Failure analysis of natural gas pipes

Incident involving failures of 6 months old API 5L X42 (NPS8) and SDR 17, 125 mm medium density polyethylene pipe (MDPE) supplying natural gas to an industrial customer has caused serious 7 h supply disruption. Study was performed to identify the most probable cause of the pipes failures. The study conducted by reviewing the existing design and construction data, visual physical inspection, pipe material analysis, structural analysis using NASTRAN and Computational Fluid Dynamics analysis (CFD) using FLUENT. Investigations revealed that high pressure water jet from leaked water pipe had completely mixed with surrounding soil forming water soil slurry (high erosive properties) formed at a close vicinity of these pipes. Continuous impaction of this slurry upon the API 5L X42 pipe surface had caused losses of the pipe coating materials. Corrosion quickly ensued and material loss was rapid because of the continuous erosion of oxidised material that occurred simultaneously. This phenomenon explains the rapid thinning of the steel pipe body which later led to its failure. Metallurgical study using photomicrograph shows that the morphology of the steel material was consistent and did not show any evidence of internal corrosion or micro fractures. The structural and CFD simulation results proved that the location, rate and the extent of erosion failures on the pipe surfaces can be well predicted, as compared with actual instances.     

Experimental and numerical studies on corrosion failure of a three-limb pipe in natural gas field

A bursting incident occurred in a three-limb pipe, having 16Mn steel for the main pipe and 316L + L416 composite metal for the branch pipe, in a natural gas field. The failure analysis was performed by means of inspection, experiments and computational fluid dynamics (CFD) simulation. The CFD results indicated the radical change in the flow characteristics inside the three-limb pipe due to its upright structure and the formation of a low vortex in the downstream near the junction, which indicated the condensation of water vapor containing high salinity. The condensed brine saturated with CO₂ adhered to the inner wall surface of the main pipe. In such a corrosive medium, 16Mn steel acts as an anode and is preferentially corroded due to galvanic corrosion. In addition, the downstream area, covered by low vortex, exhibited high shear stress and droplet impingement stress, resulting in an increase in flow erosion. Thus, the failure of the three-limb pipe can be attributed to the synergistic effect of galvanic corrosion and flow erosion.     

Failure analysis of multiphase flow corrosion–erosion with three-way injecting water pipe

Water injection is an essential component in a reaction effluent air cooler (REAC) system because its primary function is to dissolve the generated ammonium salt, which leads to deposition or blockage accidents. A damage incident in a three-way pipe made of carbon steel under the multiphase flow field was investigated. The failure analysis was performed by means of scanning electron microscope (SEM) inspection and computational fluid dynamics (CFD) simulation. CFD results show that a large velocity gradient exists near the area of 5–12d at the bottom of the main pipe. This gradient results in a region of low flow velocity, high wall shear stress, and high turbulent kinetic energy. The flow state becomes very chaotic, and the non-uniformity coefficient of velocity is high. The corrosive medium (NH₄Cl or H₂S) dissolving in water increases the causticity of fluid medium and aggravates the flow corrosion. The high risk area from the CFD simulation coincides with the breakage area of the three-way pipe on the spot. This failure incident is attributed to the flow corrosion–erosion.     

Erosion rate correlations of a pipe protruded in an abrupt pipe contraction

Erosion is one of the most serious problems in various gas and liquid flow passages such as flow in pipes, pumps, turbines, compressors and many other devices. Sand presence causes loss of pipe wall thickness that can lead to pipe erosion, frequent failures and loss of expensive production time. The importance of this problem is mainly due to many related engineering applications, viz. heat exchangers. In order to reduce the frequency of such pipe erosions, caps in the form of replaceable pipes are protruded in the sudden contraction regions which are exposed to most of the serious erosion rates. In the present work, numerical investigation of the erosion of a pipe protruded in a sudden contraction is presented. The turbulent, steady, 2-D axi-symmetric flow inside an axi-symmetric abrupt contraction pipe with a pipe protrusion embedded in it was solved by steady-state time averaged conservation equations of mass and momentum along with two equation model for turbulence. Particles are tracked using Lagrangian particle tracking. An erosion model was employed to investigate the erosion phenomena for the given geometry. The influence of the different parameters such as the inlet flow velocity (3–10 m/s), the particle diameter (10–400 μm), the protruded pipe geometry (thickness T=1–5 mm and depth H=2–5 mm) and the pipe contraction ratio (Cr=0.25–0.5) on the erosion of pipe protrusion was investigated. Correlations for the influence of inlet flow velocity, depth and thickness of the protruded pipe on the erosion rate are presented.     

