Cavitation And Net Positive Suction Head Npsh A Vapor Lock and Cavitation

As previously mentioned, a centrifugal pump increases the fluid pressure by first imparting angular momentum (or kinetic energy) to the fluid, which is converted to pressure in the diffuser or volute section. Hence, the fluid velocity in and around the impeller is much higher than that either entering or leaving the pump, and the pressure is the lowest where the velocity is highest. The minimum pressure at which a pump will operate properly must be above the vapor pressure of the fluid; otherwise the fluid will vaporize (or "boil"), a condition known as cavitation. Obviously, the higher the temperature the higher the vapor pressure and the more likely that this condition will occur. When a centrifugal pump contains a gas or vapor it will still develop the same head, but because the pressure is proportional to the fluid density it will be several orders of magnitude lower than the pressure for a liquid at the same head. This condition (when the pump is filled with a gas or vapor) is known as vapor lock, and the pump will not function when this occurs.

However, cavitation may result in an even more serious condition than vapor lock. When the pressure at any point within the pump drops below the vapor pressure of the liquid, vapor bubbles will form at that point (this generally occurs on or near the impeller). These bubbles will then be transported to another region in the fluid where the pressure is greater than the vapor pressure, at which point they will collapse. This formation and collapse of bubbles occurs very rapidly and can create local ''shock waves," which can cause erosion and serious damage to the impeller or pump. (It is often obvious when a pump is cavitating, because it may sound as though there are rocks in the pump!)

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