When the pressure falls below the vapour pressure of the liquid at a given temperature, boiling occurs and small bubbles of vapour are formed.These bubbles will grow in the low-pressure area and implode when they are transported to an area of pressure above vapour pressure. The term given to this local vaporization of the fluid is Cavitation.
The collapsing of the bubbles is the area of cavitation we are concerned with, as extremely high pressures are produced, which causes noise and erosion of the metal surface. The area of pipeline where cavitation mainly occurs is the pump suction, where the liquid is subjected to a rapid rise in velocity, and hence a fall in static pressure.
This cavitation effect on the pump can cause damage on the casing and impeller. During cavitation, a liquid/vapour mixture of varying density is produced. This results in fluctuations in pressure (caused by the liquid column being drawn in), and causes fluctuations in the discharge pressure, pump power absorbed (shown on the ammeter), and hence pump revolutions.
To reduce Cavitation we must reduce the 'losses' on the suction side, hence reduce the pipeline friction and NPSH. This means reducing the pump flow rates. To reduce 'losses' on starting, the pump should be started against a closed discharge valve.
The collapsing of the bubbles is the area of cavitation we are concerned with, as extremely high pressures are produced, which causes noise and erosion of the metal surface. The area of pipeline where cavitation mainly occurs is the pump suction, where the liquid is subjected to a rapid rise in velocity, and hence a fall in static pressure.
This cavitation effect on the pump can cause damage on the casing and impeller. During cavitation, a liquid/vapour mixture of varying density is produced. This results in fluctuations in pressure (caused by the liquid column being drawn in), and causes fluctuations in the discharge pressure, pump power absorbed (shown on the ammeter), and hence pump revolutions.
To reduce Cavitation we must reduce the 'losses' on the suction side, hence reduce the pipeline friction and NPSH. This means reducing the pump flow rates. To reduce 'losses' on starting, the pump should be started against a closed discharge valve.
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