In a hydraulic circuit, net positive suction head (NPSH) is the difference between the actual pressure of a liquid in a pipeline and the liquid's vapor pressure at a given temperature.

NPSH is an important parameter to take into account when designing a circuit: whenever the liquid 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, as well as damage the system.

Centrifugal pumps are particularly vulnerable especially when pumping heated solution near the vapor pressure, whereas positive displacement pumps are less affected 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. Careful design is required to pump high temperature liquids with a centrifugal pump when the liquid is near its boiling point.

The violent collapse of the cavitation bubble creates a shock wave that can carve material from internal pump components (usually the leading edge of the impeller) and creates noise often described as "pumping gravel". Additionally, the inevitable increase in vibration can cause other mechanical faults in the pump and associated equipment.

Considering the circuit shown in the picture, in 1-1 NPSH is:

where is the head loss between 0 and 1, is the pressure at the water surface, is the vapour pressure (saturation pressure) for the fluid at the temperature at 1, is the difference in height (shown as H on the diagram) from the water surface to the location 1, and is the fluid density, assumed constant, and is gravitational acceleration.

Read more about NPSHA Somewhat Simpler Informal Way To Understand NPSH…, Some General NPSH Examples

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