Hydrocarbon Processing - October 2021 - GP-27

TURBOMACHINERY
Turboexpander repair and maintenance
for sustained performance and profitability
L. E. RODRÍGUEZ and R. HOCH, L.A. Turbine, Los Angeles, California
Turboexpanders are critical equipment for many applications
including gas processing, industrial gas, power recovery
and power generation, and their longevity and sustained performance
is often key to the profitability of a business. This
article provides an overview of the major elements involved in
the work of maintaining turboexpanders. The discussion traces
issues related to the design and field operation of the machine,
as well as shop repairs. Users will find valuable insights and
suggestions made by practitioners with an original equipment
manufacturer (OEM) and aftermarket perspective.
What is a turboexpander? A turboexpander is a rotating
machine equipped with a radial inflow expansion turbine and
a loading or breaking device, such as a centrifugal compressor
stage, a generator or an oil brake. Unlike a power generation
or mechanical drive turbine, which works mainly to generate
power, an expander's main purpose is to increase the efficiency
of a gas expansion process. FIG. 1 provides a schematic of a typical
turboexpander operating in a gas liquification process.1
Where are turboexpanders used? More than 5,000 turboexpander
units are in operation across the globe in a variety
of applications. The equipment is standard in the natural gas
industry for liquification and dewpoint control, and it is also
commonly employed in the petrochemical industry for ethylene
plants, air separation and refrigeration.1
In all of these processes, there is a need to change the state
of a gas to a given pressure and temperature. For example, a
refrigeration cycle requires that the gas be greatly expanded to
reduce its temperature. This is referred to as a Joule-Thompson
(J-T) effect, which can be accomplished with a throttling
valve (gate or otherwise) that achieves a constant enthalpy expansion
adiabatically, with no work output.
Turboexpanders can accomplish the same sharp pressure
drop more efficiently because the turbine extracts additional
work from the gas expansion due to the mechanical motion.
In cases in which the loading device is a compressor stage or a
generator, the expander provides power recovery.
What are the components of a turboexpander? A typical
turboexpander comprises three main sections-namely, the
compressor case, the expander case and the mechanical center
section (FIG. 2).
Turboexpander cases are usually radially split and grouped
in three distinct parts. The expander casing is usually stainless
steel, due to the low-temperature gas it contains. In many
designs, it houses the inlet guide assembly. The compressor
casing is usually made of carbon steel. It forms the diffuser
part of the compressor stage, as well as the collector or volute,
depending on the design.
FIG. 3 focuses on the mechanical center section (MCS),
which is the heart of the machine. The bearing housing is
usually carbon steel and contains almost the entire machine,
from the expander wheel to the compressor wheel. It includes
the rotor, bearings and seals. The following is an overview
of the key components of the MCS and their functions and
main design features.
Rotor. The rotating element is comprised of a shaft (often
made from stainless steel) with a radial inflow turbine wheel attached
on one end and the loading device, which in FIG. 3 is a centrifugal
compressor wheel, but it could be a generator or a dyno.
J-T valve
To pipeline or
compressor
Cold
Gas/gas
exchanger
Expander
inlet
separator
Liquids out
FIG. 1. Typical natural gas liquefaction process.
separator
Compressor
Expander
FIG. 2. Turboexpander cross-section. Image courtesy of L.A. Turbine.
Gas Processing & LNG | SEPTEMBER/OCTOBER 2021 27
Hydrocarbon Processing - October 2021

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