Failure analysis of a natural gas pipeline

In the present work, the failure investigation of a 30 in. diameter gas transmission pipeline (API 5L X-60 grade steel) has been described. The failure was due to a longitudinal crack developing in the centerline of longitudinal weld joint. Mechanisms and morphology of crack initiation and propagation were studied through different tests including: thickness measurement, chemical composition analysis, metallographic inspection, mechanical property testing, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The experiments resulted to the conclusion that some factors associated with Sulphide Stress Cracking (SSC) and metallurgical defects cause the failure of weld joint pipe. Detailed examination revealed that these factors are inappropriate welding parameters, pitting corrosion on longitudinal weld, and hydrogen permeation to the weld metal.     

Failure analysis of a natural gas pipeline

A L390 natural gas pipeline exploded in 2011 in China. Macrofracture examination, thickness measurement, chemical composition analysis, metallographic inspection, mechanical property testing, fracture scan electronic microscopy examination and environment analysis were conducted to the burst pipeline. The results showed that Stress Corrosion Cracking (SCC) generated in the mechanical damaged zone of the pipeline was the reason of the failure. This kind of cracking was induced by Cl⁻ and HCO₃⁻ in near neutral corrosive medium of pH value. The stress concentration was induced by the groove damage under inner pressure.     

Failure prediction analysis for polyethylene flawed pipes

Through this paper limit load analysis and the EPRI/GE procedure were applied to predict instability conditions for medium density polyethylene flawed pipes. Predicted values for internally pressurized cylinders with axial cracks and cylinders with circumferential cracks under remote tension were compared to experimental results obtained from tests conducted on full scale structures. For the pipes under internal pressure, both schemes led to critical pressure values in agreement with actual burst pressures, despite plastic collapse having been observed in the failure of these structures. For the pipes with circumferential internal cracks subjected to remote tension, the predicted loads from the EPRI procedure do not agree with experimental values whereas limit load predictions are quite satisfactory. On the other hand, for the circumferentially externally cracked pipes both predictions reasonable agree with experimental data.     

Failure analysis and faults diagnosis of molecular sieve in natural gas dehydration

Natural gas dehydration by molecular sieves is one of the most promising processes. However, the sharply declined dehydration capability of the regenerative molecular sieve has become a serious problem for molecular sieve recycling. A series of experiments is carried out to investigate the failure factors, such as pH analysis, thermogravimetry, differential thermogravimetry, scanning electron microscope, X-ray diffraction, and Fourier transform infrared spectrum characterizations. The results indicate that there are four main aspects for the unavailability of the regenerative molecular sieve. First, the pH values and surface alkalinity decrease. Second, the secondary pore structure is diminished and blocked. Third, the crystallite size increases, and the crystals are agglomerated. Finally, not only are the hydrocarbons adsorbed, but also alcohols, ethers and carbonyl sulfides are generated in pores.     

CFD analysis of gas explosions vented through relief pipes

Vent devices for gas and dust explosions are often ducted to safe locations by means of relief pipes. However, the presence of the duct increases the severity of explosion if compared to simply vented vessels (i.e. compared to cases where no duct is present). Besides, the identification of the key phenomena controlling the violence of explosion has not yet been gained. Multidimensional models coupling, mass, momentum and energy conservation equations can be valuable tools for the analysis of such complex explosion phenomena. In this work, gas explosions vented through ducts have been modelled by a two-dimensional (2D) axi-symmetric computational fluid dynamic (CFD) model based on the unsteady Reynolds Averaged Navier Stokes (RANS) approach in which the laminar, flamelet and distributed combustion models have been implemented. Numerical test have been carried out by varying ignition position, duct diameter and length. Results have evidenced that the severity of ducted explosions is mainly driven by the vigorous secondary explosion occurring in the duct (burn-up) rather than by the duct flow resistance or acoustic enhancement. Moreover, it has been found out that the burn-up affects explosion severity due to the reduction of venting rate rather than to the burning rate enhancement through turbulization.     

钻杆的失效分析

对某油井连续发生12次刺穿断裂事故进行了调查,用力学试验、化学分析、金相、扫描电镜、能谱分析等手段并结合现场实际情况对刺穿钻杆进行了失效分析.结果表明.由空气钻井改用泥浆钻井时,由于只改变了钻井方式和钻头,未改变钻具组合而导致钻具与井壁之间的间隙过大,从而导致钻杆的疲劳断裂.     

