NPSH

  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.

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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. Apart from that, cavitation results in elevated noise levels and possible damage to the pump. Centrifugal pumps are particularly vulnerable, whereas positive displacement pumps are immune to that and they are generally able to pump mixtures of gases and liquids.

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

NPSH = P₀ + H − Y − V_t

(to be solved with coherent measuring units), where Y is the friction loss between 0-0 and 1-1, and V_t 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) Net Positive Suction Head (required), where NPSH(a) is computed at pump inlet port, and NPSH(r) is the NPSH limit the pump can withstand without cavitating.