By analyzing the failure reason of the impeller of the QW double-channel submersible sewage pump, the main reason of the wear of the submersible sewage pump impeller is that the wear of high-speed solid particles-containing flow to the overcurrent component and the combined destruction of cavitation and wear As a result, the corresponding solution is proposed based on the analysis result. According to this plan, after the pump was comprehensively transformed, after half a year of disassembly and observation, there was no obvious corrosion of the impeller and the effect was good and the expected result was achieved. Introduction QW submersible sewage pump is a pump and motor together, together submerged pump work. Compared with the general horizontal or vertical sewage pump, submersible sewage pump obviously has the following advantages: (1) compact structure, small footprint. Due to the submarine man working under pressure, the pump can be directly installed in the sewage tank without the need to build a special pumping station to install pumps and motors, saving a lot of land and infrastructure costs. (2) easy installation and maintenance. Small submersible sewage pump can be freely placed, large sewage pumps are generally equipped with automatic coupling device can be automatically installed, installation and maintenance quite convenient. (3) continuous operation for a long time. Submersible sewage pump due to pump and motor coaxial, short shafts, rotating parts light weight, so the bearing load (radial) is relatively small, longer life expectancy than the average pump. (4) small vibration and noise, low temperature motor, no pollution to the environment. It is because of the above advantages, submersible sewage pump by people of all ages, the use of more and more widely used by the original purely for the transport of fresh water can now convey a variety of domestic sewage, industrial wastewater, construction site drainage, liquid feed, etc. . In municipal engineering, industry, hospitals, buildings, restaurants, water conservancy construction plays an important role in all walks of life. 1, the impeller and shell materials and conditions A QW double-channel submersible sewage pump used in a sewage treatment plant using 7 to 9 months, the pump operating efficiency decreased significantly, after disintegration found impeller damage serious, and the shell Part of the entire surface corrosion is not serious. Pump design parameters: speed 1450r / min, flow 200mVh, head 22m, power 22kW, impeller and shell material for the HT200, the measured chemical composition is shown in Table 1. Working conditions when the working medium for domestic sewage, acidic microstrip; containing solid particles. Table 1, the chemical composition of cast iron (by mass,%) 2, the corrosion characteristics 2.1, impeller corrosion Submersible sewage pump impeller corrosion situation shown in Figure 1, only a few months run the average wear depth of the impeller has reached 3-5mm, steam Erosion pits 4-6 mm deep, the blade wear short 5mm above the impeller flow thinning serious, thinning 2-4mm, both sides of the scaly pattern and honeycomb pattern interaction, the front pattern large and deep, the back of the small pattern The shallow, scaly pattern of the direction of the direction of flow of water flow. A semicircular scour hole appears on the radial circumference of the impeller mouth ring. Impeller water side of the leaves and partitions thinning serious, the edge was uneven knife-shaped. 2.2, shell corrosion Inside the shell there are impact wear ring groove marks, the original wall thickness of 8mm, thinning to 6mm after erosion, the damage is not very serious, but the impact of wear and tear at the entrance is more serious. In some places appear deep grooved stripes. Case corrosion shown in Figure 2. Analysis of impeller failure From the macroscopic analysis, the impeller failure is the direct cause of serious thinning of the blade, through the impeller conditions and damage analysis shows that the cause of thinning of the blade is due to the emergence of a serious wear and tear Corrosion caused by the high velocity of solid particles in the sewage grinding effect of accelerating the corrosion of the material surface can be seen from Figure 1, the impeller there is a clear grinding zone, impact zone and cavitation zone. Due to different flow velocities and flow conditions in different parts of the impeller, the mechanical force on the impeller is also not the same size and direction, resulting in uneven abrasion corrosion morphology on the surface. In this main performance for the two forms of erosion and cavitation. Fig.1 Ablation failure of impellerFig.2 Ablation damage of pump body 3.1 Effect of solid particles on abrasion The solid particles in sewage are the main reason for accelerating the erosion of overcurrent components of submersible sewage pump. Abrasive damage is caused by the relative velocity of water flow and cavitation . Hard particles with a certain kinetic energy in high-velocity solid-particles-containing water flow repeatedly wear and tear on the surface of the overcurrent component and wear and tear. The wear amount of the solid particles to the overcurrent component can be