Net Positive Suction Head (often abbreviated as NPSH) shows the difference, in any cross-section of a generic hydraulic circuit (cycle) containing liquid, between the pressure and the liquid vapor pressure in that section.
NPSH is an important parameter, to be taken into account when designing a circuit. Whenever the liquid stagnation pressure drops below the vapor pressure, liquid boiling occurs, and the final effect will be cavitation: vapor bubbles may reduce or stop the liquid flow. Centrifugal pumps are particularly vulnerable, whereas positive displacement pumps are less effected by cavitation, as they are better able to pump two-phase flow (the mixture of gas and liquid), however, the resultant flow rate of the pump will be diminished because of the gas volumetrically displacing a disproportion of liquid.
The violent collapse of the cavitation bubble creates a shock wave that can literally carve material from internal pump components (usually the leading edge of the impeller) and creates noise that is most often described as "pumping gravel". Additionally, the inevitable increase in vibration can cause other mechanical faults in the pump and associated equipment.
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