【Mechanical Equipment Technology Exchange Edition】Mechanical Equipment 【Daily Question】20201104

Choose A, cavitation. Explanation: The concept and causes of cavitation: When the installation height of a centrifugal pump increases, the pressure inside the pump decreases. The lowest point of pressure inside the pump is usually located slightly downstream of the inlet of the impeller blades. When the pressure here drops to the saturated vapor pressure of the liquid being transported at its current temperature, boiling occurs; the steam bubbles formed then rapidly condense as the pressure increases sharply as the liquid flows from the inlet toward the periphery. This causes the liquid to rush toward the center of the bubble at high speed, resulting in shock waves with very high frequencies and intense instantaneous pressures. Root cause: The inlet pressure is lower than the saturated vapor pressure at the fluid transfer temperature. 　　Harm: The shock waves transmitted to the impeller and pump casing during cavitation, combined with the chemical corrosion of the metal caused by trace amounts of oxygen dissolved in the liquid, can over time cause scars and cracks to appear on their surface, leading to gradual flaking away in a sponge-like manner ； When cavitation occurs, noise is also generated, which in turn causes the pump body to vibrate ； At the same time, the generation of steam reduces the apparent density of the liquid, which in turn leads to a decrease in the actual flow rate of the liquid, the outlet pressure, and the efficiency; in severe cases, it can result in no liquid being able to be discharged at all.