Page 285 - Centrifugal Pumps Design and Application

254 Centrifugal Pumps: Design and Application

inlet velocity (Ci) and the runner inlet vane angle (01). Refer to Figure
14-4 for illustration.

Outlet loss. The outlet loss component of the total dynamic head is calcu-
lated as the velocity head due to the absolute outlet velocity (€2) times an
2
appropriate loss coefficient (KC2 /2g). The loss coefficient may be
taken as unity for many designs without serious error. Refer to Figure
14-4 for illustration.

Power loss. The power losses due to internal leakage, disc friction,
bearings, and shaft seals applicable to pumps are also applicable to hy
draulic turbines.

Turbine performance characteristics. The total dynamic head is the
sum of the individual heads due to the friction losses, absorbed head,
shock loss, and outlet loss. The hydraulic power is based on the total
available energy to the turbine. The turbine output power is the absorbed
head minus the power losses due to internal leakage, disc friction, bear-
ings, and shaft seals. The turbine overall efficiency is the ratio of the
output power to the hydraulic power. Refer to Figure 14-4 for illustration
of terms,

Predicted performance vs. test results. Figure 14-5 shows a typical
comparison of the turbine performance characteristics determined by the
preceding calculation procedure and the results obtained by actual test.
Identical nozzle sizes were used.

Turbine Performance Prediction by Factoring

The performance characteristics of a hydraulic turbine may be quite
accurately predicted by size factoring from a known performance at a
specified specific speed. The rules that apply to pumps (described in
Chapter 2) also apply to turbines.

Optimizing and Adjusting Performance Characteristics

The inlet and outlet velocity triangles as illustrated by Figure 14-4 are
used to predict, adjust, and optimize the turbine performance. These give
an instant picture of whether the turbine performance characteristics are
expected to be optimum, satisfactory, marginal, or unsatisfactory. The
optimum overall efficiency will generally be achieved when the shock
loss at the inlet to the runner is near zero and the absolute velocity (€2) at
the outlet from the runner is near a minimum value.