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Hydraulic power recovery turbine (HPRT)
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A centrifugal pump rotating in reverse direction from its normal rotation and
whose function is to recover energy from, rather than expend energy to, a liquid
stream is called a hydraulic power recovery turbine (HPRT). Energy is recovered
within the HPRT by converting pressure energy to motive power to drive another
rotating equipment such as another pump, a compressor, generator, or a motor
to take part of its load.

In many industrial processes the pressure of a liquid stream in one phase has to
be reduced significantly before being moved into the next phase of the process.
Instead of breaking down its pressure across a pressure reducing valve it may
become more cost effective to invest in HPRT to recover the energy that would
otherwise be wasted. Some liquids that have been used in HPRTs include
hydrocarbon, rich oil, lean oil, gasoil, mea solution, rich catacarb solution,
selexol, monoethonolamine, triethanelamine, amine solution, solvent, and many
others.

The popularity of centrifugal pumps being used as, or rather being converted to,
HPRTs emanates from the fact that centrifugal pumps are easily, widely, and
inexpensively available commercially. Many types of centrifugal pumps can be
used with few design changes - single stage or multistage, overhung or between-
bearing type, with axially or radially split casing. Horizontal pumps are the most
preferred design because they can be coupled directly to their driven
equipment. HPRTs do not require special metallurgy - carbon steel, stainless
steel, or chrome, are commonly used materials.

A process liquid may not be available always on a constant basis, or its pressure
may be variable thus, in such instances, the HPRT is typically not used as
standalone driver but is used in a train arrangement that includes the driven
equipment, an electric motor as primary driver, and an HPRT as secondary
driver to unload the electric motor to reduce its power consumption.

A typical train arrangement may consist of a sequence of: the driven equipment,
an electric motor with double-extended shaft, a clutch, and an HPRT. A clutch is
included so the HPRT can be disconnected from the motor in case the process
liquid becomes unavailable, or its pressure becomes too low, to avoid the HPRT
from becoming a drag to the system.

Some trains may have the following alternate arrangement: an HPRT, a clutch,
the driven equipment with double extended shaft, and a motor. In this
arrangement, it must be ensured that both the HPRT and the pump are  
accessible for inspection, repair, and maintenance without physically removing
the clutch. In one such train arrangement, a company supplied a radial split
case pump (driven equipment) between a clutch and a motor. It turns out that
the rotor of the radial split case pump cannot be removed for servicing without
removing the clutch – significant errors in the selection of both the pump and
HPRT and in the design of the train arrangement. Similar considerations should
be kept in mind when selecting double barrel pumps as HPRTs in services that
involve high pressure and temperature.

The design and selection of HPRT are basically the same as that of centrifugal
pump except for some changes due to its function, its reverse rotation, and high
inlet pressure. Some examples:

  • In HPRT, its threaded parts, like the shaft and shaft sleeves, are threaded
    in reverse direction from that of a standard pump to prevent the parts
    from becoming loose during operation.

  • Its bearing type, and the manner of its hydraulic thrust balancing, may  
    require some changes depending on its pressure.

  • The nozzles of an HPRT are the reverse of that of a pump - the inlet and
    outlet of an HPRT are the discharge and suction nozzles, respectively, of
    a pump.

  • In a multistage pump the first stage and last stage impellers become the
    last stage and first stage impellers in an HPRT.

The performance of a hydraulic power recovery turbine is different from that of a
pump. A pump can operate from its minimum continuous stable flow (MCSF) and
above, or within its preferred operating region (POR); an HPRT must operate in
a narrow range near its best efficiency point (BEP). At some point below its BEP,
the capability of an HPRT to recover energy is greatly diminished and, farther
below that, it stops recovering energy and the HPRT becomes a drag to the
system.

The power recovered by HPRT is calculated from the equation:


Q  x  H  x  SG  x  E
HP = ------------------------
          3960

where:

HP
 - energy recovered by turbine, horsepower
Q    - turbine capacity, gallons per minute
H    - differential head across turbine, feet
SG - specific gravity of liquid
E    - turbine efficiency, decimal

For simplicity the U.S. customary units are used in this article.

The above equation accounts only for the liquid flow across the HPRT and
excludes any additional energy recovery due to vapour or gas that may be
released from the liquid. Some HPRT vendors tend to ignore this extra energy
recovery in the HP calculation and consider it a safety margin in the event of a
miss in the estimated turbine efficiency, just like a pump is subject to normal
efficiency tolerance.

Users or purchasers of hydraulic power recovery turbines, who want to include
the effect of gas or vapor release in the estimated power recovery, must advise
the vendor if a portion of the process stream can be released in the form of gas
or vapor during the pressure breakdown across the HPRTs so that it can be
properly accounted for in the energy recovered calculations.

There is no industry guideline on the acceptable efficiency or power tolerance
on HPRTs, unlike in centrifugal pumps. CENTRIFUGALPUMP.COM and The
EMA Project recommend that HPRTs become subject to the same negative
efficiency or power tolerance that apply to centrifugal pumps. Users and
purchasers should take into consideration the effect of negative tolerance when
sizing HPRTs as standalone driver, and not as an auxiliary driver.

A typical hydraulic turbine performance curve is shown above [ * ]


The second part of this article [ * ] provides the answers to these questions:

  • How is a pump selected as hydraulic power recovery turbine?
  • How is a pump curve converted into an HPRT performance curve?
  • In controlling the operation of HPRT with throttle valve, should the valve
    be installed upstream, or downstream, of the HPRT, and why?

[ * ] Some information are excluded in this article.


Related articles

  • Centrifugal pumps used as turbines
  • Feasibility of using large vertical pumps as turbines for small-scale
    hydropower
  • Gas release in hydraulic turbines
  • Selection of hydraulic turbines
  • Testing of hydraulic turbines
  • Test procedure for hydraulic turbines
  • The Application of hydraulic power recovery turbines in process plant


[This is a raw article that is undergoing revision. Please bookmark this page and come back
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The EMA Project
being re-built as hydraulic power recovery turbine (HPRT).