introduction
QW submersible sewage pump is a pump and motor together, together into the submerged work of the pump. 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 pump room 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 the scope is also more and more widely used to transport pure water from the original can now transport a variety of domestic sewage, industrial waste water, 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, impeller and shell materials and conditions
When a QW double-channel submersible sewage pump for a WWTP is used for 7-9 months, the pump performance obviously drops. After disintegration, the impeller is seriously damaged and the entire surface of the casing is not seriously damaged. 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 cast iron chemical composition (by mass,%)
2, corrosion characteristics
2.1, impeller corrosion
Submersible sewage pump impeller corrosion situation shown in Figure 1, only a few months running the average wear depth of the impeller has reached 3-5mm, cavitation pits 4-6mm deep, the blade wear short 5mm short above the impeller diversion thinning serious, The amount of thinning 2-4mm, both sides of the scaly pattern and honeycomb pattern interaction, the front pattern is large and deep, the back of the pattern is small and shallow, scaly pattern direction of the flow direction. 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
The inner side of the shell has an annular wear-resistant groove mark. The thickness of the shell is 8 mm. After erosion, the thickness is reduced to 6 mm. The damage is not very serious, but the impact wear on the inlet is rather serious. In some places appear deep grooved stripes. Case corrosion shown in Figure 2.
3, impeller failure analysis
From the macroscopic analysis, the direct cause of the impeller failure is the serious thinning of the blade. Through the analysis of the condition and damage of the impeller, the reason of the thinning of the blade is caused by the serious abrasion and corrosion, that is, The grinding effect of solid particles in high-velocity sewage accelerates the corrosion of the material surface. As can be seen from Figure 1, the impeller has obvious 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.
Figure 1 impeller erosion damage live map
2 pump damage to destroy the live
3.1, the impact of solid particles on the erosion
The solid particles in the sewage are the main reasons for accelerating the erosion of the overcurrent component of the submersible sewage pump. The erosion damage is caused by the relative velocity of the water flow and the cavitation effect. Hard particles with high kinetic energy in solid particles flow at high speed repeatedly impact on the surface of the overcurrent component and cut and form wear. The wear amount of the solid particles to the overcurrent component can be estimated by the following formula
equation
It can be seen from the above equation that the wear of the solid particles on the overcurrent component is mainly caused by the action of the relative velocity 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, the impact of cavitation
The cavitation zone is located on the backwater side of the impeller blade exit area. This area sponge-shaped honeycomb, there are intensive horseshoe-shaped pit and long furrows, smooth pit wall, the top is 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, cavitation pits were continuously enlarged and merged. The staggered two cavitation pits formed a blade-like structure at the top of the pits, and the inner wall of the cavitation pits was smooth.
4, improve the program
From the above analysis shows that the main factors 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 the solid particles on the flow surface of the wear process is a composite wear, that is, cutting wear and deformation wear exist, the total amount of wear:
figure 2
image 3
From the above equation shows that the greater the speed, the less plastic material to be worn, the greater the total wear and tear, and therefore in the design process, to ensure flow, lift the same circumstances, increased impeller diameter, reducing water flow Relative speed, improved flow conditions, reduced turbulence, and appropriate thickening of the geometry of the impeller front and rear covers.
(B) is to improve the surface finish of the impeller to reduce the formation of cavitation and cavitation.
(C) is the use of anti-wear materials with good corrosion resistance, wear and tear due to corrosion of metal materials is the result of wear and tear caused by mechanical forces and medium corrosion. Therefore, the use of both corrosion and wear-resistant materials will be an effective preventive measures, based on a comprehensive analysis, the choice of martensitic stainless steel 2Cr13, after quenching, high hardness, good corrosion resistance.
(D) Epoxy diamond coating on the impeller surface protection, by the epoxy resin, curing agent, toughening agent, thinner and filler mixing mortar, smear on the surface of the impeller and volute. Bottom and base metal knot strength 400 ~ 450kg / cm, the middle layer of tensile strength 130kg / cm2, compressive strength 500kg / cm2, shear strength 300kg / cm. Emery as adjuvant, epoxy resin as the main agent of the bonding material, and the wear resistance of emery is better, so the combination of both showing a strong adhesive force, water resistance and anti-sediment wear resistance, and Has a strong impact resistance.
5 Conclusion
According to this program, the pump is rebuilt. After half a year of operation and after disassembly, as shown in Fig. 3, the whole impeller has no obvious corrosion and the effect is good. Only the surface of the impeller has a protective coating falling off locally, but does not affect the pump's normal Use, to achieve the desired effect. In summary, the main reason for the submersible sewage pump abrasion is the abrasion damage of the overcurrent components caused by high-speed solid particles flow, followed by the joint destruction of cavitation and wear. Therefore, the fundamental measure to prevent the erosion of the pump is to try to reduce the relative velocity of the solid particles on the surface of the overcurrent component. Supplemented with the necessary anti-erosion material and protective coating, the effect is more obvious. For cavitation damage, this can be prevented by optimizing the hydraulic design.
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