In a hydraulic circuit, net positive suction head (NPSH) may refer to one of two quantities in the analysis of cavitation:
The Available NPSH (NPSHA): a measure of how close the fluid at a given point is to flashing, and so to cavitation. Technically it is the absolute pressure head minus the vapour pressure of the liquid. The Required NPSH (NPSHR): the head value at the suction side (e.g. the inlet of a pump) required to keep the fluid from cavitating (provided by the manufacturer). NPSH is particularly relevant inside centrifugal pumps and turbines, which are parts of a hydraulic system that are most vulnerable to cavitation. If cavitation occurs, the drag coefficient of the impeller vanes will increase drastically—possibly stopping flow altogether—and prolonged exposure will damage the impeller.
NPSH in a pump
In a pump, cavitation will first occur at the inlet of the impeller. Denoting the inlet by i, the NPSHA at this point is defined as:
NPSH A = ( p i ρ g + V i 2 2 g ) − p v ρ g {\displaystyle {\text{NPSH}}_{A}=\left({\frac {p_{i}}{\rho g}}+{\frac {V_{i}^{2}}{2g}}\right)-{\frac {p_{v}}{\rho g}}}
where p i {\displaystyle p_{i}} is the absolute pressure at the inlet, V i {\displaystyle V_{i}} is the average velocity at the inlet, ρ {\displaystyle \rho } is the fluid density, g {\displaystyle g} is the acceleration of gravity and p v {\displaystyle p_{v}} is the vapor pressure of the fluid. Note that NPSH is equivalent to the sum of both the static and dynamic heads – that is, the stagnation head – minus the equilibrium vapor pressure head, hence "net positive suction head". Applying the Bernoulli's equation for the control volume enclosing the suction free surface 0 and the pump inlet i, under the assumption that the kinetic energy at 0 is negligible, that the fluid is inviscid, and that the fluid density is constant:
p 0 ρ g + z 0 = p i ρ g + V i 2 2 g + z i + h f {\displaystyle {\frac {p_{0}}{\rho g}}+z_{0}={\frac {p_{i}}{\rho g}}+{\frac {V_{i}^{2}}{2g}}+z_{i}+h_{f}}
Using the above application of Bernoulli to eliminate the velocity term and local pressure terms in the definition of NPSHA:
Net Positive Suction Head A = p 0 ρ g − p v ρ g − ( z i − z 0 ) − h f {\displaystyle {\text{Net Positive Suction Head}}_{A}={\frac {p_{0}}{\rho g}}-{\frac {p_{v}}{\rho g}}-(z_{i}-z_{0})-h_{f}}
This is the standard expression for the available NPSH at a point. Cavitation will occur at the point i when the available NPSH is less than the NPSH required to prevent cavitation (NPSHR). For simple impeller systems, NPSHR can be derived theoretically, but very often it is determined empirically. Note NPSHAand NPSHR are in absolute units and usually expressed in "m" or "ft," not "psia". Experimentally, NPSHR is often defined as the NPSH3, the point at which the head output of the pump decreases by 3 % at a given flow due to reduced hydraulic performance. On multi-stage pumps this is limited to a 3 % drop in the first stage head.
NPSH in a turbine The calculation of NPSH in a reaction turbine is different to the calculation of NPSH in a pump, because the point at which cavitation will first occur is in a different place. In a reaction turbine, cavitation will first occur at the outlet of the impeller, at the entrance of the draft tube. Denoting the entrance of the draft tube by e, the NPSHA is defined in the same way as for pumps:
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