First, the pump cavitation process Pump in operation, if the over-current part of the local area (usually the impeller blade inlet somewhere later), for some reason, the absolute pressure of pumping liquid down to the current temperature When the pressure is vaporized, the liquid begins to vaporize there, causing cavitation and bubbles. These bubbles flow forward with the liquid. When a high pressure is reached, the high-pressure liquid around the bubbles causes the bubbles to abruptly shrink and become ruptured (coagulation). At the same time as the bubbles are condensed, the liquid particles will fill cavities at a high speed and collide with each other to form Water hammer, this phenomenon occurs in the solid wall will make the overcurrent components corroded and destroyed, the bubble and bubble rupture flow component is destroyed during the pump cavitation process. Second, the phenomenon of pump cavitation 1. Produce noise and vibration As the pump cavitation, the bubble suddenly occurred in the high pressure area suddenly burst, and accompanied by a strong water hammer, and the noise and vibration, you can hear like a popcorn like The crackling noise can detect the nascent cavitation based on the noise, however, it is quantitatively determined that the cavitation noise is distinguished from the noise in the environment and the noise generated by the internal impact of the flow in the machine to a considerable extent Difficult, in this case, a small amount of air is injected to dampen noise, vibration and damage to the metal. 2. Corrosive damage to over-current parts When the pump is operated under cavitation for a long period of time, parts of the over-current part of the pump are subject to corrosion damage due to the metal surface being subjected to blade-like high-frequency 600 ~ 25000Hz) strong impact, the pressure reaches 49MPa, resulting in pitting and perforation of the metal surface, serious metal grains loose and exfoliated and presented honeycomb, cavitation damage in addition to the mechanical force of the outside pieces of electrolysis, chemical corrosion and more A very complex role. Practice has proved that the erosion of cavitation corrosion site, it is the disappearance of the bubble, it is often found in the impeller exit and pressurized water inlet parts of the site to find traces of damage, but you can not forget the birthplace of cavitation is the impeller at the entrance to radical steam Erosion must prevent air bubbles at the impeller inlet. High-speed axial-flow pumps and diagonal-flow pumps often fail on the blade's back and periphery. 3. Performance degradation Pump impeller cavitation when the liquid energy exchange is disturbed and destroyed, the performance of the external characteristics of the flow - lift curve, flow - shaft power curve, flow - efficiency curve decreased, serious pump In the initial stage of cavitation pump, the characteristic curve did not change significantly, sometimes due to the generation of bubbles cover the flow surface part of the flow to form a smooth layer so that the pump efficiency slightly increased, the pump Cavitation has developed to a certain extent when the characteristic curve shows obvious changes. Different speed than the pump, cavitation caused by the decline in the performance of different forms, low specific speed pump, due to narrow and long flow path between the blades, so in the event of cavitation, the bubble is easy to fill the entire flow path, so the performance curve was suddenly decreased form , With the increase of specific speed, the trend of broad and short foliage changes. Therefore, it takes a transition from the occurrence of bubble to the full flow path. The corresponding pump performance curve begins to decline slowly and then increases to a certain flow rate When the axial flow pump blades less overlap between the leaves, there is always part of the high pressure, so the performance curve is only slowly decline in the entire range, in the multi-stage pump, due to cavitation occurred in the first level, So the performance curve is lower than single-stage pump.
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