Like pumps, turbines can also cavitate when the pressure at point 1 in Fig. P11.98 drops too low. With NPSH defined by Eq. 11.20, the empirical criterion given by Wislicenus [Ref. 4] for cavitation is Use this criterion to compute how high (z1 − z2) the impeller eye in Fig. P11.98 can be placed for a Francis turbine, with a head of 300 ft, Nsp = 40, and pa = 14 psia, before cavitation occurs in 60°F water.

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(rpm)(gal/min)12 INS 2 11.000