NPSH

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Hydraulic circuit
Hydraulic circuit

NPSH is an acronym for Net Positive Suction Head. It shows the difference, in any cross-section of a generic hydraulic circuit, 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 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.

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.


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

NPSH = P0 + H − Y − Vt

(to be solved with coherent measuring units), where Y is the friction loss between 0-0 and 1-1, and Vt the liquid vapour pressure at the actual temperature in section 1-1.

In pump operation, two aspects of this parameter are called respectively NPSHA or NPSH (a) Net Positive Suction Head (available) and NPSHR or NPSH(r) or NPSH-3 Net Positive Suction Head (required), where NPSH(a) is the suction pressure presented at the pump inlet port, and NPSH(r) is the suction pressure limit at which the pump's total differential head performance is reduced by 3% due to cavitation. It's important to note that cavitation occurs at suction pressure levels above the NPSH-3 level and pump damage can occur from cavitation even though the pump may continue to provide the expected hydraulic performance.