Spherical dendritic particles formed in cavitation erosion

Micro spherical particles were found in a vibrating cavitation erosion experiment. Examination of the spherical particles reveals the dendritic pattern on the surface and the hollow structure of the interior. The surface and interior structures of the particle are also related to the particle’s size. For smaller particle, the dendritic structure is replaced by the fine cell and the interior becomes solid. Such special structures are considered to be the result of particle’s solidification from molten state at a rapid cooling rate, which happens in a special transient environment with transient high temperature and high pressure provided by cavity collapsing. The specific area and surface tension force are the main reasons for the different structures of the particles in different size.     

Research on cavitation erosion and wear resistance performance of coatings

Depending on the nature of the working medium and working conditions, corrosive and cavitation damage shall arise to pump’s components. In industrial applications the corrosion-reducing coatings are sprayed on hydraulic components. But it is questionable whether such products actually do help under wear or cavitation loads or not. Abrasive jet wear tests were carried out to determine the wear resistance of coating materials: polymers and ceramics, cast iron, and steel of various types. The samples were loaded for five hours, and finally the wear depth was measured as a determining indicator of the sample’s wear resistance. Results of investigation on anti-erosion performance of epoxy resin, ceramic and Polyurethane (PU) coatings brushed on alloy steel surface were also presented. Cavitation erosion tests were performed on the ultrasonic rig. The mass loss and surface morphology of the specimens were examined by balance analysis and 3-D laser microscopy, respectively. The investigations showed excellent wear-resisting performance of ceramic coatings, which is better than wear-resistance of stainless steel, cast iron and high chrome alloy steel. But the excellent wear-resisting performance could not guarantee a good erosion-resisting performance. The ceramic coatings’ anti-erosion performances were inferior to that of gray cast iron, and hardly comparable to those of stainless steels. The basic factors that influenced coating’s cavitation erosion endurance were adhesion and thickness of coatings. Analysis of coating’s degradation mechanism showed that PU coatings could withstand longer incubation period thus enhancing the materials’ cavitation erosion resistance. Several practical cases were analyzed, showing some guidance for coatings’ application.     

Laser surface modification of UNS S31603 stainless steel. Part II: cavitation erosion characteristics

Austenitic stainless steel UNS S31603 was laser surface alloyed with various elements (Co, Ni, Mn, C, Cr, Mo, Si) and alloys/compounds (AlSiFe, Si₃N₄ and NiCrSiB) as presented in Part I together with the microstructures and the corrosion characteristics of the alloyed specimens. In Part II, the cavitation erosion characteristics of the alloyed specimens in 3.5% NaCl solution at 23°C were studied by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm. The hardness profile and the compositional profile of the alloyed layers were investigated by a Vickers hardness tester and by EDX respectively. The cavitation erosion resistance of specimens alloyed with AlSiFe, C and NiCrSiB were highest, reaching 11.1, 10.5 and 7.9 times that of the substrate respectively. The damage mode was identified to be ductile fracture for specimens containing austenite as the major phase, and brittle fracture when the major phase was ferrite or intermetallic. Cavitation erosion was initiated at the phase boundaries where there was an abrupt change in mechanical properties (e.g. hardness) and then propagated into the weaker phase. It was also noted that large improvement in cavitation erosion resistance and corrosion resistance could not be simultaneously achieved in the present study.     

Hydrogen effect on the cavitation erosion resistance of AISI 316L stainless steel laser surface-modified with NiTi

AISI 316L stainless steel was laser surface-modified with NiTi for improving cavitation erosion resistance as reported in a previous study. The present study aimed at investigating the effect of hydrogen charging on the cavitation erosion resistance of the NiTi-modified layer, in comparison with 316L substrate and bulk NiTi plate. To compare the hydrogen effect, the three types of samples were subjected to the same electrolytic charging intensity. The change in surface morphology and in the phases present was studied by scanning-electron microscopy and X-ray diffractometry. The indentation properties were studied using Vickers microhardness test and nanoindentation test. Cavitation erosion test indicated that hydrogenation resulted in different degrees of decrease in erosion resistance in all the three types of samples, which could be attributed to different mechanisms. For 316L, the hydrogen effect was mild, congruent with the small change in indentation properties. For NiTi-modified 316L, the decrease in resistance was due to a drop in hardness and elasticity, while for bulk NiTi, the decrease was mainly attributable to the formation of hydrides leading to the presence of surface cracks.     

