Failure analysis and mechanical performance of an oil pipeline

This paper presents a work scheme for failure analysis of buried oil pipelines. The scheme provides answers for two main questions at least: what were the causes that made the system fail and why the system failed in the way it did. The scheme is based on five stages and the results of these stages are related to each other and consistent with the type of failure, thereby providing objective conclusions. The proposed scheme was successfully applied to a case of a buried oil pipeline whose failure was a result of external corrosion. We established the occurrence of accelerated corrosion characterized by two corrosion mechanisms: one involving iron oxides and the other, iron sulfides. Furthermore, the ductile type failure has proved to be due to leakage, which is consistent with the reported evidence.     

MVR系统中离心式蒸汽压缩机与蒸发器的匹配特性研究

为了保证机械式蒸汽再压缩（mechanical vapor recompression, MVR）系统的运行经济性和稳定性,对MVR系统中离心式蒸汽压缩机与蒸发器的匹配特性进行研究。针对蒸发器换热系数在新投、工作和结垢工况下的变化,提出了蒸发器运行温阻特性线的概念,并将其与离心式蒸汽压缩机的温升特性线叠加,从而开展离心式蒸汽压缩机与蒸发器的匹配分析。通过分析发现,离心式蒸汽压缩机的设计流量偏大或蒸发器的换热面积过小会导致匹配不足,易发生喘振,从而影响MVR系统的运行稳定性。而离心式蒸汽压缩机的设计流量偏小或蒸发器的换热面积过大会导致匹配过度,致使MVR系统的运行经济性差,甚至可能造成MVR系统无法建立热力自循环。结果表明,离心式蒸汽压缩机在MVR系统启动过程中会出现不稳定的喘振现象,可以通过系统参数的临时调节或采取辅助措施来避开不稳定区。设计时应保证离心式蒸汽压缩机的喘振裕度大于20%,蒸发器换热面积的设计裕度为30%;MVR系统运行时实际蒸发温度与设计温度的偏差应控制在±5 ℃以内。研究结果可为MVR系统的设计和调试提供参考。     

失效分析技术(续)

断口是断裂失效中两断裂分离面的简称。由于断口真实地记录了裂纹由萌生、扩展直至失稳断裂全过程的各种与断裂有关的信息。因此,断口上的各种断裂信息是断裂力学、断裂化学和断裂物理等诸多内外因素综合作用的结果,对断口进行定性和定量分析,可为断裂失效模式的确定提供有力依据,为断裂失效原因的诊断提供线索。     

失效分析技术(续)

裂纹是材料表面或内部完整性或连续性被破坏的一种现象,是断裂的前期;断裂则是裂纹发展的结果。裂纹分析包括裂纹的无损检测、表面分析、光学金相分析及裂纹打开后的断口分析等内容。     

Application of Casting Simulation in Failure Analysis of Impeller

Journal of Failure Analysis and Prevention - Commercial programs using finite element analysis (FEA) have been developed for casting design process. However, their applications have been limited...     

Failure of Silicon Bronze Impeller from Dealloying Via Desiliconification

A cast silicon bronze (UNS C86700) impeller that had been severely corroded was submitted for failure analysis. The customer had informed that this impeller was used to pump potable water, but the service length and chlorine content of the water were unknown. The impeller displayed a Cu-rich red phase on its surfaces and showed a pattern very similar to dezincification. Further investigation to determine the cause of damage using various techniques, including light microscopy, scanning electron microscopy combined with energy dispersive x-ray spectroscopy, revealed that the microstructure consisted of multiple phases and that a Si-rich phase was being preferentially attacked, leading to a porous structure. After a thorough examination, it was concluded that this part had failed due to dealloying via desiliconification.     

Failure analysis of an aircraft propeller

Safe operation of an aircraft depends strongly on periodic inspection of high risk parts, particularly rotating parts that are subjected to complex dynamic loads. For the highest level of flight safety, maintenance rules, such as limits and intervals should be applied carefully.

This paper describes the root cause of a fracture mechanism in an Cessna-185 aircraft propeller blade due to FOD induced fatigue crack propagation.     

Dynamic mechanical and thermal analysis of aligned vapor grown carbon nanofiber reinforced polyethylene

Dynamic mechanical and thermal analysis of aligned vapor grown carbon-nanofibers (VGCNFs)-reinforced high-density polyethylene (HDPE) was performed. High-shear mixing was used to disperse and distribute the nanofibers. Extensional flow was used to obtain anisotropic nanoreinforced composite tapes. Dynamic mechanical analysis showed dual increase of storage modulus and loss modulus with different draw ratios. The modulus and complex viscosities of the drawn samples converged to that of pure PE at high temperatures, indicating that in the melt, the behavior is dominated by the semicrystalline matrix. Additionally, an increase in thermal stability was observed for the composites compared to PE matrix. Differential scanning calorimetry analysis showed that the inclusion of nanofibers hindered the structure evolution of PE upon drawing.     

Failure analysis of heat exchanger fluid vapor phase vent lines in a Polyester plant

A Polyester plant is using Proprietary heat exchange fluids (26.5% biphenyl, 73.5% diphenyloxide). The plant hot condensate tank, condensate feed tank, and heat exchanger vapor phase dead end and ventable lines (ASTM 106 Grade B) with isolation valves were suffering frequent leakages with severe gouging and pitting corrosion. Lustrous (shiny) coke deposit was prominent inside the cut sections due to coke formation by low boiler products. Some adhered brown iron oxide (rust) deposits were also visible. A sludge sample was analyzed by XRD which revealed that carbon was the major constituent with γ-Maghemite-C (an iron oxide) as second mineral in order of abundance. The failure was diagnosed as severe localized corrosion and gouging at temperatures of about 200 °C and 53 psi by non-condensable low boilers decomposed corrosive products. The remedy was suggested which has been validated over the 2 years trouble free performance.     

