Analysis Of The Main Causes Of Centrifugal Pump Cavitation

The factors that affect the cavitation of centrifugal pumps must be considered in the design and use of centrifugal pumps. In recent years, a lot of research has been done at home and abroad. However, due to the different focuses of the research, and most of them are research on a certain parameter that affects the cavitation of centrifugal pumps, the research results are scattered, and some viewpoints are contradictory. This paper synthesizes a large number of domestic and foreign literature, compares and analyzes the relevant research results on the factors affecting the cavitation of centrifugal pumps, and obtains a relatively comprehensive main factor affecting the cavitation of centrifugal pumps.

1. Influence of fluid physical properties

The influence of fluid physical properties on centrifugal pump cavitation mainly includes the purity, pH value, and electrolyte concentration of the transported fluid, the amount of dissolved gas, temperature, kinematic viscosity, vaporization pressure, and thermodynamic properties.

(1) The influence of purity (concentration of contained solid particles) The more solid impurities contained in the fluid, the more the number of cavitation nuclei will increase. Thereby accelerating the occurrence and development of cavitation.

(2) The influence of pH value and electrolyte concentration The cavitation mechanism of centrifugal pumps that transport polar media (such as general water pumps) and centrifugal pumps that transport non-polar media (pumps that transport organic substances such as benzene and alkanes) are different of. The cavitation damage of centrifugal pumps conveying polar media may include mechanical action, chemical corrosion (related to fluid PH value), electrochemical corrosion (related to fluid electrolyte concentration); and cavitation damage of centrifugal pumps conveying non-polar media Probably only mechanical action.

(3) Effect of gas solubility Foreign studies have shown that the dissolved gas content in the fluid promotes the generation and development of cavitation nuclei.

(4) Effect of gasification pressure The research shows that with the increase of gasification pressure, the cavitation damage first increases and then decreases. Because with the increase of gasification pressure, the number of unstable bubble nuclei formed in the fluid is also increasing, resulting in an increase in the number of bubble ruptures, an increase in the intensity of shock waves, and an increase in the cavitation rate. However, if the gasification pressure continues to increase, the number of bubbles increases to a certain limit and the bubble group forms a "layer gap", which prevents the shock wave from traveling and weakens its strength, and the degree of damage to cavitation will gradually decrease.

(5) Effect of temperature The change of temperature in the fluid will lead to large changes in gasification pressure, gas solubility, surface tension, and other physical properties that affect cavitation. It can be seen that the influence mechanism of temperature on cavitation is relatively complicated, and it needs to be judged in combination with the actual situation.

(6) Effect of surface tension When other factors remain constant, lowering fluid surface tension can reduce cavitation damage. Because as the surface tension of the fluid decreases, the intensity of the shock wave generated by the bubble collapse decreases, and the cavitation rate decreases.

(7) Influence of liquid viscosity The greater the viscosity of the fluid, the lower the flow velocity, the fewer the number of bubbles reaching the high-pressure area, and the smaller the intensity of the shock wave generated by the bursting of the bubbles. At the same time, the greater the viscosity of the fluid, the greater the weakening of the shock wave. Therefore, the lower the viscosity of the fluid, the more severe the cavitation damage.

(8) Effect of liquid compressibility and density With the increase of fluid density, the compressibility decreases, and the cavitation loss increases.

2. The influence of the material characteristics of the wetted parts

The cavitation damage of the pump is mainly reflected in the damage to the material of the flow-passing parts. Therefore, the material properties of the flow-passing parts will also affect the cavitation of the centrifugal pump to a certain extent, and the use of materials with good cavitation resistance to manufacturing the flow-passing parts is an effective measure to reduce the impact of cavitation in centrifugal pumps.

(1) Hardness of the material Taking the impeller made of AISI304 as an example, cavitation will cause work hardening and phase transformation of the impeller material to induce martensitic steel, and this change will in turn prevent further cavitation of the material. The cavitation resistance of work hardening and phase transformation-induced martensitic steel mainly depends on the hardness of the impeller material.

(2) Work hardening and fatigue resistance The higher the work hardening index of the material, the better the fatigue resistance, and the better the cavitation resistance of the material.