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.     

Failure analysis on abnormal wall thinning of heat-transfer titanium tubes of condensers in nuclear power plant Part II: Erosion and cavitation corrosion

In Part I of the failure analysis on abnormal wall thinning of heat-transfer titanium tubes used in condensers in nuclear power plant, we analyzed the causes and mechanisms of abnormal thinning that commonly happened at the contact part between the tubes and the support plates. This kind of failure was the mainstream failure type in our case and the main causes were found to be eccentric contact wear and three-body contact wear rooted in processing defect of internal borings, corrosion products deposit and sagging, and foreign particles. However, there were still some individual failure tubes with different failure sites and modes and were located under the bypass pipes at the shoulder of the tube tower instead of in its lower part, obviously telling another failure story. In Part II of the failure analysis, material analysis, metallographic examination, mechanical performance tests, macro- and microstructure analysis and composition analysis were conducted. The failure causes were found to be erosion and cavitation corrosion and the synergetic effect of them. Finally, corresponding countermeasures were suggested.     

Degradation of alachlor in aqueous solution by using hydrodynamic cavitation

The degradation of alachlor aqueous solution by using hydrodynamic cavitation was systematically investigated. It was found that alachlor in aqueous solution can be deomposed with swirling jet-induced cavitation. The degradation can be described by a pseudo-first-order kinetics and the degradation rate was found to be 4.90 × 10⁻² min⁻¹. The effects of operating parameters such as fluid pressure, solution temperature, initial concentration of alachlor and medium pH on the degradation rates of alachlor were also discussed. The results showed that the degradation rates of alachlor increased with increasing pressure and decreased with increasing initial concentration. An optimum temperature of 40 °C existed for the degradation rate of alachlor and the degradation rate was also found to be slightly depend on medium pH. Many degradation products formed during the process, and some of them were qualitatively identified by GC–MS.     

Erosion characteristics and failure mechanism of reservoir rocks under the synergistic effect of ultrasonic cavitation and micro-abrasives

Ultrasonic cavitation erosion of reservoir rocks in distilled water with and without SiO₂ micro-abrasives (mean diameter 0.5 μm, mass concentration 1–7 wt%) are conducted. The erosion characteristics of eroded rocks are measured and analyzed. Both the area of cavitation damage zones and the surface roughness are increased when rocks are eroded in water-abrasive mixture. Compared with distilled water, the mass loss in 3 wt% water-abrasive mixture has improved by 69.66–104.23% (average 81.41%) for sandstone, 495.05–665.15% (average 557.38%) for shale, and 158.49–211.92% (average 188.16%) for granite during an erosion time of 5 ∼ 60 min. The promoted erosion can be explained by the synergistic effect of cavitation and micro-abrasives. The failure mechanisms are elucidated for different types of rocks. The erosion of reservoir rocks results from the peeling-off of mineral grains and the breakdown of large mineral crystals. Numerous micro-fractures and pores are produced in the surface of the eroded rocks, creating additional permeable channels for fluid flow. The results demonstrate that the presence of micro-abrasives can increase the ultrasonic cavitation erosion of reservoir rocks, indicating that the addition of micro-abrasives is feasible to enhance the performance of ultrasonic treatment for oil wells in petroleum industry.     

