| Hydraulic Institute Pump FAQs January 2008 |
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| Written by Hydraulic Institute | |
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Page 2 of 2
For preliminary selection, a rough approximation procedure can be used to estimate the turbine performance from known pump performance. QT = Qp Ht = Hp n n Where: Qt = Rate of flow as turbine Qp = Rate of flow as pump Hp = Total head as pump Ht = Total head as turbine η = Efficiency
Most centrifugal pumps are suitable and capable of operating as turbines. Because of the reverse rotation, be sure that the bearing lubrication system will operate in reverse, and threaded shaft components, such as impeller locking devices, cannot loosen. Pumps operated in reverse as turbines tend to have relatively narrow operating bands compared to variable nozzle turbines. At constant speed, the power developed and efficiency drop to zero at approximately 40 percent of the hydraulic turbine best efficiency rate of flow (see Figure 1.52).
Figure 1.52
These facts, coupled with the difficulty in predicting hydraulic turbine performance from pump performance, results in some uncertainty when applying a pump to a power recovery turbine application unless actual test data is available on the specific pump running in reverse as a turbine. Some of the other factors which affect the use of pumps as turbines are:
Q. Is there a simple way to determine the minimum submergence required for a large vertical turbine pump to prevent the formation of surface vortices and the entrance of air into the pump? A. This answer provides the recommended minimum submergence of a vertical pump inlet bell to reduce the probability that strong free-surface air core vortices will occur. If a submergence greater than recommended here is needed to provide the required NPSHR for the pump, the greater submergence should be used. Approach-flow skewness and the resulting circulation have a controlling influence on free surface vortices in spite of adequate submergence. The recommended minimum submergence given here is for a reasonably uniform approach flow to the pump suction bell. Highly non-uniform approach flows will require the application of vortex suppression devices. Experimental analysis and field experience have resulted in the following empirical relationship:
S = D + 0.574Q/D1.5
Where: S is submergence in inches D is bell diameter in inches Q is rate of flow in gpm The required minimum submergence can also be determined from Figure 9.8.26B taken from ANSI/HI 9.8 Pump Intake Design.
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Figure 9.8.26B 
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