Investigation on failure process and structural optimization of a high pressure letdown valve

High pressure letdown valve in direct coal liquefaction is used to adjust the flow rate of coal–oil slurry that enters into the downstream separator. Severe erosion–cavitation wear is found on the valve spool, seriously affecting the safety and reliability of unit. The majority of this paper investigates the failure process of valve spool and proposes a corresponding structural optimization via computational fluid dynamics (CFD) methodology. Three geometries of different failure states are selected as the computational domains in the numerical simulation. The Schneer–Sauer model, particle rebound-velocity model and erosion model are employed to calculate the cavitation phenomenon and erosion rates distribution. Experiments of flow rates and cavitation on valve model under different pressure drops are conducted to validate the accuracy of numerical approach. Results showed that the damage development of valve spool aggravates the erosion–cavitation wear. The maximum erosion rates are located on the top of spool head in all the three states. The erosion rates on spool arc surface are two orders of magnitude higher than that on parabolic surface. The decrease in radius of spool head reduces the intensities of erosion–cavitation wear. The numerical results are in agreement with actual failure morphologies of valve spool in different states.     

Experimental and computational failure analysis of a high pressure regulating valve in a chemical plant

A high pressure regulating valve for producing ethylene vinyl acetate copolymer in a chemical plant failed after three years in service. The failure analysis of valve was performed by means of visual inspection, stereo microscopy, optical microscopy, scanning electron microscopy and computational fluid dynamics (CFD).The morphology analyses showed that the craters on the valve plug and cage, especially the end of the valve plug, were mainly caused by cavitation erosion, and the ditch damage resulted from the erosion due to impingement and scouring of the high velocity flow. The corrosivity of vinyl acetate was proved to have few effects on the materials damage. The failure reasons were further demonstrated by the simulation results, which presented the distribution of the sites suffering the high velocity flow erosion and possible cavitation erosion. The computed numerical results showed a good agreement with morphology analyses.     

多级降压疏水调节阀流致噪声数值模拟研究

特殊工况用高参数调节阀在使用过程中出现的高噪声是阀门参数化设计和优化设计必须考虑的重要问题之一。建立高压降多级降压疏水调节阀三维模型,以流致噪声理论为基础,结合RNG k-ε和声学边界元方法(BEM),研究了套筒式减压结构不同结构设计参数对噪声特性的影响规律。研究表明:套筒节流区域的流体压力脉动程度最强,是诱发噪声的主要区域;不同套筒结构参数的调节阀噪声频谱均呈现明显的宽频特性;声压级随套筒孔径的增大而增大,较小的套筒孔径对调节阀的流致噪声有更强的抑制作用;声压级随套筒间隙的增加呈先递减后递增的变化规律,在套筒间隙为8 mm时声压级达到极小值51.02 dB(A)。     

Fretting fatigue failure mechanism of automotive shock absorber valve

Fretting fatigue is a complex mechanical failure phenomenon, in which two contact surfaces undergo a small relative oscillatory motion due to cyclic loading. This study proposes a methodology to analyze the fretting fatigue failure mechanism of automotive shock absorber valve by means of experimental and numerical approaches. A servo hydraulic test set-up is used to simulate fretting fatigue under real working conditions. Moreover, a 3-D finite element model is developed to analyze the contact status and stress distribution at contact interface between connected components, i.e. washer-disc contact. The experimental test results depict that fretting damage appears at contact interface between washer and disc, which causes the initial crack nucleation and advancing the crack up to the final fracture of valve disc. Stress field, obtained by numerical simulation, is used to monitor some fretting fatigue features such as the distribution of relative slip amplitude, contact pressure and different stress fields at contact interfaces. Eventually, the crack initiation site is estimated by monitoring variation of equivalent multiaxial damage stress at contact interface.     

Study of the failure of a duplex stainless steel valve

A study has been performed to determine the reason for the failure of a group of duplex stainless steel valves after 15 years in service. Chemical analysis did not agree with the required quality. Metallographic analysis revealed some small pits with corrosive products inside which were blamed to be the origin of long cracks and consequently of the failure. Pitting and cracks progressed along the ferrite–austenite interface where carbides and brittle phases are precipitated. This precipitation drags a significant content of chromium whose percentage in the surrounding matrix is depleted losing its pitting resistance being attacked by the circulating fluid. The use of a steel with higher pitting resistance and avoid working in the temperature range where both carbides and brittle phases are formed is recommended.     

Metallurgical investigation on fatigue failure of stainless steel chain in a continuous casting machine

Stainless steels strips (chains) are used for the connection of dam blocks in belt casting machines. Thermal cycling and repetitive stressing under complex loading conditions due to tension and bending are the most frequent function modes during production. Samples from fractured stainless steel strips used for the connection of dam blocks in a copper rod continuous casting line, were sent for failure investigation. Optical and scanning electron microscopy for structural and fractographic evaluation along with mechanical testing are used as the principal analytical techniques in the context of the present investigation. Failure analysis findings suggest strongly that the failure was caused by bending fatigue which assisted also by thermal cycling, initiated from the strip surface and followed by ductile final overload fracture. Final fracture occurred via ductile failure, when the remaining strip cross sectional area reaches a critical size, becoming unable to sustain the operating load. Review of the service history (operating conditions, e.g. process design, applied loads, thermal cycles), in combination to the examination of a potential substitution of the material to a more heat (and fatigue) resistant one are suggested as further fatigue damage preventive actions.     

