If the centrifugal cryogenic pump produces noise and vibration during operation, and is accompanied by a reduction in flow, head and efficiency, and sometimes even fails to work, and during maintenance, it is often found that there are pits or honeycomb damage near the blade inlet edge, In severe cases, the entire blade has this phenomenon, and even the blade is penetrated, which is the damage caused by cavitation.
The reason for the cavitation of the centrifugal cryogenic pump: the pump does work on the liquid through the rotating impeller, which increases the energy of the liquid. During the interaction, the velocity and pressure of the liquid change. Typically, the inlet to the impeller of a centrifugal cryopump is where the pressure is lowest. If the pressure in this place is equal to or lower than the vaporization pressure of the liquid at that temperature, there will be a large amount of vapor and gas dissolved in the liquid escaping from the liquid, forming many small bubbles of vapor mixed with the gas. When these small bubbles flow to the high pressure area with the liquid, the pressure difference is generated because the vaporization pressure in the bubbles is greater than the vaporization pressure around the bubbles. Under the action of this pressure difference, the bubbles are ruptured and re-coagulated. During the condensation process, the liquid particles accelerate from the surrounding to the center of the bubble. At the moment of condensation, the particles collide with each other, resulting in a high local pressure. If these bubbles burst and condense near the metal surface, the liquid particles are like countless small warheads, hitting the metal surface continuously. Under the continuous blow of high pressure and high frequency, the metal surface is gradually damaged due to fatigue, which is usually called erosion. The generated bubbles are also mixed with some active gases (such as oxygen, etc.), which can chemically corrode the metal with the help of the heat released when the bubbles condense. The combined effect of chemical corrosion and mechanical erosion makes the metal damage faster. This phenomenon is called cavitation damage.
When the centrifugal cryogenic pump begins to cavitation, the cavitation area is small, which has no obvious influence on the normal operation of the pump, and there is no obvious reflection on the pump performance curve. However, when the cavitation develops to a certain extent, a large number of air bubbles will be generated, which will affect the normal flow of the liquid, and even cause the liquid flow to be interrupted, resulting in vibration and noise. At the same time, the flow rate, head and efficiency of the pump decrease significantly, which is also evident on the pump performance curve. . In severe cases, the pump cannot work. In order to avoid cavitation as much as possible, in the process design, the liquid should have a certain degree of subcooling before entering the pump, and the pump body should be installed at a lower position, so that the liquid inlet has a certain degree of subcooling. certain static head. In addition, attention should be paid to cold preservation and minimize cold losses.