叶轮空蚀状态下离心泵振动特性分析

空蚀是指空化过程中产生的空泡溃灭引起过流表面材料损坏的现象。为研究离心泵叶轮空蚀后的振动信号特征,选用IS-50-160-00单级单吸离心泵为试验对象,基于虚拟仪器技术搭建试验泵系统。测得离心泵空蚀条件下的振动信号,采用均方根(RMS)分析、峭度(K)分析两种统计方法对发生空蚀后的离心泵振动信号的平均能量、冲击波能量进行分析,采用短时傅里叶变换(STFT)分析了振动信号的时频域特性。分析结果表明:空蚀条件下整体来看基座方向和轴向方向振动幅值较大且都是无规则振动,而横向方向和纵向方向上的振动信号振幅相对较小;振动信号的能量随着流量的增大呈现先减小后平稳再增加的趋势,空蚀增加了振动信号的能量值;空蚀加剧了液体对离心泵的冲击使得振动信号峭度值增加,且基座方向峭度值大于3可作为空蚀故障的诊断参考标准;通过时频谱分析可知空蚀发生后流体可能对离心泵存在冲击波及冲击波导致的脉冲信号,且空蚀后产生了高频振动信号,高频带的振动信号可为离心泵空蚀故障诊断提供参考。研究叶轮空蚀后离心泵振动信号的特征有助于及时发现离心泵空蚀故障的发生,从而调整运行参数,以免造成严重后果。     

Failure analysis of bursting on the inner pipe of a jacketed pipe in a tubular heat exchanger

In order to identify the causes of a bursting incident that occurred on the inner pipe of a jacketed pipe in a tubular heat exchanger for synthesis of high pressure polyethylene, series of characterization analysis were conducted. Metallurgical structure and chemical composition of the pipe’s metal matrix were inspected by metallographic microscope (MM) and photoelectric direct reading spectrometer; scanning electron microscope (SEM) and energy dispersive spectroscope (EDS) were applied to observe the microscopic morphology and micro-area composition on the ruptured surface; compositions of the coolant, i.e. the circulating cooling water were examined by inductively coupled plasma-atomic emission spectrometry (ICP-AES), ion chromatography (IC) and Fourier transform infrared spectroscopy (FTIR). In addition to these, finite element analysis (FEA) was employed to study the erosive effect of on the pipe. Analysis results revealed that interaction between corrosion and erosion both led by scaling, was the main cause that accelerated its thinning and eventually resulted in its premature failure. Finally countermeasures and suggestions were proposed.     

Non-uniform hydrogen attack cavitation and the role of interaction with creep

Hydrogen attack (HA) is the development of grain-boundary porosity by cavities filled with high-pressure methane that originates from the reaction of carbides with hydrogen at high temperatures. The cavities grow by grain-boundary diffusion and by creep of the adjacent grain material till they coalesce with neighbouring cavities to form a microcrack. Earlier work on HA has focussed on unit cells containing a single cavity, using average cavitation properties. Here, non-uniform cavitation properties on the grain-size scale are assumed in a polycrystalline aggregate, and unit cell analyses are performed to investigate the influence of the adjacent grains on the development of the grain-boundary HA. The numerical results are explained in terms of two simplified models which highlight the key parameters governing the grain deformation-grain boundary cavitation interaction process.     

The effect of duplex stainless steel microstructure on its cavitation morphology in seawater

An ultrasonically induced cavitation facility was used to study the effect of a cast duplex stainless steel (DSS) microstructure on its corrosion behavior in seawater. Under cavitation conditions, small cavities initiated in the ferritic matrix and at the ferrite–austenite boundaries. With the progress of cavitation, the attack concentrated in the austenitic phase and then spread to the ferritic phase and was associated with cleavage-like facets, ductile tearing, river patterns and crystallographic steps at later stages. Cross-sections of specimens revealed microcracks initiating from the ferritic matrix at the bottom of cavities. Crack propagation into the matrix was impeded by the austenitic islands.     

Comparison of cavitation flows in He I and He II

This paper describes some of the results of an experimental study on the cavitation phenomena in He II and He I by the visualization and the measurements of the pressure and the temperature. The cavitation flow is generated in liquid helium driven by a bellows pump in the downstream region of the throat in a Venturi channel with a rectangular cross section. It is found that there are some definite differences in the appearance of cavitation between He I and He II flows, and that the λ-phase transition from He I to He II is sometimes induced because of the temperature drop in cavitating He I flow. In addition, the expression for the relation between the amount of the temperature drop induced by cavitation and the void fraction was described, which was found to be qualitatively in good agreement with the experimental result.     