Impact of Internal Pipe Grooves on Flow-Accelerated Corrosion of Small-Bore A-106 Carbon Steel Pipes

Segments of welded small-bore A-106 carbon steel from a piping system that experienced severe flow-accelerated corrosion were characterized for surface damage due to flow-accelerated corrosion (FAC). A computational fluid dynamics (CFD) analysis was done to compare findings of CFD model versus observed surface FAC damage inside the pipes. CFD results expressed in terms of turbulence intensity showed good agreement with actual surface damage due to FAC. It was concluded that the presence of internal grooves would cause turbulent flow regime, and therefore, it would cause pipe material damage.     

Failure analysis of IEU drill pipe wash out

Many 127.0×9.19 mm IEU G105 drill pipe failures of wash out occurred after 2367 h of pure drilling time and 8726 m of penetration footage. This paper gives a detailed investigation on these failures and a systematic analysis is carried out on service and loading conditions of the drill pipes. Measurement and inspection were performed on configuration dimensions, chemical composition, mechanical performance, metallography, macro-fractography, micro-fractography, and corrosion products. Configuration stresses at the crack positions of the drill pipe were calculated by FEA. Crack extending velocity of the drill pipe material under corrosion medium was also measured. It is thought from test and analysis results that the drill pipe wash out or fracture accidents were premature corrosion fatigue failure accident. The failure courses were as the following: corrosion pits occurred first on the internal surface at the stress concentrating area of the drill pipe, and then fatigue cracks initiated in pit bottoms, and washed out or fractured subsequently as cracks penetrated through the wall thickness of the drill pipe. The reasons of drill pipe wash out were related to configuration, material quality, and load condition of the drill pipe string.     

Experimental testing and numerical simulation to analyze the corrosion failures of single well pipelines in Tahe oilfield

Frequent corrosion failures occurred on single well gathering lines in a section of the Tahe oilfield. The corrosion failures were investigated by means of experimental lab testing and numerical simulation. The electrochemical test results showed that the corrosion of the pipelines were controlled by two different mechanisms, which were influenced mainly by excessive dissolved oxygen (DO), higher temperature, lowered flow velocity and increased water cut. One mechanism was influenced by the excessive DO concentration and the higher temperature of injection water near the pipe entrance region. The corrosion rate noticeably increased at the entrance at a high temperature until DO was exhausted. The other mechanism resulted from the oil-water separation due to decreased flow velocity at the terminal climbing pipe sections. The change in flow led to the suspended particles being deposited, which caused under-deposit corrosion. The statistical analysis of the field corrosion data supported the two corrosion mechanisms.     

Experimental method for determining through thickness residual hoop stresses in thin walled pipes and tubes without inside access

The determination of through thickness residual stresses in pipes and pressure vessels is of growing interest because of emphasis placed on life prediction, design, and failure analysis of piping systems. Most of the through thickness residual stress measurement techniques require the placement of gauges on the outside and inside of the pipe. These methods are severely hampered when gauges cannot be placed on the inside of the pipe. This constraint could arise for small diameter pipes, long pipes or for pipes that have been used in a service condition causing corrosion or fouling of the inner surface.
This paper focuses on the first step of a three step procedure for determining residual hoop stresses in thin walled pipes and tubes. The method described is designed for cases where it is impossible to place gauges on the inside of the pipe. The method yses biaxial strain gauges on the outside of the pipe and involves a through thickness axial cut of the pipe. Based on the change in strain on the outside of the pipe, changes in the hoop residual stress distribution due to the axial cut are obtained with the method presented here. The method provides a means to evaluate changes in stresses on both the outside surface and the inside surface of the pipe as well as an evaluation of the change in through thickness hoop stress distribution at any location in the pipe cross section. This paper further demonstrates that the problem of shortening long pipes to enable placement of gauges on the inside of the pipe can result in the loss of significant residual stress information.     

CFD modeling of erosion wear in pipe bend for the flow of bottom ash suspension

In the present study, erosion wear behavior of slurry pipeline due to solid–liquid suspension in the pipeline has been investigated using commercial computational fluid dynamics (CFD) code FLUENT. A multiphase Euler–Lagrange model was adopted to predict the solid particle erosion wear in a 90° pipe bend for the flow of bottom ash–water suspension. A standard k–ε turbulence modeling scheme was used to simulate the flow through the pipeline. Water and bottom ash were taken as liquid and as a dispersed phase of solid–liquid mixture, respectively. A simulation study for erosion wear in a pipe bend was carried out to investigate the influence of various parameters including velocity, solid concentration, and particle size. The velocity of the bottom ash–water suspension varied from 0.5 to 2.5?m/s for solid concentrations with a range of 2.5 to 10.0% (by volume). The particle diameters of the bottom ash were 162 and 300?µm. The simulation results agree with the results of previous studies.     

