Steel Elbow Research Articles

Failure investigation of welded high nickel stainless steel flange-elbow joint

Abstract Failure of a welded joint consisting of a high nickel stainless steel elbow joined to flanges on both sides is investigated. In one batch of the samples, after the welding procedure cracks in heat affected zone of the flange were observed before installing the joint into the field application. The welding cracks were persistent in hydrostatic test after the repair weld procedure. To assess the cause of weld joint failure, a range of analytical, microstructural and mechanical characterization tests were carried out. The susceptible microstructure of the flange led to its failure through hot cracking and secondarily stress relaxation cracking of the coarse grain flange under weld residual stresses. A better heat-treatment with finer grain size is suggested for flange steel material.

Life Prediction for Stress Corrosion Behavior of 316L Stainless Steel Elbow of Nuclear Power Plant

Life Prediction for Stress Corrosion Behavior of 316L Stainless Steel Elbow of Nuclear Power Plant

Effect of flow velocity on erosion–corrosion of 90-degree horizontal elbow

Erosion–corrosion is a serious problem in oil and gas gathering and transportation pipeline systems; specifically, the elbow is the weak part. Erosion–corrosion at different locations of a horizontal steel elbow through which a sand slurry was passed at different velocities was studied using weight loss measurement. Erosion samples were characterized using three-dimensional confocal microscopy and computational fluid dynamics was used to characterize the flow patterns and distribution of sand particles in the elbow. The erosion–corrosion rate increased most as the fluid velocity increased from 3.5 to 4.0m/s. Increasing particle velocity increased the mechanical effects of the particles and induced secondary flow, leading to differing velocity contours in different cross-sections of the elbow, and thereby differing rates of erosion–corrosion. The maximum rate appeared in the outer part (annular angles φ=45, 90, and 135 degrees) and bottom of the inner part (annular angles φ=225, 270, and 315 degrees) of the elbow outlet with an axial angle between 75 and 90 degrees.

Comparison Between Corrosion Behavior of Copper and Stainless Steel 90° Elbow and Failure Investigation of 90° Copper Elbow

Frequently found in elbow, erosion failures may lead to the leakage of pipes and even damage the whole system. Erosion is a form of material degradation that involves electrochemical corrosion and mechanical wear processes encountered on the surface of metal pipes. Research on the erosion–corrosion mechanism indicates that the erosion mainly results from the interactions between the elbow surface and the fluid traveling along the surface. Corrosion behavior of 90° copper and stainless steel elbow was studied. Scanning electron microscopy was processed on the 90° copper elbow to show the surface morphology of the failed copper elbow. Failure investigation was carried out on 90° copper elbow to determine failure location and failure causes.

Failure criterion for steel pipe elbows under cyclic loading

Abstract Steel elbow components are considered to be critical parts in industrial piping system due to their probability of collapse or failure. Therefore, the structural behavior of elbows is considered with respect to failure criteria through experiments and corresponding numerical models. Thirty-eight sets of experiments were conducted on three inch pipe elbow specimens. The numerical simulation results of the specimens are in good agreement with the test results. Damage indices available in the literature are used for failure estimation of the elbows. We suggest that the damage calculated using the Park and Ang damage index, and the Banon damage index, based on only one failed specimen under any constant amplitude cyclic loading, can be defined as the failure point and used to predict the failure of the component under other loading amplitudes. Therefore, the low cycle fatigue curve of an elbow can be derived using these simulation results. We also found that the calculated damage of an elbow component under constant, non-constant, and fully or partial amplitude reversals is quite similar.

Erosion-corrosion of stainless steel at different locations of a 90° elbow

Erosion-corrosion (E-C) behaviour of stainless steel at different locations of a 90° elbow was studied by array electrode technique. It is demonstrated that more negative corrosion potential and pitting potential, higher passive current density, and higher E-C rate appear at the outer wall of elbow outlet. The electrodes at the outer wall of elbow inlet and at the inner wall act as cathodes while the electrodes at the outer wall of elbow outlet as anodes after coupling. The passivation behaviour and galvanic effect of stainless steel elbow are associated with the hydrodynamics and sand particles distribution at the elbow.