(3) Effect of crystal structure In the case of certain other conditions, the anti-cavitation rate is a function of the microstructure. In the cubic crystal system, due to the high strain rate sensitivity of the body-centered cubic lattice metal, when the strain rate increases, it will cause rapid transgranular brittle fracture and cleavage fracture, and lead to the formation of pitting corrosion, resulting in higher High abrasive rate. For metals with a close-packed hexagonal lattice, when the axial ratio is close to the ideal and in a cavitation environment, all six slip systems are activated, and quickly transform into a stable FCC, absorbing the work done by the cavitation stress, and making the erosion rate decline. For metals with a face-centered cubic lattice, there are many slip systems, and plastic rheology will occur under high stress. Therefore, the incubation period is long and the abrasion rate is reduced. In short, during the cavitation process, the transition from BCC to HCP or FCC to HCP will improve the cavitation resistance.

(4) Effect of grain size The smaller the grain size of the metal material used in the impeller, the better the cavitation resistance. Because the grain size of the metal is smaller, the fine grain increases the grain boundary, the dislocation slip is hindered, and the resistance of the crack in the expansion increases, which prolongs the abrasion life.

3. The influence of centrifugal pump structure design

In terms of centrifugal pump structure design, the main influence on pump cavitation characteristics can be divided into two aspects: pump body design and impeller design. Studies have shown that the direct factor affecting the cavitation performance of centrifugal pumps is the local flow uniformity at the impeller inlet, so the design of the impeller structure has a greater impact on the cavitation of centrifugal pumps than the design of the pump body, and is the main influencing factor.

(1) Effect of impeller structure on cavitation performance of a centrifugal pump

The impeller structure of a centrifugal pump has an important influence on the cavitation performance of the pump, and a reasonable impeller structure can improve the cavitation performance of the pump.

1) Blade inlet thickness. The displacement of the blades increases the velocity of the fluid at the inlet, resulting in a pressure loss. Choosing a smaller blade inlet thickness can reduce the impact of the blade on the liquid flow, increase the flow area at the blade inlet, reduce the displacement of the blade, thereby reducing the absolute and relative speed of the blade inlet, and improve the anti-cavitation performance of the pump.

2) The surface roughness of the impeller inlet flow channel. The surface roughness of the impeller inlet flow channel of the centrifugal pump can be divided into two categories: one is isolated rough protrusions (such as obvious slag inclusions on the surface of the protruding flow channel or obvious machining and non-processing transition edges, etc.), the other is Classes are rough protrusions that are uniformly distributed along a portion of the entire surface. Studies have shown that isolated rough protrusions will cause additional impacts and vortices in the liquid flow, so the risk of cavitation is much smaller for asperities uniformly distributed along the entire surface than for isolated rough protrusions of the same height. It can be seen that the necessary grinding of the surface of the rough flow channel, especially the surface with isolated rough protrusions, is an effective measure to improve the anti-cavitation performance of the centrifugal pump.

3) The blade inlet throat area. The throat area of the blade inlet has a great influence on the cavitation performance of the centrifugal pump. If the throat area of the blade inlet is small, even if the ratio of the flow area at the blade inlet to the cross-sectional area of the impeller inlet is designed reasonably, the ideal cavitation performance may still not be achieved. If the inlet throat area of the impeller blade is too small, the absolute velocity of the liquid flow at the blade inlet will increase, resulting in a decrease in the anti-cavitation performance of the centrifugal pump.

4) Number of leaves. The number of blades in the centrifugal pump impeller has a great influence on the lift, efficiency, and cavitation performance of the pump. Of course, the use of fewer impeller blades can reduce the friction surface and make it easier to manufacture, but its guiding effect on the fluid has become worse, and the use of more blades can reduce the load on the blades and improve the initial cavitation characteristics, but the blades If the number is too large, the degree of displacement will increase, and the width between adjacent blades will decrease, which will easily form bubble groups to block the flow channel, resulting in poor cavitation performance of the pump. Therefore, when selecting the number of impeller blades, on the one hand, it is necessary to minimize the displacement and friction surface of the blades, and on the other hand, it is necessary to make the blade path have sufficient length to ensure the stability of the liquid flow and the full effect of the blades on the liquid. At present, there is no definite and generally accepted rule for the value of the number of leaves. However, a large number of studies have shown that for the specific centrifugal pump design, the application of the CFD flow field numerical simulation method can effectively determine the optimal range of the number of impeller blades.

(2) Effect of impeller suction parameters on cavitation performance of a centrifugal pump

The parameters of the impeller suction inlet are the relevant structural parameters that determine the area of the impeller blade inlet, including the blade inlet angle of attack, the impeller inlet diameter, the width of the blade inlet flow channel, and the diameter of the hub.

