1. Cavitation phenomenon
From the working principle of the pump, it can be known that the suction process of the pump can only be carried out when a vacuum degree (relative to the surface pressure of the suction tank) is generated inside the pump, and the vacuum degree has a certain limit. An increase in vacuum degree indicates a decrease in absolute pressure. When the vacuum degree reaches a certain level, the absolute pressure drops to a certain degree, and a "cavitation" phenomenon occurs during the operation of the pump.
When the pump is running and the pressure at the impeller is equal to or lower than the saturated vapor pressure of the liquid being transported at the operating temperature, the liquid will vaporize and form many bubbles. At the same time, the gas that was originally dissolved in the liquid will also escape. These bubbles are immediately carried into the high-pressure zone inside the impeller by the liquid flow, and under the action of high pressure, the bubbles are compressed and re condensed into liquid. During the condensation process, the volume rapidly shrinks, as if forming a cavity. At this time, the surrounding liquid rushes towards the cavity at a high speed, causing the liquids to collide with each other. Due to the mutual impact of liquid particles, high instantaneous local impact pressure is generated, which continuously hits the surface of the blade. The metal surface of the blade will be eroded into pockmarks, honeycomb sponge like shapes due to impact fatigue. The above-mentioned liquid vaporization, condensation, and impact form high-speed, high-pressure, and high-frequency impact loads, causing mechanical cracking of metal materials, which is called cavitation phenomenon.
2. The hazards of cavitation
(1) Cavitation causes damage to the impeller lining. Typically, the location of cavitation damage in centrifugal pumps is first near the blade population and then extends to the impeller outlet. At first, there are pockmarks on the metal surface, followed by sponge like, honeycomb like, and fish scale like cracks on the surface. In severe cases, it can cause perforation of the front and rear cover plates of the blades or impeller, and even rupture of the impeller.
(2) Cavitation causes a decrease in pump performance. In the initial stage of cavitation, there is no significant impact on the normal operation of the pump. When cavitation develops to a certain extent, a large number of bubbles are generated, blocking the flow channel and disrupting the continuity of liquid flow inside the pump. Therefore, the pump performance curve shows a sudden decline.
(3) FAW corrosion causes noise and vibration in the pump. Due to the continuous generation and collapse of bubbles, liquid particles collide with each other, producing various frequencies of noise. When cavitation is severe, a "cracking" explosion sound can be heard inside the pump, which also causes vibration of the pump. The larger the pump, the more severe the noise and vibration caused by cavitation.
3. Prevention of cavitation
Cavitation is an extremely abnormal phenomenon during the operation of centrifugal pumps. In order to avoid cavitation during pump operation, the following methods are generally adopted:
(1) To increase the operating pressure P and increase the pressure P at the pump inlet, the following measures can be taken: increase the diameter of the suction pipe; Shorten the length of the suction pipe section; Reduce bends and accessories: lower the installation height of the pump AZOS, and if necessary, set up an underground pump room: increase the pressure Po inside the suction container; reduce the operating flow rate of the pump; Open the inlet regulating valve of the pump. (Reduce [0h] or increase (H) at the same speed and flow rate, using double suction impellers, or using pumps with induced impellers.
(3) Using anti cavitation materials - generally, when limited by usage conditions, it is not allowedCan completelyTo avoid cavitation, impellers should be made of materials with high strength, hardness, toughness, and chemical stability to extend their service life. Generally, 2Cr13 and high nickel chromium alloys are used.