Slurry pump manufacturers cavitation phenomenon Shijiazhuang Tiangong pump cavitation phenomenon Definition: During the pump operation, if the local pressure in the pump is lower than the saturation vapor pressure of the water at this temperature, the impurities in the water body, tiny solid particles, or Gaseous nuclei present in the gaps between the liquid and solid contact surfaces rapidly grow into macroscopic cavities which collapse when the water reaches the high pressure zone and is squeezed by the surrounding water. Cavitation generation, collapse process involves many physical and chemical phenomena, will produce noise, vibration, and flow components of the material erosion. This phenomenon is called the pump cavitation phenomenon. Saturated water vapor pressure refers to a certain water temperature boiling water when the steam pressure head, commonly used pa that water saturated steam pressure head (referred to as pressure head), the unit is m (Mh2O). Different water temperature saturated steam head pressure water table 2 water temperature /. C 0 5 10 20 30 40 50 60 70 80 90 100 Saturated steam pressure head hva / m 0.06 0.09 0.12 0.24 0.43 0.75 1.25 2.02 3.17 4.82 7.14 10.33 Harm of cavitation There are mainly three hazards in pump cavitation: Deterioration of pump performance Cavitation will produce a large number of cavitations, the water contains a large number of cavities destroyed the normal flow of water, so that the effective cross-sectional area of ​​the leaf slot decreases, the flow direction changes, the energy loss increases, causing the pump flow , Head and the rapid decline in efficiency, cavitation serious or even cut off. This performance change for different than the speed of the pump have different consequences, as shown in Figure 3. Centrifugal pumps at medium and low specific speeds have a long and narrow vane slot, and cavitation can have a significant effect on the performance of the pump. However, the axial flow pump with a high specific speed has a large vane slot and cavitation can not easily spread to the entire vane slot after cavitation. The performance curve slowed down so that there was no apparent breaking point. 2. Damage flow components data show that when the bubble is brought to the high pressure zone by the rapid condensation, collapse, the water particles at high speed to the bubble center impact, resulting in a strong impact, the impact of the pump shell and the pump Impeller wall, the moment of partial pressure, can reach tens to hundreds of megapascals, and the role of frequency up to 2-3 million times per minute. Pump wall in the repeated impact of high-intensity impact, the metal surface local deformation and hardening brittle, resulting in fatigue phenomenon, the metal rupture and spalling. In addition to mechanics, but also mixed with the living water from the gas (such as oxygen) to the metal chemical corrosion and electrochemical corrosion of water on the metal, etc., under the combined effect, the pump wall appears at first pitting, and then change Into a honeycomb. In severe cases, the wall will be broken down in a short period of time. 3. Generation of vibration and noise Cavitation occurs, with the collapse of the bubble burst of water particles between the impact of each other, as well as high-intensity impact on the pump shell and impeller repeated impact, the pump will produce vibration and strong noise. When the frequency of the cavitation vibration is close to the vibration frequency of the pump itself, resonance of the pump may be caused, thereby greatly increasing the amplitude of the vibration and causing the base and the foundation to vibrate and cause damage. Therefore, noise and vibration are also one of the main bases for judging whether cavitation occurs or disappears. The location of cavitation According to the different parts of the pump cavitation, cavitation can be divided into the following four categories: 1. Leaf cavitation is the cavitation occurs in the blade surface, mainly because of the pump installed too high, or flow deviation Cavitation produced when the design flow is too large. The formation and collapse of vacuoles occur mostly at the front, back or front inner surface of the blade. 2. Gap cavitation pump flow through the water suddenly narrowing the gap, the speed increases, the local pressure drop, will also produce cavitation. Such as the axial flow pump blade outer shell and the gap between the pump shell, centrifugal pump seal ring and the impeller inlet outer edge of the gap, due to the impeller inlet and outlet side of the pressure difference is large, resulting in high-speed reflux, resulting in local pressure Drop, causing gap cavitation. Vortex cavitation due to poor water collection pool, inflow or water pump design in non-design conditions may also be generated below the impeller vortex ribbon (bottom vortex), when the vortex center pressure Below the vapor pressure, the vortex becomes a cavitation band. Rough cavitation water flow through the pump within the bumpy inner wall and flow components, downstream of the protrusions are also prone to local negative pressure and lead to cavitation, the cavitation is called rough cavitation. The location of pump cavitation occurs as shown in Figure 5-2, Figure 5-3 and Figure 5-4. How to effectively reduce the phenomenon of cavitation 1, the installation of the pump height, the higher the pump installation height, the lower the pump inlet pressure, reducing the pump installation height can increase the pump inlet pressure. Therefore, a reasonable determination of the pump installation height to avoid pump cavitation. 2, pipette resistance, set in the pipette elbows, valves and other pipe fittings, the greater the resistance of the pipeline, the lower the inlet pressure of the pump. Therefore, try to reduce some of the unnecessary pipe fittings or as much as possible to increase the pipette diameter, reduce pipe resistance, can prevent pump cavitation. 3, the pump geometry, the liquid at the pump inlet with the kinetic energy and static pressure can be converted to each other, its value remains unchanged. When the inlet liquid flow velocity is high, the pressure is low, and when the flow velocity is low, the pressure is high. Therefore, the flow area of ​​the pump inlet is increased and the inlet velocity of the impeller is decreased. Pump cavitation can be prevented. 4, the density of liquid. Pumps with higher density deliver less height to the pump, and pump cavitation may occur when using larger, less densely packed pumps to deliver cavitations, but with a higher delivery density Large liquid pump to send less dense liquid, the pump inlet pressure is higher, does not produce cavitation. 5, the temperature of liquid transport. The saturated vapor pressure of the liquid increases as the temperature increases. When the inlet pressure of the pump is constant, the saturated vapor pressure of the liquid may rise to equal or exceed the inlet pressure of the pump as the temperature of the delivered liquid increases, and the pump will cavitate.
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