高频交变压力下先导式溢流阀响应特性研究

为研究液压激振系统中高频交变压力下先导式溢流阀的响应特性,建立了先导式溢流阀数学模型和AMEsim仿真模型,仿真研究了高频交变压力下先导式溢流阀开启情况,并进行试验对比研究。仿真结果表明：交变压力下先导式溢流阀主阀口存在异常开启现象,增大了系统能量损失;主阀口异常开启量随交变压力幅值增大而增大;当交变压力频率达到200 Hz时,溢流阀主阀口未出现异常开启。试验结果与仿真结果基本吻合。研究结果对高频交变压力下先导式溢流阀的使用有重要的理论指导意义和实际工程意义。     

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.     

Failure analysis of a high pressure differential regulating valve in coal liquefaction

Under harsh working conditions-high pressure differential, solid concentration and high velocity of a regulating valve in coal liquefaction, the valve plug damages easily. Its longest service life is less than 2000 h, which seriously affects the running safety. Failure analysis of valve plug is conducted via computational fluid dynamics (CFD) by using the actual physical parameters. The results show that the damage of valve plug results from a synergistic effect of cavitation erosion and abrasion. Two cavitation regions exist on the wall of valve bushing and plug, and a high-speed reflux appears on the plug head where the pressure is higher than the saturation pressure. Driven by the reflux, the cavitation bubbles and solid particles move toward the plug head, thus the most severe cavitation erosion and abrasion occur on the plug head because of the bubbles collapse and particle impacts. The decrease of valve opening tends to aggravate the valve plug damage caused by the combined effects of cavitation erosion and abrasion. Compare with the actual corrosion morphology, the accuracy of failure analysis is verified.     

Analysis of a crankshaft fatigue failure

The reason of the crankshaft fracture of the air compressor has been analyzed through the chemical composition, mechanical properties, macroscopic feature, microscopic structure and theoretical calculation methods. The analysis results show that the crankshaft which has obvious fatigue crack belongs to fatigue fracture. The fatigue crack initiated from the fillet region of the lubrication hole because of the high bending stress concentration which is caused by both the small fillet and the misalignment of main journals. The crankshaft fatigue fracture was only attributed to the initiation and propagation of the fatigue cracks on the lubrication hole under cyclic bending and torsion. The high bending loading bending level is the root cause of the failure.     

Causes of fatigue failure in the main bearing of an aero-engine

Fatigue failure in the central main bearing (CMB) of the compressor shaft of an aero-engine resulted in an air-crash. The cage of the CMB broke due to fatigue, got stuck between the bearing balls and the outer race, misaligned them resulting in severe wear of the components and damaged the function of the engine. Simulated laboratory tests are conducted and the source of cyclic stresses is discussed.     

Thermo-mechanical fatigue damage and failure of modern high performance diesel pistons

This study resulted from an engineering failure investigation related to diesel engine piston failures which occurred during a bench dynamometer engine durability test programme. The test programme aimed at evaluating the effects of various fuel types on the durability of fuel system components in passenger car diesel engines. A number of unexpected cylinder head, turbocharger and piston failures were experienced during the course of the test programme. This study focused on the cause of the piston failures experienced during these tests.Analyses of the fractured pistons revealed that thermo-mechanical fatigue initiation occurred as a result of primary silicon phase cracking and subsequent micro-crack formation due to excessive thermo-mechanical loading. Progressive formations of such micro-cracks lead to flaws that were of sufficient magnitude to initiate propagation by high cycle fatigue mechanisms.The investigation also revealed that the excessive thermo-mechanical piston loading was caused by over-fuelling and a combination of elevated and poorly controlled post intercooler air temperature. There was no evidence to suggest that the failures were related to the test fuel formulations.     

Probability of fatigue failure as a statistic

The probability of failure is treated as a statistic from the viewpoint that the probability can be determined only through experimental data. On the basis of a statistical theory for large samples, an asymptotic distribution function for the probability of fatigue failure under stationary random external loading is given and then a simple policy for fatigue-proof design is proposed.     

Analysis of premature failure of a tie bar in an injection molding machine

Premature failure of a tie bar made of AISI 4140 steel in a 150 tonne plastic injection-molding machine has been analyzed. Although the nominal tensile stress acting on the tie bars (95.5 MPa) is far lower than the yield strength of this material (750–900 MPa), the tie bars are subjected to a pulsating cyclic loading during the plastic molding process. The failure was found to occur at the root of the first thread by transverse fatigue fracture induced by a pulsating tensile stress with multiple points of high stress concentration. The high stress concentrations appear to have been introduced by a combination of improper molding parameters resulting in uneven tensions in the four tie bars and aggravated by the presence of some material defects. The material defects observed are inclusions, presence of some retained austenite and fine cracks. A hydraulic clamping mechanism rather than a toggle clamp mechanism for the mould will minimize the cyclic strain on the tie rods.