at lower slurry concentrations; conversely, a local maximum protection current density is reached at higher slurry concentrations in lower flow velocities. The contrast in local maximum protection current densities likely occurs due to additional particle‐particle collisions at higher velocities causing the maximum mass transfer rate of oxygen to be reached at lower slurry concentrations. SEM micrographs show that wear becomes more evenly distributed with increasing flow velocity due to the homogeneous distribution of particles in the flowing medium.

Experimental and numerical simulation of erosion-corrosion of 90° steel elbow in shale gas pipeline

Coexistence of the solid particle erosion and acid liquid corrosion is common in shale gas gathering and transportation pipelines. It is conceivable that the synergistic effect of erosion and corrosion accelerates the pipeline damage process. In this research, a novel experimental device is designed to study the erosion-corrosion characteristics of 90° steel elbow in the gas-liquid-solid three-phase flow. A total of 28 group experiments are designed including 4 gas velocities, 3 liquid pH values, and 150 μm average sand particles. Moreover, a numerical simulation model is built based on the FLUENT code to predict the erosion-corrosion behavior. Experimental and simulation results show that the most severe erosion-corrosion region of the pipe bend occurs at the axial angle of the elbow between 20° and 50° and at the radial angle of the elbow between −45° and 45°. At the bottom of the elbow and the outer wall surface close to the outlet, the synergistic effect of erosion and corrosion accelerates the wear of the pipeline, but the opposite phenomenon exists at the position of the elbow with an axial angle of 50–70°.

Study of CAP1400 secondary pipe wall thinning rate under flow accelerated corrosion

Wall thinning rate is an important issue during design of CAP1400 nuclear power plant (NPP) secondary loop. A single-phase flow accelerated corrosion (FAC) test loop was established to simulate the typical working condition of the secondary loop. The wall thinning rate of A335P11 and 20# steel elbow was measured using high precision ultrasonic sensor on the test loop. The results show that elbow part has higher thinning rate than that at straight part, especially at intrados of elbow. Wall thinning rate of 20# steel elbow is about three to four times of that A335P11 steel because of the high contents of Chromium at A335P11. According to the FAC test results it shows that A335P11 could be one of the preferred pipe material of CAP1400 secondary loop.

Ratcheting assessment of corroded elbow pipes subjected to internal pressure and cyclic bending moment

The present study aimed to study ratcheting strains of corroded stainless steel 304LN elbow pipes subjected to internal pressure and cyclic bending moment. To this aim, spherical and cubical shapes corrosion are applied at two depths of 1 mm and 2 mm in the critical points of elbow pipe such as symmetry sites at intrados, extrados, and crown positions. Then, a Duplex 2205 stainless steel elbow pipe is considered as an alternative to studying the impact of the pipe materials, due to its high corrosion resistance and strength, toughness, and most importantly, the high fatigue strength and other mechanical properties than stainless steel 304LN. In order to perform numerical analyzes, the hardening coefficients of the materials were calculated. The results highlight a significant relationship between the destructive effects of corrosion and the depth and shape of corrosion, so that as corrosion increases, the resulting destructive effects increases as well, also, the ratcheting strains in cubic corrosions have a higher growth rate than spherical corrosions. In addition, the growth rate of the ratcheting strains in the hoop direction is much higher across the studied sample than the axial direction. The highest growth rate of hoop strain was observed at crown and the highest growth rate of axial strains occurred at intrados position. Altogether, Duplex 2205 material has a better performance than SS 304LN.

Effect of distance between two elbows in series on erosion for gas dominated conditions

Solid particle erosion is a potential problem in pipelines for the oil and gas processing industries. Solid particle erosion usually occurs in piping components or equipment when the direction of the flow changes, such as in elbows and tees. The distance between two elbows effects on the behavior of solid particle erosion on the second elbow has been investigated experimentally and computationally. Firstly, paint removal experiments were carried out to investigate the erosion pattern in a standard 3-inch (76.2 mm) elbow with a bend radius to pipe diameter (r/D) ratio equal to 1.5 for a gas-sand condition with different particle sizes. The investigated distances between the two elbows are 2D and 12D. Secondly, the erosion magnitudes in a standard 3-inch (76.2 mm) stainless steel elbow were measured using a state-of-the-art ultrasonic technique. These results have been compared with the previous experimental data in a 4-inch (101.6 mm) elbows to investigate the pipe size effect. Thirdly, a commercial CFD code was used to simulate the experimental conditions and compare erosion predictions with the experimental results. Different turbulence and particle rebound models were applied in the CFD simulations to examine their influence on the predicted erosion on the elbows. Finally, the effect of distance between the two elbows on the particle trajectories were investigated using the CFD approach.

Using XFEM to Predict the Damage with Temperature of the Steel Pipe Elbows under Bending and Pressure loading

The pipes, during their service, are subjected to accumulated loads such as internal pressure and that of the soil. The latter considerably accelerate their damage. In this work, the bending moment stress of API 5L X70 category steel elbows under thermo-mechanical behavior and in the presence of pressure were studied. We used FEM (finite element method) through the numerical calculation code ABAQUS and the XFEM technique for structural damage while using solid elements as a structure. Our objective is to evaluate the response and resistance capacity of the steel elbow by its location in the tube–elbow-tube system under a mixed loading of pressure and moment for all scenarios. It is based on a single standardized dimensioning of the elbow (diameter and thickness). The effect of several parameters has been studied such as the type of loading and the pressure levels, which are clearly conditioned by the level of damage. Numerical damage results are presented by moment-rotation curves. They illustrate the variation in damage as a function of these effects which act simultaneously.