When cavitation occurs in the submerged slurry pump, a large number of bubbles are generated, developed and collapsed, which destroys the normal flow law of the water flow. The overflow area in the flow channel is reduced and the flow direction is changed. Therefore, the energy exchange between the impeller and the water flow is disturbed and damaged, and the energy loss increases, resulting in the rapid decline of the flow, head and efficiency of the water pump, and even the cut-off state.
The characteristics of performance degradation caused by cavitation are different for submerged slurry pumps with different specific speeds. For low specific speed centrifugal pumps, the flow channel between blades is narrow and long, and the width is small. After cavitation begins, the bubble area first appears at the inlet of the blade flow channel, and quickly expands to the whole width of the flow channel, "blocking" the whole flow channel, damaging the continuity of water flow, and the performance curve of submerged slurry pump shows a sharp decline, A "fracture" condition occurs. For medium and high specific speed centrifugal pumps and mixed flow pumps, because the impeller channel is relatively wide, it is not easy to be "blocked" by bubbles, so the performance curve of submerged slurry pump first decreases slowly. When the flow increases and cavitation develops to a certain extent, the "fracture" working condition occurs. For high specific speed axial flow pump, due to less blades, less overlap between blades and wide flow channel, the cavitation area is not easy to expand to the whole flow channel after cavitation begins. Therefore, the performance curve not only decreases slowly, but also there is no "fracture" condition.
When the bubble is brought to the high-pressure area by the water flow and quickly condenses and collapses, the water particles around the bubble concentrate to the center of the bubble at high speed, resulting in a strong impact. The observation data show that the bubble collapse can be completed in milliseconds or even microseconds. Therefore, it produces a great impact force, and the local pressure can reach even hundreds of MPa. If the bubble collapses near the surface of the flow passage part, it will strike the flow passage part. Under the constant impact of such high frequency and high pressure, the metal materials of flow passage parts cause plastic deformation and local hardening, fatigue and brittle performance, and cracks and spalling will soon occur, resulting in the formation of honeycomb holes. The further action of cavitation makes the cracks penetrate each other and connect the holes until the impeller or pump shell is corroded or even broken. This is the mechanical denudation effect of cavitation on the metal surface of flow passage parts.
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Harm of cavitation of slurry pump