Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

280 Centrifugal Pumps: Design and Application
will speed up. In speeding up, the turbine will produce more shaft horsepower,
which the driven pump or compressor must absorb at the new
speed. Finally, the horsepower will be balanced, but the speed of the
driven unit may be off design.
If speed control is necessary, throttling some of the turbine's driving
fluid across a valve bypassing the turbine allows it to satisfy the horsepower-capacity-speed
requirements of the driven unit. If the amount of
fluid available to the turbine is less than that needed for the design conditions
of the driven unit, the turbine will slow down and try to shed some
of the load. Here speed control can be achieved by throttling the available
pressure so that the turbine sees only that portion of the available head
needed to satisfy its head-capacity-speed relationship at the desired
speed.
When a power recovery turbine is combined with a makeup driver, except
at a single point, the recovery turbine always requires either flow
bypassing or inlet pressure throttling. The balance point is always determined
by the power-speed characteristics of the driven unit. If the driven
unit can use all the generated horsepower, such as a floating electric generator
would, capacity control and pressure throttling may not be needed.
When a speed-controlling, variable-horsepower helper driver such as an
electric motor or steam turbine is used it will hold the speed constant and
make up just enough horsepower to permit the power recovery pump turbine
to satisfy its head-capacity curve at virtually any flow rate.
Split-range liquid-level controllers are typically used to regulate the
available flow to HPRT's. The split-range liquid-level controllers and
pressure-control valves are usually furnished and installed by the purchaser.
The pressure-control valve is usually located at the inlet side of
the hydraulic turbine to prevent an excess pressure condition from occurring
at the turbine shaft seals by a closed valve. Also, the low shaft sealing
pressure usually results in a lower initial cost and reduced maintenance.
The signal from the controller is used to adjust the
pressure-control valve when too much head is available for the capacity
and speed and to bypass excess capacity from the system when more liquid
is available to the HPRT than needed to satisfy the relationship. When
an HPRT is provided with adjustable guide vane nozzles for performance
variation, a proportional range controller will provide the operator signal
to appropriately adjust the guide vane setting for optimum conditions.
An over-speed trip device is often furnished with the hydraulic turbine,
This device is typically used to provide a signal to operate an over-speed
alarm or to close the pressure-control valve for minimum flow turbine
operation. The sensing device may be a pneumatic or electronic transmitter
or a mechanical trip mechanism installed to sense the turbine shaft
speed.