Effect of Hydrodynamics on the Inhibition Effect of Thioureido Imidazoline Inhibitor for the Flow Accelerated Corrosion of X65 Pipeline Steel

Effect of hydrodynamics on the inhibition effect of thioureido imidazoline inhibitor (TAI) for the flow accelerated corrosion (FAC) of X65 carbon steel elbow was studied by electrochemical measurem...

Fracture studies on carbon steel elbows having part-through notch with and without internal pressure

Elbows are one type of components widely used in various industrial applications in conventional and nuclear power generating plants, chemical processing plants and petroleum refineries. Due to the complexities of geometric structure, elbows are one of the piping components that are prone to failure in the piping system of nuclear power plants (NPPs). In service, the piping components are subjected to internal pressure in addition to bending loads, and the internal pressure is known to have significant effect on the load carrying capacity of these components. Hence, fracture studies were carried out on 406 mm diameter elbows with and without internal pressure. The investigations have shown that the location of notch plays an important role in the load carrying capacity of the elbows and their failure behaviour. The internal pressure reduced the load carrying capacity of the elbow when the notch was located at the intrados in circumferential direction and the behaviour is vice-versa when the notch was located at the intrados in an axial direction, when subjected to opening moment, in both the cases.

Laser based impedance measurement for pipe corrosion and bolt-loosening detection

This study proposes a laser based impedance measurement system and impedance based pipe corrosion and bolt-loosening monitoring techniques under temperature variations. For impedance measurement, the laser based impedance measurement system is optimized and adopted in this paper. First, a modulated laser beam is radiated to a photodiode, converting the laser beam into an electric signal. Then, the electric signal is applied to a MFC transducer attached on a target structure for ultrasonic excitation. The corresponding impedance signals are measured, re-converted into a laser beam, and radiated back to the other photodiode located in a data interrogator. The transmitted impedance signals are treated with an outlier analysis using generalized extreme value (GEV) statistics to reliably signal off structural damage. Validation of the proposed technique is carried out to detect corrosion and bolt-loosening in lab-scale carbon steel elbow pipes under varying temperatures. It has been demonstrated that the proposed technique has a potential to be used for structural health monitoring (SHM) of pipe structures.

Inhibition effect of thioureidoimidazoline inhibitor for the flow accelerated corrosion of an elbow

Inhibition effect of thioureidoimidazoline inhibitor (TAI) for the flow accelerated corrosion (FAC) at different locations of X65 carbon steel elbow was studied by array electrode and computational fluid dynamics (CFD) simulation. It is demonstrated that TAI is an anodic type inhibitor by remarkably inhibiting the anodic process. The inhibition efficiency at the inner wall is lower than that at the outer wall, which is associated with higher flow velocity, shear stress and turbulent kinetic energy at the inner wall of the elbow. The distribution of inhibition efficiency is in good accordance with the distribution of hydrodynamic parameters.

Low-Cycle Fatigue of Pressurized Steel Elbows Under In-Plane Bending

The present study examines the mechanical behavior of steel process piping elbows, subjected to strong cyclic loading conditions. The work is numerical, supported by experimental data on elbow specimens subjected to in-plane cyclic bending, with or without internal pressure, resulting in failure in the low-cycle-fatigue range. The investigation of elbow behavior is conducted using rigorous finite element analysis accounting for measured elbow geometry and the actual material properties. An advanced cyclic plasticity material model is employed for the simulation of the tests. Emphasis is given on the value of local strain and its accumulation at the critical elbow location where cracking occurs. Based on the cyclic stress–strain curve of the material and the strain-based fatigue curve from the test data, the use of Neuber's formula leads to a fatigue analysis and design methodology, offering a simple and efficient tool for predicting elbow fatigue life.

