Page 282 - Centrifugal Pumps Design and Application

Hydraulic Power Recovery Turbines 251

Configuration

The configuration of an HPRT is a function of the N s, NPDHA or
TAEH, BMP, RPM, installation requirements, and preferable vibration
and noise levels. Similar to pumps, the specific speed, N s, controls the
number of stages for a given head capacity and RPM. The specific speed
also specifies either single- or multi-stage HPRT's, which can be either
the horizontal or vertical configurations.
The available net positive discharge head or total available exhaust
head of the system for a given capacity will limit the RPM of the HPRT
and determine whether the runner will be the single- or double-eye con-
struction. Space requirements will specify the length of a horizontal unit
or will call for the vertical installation. Energy-level requirements will
limit the BMP per stage and may result in use of multi-stage units,

Turbine Performance Prediction

Prediction by Approximation

The performance characteristics of centrifugal pumps operating as hy-
draulic turbines may be approximated from pump performance charac-
teristics. Typically, the capacity and head at the best efficiency point
(BEP) will be greater for the turbine operation than for operation as a
pump. The amount of shift from pump performance generally varies ac-
cording to the specific speed. From tests, curves are developed that give
ratios versus specific speed that are used to give the percent shift from
pump to turbine performance.
Another procedure that is used to estimate the turbine performance
from known pump performance characteristics is to simply divide the
pump capacity and head values at the BEP by the pump efficiency at that
point. This will give a rough approximation of the turbine head and ca-
pacity at the turbine BEP. Since there can be considerable error using
these estimating procedures, they should only be used for preliminary se-
lection of candidates for a particular application.

Prediction by Analysis

The performance characteristics of centrifugal pumps operating as hy-
draulic turbines and other turbomachines used exclusively as hydraulic
turbines can be readily predicted with reasonable accuracy by use of a
relatively simple analysis procedure. Only minor adjustments are to be
expected to obtain the required performance on actual tests compared to