Failure analysis of gas turbine buckets

The failure of a gas turbine first stage bucket was investigated by visual inspection and finite element analysis. The failure of a major bucket cooling passage was a critical cause of the separation of a bucket segment and caused microstructural deterioration of the neighboring regions by serious thermal load. Changes of microstructural morphologies of the damaged buckets under the thermal and mechanical stress were observed. After coating stripping, the bucket surface condition was evaluated through visual inspection and finite element analysis. The TMF (thermal-mechanical fatigue) cracking of surface coatings on the suction and pressure sides of the bucket was described.     

Failure analysis of an exploded gas cylinder

This paper reports the major activities carried out during the failure analysis of an exploded cylinder containing hydrogen. The general cracking pattern of the cylinder, the fractographic features, and the stress analysis results were all indicative of an internal gaseous detonation. Accordingly, several specific characteristics of detonation-driven fracture of closed-end cylindrical tubes were identified. These characteristics were analyzed through detailed examinations of the fracture surfaces, cracking patterns, and dynamic stress analysis of the cylinder using a transient analytical model. Based on the size and location of special markings found on the shear lips, and using the time duration of flexural waves, the crack growth increments and speed were computed. Consequently, the basic features of the gaseous detonation and the composition of the original gas mixture were identified. The results indicated that the detonation of a low-pressure oxygen-rich mixture of hydrogen and oxygen was the cause of this failure. The presence of oxygen was attributed to an improper usage of an oxygen cylinder for hydrogen storage.     

Failure analysis of natural gas buried X65 steel pipeline under deflection load using finite element method

A 3D parametric finite element model of the pipeline and soil is established using finite element method to perform the failure analysis of natural gas buried X65 steel pipeline under deflection load. The pipeline is assumed to be loaded in a parabolic deflection displacement along the axial direction. Based on the true stress–strain constitutive relationship of X65 steel, the elastic–plastic finite element analysis employs the arc-length algorithm and non-linear stabilization algorithm respectively to simulate the strain softening properties of pipeline after plastic collapse. Besides, effects of the soil types and model sizes on the maximum deflection displacement of pipeline are investigated. The proposed finite element method serves as a base available for the safety design and evaluation as well as engineering acceptance criterion for the failure of pipeline due to deflection.     

Identification of pipes damages on gas compressor stations by modal analysis methods

During their operation the pipes of gas compressor stations are influenced by dynamic impacts caused by operating equipments as well as flowing media. Due to the occurrence of excessive vibrations on output pipes of turbocompressor after its modernization, experimental vibration analysis of the output pipe of turbocompressors was realized, which included modal analysis and vibration measurements during operating of turbocompressor. Objects of measurements were two compressors of the same type. One of them has been an origin and the second was after modernization. In the paper there is described the methodology of eigenfrequencies measurement of pipes and measurement of their vibration velocities during operation of compressors. The aims of the measurements were to determinate a source of an excessive vibration and proposal of measures leading to reducing vibration to an acceptable level defined by a relevant standard.     

Failure analysis of an HCl gas cylinder valve

The fouling failure of an anhydrous HCl gas cylinder valve was investigated after the failure led to an accidental release of HCl gas. It is surmised that water penetrated into the valve by improper purging or valving, and created a severely corrosive environment. The aluminum bronze valve body underwent general corrosion. The corrosion products, primarily nantokite (CuCl), built up within the valve and led to the fouling failure. Dezincification was observed in a leaded nickel silver component of the valve. The Monel 400 valve stem was intact. The cylinder and valve testing procedures that led to the gas release incident are also examined.     

客车空调压缩机排气管路失效分析与结构改进

采用ANSYS Workbench软件对客车空调压缩机排气管路进行失效分析和结构改进。以3种典型管路为研究对象,在压缩机端施加位移载荷进行管路失效分析,模拟结果与管路实际断裂位置一致。通过添加弯头、调整管路角度以及增加不锈钢波纹管长度等改进措施,可以降低管路的最大等效应力,延长管路的使用寿命。     

Failure analysis of a high pressure natural gas pipe under split tee by computer simulations and metallurgical assessment

Crack failure of a 36 inch high pressure gas pipe observed during regular inspection of a station has been investigated and the results are presented in this paper. The crack, approximately one meter long, was initiated from a notch inside the hot tapped hole in a pipeline installed about 30 years ago. The study was conducted by reviewing the design history and construction data, visual inspection, pipe material characterization, stress and modal analysis by using finite element method. Investigations revealed that the valve, directly connected to the split tee, faced large dynamic periodic forces due to a pressure drop between two pipelines. Metallurgical evaluation of the pipe material by optical microscope and fractography of the crack surface by scanning electron microscopy indicated the presence of elongated inclusions in the steel microstructure together with some indications of fatigue fracture as a poorly formed sawtooth profile. Based on dynamic analysis, it was found that the first mode shape, the maximum displacement and, therefore, the maximum stress were exactly situated within the crack initiation zone. It was concluded that the notch effect in the hot tapped hole, the position of the supports under the split tee and the presence of a large periodic stress were responsible for the initiation and fatigue propagation of the crack in the gas pipe.     