1) The blade inlet attack angle Δβ generally takes the positive attack angle (3°~10°). Due to the positive attack angle, the inlet angle of the blade is increased, so that the bending of the blade can be effectively reduced, the flow area of the blade inlet is increased, and the displacement of the blade is reduced. These factors will reduce v0 and ω0 and improve the anti-cavitation performance of the pump. And when the flow rate of the centrifugal pump increases, the relative flow angle of the inlet increases, and the positive attack angle can avoid the negative attack angle when the pump operates under high flow rate, causing λ2 to rise sharply (as shown in the figure below). A large number of studies have shown that increasing the blade inlet angle and maintaining the positive attack angle can improve the anti-cavitation performance of the pump, and have little effect on the efficiency. However, the choice of the angle of attack has an optimal value for the anti-cavitation performance of the centrifugal pump. It is not that the larger the angle of attack, the better. It should be analyzed and selected according to the actual situation.

2) The impeller inlet diameter. In the case of constant flow, the absolute and relative velocity of the liquid flow at the impeller inlet is a function of the suction pipe diameter. Therefore, for improving the anti-cavitation characteristics of the centrifugal pump, there is an optimum value of the impeller inlet diameter. When the impeller inlet diameter is smaller than the optimal value, the flow velocity at the inlet decreases with the increase of the impeller diameter, and the cavitation performance of the centrifugal pump continues to improve. However, when the impeller diameter exceeds the optimum value, for a given flow rate, as the inlet diameter increases, a stagnation zone, and reverse flow will be formed at the impeller inlet, which will gradually deteriorate the cavitation performance of the centrifugal pump.

3) The width of the blade inlet runner. When the working conditions of the centrifugal pump remain unchanged, increasing the width of the flow channel at the blade inlet will reduce the axial surface component velocity of the absolute velocity of the liquid flow, thereby improving the cavitation characteristics of the centrifugal pump and affecting the hydraulic pressure of the centrifugal pump. Efficiency and volumetric efficiency are less affected.

4) Hub diameter. Reducing the diameter of the hub of the impeller will increase the actual inlet area of the impeller flow channel, thereby improving the cavitation performance of the centrifugal pump.

5) The radius of curvature of the impeller front cover. When the fluid flows from the suction port of the centrifugal pump to the inlet of the impeller, due to the constriction of the channel, the fluid velocity increases, resulting in a certain pressure loss. At the same time, since the direction of fluid flow changes from axial to radial during this process, some pressure loss will also occur due to the uneven flow field at the turn. It can be seen that the radius of curvature of the impeller front cover directly affects the pressure loss, and then affects the cavitation characteristics of the centrifugal pump. Using a larger curvature radius can reduce the change of flow velocity at the turning point of the liquid flow at the front cover, make the flow velocity uniform and stable, and improve the cavitation performance of the centrifugal pump.

4. Other influences:

1. Interaction of parameters

So far, the research on the influence factors of centrifugal pump cavitation is only for a certain parameter, and the mutual influence among various parameters is seldom studied. However, the influence of structural parameters is a unified whole, and they are mutually restricting and influencing each other. Future research should develop in the direction of comprehensive influencing factors.

2. Operating conditions of the centrifugal pump

During the actual use of the centrifugal pump, due to the extremely complex operating conditions, the pump inlet flow and pressure are constantly changing. Therefore, the actual working conditions of the centrifugal pump often deviate greatly from the experimental and designed working conditions. The possibility of cavitation is far beyond the experiment's prediction.

summary

Because the mechanism of cavitation is very complex, there are many factors affecting the cavitation of centrifugal pumps, and various factors do not act in isolation, and there are interactions and mutual influences between different influencing factors. Therefore, when studying the cavitation performance of centrifugal pumps, the mechanism and factors affecting pump cavitation should be considered comprehensively in combination with the actual situation. In recent years, with the development of CFD technology, the numerical simulation of the flow field in the centrifugal pump has provided a new means for studying the cavitation performance of the centrifugal pump under the influence of various factors. However, at present, most CFD numerical simulations of centrifugal pump cavitation are still limited to the study of the influence of a single factor on pump cavitation performance. Further research should pay more attention to the influence of the interaction of different factors on the anti-cavitation performance of centrifugal pumps.