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Cavitation and NPSH

If the pressure at the pump inlet gets too low, the liquid boils and forms vapour bubbles that collapse violently. This is called cavitation and destroys the impeller. The available NPSH must be larger than what the pump requires.

NPSHA=pin−pvρgNPSH_A = \frac{p_{in} - p_v}{\rho g}net positive suction head available
NPSHA>NPSHRNPSH_A > NPSH_Rrequirement to avoid cavitation

Symbols

pinp_{in}total pressure at the inletPa
pvp_vvapour pressurePa
NPSHRNPSH_Rpump requirement (from data sheet)m

Example

Water at 20 °C (pv≈2.3p_v \approx 2.3 kPa), inlet pressure 50 kPa:

NPSHA=47 700/9810≈4.9NPSH_A = 47\,700/9810 \approx 4.9 m.

Place the pump low and keep the suction line short and wide to get a high NPSH.
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← System curve and operating point

Part of Fluid Mechanics: Pumps and pipe systems.