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

Hydraulic Power Recovery Turbines 281
If the hydraulic turbine should operate at runaway conditions (zero
torque) due to no load, the turbine shaft speed will generally increase to
within the range of 120% to 155% of the normal design speed with 100%
normal design head. The overspeed amount depends on the specific
speed characteristics of the machine. Should an upset condition occur
where there is a large amount of vapor present with a loss of liquid level
and with full differential pressure across the turbine, a very high runaway
speed could occur. This is due to the low-density vapor producing a
high differential head and a high-volume flow.
HPRT's should be brought up to ftill operating speed as rapidly as possible,
because they not only fail to generate power but actually consume
power until they attain about 40% of the design capacity.
The installation of the previously mentioned over-running automatic
free-wheeling clutch between turbine and the driven pump or compressor
is a good solution. The to-be-driven machine does not have to turn until
fluid is available to the HPRT, which is not connected to the to-be-driven
unit until it tries to run faster and puts out power. Using this arrangement,
the start-up sequence can be selected so that the HPRT goes from
zero speed to full operating speed along the zero torque curve,
Conclusion
In view of the significant power savings possible by use of power recovery
turbines, energy users should take advantage of every opportunity
to investigate the economics involved. Justification is based on the
value of the energy saved during a projected life of the turbine versus the
projected cost of purchasing, installing, and maintaining the machine for
the same period of time.
The effects of changes to the operating conditions, such as available
flow capacities and differential pressures for the HPRT's and driven machines
need to be considered. Since the most commonly used turbine
types have fixed performances, changes to the operating conditions may
cause a significant change to the power output from the turbine unless
modifications to the turbine internal nozzle sizes are made. HPRT's with
internally or externally adjustable guide vane assemblies are desirable
when changes to performance characteristics are expected.
Another consideration for selecting a hydraulic turbine as a driver in
place of an electric motor or steam turbine is the fact that the hydraulic
turbine does not have the incremental costs in energy. Experience with
HPRT's in actual operating installations shows that these machines are
very reliable, they perform the design requirements, and the operating
costs are minimal. The hydraulic and mechanical performances are readily
predictable.