Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
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Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

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Hydraulic Power Recovery Turbines 249 The following criteria should be considered since they will help in specifying and classifying the HPRT. Specific Speed HPRT's are classified by their specific speed (N s ) which is a dirnensionless quantity that governs the selection of the type of runner best suited for a given operating condition. where N = Revolutions per minute BMP = Developed power in horsepower H = Total dynamic head in feet across turbine at best efficiency point (BEP) The physical meaning of specific speed is: Revolutions per minute at which a unit will run if the runner diameter is such that running at 1-ft head it will develop 1 BMP. The customary specific speed form used for pumps for classification of impeller-type characteristics is also applicable for HPRT (basically for reverse running pumps). The values will be similar to those for pumps. The impulse Pelton wheels have very low specific speeds as compared to propellers (Kaplan) having high specific speeds. Francis-type runners cover the N s range between the impulse and propeller types (Figure 14-3). Net Positive Discharge Head Net positive discharge head required (NPDHR) applies to an HPRT as does NPSHR to a pump to preclude cavitation and its attendant physical damage effects. Some literature refers to the term "total required exhaust head" (TREH) rather than NPDHR. Test data have indicated that the NPDHR or TREH of a machine for the turbine mode is less than the NPSHR of the same machine for the pump mode at the same flow rate. The available net positive discharge head (NPDHA) or total available exhaust head (TAEH) at the installation side of the HPRT has to be higher than or at least equal to the NPDHR or TREH. This applies only to the reaction-type HPRT, since the impulsetype is a free jet action and is therefore not subject to low-pressure areas.

250 Centrifugal Pumps: Design and Application Figure 14-3. Turbine-type vs. specific speed. The ratio between N S , Q and Ng )BH p is approximate only, since N S ,BHP is a function of turbine efficiency. Power Output and Affinity Laws Power output is the rotational energy developed by the HPRT. Its value in BMP is calculated in a similar manner as for pumps except for the efficiency term. where Q = Capacity GPM H = Total head in feet sp gr = Specific gravity Et = Overall Efficiency at the turbine mode Variations in capacity, head, and BHP due to RPM (N) changes can be determined within reasonable limits by using the affinity laws, which normally are used for pumps but also apply to HPRT's (described in Chapter 2).

250 <strong>Centrifugal</strong> <strong>Pumps</strong>: <strong>Design</strong> <strong>and</strong> <strong>Application</strong><br />

Figure 14-3. Turbine-type vs. specific spe<strong>ed</strong>. The ratio between N S , Q <strong>and</strong> Ng )BH p<br />

is approximate only, since N S ,BHP is a function of turbine efficiency.<br />

Power Output <strong>and</strong> Affinity Laws<br />

Power output is the rotational energy develop<strong>ed</strong> by the HPRT. Its value<br />

in BMP is calculat<strong>ed</strong> in a similar manner as for pumps except for the efficiency<br />

term.<br />

where<br />

Q = Capacity GPM<br />

H = Total head in feet<br />

sp gr = Specific gravity<br />

Et = Overall Efficiency at the turbine mode<br />

Variations in capacity, head, <strong>and</strong> BHP due to RPM (N) changes can be<br />

determin<strong>ed</strong> within reasonable limits by using the affinity laws, which<br />

normally are us<strong>ed</strong> for pumps but also apply to HPRT's (describ<strong>ed</strong> in<br />

Chapter 2).

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