Page 280 - Centrifugal Pumps Design and Application

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 dirnen-
sionless 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 effi-
ciency 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 impulse-
type is a free jet action and is therefore not subject to low-pressure areas.