Page 303 - Centrifugal Pumps 2E

Hydraulic Power Recovery Turbines 271

Operating Considerations

The product handled by a hydraulic turbine may be a single-phase liq-
uid, a multiphase liquid-gas mixture, or a slurry composition.
Hydraulic turbines have been extensively used for two-phase, liquid-
gas flow streams where there is a potential for a substantial amount of
gas released as the product passes through the turbine. There may also be
small amounts of "free" gas at the turbine inlet. With a decrease in pres-
sure, gas is subject to be released from the liquid with a resultant increase
in volumetric flow. The effects of the potential vaporization at the vari-
ous turbine stage pressures is evaluated to assure proper turbine perfor-
mance. Generally, this may be accomplished by limiting the two-phase
flow velocities at the runner (impeller) outlet eye to a reasonable value.
It is also appropriate to give consideration to the runner (impeller) design
to assure proper vane angles and eye sizes to accommodate any potential
vapor release from the fluid stream. Actual field experience known to
the author has shown that calculated two-phase flow velocities at the tur-
bine outlet runner eye up to 150 ft/sec can be accommodated with no
adverse effects. This velocity is suggested as a guideline for HPRT's
whether they be single- or multi-stage types. Using this limit, the two-
phase flow rate by volume can be at least three or four times the single-
phase flow rate for many applications.
Theoretically higher output horsepower should be achieved by gas ex-
pansion through the turbine since the increase in volume means more
work done. However, many reports have indicated that the expected ad-
ditional power has not been realized. One explanation may be that the
product passes through the turbine too fast for vapor-equilibrium to be
obtained. For example, consider the time it takes for the carbon dioxide
to escape from a bottle of carbonated beverage when the cap is removed.
It does not all escape instantly. Another reason may be the fact that as the
gas expands, the product velocity increases, and causes additional losses
to occur.
For multi-stage hydraulic turbines, the nozzles may be sized differently
from stage to stage to accommodate any theoretical increase of the volu-
metric flow as the pressure is reduced.

Performance Testing

Performance tests for hydraulic turbines may be accomplished by use
of a centrifugal pump to furnish the head and flow capacity necessary to
drive the turbine and to verify the turbine performance throughout its op-
erating range. An induction motor excited by AC power from the utility
system is used as an induction generator to absorb the output from the