Failure analysis of an aero-engine combustor liner

The paper presents a failure case of an aero-engine combustor liner. Macroscopic and microscopic observations, microstructure investigation, chemical analysis and hardness measurement have been performed to investigate the damage mechanism and failure causes of notches and cracks. The results show that the failure modes of notches, axial-direction crack along the combustor liner and crack at the edge of welding spot are respectively attributed to high-temperature ablation, mixed crack of ablation and thermal fatigue crack. The bad configuration of crossover tube is the main reason for the failure of combustor liner.     

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 an automobile valve spring

This paper presents the failure analysis of an automobile valve spring which failed in service. The fractured surfaces as well as the surface of the spring material close to the fractured surface were examined in a scanning electron microscope at suitable magnifications. Optical microscopy was performed to evaluate the basic microstructure of the as-received material. Detailed electron microscopic studies have indicated that the failure was due to the presence of non-metallic inclusions near the surface of the spring material.     

Failure analysis of an abrasive cut-off wheel

The relationship between composition, structure, properties and performance is central to the design, use and failure analysis of engineering components. Thermogravimetric analysis (TGA) and wavelength X-ray fluorescence (WRXF) were applied in evaluating the elemental composition and thermal properties, respectively, of a failed abrasive cut-off wheel. The cutting tool, consisting of a bonded abrasive disc, failed prematurely during the routine sectioning of a steel member. The operator was injured as a result. Analysis indicated an improper mixture of the organic constituents comprising the wheel bonding material. Instead of the uniform wear anticipated under normal cutting conditions, the improper mixture resulted in a degradation of the wheel’s mechanical properties, and catastrophic brittle fracture.     

高速离心式压缩机叶片断裂失效分析

某高速离心式压缩机叶片在运行过程中发生断裂。通过对叶片断口及冲蚀表面进行宏观观察、材料化学成分分析、力学性能测试、断口扫描电镜观察、能谱分析和显微组织分析,找出了叶片的断裂失效原因。结果表明:压缩机叶片断裂主要是由于级间冷却器流体布局设计不合理,致使叶片在运行时不断受到冷却器管束铝翅片微粒高频脉动的冲刷磨损作用,在局部叶片迎风表面形成垢层,产生了高周疲劳载荷,使位于其对称位置的叶片在相对薄弱的顶部萌生裂纹并逐渐扩展,最终导致叶片高周疲劳断裂失效;另叶片材料冲击韧度低加速了疲劳裂纹的扩展。     

Premature Failure of Ductile Iron Pump Impeller in Cooling Tower System

The root cause of corrosion of a pump impeller in a cooling water system is investigated. The impeller material was made of ductile cast iron. The pump failure was encountered after eight months of operation. A detailed visual examination, microstructure examination, and water analysis were carried out to ascertain the probable cause of failure. Finally, the obtained results infer that the solid-particle containing fluid flow was responsible for erosion–corrosion in the impeller failure.     

Failure analysis of an aero engine ball bearing

An aero engine failed due to the misalignment of the ball bearing fitted on the main shaft of the engine. The aero engine incorporates two independent compressors: a six-stage axial flow low-pressure compressor and a nine-stage axial flow high-pressure compressor. The bearing under consideration is a high-pressure-location bearing and is fitted at the rear of the nine-stage compressor. It was supposed to operate for at least 5000 h but failed catastrophically after 1300 h of operation and rendered the engine unserviceable. Unusually high stresses caused by misalignment and uneven axial loading resulted in the generation of fatigue crack(s) in the inner race. When the crack reached the critical size, the collar of the race fractured, causing subsequent damage. The cage also failed due to excessive stresses in the axial direction, and its material was smeared on the steel balls and the outer race.     

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.     

Failure analysis of an aircraft piston engine components

In this study, failed parts from the piston engine of TB-20 Trinidad aircraft which declared an emergency landing to Anadolu University Airport, Eski?ehir, Turkey due to the overspeed of propeller during a training flight were investigated. Fortunately, landing was successful, however, most of the engine parts were found to be damaged. Some of the failed parts such as propeller governor idler gear and cam followers were examined by using a variable pressure scanning electron microscope (SEM) attached with an energy dispersive X-ray spectrometer (EDX) to identify the cause of failure in the engine.     

Failure analysis of an EDC incinerator quench nozzle

The quench nozzles (Hastelloy C-276) installed in the firebox of an incinerator in the EDC unit of a Saudi petrochemical plant experienced frequent premature failures. One damaged quench nozzle (male and female parts), therefore, was investigated by chemical analysis, optical microscopy, and EDS techniques in order to find out the cause and preventive solutions. The study indicates that the quench nozzle was corroded likely due to hydrochloric acid (HCl) formed in the hot service following the failure of the refractory lining. The presence of both moisture and chloride (from EDC) made HCl formation possible. Upgrading the construction material to Alloy C-59 is recommended. Alternatively, ceramic coatings of SiC can be applied.     

Failure analysis of an automobile damper spring tower

The cause of a passenger car’s damper spring tower early failure is investigated in this paper. Inspection of the road surface, tire inflation pressure, suspension, and service load are firstly done in order to determine the further test procedures and analysis methods. The static stress of the spring tower caused by the body weight is calculated by finite element model. Public road tests with an equipped car are carried out to simulate the real usage by the customers. With the measured strain signals of different test conditions and local strain–life method, fatigue life prediction is made. The calculated fatigue life coincides with the actual failure mileage, and it turns out that the broken spring damper causes the early failure of the spring tower. It is suggested that more emphasis should be taken on the durability design and test of the spring damper.