According to almost every fluid text book, the formula for calculating the available NPSH is: NPSHa = Pi/(rhog) + Vi2/(2g) - Pv/(rhog) where i is the point at pump inlet and Pv is the vapor pressure of the liquid. Why do we add the dynamic head to the pressure head? If the static pressure goes below the vapor pressure, the liquid will evaporate no matter how large the dynamic head is. If, for example, Pi = 0.1 bar and Pv = 0.12 bar, the liquid will evaporate even if Vi=20 m/s. So why do we added it any way? I have looked into many books and couldn't find an answer.
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