Page 294 - Centrifugal Pumps Design and Application

262 Centrifugal Pumps: Design and Application

The eye area at the outlet of the runner is extra large and the eye vane
angles are carefully selected to accommodate any potentially large
amounts of vapor that may evolve out of solution by expansion through
the turbine. The combination of the number of nozzles in the guide vane
assembly and the number of vanes in the runner is selected to preclude
in-phase torque pulses. For higher working pressures or temperatures or
lower specific gravities, this type of HPRT can'be supplied in a radially
split and/or centerline-mounted volute case. For most applications, how-
ever, the axial-split-type is sufficient and preferred, basically because of
the ease of maintenance and inspection of the rotating element,
The runner is essentially balanced in both radial and axial directions,
The thrust bearing in the outboard bearing housing will take the axial
thrust resulting from upset conditions such as unequal amounts of vapor
in the two eye areas of the runner. For higher-speed applications, these
HPRT's are furnished with Kingsbury thrust and sleeve radial bearings.
For higher differential pressure and lower capacity, multi-stage
HPRT's with guide vane assemblies are available. These are generally
lower-specific-speed turbomachines with single eye and narrow runners
to avoid large bearing spans. In general, the mechanical design criteria
are the same as used for centrifugal pumps running in reverse.

Turbine Design with Internally and Externally Adjustable Guide
Vanes

Specially designed HPRT's include the feature of an adjustable guide
vane assembly, which can be furnished for a single-stage or multi-stage
HPRT. The method of adjusting the guide vane assembly is made possi-
ble by an internal or external design feature. The advantage of this varia-
ble vane assembly is the capability of operating more efficiently over an
extended flow range compared to an HPRT or a reverse running pump
with fixed inlet guide vanes.
The performance characteristics of an HPRT can be varied over a con-
siderable range by changes to the velocity of the liquid passing through
the guide vane assembly. For optimum performance, this is best accom-
plished by changes to the flow-cross-section area formed by the vanes
and the side walls of the assembly when aligned at a proper angle to the
runner. A decrease in the flow-cross-section area will generally shift the
optimum efficiency to a lower flow range.
The typical performance characteristic curve for an HPRT with fixed
guide vanes is illustrated in Figure 14-11. The hydraulic turbine is essen-
tially like an orifice in a fixed-pressure-differential system. The operat-
ing point will be where the particular head, capacity, speed, and power
relationship is satisfied.