Page 313 - Centrifugal Pumps 2E

278 Centrifugal Pumps: Design and Application

age systems. Hydraulic turbines in power recovery applications may be
used to drive a pump, compressor, or other types of rotating equipment
either as a sole driver or as a helper driver in tandem with another driver
such as an electric motor or steam turbine. Hydraulic turbines may also
be used to drive electric generators.
When the hydraulic turbine is used in tandem with another driver to
drive a pump, consideration must be given to the available starting load
requirements and operating load conditions. If the hydraulic turbine is
able to bring the pump up to a speed with a reduced flow capacity
through the pump, such as at pump minimum flow where the required
HP is less, it is possible to use a reduced size electric motor or steam
turbine driver to make up the horsepower difference required for normal
pump operating conditions. This is not usually done, however, because
plant operating conditions may cause an upset in the flow capacity to the
hydraulic turbine with a resultant potential overload on the partial-sized
drivers; the pump system would malfunction. The driver used in con-
junction with the hydraulic turbine is usually foil sized to run the pump
by itself and in addition to accommodate the low flow input horsepower
requirements for the hydraulic turbine.
On tandem-drive pump units, an over-running automatic free-wheeling
clutch is often used that will permit the hydraulic turbine to be dis-
engaged from the drive operation for simplified start-up procedures, sys-
tem operating upsets, and maintenance. The use of the over-running
clutch will also permit a lower flow capacity to the hydraulic turbine
when it is operating at minimum flow conditions.
The arrangements of the drive train components for tandem-drive units
depend on the disassembly requirements for the components.
When an electric motor is used in conjunction with a hydraulic turbine
in tandem-drive arrangements, a double-extended motor shaft with the
pump on one end and the turbine on the other end, is most common. An
over-running clutch may be used between the motor and the hydraulic
turbine when desired. The full-sized motor acts as an excellent speed
governor for the hydraulic turbine. The motor may be essentially idle or
it may even function as an electric generator with no adverse effects on
the electric utility system should the RPM reach or slightly exceed syn-
chronous speed.
When a steam turbine is used in conjunction with a hydraulic turbine in
tandem arrangements, the pump is typically installed between the steam
turbine and the hydraulic turbine since the steam turbine is usually not
available with a double extended shaft.
A steam turbine is capable of acting as a good governor for speed regu-
lation, provided the hydraulic turbine power rating does not significantly
exceed that required by the pump (or other driven equipment). Power re-