estimated by the following equation. From the above formula, The wear of the overcurrent component is mainly caused by the action of the relative speed of the water flow. Solid particles increase the incidence of cavitation and reduce the cavitation resistance. First of all, the physical and chemical properties of solid particles and fluidity are very different from that of clear water. The viscosity of solid particles containing water is reduced, the fracture resistance is poor and the probability of cavitation is increased. Secondly, the entrapped bubbles in the solid particles will increase the water vapor in the water, increasing the probability of cavitation. In addition, there is a significant difference between the solid particles and the mass and inertia force of the water. The trajectories of the solid particles deviate from the water flow lines to distort the flow field. Due to the additional velocity of the solid particles caused by the flow resistance and the inertial force, Causing local pressure to reduce, increasing the probability of cavitation occurs. Cavitation pits can severely damage the smoothness of the surface of the overcurrent component and aggravate the wear of the solid particles. Domestic and foreign scholars have done a lot of experimental studies on the relationship between solid particles and cavitation. The simulation results show that the cavitation strength of solid particles is 4-16 times of the cavitation strength of water when other conditions are the same. The analysis of operating data in fresh water and muddy water also confirms this conclusion. In the guide part of the blade, the angle between the water flow direction and the blade is less than 45 degrees, and the grinding effect is obvious. The characteristics of this area are that the blade is uniformly thinned, the fish-scale stripes with a certain orientation and the furrow-shaped stripes ; At the leading end of the inlet side of the impeller, the angle between the water flow direction and the metal surface is greater than 45 degrees. This area is characterized by a serious loss of local material at the blade-front plate junction due to the high velocity Repeated impact, the weak part of the material will be the first to be destroyed to form holes, pits, or because part of the small flaky sand grains along the intergranular spread of micro-cracks on the grain boundaries, and eventually developed into the distribution of intergranular fatigue cracks, Causing the grain fatigue spalling, thus exacerbating the loss of materials. The edge of the blade is uneven blade, for three reasons: First, the high-speed movement of the impeller to the solid particles in the sewage thrown to the tail edge of the blade, resulting in blade edge solid particle concentration is greater than the average concentration; the second is the impeller The tail line speed is greater, the test shows that the fluid in the solid phase particles relative to the impeller surface movement speed is higher, the material loss will be more serious; third is affected by cavitation in this area, the next step to accelerate the blade tail Area wear and tear corrosion. 3.2 Cavitation The cavitation area is located on the backwater side of the impeller vane exit area. This area sponge-shaped honeycomb, there are intensive horseshoe pit and long deep furrows, smooth pit wall, the top was blade-shaped. When the impeller turns at a high speed, a large number of small vacuum bubbles are generated due to the local negative pressure on the back of the blade. After the negative pressure is reduced, small bubbles annihilate and release energy, and some of the energy is directly transferred to the metal matrix while the other part is transferred to the solid particles. That part of the energy passed to the metal is continuously absorbed by the material to develop microcracks into fatigue cracks, which in turn cause the grains to loosen and fall off, forming early pocketed cavities. The microcracking of the material surface is mainly caused by the material's own defects, impacts and penetration of solid particles. The energy transferred to the solid particles accelerates the solid particles, and the partially accelerated solid particles impact or slide across the metal surface at various angles to form small-angle grinding and high-angle impact damage. The sand particles entering the cavitation pits are The preformed cavitation pits formed a "reciprocating" failure. As a result, the cavitation pits were continuously enlarged and merged. The staggered two cavitation pits formed a blade-like structure at the top of the pit walls, and the inner wall of the cavitation pits was smooth. 4, improve the program From the above analysis shows that the main factor affecting the service life of the impeller is the design parameters of the impeller and materials. Without reducing the corrosion resistance of the impeller, it is the most effective way to prolong the service life of the impeller by improving the wear resistance and corrosion resistance and optimizing the design parameters of the impeller. (A) in the design, due to solid particles on the flow of the surface wear process is complex wear
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