Hydraulic transport of solids in pipes: a simple method for the prediction of pressure drop

Suspensions such as grouting slurries, concrete and dredge muds exhibit a complex flow behaviour in pipes which is not fully understood. This flow in cement systems is often complicated by the time-dependent behaviour of the material and the apparent slippage at the wall of the pipe. Most of the methods which have been developed to predict the flow behaviour of thick suspensions are very complex and are frequently inaccurate. In this paper a simple method is discussed which can be used to predict the pressure drop in pipes for time-dependent or time-independent materials with a yield stress. The method is based on the assumption that the sheared layer in the pipe can be simulated by a similar layer in the coasial cylinder viscometer, and the properties of the layer are only dependent on plug speed in both systems. The implication of this assumption is that the shear stresses at the surface of the pipe and at the surface of the inner cylinder of the coaxial viscometer are independent of the pipe diameter and the diameter of the cylinders. The method was tested against the authors’ own data and by other data from the literature.     

原油输送管道内腐蚀原因分析

为明确原油输送管道内腐蚀的原因,通过实际腐蚀管道失效分析,对原油中水溶性腐蚀性组分进行了分析测试,利用扫描电镜(SEM)、能谱(EDS)等分析了腐蚀产物的形貌及成分,结合管道中油水分布多相流模拟,指出:原油管道发生内腐蚀破坏的必要条件是管道内壁与管输物料之间形成可发生腐蚀的原电池。评价管输原油是否会造成管道腐蚀,首先应当明确水能否从原油中析出,析出的水能否直接接触管道表面,析出的水会在管道什么部位聚集。通过原油中水的稳定性测试,可以确定水能否从原油中析出,原油在管道表面的润湿性测试表征管道表面能否接触到腐蚀性水,多相流分析可以明确管道中聚集水的部位。     

Explosive welding of stainless steel–carbon steel coaxial pipes

Bi-metallic corrosion resistant steel pipes were produced through explosive welding process. The weldability window of the stainless steel pipe (inner pipe) and the carbon steel pipe (outer pipe) was determined by the use of available semi-empirical relations. The impact velocity of the pipes as the most important collision parameter was calculated by the finite element simulation. Direct effect of the explosive mass reduction on the bonding interface of the pipes was studied. Optical microscopy study showed that a transition from a wavy interface to a smooth one occurs with decrease in explosive load.     

反应器分布器分布管断裂分析

对断裂反应器的分布管进行宏观、金相微观组织、化学成分分析、硬度检验、相结构分析和断口分析。结果表明,在分布器的工况条件下,分布管外表面氧化腐蚀,导致表面晶界氧化裂纹萌生;管内介质NH3分解导致内壁渗氮,生成氮化物材质脆化;在震动作用下,由管外壁起裂,导致失效。     

A combined numerical-experiment investigation on the failure of a pressure relief valve in coal liquefaction

In a direct coal liquefaction unit, pressure relief valves locate on the pipeline between the atmospheric and vacuum towers. Failures of the valve components occur frequently owing to the harsh operation conditions. A combined numerical-experiment investigation on the failures of valves is conducted in this paper. The variation of relative erosion rates of WC–Co coating with impact angles, the function of relative particle velocity, and the distribution of particle diameters are obtained from the high-temperature erosion experiments. Furthermore, the erosion mechanism of WC–Co coating under large impact angles is clarified. In the numerical simulation, the evaporation–condensation, particle motion, erosion, and the modified RNG k-ε turbulence models are used to analyze the phase transition and particle erosion in the valves. Results showed that: due to the high pressure drop and convergent–divergent structure of angle valve, the coal-oil slurry flashes as it enters into the valves. The evaporation of liquid oil produces a large amount of vapor oil, and results in a rapid increase in flow velocity. High concentration solid particles, driven by the high-speed stream, tend to erode the inner surface of valves. Severe erosion can be found in the spool of angle valve, downstream bushings at the angle valve and ball valve. The calculation results agree well with actual failure morphologies, verifies the accuracy of the present prediction method.     

The Effect of Phosphate on the Properties of Copper Drinking Water Pipes Experiencing Localized Corrosion

Extensive localized or pitting corrosion of copper pipes used in household drinking water plumbing can eventually lead to pinhole water leaks that may result in water damage, mold growth, and costly repairs. Water chemistry has been recognized as the cause of some community-wide copper pinhole leak outbreaks. A large drinking water system in Florida recently switched from pH adjustment and orthophosphate addition to a blended ortho-polyphosphate chemical to address this problem. The objective of this study was to examine the impact of phosphates on the morphology and elemental composition of the interior surface of failed copper pipes removed from homes in the community. Scanning electron microscopy (SEM) and energy dispersive spectroscopy analysis of pipe surfaces revealed the build-up of phosphorus over time. Phosphorus was most greatly concentrated over areas of localized corrosion attack. Examination of the corrosion by-product mounds that covered corroding pits showed that phosphorus had migrated to the region adjacent to the copper pipe wall. Distinct copper–phosphorus solids were identified under SEM magnification; however, no crystalline copper–phosphate compound was identified by x-ray diffraction analysis.     

