Chemical Engineering April 2021 - 30

Sebastiano Giardella
FIGURE 3. The diagram shows an example of an overall process arrangement for an expander generator
affecting turbine wheel design include
inlet/outlet pressure, inlet/outlet temperature,
amount of volumetric flow and
fluid
characteristics.
Turboexpanders
with several turbine wheels are required
when the pressure ratio is too large to
reduce via a single stage. Both radial
and axial turbine wheels can be designed
with multiple stages, but the axial
turbine wheel has a much shorter axial
length and thus is more compact. A radial
multi-stage turbine requires the gas
to flow from axial to radial, then back to
axial again, and thus produces higher
friction losses than the axial turbine.
Bearings. Bearing design is critical for
efficient turboexpander performance.
Bearing types associated with turboexpander
designs vary widely and can
include oil lubrication, fluid film, conventional
ball and magnetic. Each has its
own advantages and disadvantages, as
summarized in Table 1.
Many turboexpander manufacturers
are transitioning to magnetic bearings
as the " bearing of choice, " due to their
distinct advantages. Magnetic bearings
provide
frictionless
operation
for
the
dynamic components of the turboexpander,
dramatically reducing operating
and maintenances costs over the life of
the machine. They are also designed to
adapt to a wide range of axial and radial
loads and surge conditions. Their higher
upfront costs are offset by much lower
lifecycle costs.
Generator. The generator takes the rotational
energy from the turbine and converts
the energy into useable electricity
30
through an electromagnetic generator,
which can be either an induction or
permanent magnet (PM) generator. Induction
generators are rated for lower
speed, and thus require gearboxes
for high-speed turbine applications,
but are capable of being designed
to match the frequency of the grid,
thereby eliminating the need for a variable
frequency drive (VFD) to transfer
the generated electricity. PM generators,
on the other hand, can be directly
shaft-coupled to the turbine, and can
deliver electricity to the grid via a VFD.
The generator can be designed to output
the maximum amount of electricity
according to the available shaft power
of the system.
Seals. Sealing is also a critical component
in designing a turboexpander
system. To maintain high efficiency and
meet environmental standards, systems
must be sealed to prevent potential
process gas leaks. Turboexpanders
can be designed with dynamic or static
seals. Dynamic seals, such as labyrinth
seals and dry gas seals, provide sealing
around the rotating shaft, usually
between the turbine wheel and the rest
of the machine where the bearings and
generator are located. Dynamic seals
wear over time and require regular
maintenance and inspection to ensure
they are working properly. Static seals
may be used when all components of
the turboexpander are contained inside
one housing, protecting any lead wires
exiting the housing, including leads for
generators, magnetic bearing actuators
or sensors. These hermetic seals
provide permanent protection against
gas leaks and do not require any maintenance
or servicing.
Expander process arrangement
From a process point of view, the basic
requirements for an expander installation
are a high-pressure compressible
(non-condensable) gas being delivered
to a lower-pressure system at a sufficient
flowrate, a pressure differential and
a use factor to maintain the equipment's
working parameters within safe and efficient
levels.
From a pressure-reduction function,
expanders can be used to replace a
Joule-Thomson (J-T) valve, also known
as a throttling
valve. Given that the
J-T valve follows an isenthalpic path,
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