# Available NPSH in turbine (Net Positive Suction Head)

## Description

In a hydraulic circuit, net positive suction head (NPSH) may refer to one of two quantities in the analysis of cavitation:

1. The Available NPSH (NPSHA): a measure of how close the fluid at a given point is to boiling, and so to cavitation.

2. The Required NPSH (NPSHR): the head value at a specific point (e.g. the inlet of a pump) required to keep the fluid from cavitating.

NPSH is particularly relevant inside centrifugal pumps and turbines, which are parts of a hydraulic system that are most vulnerable to cavitation.

The calculation of NPSH in a reaction turbine is is based on the fact that the cavitation will first occur at the outlet of the impeller, at the entrance of the draft tube.

## Variables

NPSH | Available NPSH in a turbine (m) |

p_{0} | Total pressure at the lower free surface (m^{-1}*kg*s^{-2}) |

ρ | Density of the fluid (kg/m^{3}) |

g | Standard gravity |

p_{v} | Pressure of vapor (m^{-1}*kg*s^{-2}) |

z_{e} | Elevation of the tube entrance (m) |

z_{0} | Elevation of the lower free surface (m) |

h_{f} | Turbines minor losses (m) |