Failure analysis of an exploded CO2 gas cylinder

This paper presents the analysis and subsequent recommendations towards prevention of failure in an IS 7285 type 2 cylinder which is seamless gas cylinder grade steel. The region of fracture showed the presence of an oxidized zone in the inner walls of the CO₂ gas cylinder along with a fresh surface. Visual observation showed presence of multiple small cracks at the inner surface. Microstructural examination with optical and scanning electron microscopes revealed that the cracks were associated with scale indicating their pre-existence. A decarburized layer was also observed at the inner surface. The stress intensity factor (K_1C) estimated at the crack tip upon consideration of the pressure inside the cylinder surpassed the fracture toughness of the material. Exposure to direct sunlight elevated the temperature of the CO₂ gas wherein it behaved as a supercritical fluid occupying the entire volume of the cylinder with a pressure higher than ambient. At a particular density, above 48–54 °C, this pressure increased abruptly, beyond the limit offered by the pressure valve resulting in explosion. Periodic checking of pressure valves and proper quality assurance system enabled by NDT techniques were recommended for the prevention of such catastrophic failures.     

Failure analysis of austenitic stainless steel pipes in amine liquid regeneration unit of a desulfurizer

Failure investigation was carried out on the austenitic stainless steel pipes of an amine liquid purification unit. The authors investigated the causes of the pipe failure by analyzing chemical compositions of materials and corrosion products, observing microstructures, and testing the pitting corrosion resistance of materials. The results showed that the failure was caused by pitting corrosion due to the presence of chlorine ions in the working medium. Another important cause of pitting corrosion was the decrease of corrosion resistance of welds owing to the microstructural changes taken place during welding. A large amount of sulfate ions in the working medium accelerated the pits growth. At the same time, proposals were put forward to the user of the failure pipes.     

Failure analysis of carbon steel pipes used for underground condensate pipeline in the power station

In this paper, the cause of failure of an ASTM A53 Grade B carbon steel pipe was investigated. The investigated ASTM A53 Grade B pipe was used for a condensate pipeline and was found to be cracked after six months in service. The failed parts were investigated by means of stereoscopic microscope, optical microscope, scanning electron microscope equipped with energy dispersive X-ray spectroscopy micro-analysis, spark emission spectrometer, tension tester and Vickers hardness tester, in order to identify the failure causes and to suggest preventive solutions. A specially instrumented indentation tester was used for the measurement of residual stress on welding seam. The study shows that failure was mainly due to stress corrosion cracking caused by chloride and tensile residual stress on the welding seam due to insufficient heat input or low compression, etc during welding processing.     

Failure analysis and prediction of pipes due to the interaction between multiphase flow and structure

Frequently found in piping, 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 pipe surface and the fluid traveling along the surface. This paper studied multiphase flow induced erosion–corrosion in pipes, mainly focusing on the interactions between the multiphase flow and the structure. In order to evaluate the erosion effects caused by the flow and the coupled dynamics, the multiphase flow regimes, flow dynamics parameters and the distribution of phases on different positions of the boundary layer in REAC pipe inner surface were analyzed. The 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. Practically, this method can be used in optimizing the design of the inner sleeves of pipes.     

Failure analysis of gas blower shaft of a blast furnace

Failure of a gas blower shaft of a blast furnace was analyzed. Investigation showed that the loosening of the lock plate was responsible for the shaft fracture. Examination revealed that the dislodgement of the lock plate resulted in constant rubbing of this component against the shaft leading to formation of grooves and loss of material from the shaft in localized regions. The fractographic features were completely obliterated due to post fracture rubbing and hence the micro mechanism of fracture could not be determined. However, analysis suggests that the failure of the shaft had occurred most probably by fatigue. The shaft failure was secondary in nature and had resulted subsequent to the primary failure, i.e., dislodgement of the lock plate from its position.