Erosion–corrosion at different locations of X65 carbon steel elbow

Erosion–corrosion (E–C) behavior of X65 pipeline elbow was investigated by array electrodes technique and computational fluid dynamics (CFD) simulation. The four components (pure corrosion rate, pure erosion rate, erosion–enhanced corrosion rate and corrosion–enhanced erosion rate) of E–C rate at different locations of an elbow were quantified. The percentages of these four components were also determined to reveal the dominant factor and the essential difference in E–C at different locations of the elbow due to the change in hydrodynamics. The distribution of E–C rate is in good accordance with the hydrodynamics and sand particles distribution at the elbow.

A132 長時間使用された高クロム鋼エルボの観察および評価(OS2 保全・設備診断技術(2))

A132 長時間使用された高クロム鋼エルボの観察および評価(OS2 保全・設備診断技術(2))

The Ratcheting Behaviour of Stainless Steel Pressurized Piping Elbows Subjected to Dynamic Out-of-Plane Moments

In this paper the ratcheting behavior of four pairs of stainless steel elbows is studied under conditions of steady internal pressure and dynamic conditions that induced out-of-plane external moments at frequencies typical of seismic excitations. The finite element analysis with the nonlinear kinematic hardening model has been used to evaluate ratcheting behavior of the piping elbows under mentioned loading condition. Material parameters have been obtained from several stabilized cycles of specimens that are subjected to symmetric strain cycles. The direction of maximum strain is at about 45° between the hoop and axial directions. The results show that the direction of highest ratcheting is along the hoop direction rather than the direction of maximum principal strain. Also, the initial rate of ratcheting is large and then it decreases with the increasing cycles. Also, the FE method gives over estimated values compared with the experimental data.

Experimental fracture studies on carbon steel elbows with and without internal pressure

Pipe bends or elbows are commonly used components for nuclear power plant piping system. In service, these piping components are subjected to internal pressure in addition to bending loads and the internal pressure is known to have a significant effect on the load carrying capacity of these components. Hence, a systematic study was carried out to investigate and quantify the effect of internal pressure on the fracture behaviour of elbows used in nuclear power plant piping system. Fracture studies were conducted on five 219 mm diameter carbon steel elbows with and without internal pressure under in-plane opening moment. The investigations have shown that the presence of a circumferential notch at the intrados has a more detrimental effect on the fracture behaviour of the elbow, when compared with the presence of an axial notch at the intrados. It is also found that internal pressure plays a significant role in reducing the ovalization.

Investigation on the Bending Ratcheting Behavior of Pressurized Z2CND18.12N Stainless Steel Elbows

A series of ratcheting experiments and finite element analysis simulation under bending loading for Z2CND18.12N stainless steel elbows were carried out. Chaboche and modified Ohno-Wang model are applied to evaluate structural ratcheting response simulations. It is found that ratcheting strain initiates firstly in the hoop direction and increases in the axial direction with the increasing of loading. The Ratcheting strain rate grows with the increase of the reversed in-plane bending load or internal pressure for both specimens with different loadings. Comparison of simulation and experiment showed that modified Ohno-Wang model presented simulation more reasonably.

Cyclic Fracture, FCG and Ratcheting Studies on Type 304LN Stainless Steel Straight Pipes and Elbows

Abstract With an objective of understanding the cyclic fracture, fatigue crack growth and ratcheting behaviour of Type 304LN stainless steel piping components used in primary heat transport system of nuclear power plants, subjected to various operational, loading and environmental conditions, experimental studies were carried out on totally 36 straight pipes and eight elbows. The pipe dimensions were in two categories: 170 mm OD and 15 mm thickness or 324 mm OD and 25 mm thickness. These studies on actual size piping components of power plants have resulted in very valuable fatigue behaviour related data, which would form important inputs in assuring safety and damage tolerant design of power plants. An overview of the studies is presented in this paper.

