Cavitation in centrifugal pumps
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How does cavitation occur in centrifugal pumps, and what are its hazards? How to prevent cavitation? Answer: When the pressure at the pump’s suction inlet drops below the saturated vapor pressure of the liquid inside the pump at that temperature, the liquid boils, resulting in the formation of numerous bubbles. At the same time, certain gases dissolved in the liquid will also escape to form bubbles as the pressure decreases. As the bubbles enter the high-pressure area along with the liquid, they burst rapidly, creating a local vacuum. The surrounding liquid then rushes into the space occupied by the bubbles at high speed, colliding with each other and converting its kinetic energy into pressure energy, thereby generating a very high local impact force in an instant. This impact force causes significant damage to the impeller, and can result in a honeycomb or sponge-like structure forming on its surface. When cavitation occurs, the pump body is subjected to shocks that cause vibration and noise; the pump’s performance deteriorates sharply, and in severe cases flow stops and the pump can no longer function properly. Methods to prevent cavitation are: (1) the installation height of the pump must be lower than the allowable suction vacuum level. ⑵ The effective NPSH provided by the pump installation is greater than the NPSH required by the pump. ⑶ As the flow rate increases, the net positive suction head also increases, and this should be taken into account during operation. ⑷ Be careful that the operating temperature of the liquid being transported is not too high.
A malfunctioning cooling water system or an improperly designed suction pipeline can both lead to cavitation
1. Cavitation: At a certain temperature, when the pressure of a liquid is reduced to its vaporization pressure at that temperature, bubbles form in the liquid. This phenomenon of bubble formation is called cavitation. 2. Cavitation collapse: The bubbles formed during cavitation shrink in size as they move to areas of higher pressure, resulting in their destruction. This phenomenon of bubbles disappearing in the liquid due to rising pressure is called cavitation collapse. 3. Causes and hazards of cavitation: During operation of a pump, if the absolute pressure of the liquid being pumped drops to the vaporization pressure of that liquid at the given temperature in certain local areas of its flow path (usually somewhere slightly downstream of the inlet to the impeller blades), the liquid begins to vaporize there, generating large amounts of steam and forming bubbles. When this liquid containing many bubbles moves forward through the high-pressure areas inside the impeller, the high-pressure liquid surrounding the bubbles causes them to shrink rapidly and eventually burst. As the bubbles condense and burst, liquid particles fill the voids at high speeds; this creates a very strong water hammer effect. The impact stress on the metal surface can reach several hundred to several thousand atmospheres, with an impact frequency of tens of thousands of times per second. In severe cases, this can cause the wall thickness to be penetrated. 4. Cavitation process: The process in a water pump in which bubbles are formed and then burst, causing damage to the flowing components, is known as the cavitation process in that water pump. After cavitation occurs in a water pump, in addition to causing damage to the flow-through components, it also generates noise and vibration, leading to a decline in the pump’s performance; in severe cases, it can even interrupt the flow of liquid within the pump, preventing it from functioning properly. There are three measures to prevent cavitation in pumps: a) Structural measures: using a double-suction impeller to reduce the flow velocity through the impeller, thereby decreasing the pump’s net positive suction head ； A booster pump is installed in front of large high-head pumps to increase the inlet pressure ； The impeller is specially designed to improve the flow conditions at the inlet of the blades ； An inducer wheel is added in front of the centrifugal impeller to increase the pressure of the fluid flow entering the impeller. b) Installation and operation measures: Ensure that the pump’s filling height is greater than the minimum filling height ； Be careful not to let the temperature of the fluid become too high. c) Other measures: Use materials resistant to cavitation damage to manufacture the components of the pump’s flow-through parts ； Reduce the pump speed. To prevent cavitation, the following measures are adopted in the pump’s design: 1. Use of a double-suction impeller ； II. Increase the inlet area of the impeller ； III. Increase the width of the blade inlet edge ； IV. Increase the radius of curvature at the bends of the front and rear cover plates of the impeller ； V. The inlet edge of the blade extends toward the suction side ； VI. The first stage of the impeller is made of cavitation-resistant material ； VII. Install a pre-induction wheel. For existing water pumps, measures to prevent cavitation include: first, ensuring that the rate of change of the cross-sectional area in the flow passage is as small as possible, and keeping the wall surfaces smooth ； II. The suction pipe should have low resistance, and be short and straight ； III. Correctly select the suction height ； IV. Apply epoxy resin coating to the cavitation area.