Study On Erosion Wear Mechanism And Law Of Impeller In Centrifugal Compressor

Centrifugal compressor is a kind of machinery with high-speed rotating impeller working through gas medium. It is widely used in industry as the critical equipment, especially for the oil industry, chemical engineering, air separation, smelting industry, refrigeration, mine ventilation, and environment engineering. However, the solid particles in the gas make the impeller easy erosion and wear out, which would result in the reduction of impeller thickness, even breakage. Impeller is the key unit of the centrifugal compressor with special material, complex manufacturing processes, higher price and interest value. Hence, it is significant to research on the erosion wear of impeller in order to ensure the safety of operators as well as the engineering value and the interest of the company. This thesis is supported by the National Basic Research Program of China (973 Program)-"Behavior and Mechanism of Reman-parts under complex environments, critical threshold of Reman-life (Grant No.2011CB013401)". It consisted of four major points:(1) erosion environments of impeller in centrifugal compressor based on gas-solid two-phase flow, (2) the erosion characteristics and mechanisms of impeller’s material, (3) erosion rate and erosion depth computational model for impeller’s material, (4) influence of erosion wear on surface properties of impeller’s material. Finally, the mechanism of erosion wear of centrifugal compressors was studied systematically from macro to micro.According to the air-separation centrifugal compressor which suffers from erosion wear severely,3D model of the impeller channel was built firstly. And by adopting three-dimensional Reynolds time-averaged N-S control equations and standard k-e turbulence model, the distribution of air flow velocity field, pressure field and temperature field in the impeller were studied. Afterwards, the gas-solid two-phase flow in the impeller was simulated based on discrete phase model and mass erosion rate model of plastic materials. Particle motion trajectory, motion speed, mass concentration and blades erosion were also analyzed. It was found that the enrichment of particles during transport was the main reason which accounted for the trailing edge of the large blades bear erosion wear severely while the small blades were non-damage. Based on numerical calculation, the erosion environment parameters of impeller, such as impact angles, impact speeds, particle concentration, temperature etc. were confirmed, which provided data foundation for the erosion test on impeller materials.In order to find out the essence and the factors influencing materials erosion behaviors from the perspective of macro and micro, firstly, the erosion experiments by simulating the real work condition of impeller blade were conducted. And the influences of impact time, impact angle, impact velocity and particle diameter on the erosion characteristics of impeller’s material FV520B were studied systematically. It was found that both high-strength and low-strength FV520B materials exhibited the erosion characteristics of typical plastic materials. High-strength FV520B with high yield strength and surface hardness showed lower erosion rate relatively. The erosion rate increaseed rapidly along with impact speed, appearing exponential relationship, and the erosion rate also increased as particle diameter got larger, a linear relationship presented. Secondly, based on the numerical simulation of erosion process, the influences of particle shape, impact angle, impact velocity, particle diameter etc. on stress, strain, energy changing, erosion rate etc. were studied, and simulation results were also compared with the experimental results. The changing regularity of Mises-stress and erosion rate under different conditions were consistent with each other, thus the Mises-stress of the erosion area could reflect the severity of target erosion. The depth of the pit eroded by particles with different shapes change from big to small ranked in:triangular prism, quadruple prism, hexagonal prism, regular square prism, sphere, which was in the same order as the target absorbs strain energy. But the change regulation of pit depth and stress influence depth with different particles were different, for the same particle. And it’s inappropriate to estimate stress influence depth just by erosion depth. It turned out much reasonable to study sub-surface hardening mechanism in erosion process by taking yield stress as stress influence. Finally, impeller material’s erosion mechanism was discussed relying on the analysis of erosion morphology. The erosion of FV520B was a combination of micro-cutting and deformation wear. At low impact angles, the erosion was dominated by micro-cutting and micro-ploughing, while at high angle greater than 60°, the erosion was dominated by deformation wear.At present, most of the erosion rate calculation models are aimed at the erosion test under certain conditions and simplified from erosion process, which present a poor portability and lack deep researches on the erosion depth calculation models. To describe the erosion behavior of FV520B using mathematical methods, in the first, the erosion rate and erosion depth calculation model related to impact velocity, impact angle, particle size, particle shapes, properties of target were developed, which were consisted of micro-cutting wear and deformation wear. Secondly, based on regression analysis of multi-factor orthogonal erosion experiments, erosion rate and erosion depth formula of FV520B were obtained. Comparative analysis showed that the predictions were in good agreement with the results of single factor experiments. Finally, the erosion depth formula was used to estimate the thinning life of eroded blades, and the calculated life agreed with the actual life very well. This study provided theory and data bases for the erosion life evaluation of compressor impeller.Degradation of surface topography and accumulation of residual stress have a great effect on the performance and security of compressor impeller. In order to learn about the influence rule and mechanism that erosion wear process have on the surface geometry morphology, metallographic structure and property of the subsurface, the variation rules and formation mechanism of the surface characteristics such as erosion depth, surface roughness, surface metallographic structure, microhardness and residual stress were analyzed systematically, relying on incremental erosion test of FV520B material under different impact angles. The variation law of erosion penetration rate was different under different impact angles, which was caused by the different erosion mechanisms. When the impact angles were low, surface roughness decreased gradually and surface became smoother with the process of erosions, and when the impact angle was 90°, the roughness had no obvious change, while the impact angles were medium, surface roughness increased at first and then decreased, which was caused by the appearance of transverse stripe deriving from the differentiation of erosive angle after deep erosion. The erosion particle was too small to bring obvious heat effect on the target surface, so there was no significant change in the metallographic structure of the sample during erosion. But erosion wear caused a hardening layer to the subsurface whose depth varied from 5μm to 10μm, and the hardening depth increased with the increasing of impact angles, while increased slightly with the increasing of particle quality under the same impact angle. Erosion wear removed materials and changed the original stress state of the sample surface simultaneously. When the impact angles were low or medium, plastic extruding effect of micro cutting and plough cutting was outstanding, led to residual tensile stress on the surface. When the impact angle was 90°, shot peening effect held the dominate position, so residual compressive stress forms on the surface.