A methodology for risk-based inspection planning of oil and gas pipes based on fuzzy logic framework

In an efficient and effective pipe integrity management programme, maintenance engineers often use the risk-based inspection (RBI) and maintenance strategy. Unfortunately, the calculation of risk is a daunting task because in order to calculate the risk of failure, a maintenance engineer needs to predict the rate of growth of a defect, the effect of the defect on the integrity of the structure and the consequence of failure. Unfortunately precise calculation for either of these parts is quite difficult.Fuzzy logic is a mathematical tool suitable for handling imprecise information in the real world. The benefit of this approach lies in its ability to include personal experiences along with acceptable deterministic models in the calculation. The structure of the model also allows easy calibration of the model to suit a particular plant condition. This approach can thus help to reduce the dependence upon the precise data, allow modelling even when a phenomenon is incompletely understood, and reduce the difficulties arising due to the complex computation required by more traditional methods.This paper presents a proposed methodology, based on fuzzy logic framework, for the establishment of an RBI programme for pipes. The paper also presents in detail a section of the methodology that can be used for calculating the estimated rate of CO₂ corrosion in carbon steel pipes. In this technique the plant operating parameters (temperature, gas and liquid flow rates, total pressure, CO₂ partial pressure and pH) are taken as fuzzy variables and used to calculate the Predicted Rate of Corrosion. The inspected rate of corrosion and the efficiency of inspection are also considered as fuzzy variables and are used to calculate Trust in Inspection Results and Trust in Predicted Results. By combining all the modules an estimated rate of corrosion is calculated. This estimated rate of corrosion can then be used for developing the risk-based inspection programme.     

The effects of the injection moulding temperature on the mechanical properties and morphology of polypropylene man-made cellulose fibre composites

Composites made of polypropylene and man-made cellulose fibres that are intended for injection moulding applications show potential for use in sustainable and light weight engineering with high energy absorption capacity. Due to the thermal sensitivity of the cellulose fibres, process parameters play an important role during the injection moulding process. A polypropylene and a man-made cellulose fibre were chosen for this investigation. Effective melt temperatures between 200 °C and 269 °C were used to process the compounds into test specimens. Tensile, impact and colorimetric tests, as well as an SEM analysis, and a measurement of the fibre length distribution were carried out in order to characterise the mechanical and optical properties of the composites. It was observed that the fibre length becomes shorter above 256 °C and elongation at break and Charpy strength (notched) of the composites already decrease at lower temperatures than tensile strength. A direct correlation between mechanical properties and discoloration was not observed. Therefore, melt temperatures up to 250 °C are suitable for these composites.     

Failure analysis on abnormal wall thinning of heat-transfer titanium tubes of condensers in nuclear power plant Part I: Corrosion and wear

Titanium tubes used in condensers in a nuclear power plant in China encountered abnormal wall thinning, and was thus forced to temporarily stop operation or it could bring about catastrophic safety problems. Most of the wall thinning happened at quite regular positions on the tubes and these failure tubes were located similarly in the condensers, indicating some common problems. To find out the root cause and mechanism of the thinning failure, we conducted surface deposit analysis, appearance inspection, microstructure analysis and composition analysis of the samples by means of X-ray diffraction (XRD), stereo microscope, scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS). The results revealed that the wall thinning was primarily caused by eccentric contact wear and three-body contact wear rooted in processing defect of internal borings, corrosion products deposit and sagging, and foreign particles. Finally, countermeasures were proposed for repair and prevention.     

Numerical simulation of injection moulding process and the pre-modelling concept

Finite element or finite difference methods are implemented in several flow analysis programs (e.g. Moldflow^®). These filling programs rest on heavy computations to carry out an industrial parts production. In consequence, there exists a need among the polymer processing companies to develop innovative approaches in terms of grid computing and mould simulation software. There seems to be a lack of pre-modelling ‘tools’, which allow estimating an effective solution. In this direction, to simulate polymer injection in a mould cavity, we present an original approach based on a macro analysis of the propagation front during the filling phase. In addition, we propose to optimise the mould design in order to perform the most homogeneous filling and by the way, the best positioning of injection points in order to reach a minimal end-filling pressure.     

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.     

循环液氩泵汽蚀引发空分设备运行工况波动的原因及处理

介绍因循环液氩泵汽蚀造成空分设备运行工况波动的故障和原因,阐述汽蚀发生后的相关操作及对运行工况的影响,提出汽蚀故障发生时的应变措施。