Ratcheting Strain Assessment in Pressurised Stainless Steel Elbows Subjected to In-plane Bending

Abstract Ratcheting studies were carried out on Type 304LN stainless steel elbows, by applying steady internal pressure and cyclic bending. The elbows were of 168 mm nominal outer diameter and the average thickness varied from 14.7 mm to 15.1 mm. The elbows filled with water were pressurized between 27.6 MPa and 39.2 MPa. Cyclic bending load, under the opening and closing moments, was applied on the elbows at ambient temperature. During the ratcheting experiments, strains were measured at different locations in the curved and straight portions of the elbows using post-yield rosette strain gauges. Ratcheting strain and its rate of change were estimated at different locations of the elbows with increase in number of cycles. In the curved portion of the elbow, ratcheting strain was observed in the hoop direction at the intrados, crown and at a few other locations other than the extrados. No significant ratcheting was observed in both the axial and circumferential directions at the extrados location for all the specimens. The specimens failed by the occurrence of through-wall axial crack in the curved portion, at one of the crown locations, accompanied by simultaneous bulging.

Fatigue Crack Growth Behavior in Pipes and Elbows of Carbon Steel and Stainless Steel Materials

Abstract The objective of the present study is to understand the fatigue crack growth behavior and validate analytical procedures for austenitic stainless steel and carbon steel pipes, pipe welds and elbows. The study involved fatigue tests on actual components and specimens. The Paris law has been used for the prediction of fatigue crack growth life. Paris constants have been determined for pipe (base), pipe weld and pipe elbow materials by using Compact Tension (CT)/Three Point Bend (TPB) specimens machined from the actual pipe, pipe weld and pipe elbow. Analyses have been carried out to predict the fatigue crack growth life of these piping components having part through cracks on the outer surface. In the analyses, Stress Intensity Factor ( K ) has been evaluated through two different schemes. The first scheme considers the ‘ K’ evaluations at two points of the crack front i.e. maximum crack depth and crack tip end at the outer surface. The second scheme accounts for the area averaged root mean square stress intensity factor (KRMS) at deepest and surface points. In order to validate the analytical procedure/results, experiments have been carried out on full scale pipes, pipe welds with part through circumferential crack. The tests have also been conducted on the full scale carbon steel elbows having part-through circumferential notch at intrados location and axial notch at crown location on the outer surface. Fatigue crack growth life evaluated using both schemes have been compared with experimental results. Use of stress intensity factor (KRMS) evaluated using second scheme gives better fatigue crack growth life prediction compared to that of first scheme.

Ratcheting Studies on Type 304LN Stainless Steel Elbows subjected to Combined Internal Pressure and In-Plane Bending Moment

Abstract “Ratcheting” is a phenomenon which leads to reduction in fatigue life of a structural component by loss of ductility due to cycle by cycle accumulation of plastic strain. Ratcheting occurs in a structure subjected to a combination of steady/sustained and cyclic loads such that the material response is in inelastic region. Ratcheting studies were carried out on Type 304LN stainless steel elbows, subjected to steady internal pressure and cyclic bending. The elbows filled with water were pressurized between 27.6 MPa and 39.2 MPa. Cyclic bending load, under opening and closing moments, was applied on the elbows at ambient temperature. Number of cycles corresponding to occurrence of a through-wall crack was recorded. Crack was observed in the bent portion at one of the crown locations in all the four specimens. Maximum strain was observed at the intrados and crown locations of the elbows. The ratcheting strain increased with number of cycles at crown and intrados locations. However, the strain accumulation rate decreased with number of cycles. Strain was observed to be minimum at the extrados location and the same stabilized toward the end of the tests. The specimens have failed by occurrence of through-wall axial crack accompanied by simultaneous ballooning. The ballooning was found to be varying from 3.8% to 5.8% with respect to the original circumference in the bent portion. The reduction in thickness was found to be around 12%–15%.