建筑隔震金属柔性管道抗震性能试验研究

隔震层中具有大变形需求的柔性管道显著影响隔震建筑的震后功能可恢复能力,然而目前对于柔性管道抗震性能的研究还相对较少。该文以量大面广且承载重要功能的竖向安装金属柔性管道为研究对象,考虑实际工程应用现状和相关规范建议,以400 mm变形为目标,以公称内径、管道安装长度、管道设计长度为研究变量,共设计了9组27个试件,进行了抗震性能试验,研究了金属柔性管道的损伤演化模式、关键损伤状态及其变形和承载能力。结果表明:目前工程上采用的柔性管道设计方案无法满足大变形需求或存在安全隐患,规范建议的方案则可很好地满足目标;对于破坏的柔性管道主要经历两个关键状态,即外金属套网绷直和单侧完全拔出破坏,破坏后无法承受工作压强需要更换;水平变形能力主要取决于管道安装长度和管道设计长度,随两者的增大而增大;破坏主要取决于管道受拉竖向荷载分量,该分量近似正比于公称内径,破坏水平荷载随着公称内径和极限水平位移的增大而增大。该文的研究成果可为建筑隔震金属柔性管道的抗震设计和易损性研究提供重要参考。     

基于FLUENT的B10管路弯头内部流场数值模拟分析

基于FLUENT软件,建立海水管路弯头流体模型,采用数值模拟的方法对比分析了焊缝余高、海水流速、弯曲半径以及管路直管段距离对弯头内部流场的影响,采用模拟正交试验并利用极差分析法,分析了各因素对管路弯头内部冲刷腐蚀影响的主次关系。      

Failure analysis of spiral finned tube on the economizer

This article describes the results of an investigation about the failure of spiral finned tube on a newly designed and retrofitted low pressure economizer in a 300 MW pulverized-coal-fired power plant. In order to find out the failure causes and to suggest preventive measures, phase compositions and macrostructure of the tube metal surface were investigated by X-Ray Diffraction and Scanning Electron Microscope equipped with energy dispersive X-Ray spectroscopy micro-analysis. The results show that the failure was principally owing to comprehensive multiphase erosion, an interaction of the fly ash wearing, flue gas washing and sulfurous acid corrosion. Recommendations are given to minimize such failures.     

Quantitative Evaluation of Corrosion in a Thin Small-Bore Piping System Using Bobbin-Type Magnetic Camera

Nondestructive testing (NDT) methods have been developed to ensure the integrity of heat exchanger pipes. NDT systems can be used to not only locate cracks on the pipes but also evaluate the size and position of the cracks. A bobbin-type magnetic camera (BMC), an electromagnetic NDT system, was developed to inspect both the inner and outer diameters (ID and OD, respectively) for stress corrosion cracking on a small-bore piping system with a high spatial resolution at high speeds. In this paper, an algorithm that discriminates the ID cracks from the OD cracks and estimates the crack volume is proposed. Artificial ID and OD hole-type cracks which have diameter of 1–4.5 mm and depth of 0.3–27 mm were used to imitate the corrosions on a 1.27 mm thickness and 16.56 mm inner diameter copper alloy pipe to verify the proposed algorithm. The inspection results of the BMC with respect to excited frequencies from 1–9 kHz are presented. 100 % of the ID and OD cracks were discriminated, and their volumes were estimated with a standard deviation of 1.132 mm³ for volume from 1.00–9.01 mm³.     

Numerical simulation of carbon dioxide corrosion under flow conditions in an elbow

A computational fluid dynamics (CFD)‐based method combined with water chemistry and electrochemical reaction calculation is developed to investigate the effect of fluid flow on carbon dioxide corrosion in an elbow. The mass transfer of reactants and products influenced by flow during corrosion is revealed visually by distribution of main species concentration. The results show that the distributions of main species concentration and corrosion rate on the bare steel surface are in good accordance with the distribution of flow velocity. The maximum corrosion rate appears at the entrance on inner side wall of the elbow, and the corrosion rate decreases along the flow direction. Near the exit of the elbow, the secondary flow arises and enhances the mass transfer as well as the corrosion rate. According to the saturation value, the iron carbonate is more likely to precipitate at the inner side wall near the exit of the elbow. The model is validated by the good agreement between simulation results and published experimental data in prediction of hazardous corrosion region and the regional corrosion rate of the elbow.