Page 287 - Centrifugal Pumps Design and Application

256 Centrifugal Pumps: Design and Application

The inlet velocity (Q), which depends on the nozzle size, is critical to
the turbine performance characteristics. It can produce a significant
change to the turbine performance by its effect on the shock loss and the
absorbed head. Therefore, the nozzle size is usually the main control for
adjusting the turbine performance characteristics. A smaller-size nozzle
area will generally shift the best efficiency point (BEP) to lower capaci-
ties and the larger size to higher capacities. The runner diameter controls
the peripheral velocity (UO, which theoretically could be adjusted to
change the performance characteristics. Usually, however, only minor
adjustments to the runner diameter can be made without distorting the
hydraulic relationships.
Not all pump designs will make a good performing turbine without
some modifications. Quite often the existing pump impeller vane angles
at the outside diameter (runner inlet) and at the eye (runner outlet) are not
a good combination for best performance. Also existing nozzle sizes and
stationary passages may be too large or too small, which would require
an alteration.
A pump operating as a hydraulic turbine will usually have an overall
efficiency equal to or greater than the same machine operating as a
pump, provided that the internal hydraulic parameters for turbine opera-
tion are good. This depends to a great extent on the runner vane angles
and nozzle velocity considerations.
The overall efficiency of a turbine at capacities near the best efficiency
point usually is improved by shaping the inlet ends of the runner vanes to
a bullet-nose-type configuration and slightly rounding the inlet edges of
the runner shrouds. The improvement is 1% to 2% at BEP capacity
(100% capacity) and still achievable at ±20% of BEP capacity (80% to
120% capacity). The reason is the reduced turbulence of the runner inlet.
The effect of surface finish on friction losses and the effect of leakage
losses may be readily evaluated by the turbine performance prediction
procedure.

Design Features (Hydraulic and Mechanical)

Reverse-Running Pump
Most centrifugal pumps in the low- to medium-specific speed range
(N,,Q - 600 to 5,000 or N S,BHP = 9 to 75 (see Figure 14-3)) are suit-
able and capable of operating as HPRT's. Because of the reverse rotation,
one has to check that the bearing lubrication system and threaded shaft
components, such as impeller locking devices, cannot loosen. However,
most pumps nowadays are designed to withstand reverse rotation.