A pump is (same) cavitando?

We often hear references to problems related to the suction line of a centrifugal pump, attributing the cause of these problems to the "famous" cavitation.

actually, in certain cases, even a pump operating Drowned, with the losses in the suction, the liquid temperature, its specific weight and its vapor pressure due account, may exhibit malfunctions that resemble the effects of cavitation, in disagreement with the calculations.

The source of this distortion is often the trend, very common, from adopt suction pipe of the same diameter of the pump suction nozzle. In some cases, even lower gauge.

In these situations, particularly when the suction line is short, the calculated pressure loss resulting small, and it is considered removing the possibility of cavitation. However, if the velocity of the liquid in the suction line overcome 2 m/s (guidance parameter for cold water), may occur the phenomenon called hydraulic "Net column break", that is, pipe flow becomes intermittent, because the pump draws faster than the liquid can move. In consequence, pump operation becomes jerky, and vibration, sometimes severe.

Cavitation "true" occurs when fluid, By the combined effect of temperature and pressure loss in the suction, reaches the pump impeller up vaporizing, with extensive formation of microbubbles. It is the later "implosion" of those bubbles, with the increase in pressure along the rotor palettes, which causes the known noise effects, vibration and erosion.

in conclusion, besides the usual calculations of pipe and positioning the pump, in order to avoid cavitation, it is important to take into consideration the speed factor in the suction line, also preventing the liquid column breaks.

Eng.Mauro Baldini





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