【Mechanical Equipment Technology Exchange Edition】Mechanical Equipment 【Daily Question】20201129

Correct. Explanation: The part of the centrifugal pump most susceptible to cavitation damage is the outlet impeller. The reason is as follows: When a centrifugal pump is in operation, the pressure of the fluid decreases from the pump inlet to the inlet of the impeller, with the liquid pressure being lowest near the blades. Thereafter, the pressure rises rapidly as the impeller does work on the liquid. When the pressure near the inlet of the impeller blades is less than or equal to the saturated vapor pressure at the liquid transport temperature, the liquid vaporizes. At the same time, gases dissolved in the liquid may also escape, forming many bubbles. When the bubble moves with the liquid to a region of higher pressure within the flow channel, the external liquid pressure is higher than the vaporization pressure inside the bubble, causing the bubble to condense and collapse, thereby forming a cavity. In an instant, the liquid around it rushed toward the cavity at extremely high speeds, causing the liquids to collide with each other and leading to a sudden surge in local pressure (which can reach several hundred atmospheres). This not only hinders the normal flow of the fluid, but more seriously, if these bubbles collapse near the walls of the impeller, the liquid acts like countless small projectiles, continuously striking the metal surface at a high frequency (sometimes as high as 2000–3000 Hz), causing the metal surface to crack due to impact fatigue. If the bubbles contain certain reactive gases (such as oxygen), they utilize the energy released during bubble condensation (with local temperatures reaching 200–300°C) to form thermocouples and induce electrolysis, thereby causing electrochemical corrosion of the metal and accelerating the rate of its degradation.