Handbook of Civil Engineering Calculations

Turbine specific speed N 9

FIGURE 5. Cavitation constant for hydraulic turbines. (Chemical Engi-

neering.)

for hydraulic turbines. Although a pump used as a turbine will not have exactly the same
relationship, this curve provides a good estimate of crr for turbine duty.
To prevent cavitation, the total available exhaust head (TAEH) must be greater than
the TREH. In this installation, Ns = 1090 and TAEH = 20 ft (6.1 m). From Fig. 5, ar =
0.028 and TREH = 0.028(645) = 18.1 ft (5.5 m). Because TAEH > TREH, there is
enough exhaust head to prevent cavitation.

7. Determine the turbine performance at 80 percent flow rate
   In many cases, pump manufacturers treat conversion factors as proprietary information.
   When this occurs, the performance of the turbine under different operating conditions can
   be predicted from the general curves in Figs. 6 and 7.
   At the 80 percent flow rate for the turbine, or 1200 gal/min (4542 L/min), the operat-
   ing point is 80 percent of bep capacity. For a specific speed of 1090, as before, the per-
   centages of bep head and efficiency are shown in Figs. 6 and 7: 79.5 percent of bep head
   and percent of bep efficiency. To find the actual performance, multiply by the bep values,
   Or, Ht = 0.795(1290) = 1025 ft (393.1 m); Et = 0.91(74.5) = 67.8 percent.
   The bhp at the new operating condition is then bhp = 1200 (1025)(0.678)(0.52)/3960 -
   110hp(82.1kW).
   In a similar way, the constant-head curves in Figs. 8 and 9 predict turbine performance
   at different speeds. For example, speed is 80 percent of bep speed at 2880 r/min. For a
   specific speed of 1090, the percentages of bep capacity, efficiency, and power are

Cavitation constant

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