Hydrocarbon Processing - June 2021 - GP-23

TURBOMACHINERY
Introduction to oil bearing
turboexpander rotor dynamics
T. AVETIAN and L. E. RODRÍGUEZ, L.A. Turbine, Los Angeles, California
Turboexpanders are the ideal choice
for processes with large pressure drops
that require refrigeration or power recovery.
Whereas a valve or orifice also
can be used to effectively drop pressure,
turboexpanders extract useful work from
a flowing gas stream. Isentropic efficiencies
as high as 90% are readily achieved
with a turboexpander, resulting in much
lower discharge temperatures.
Turboexpanders were introduced in
the mid-1930s when the first machine
was designed and implemented for air
separation. The first unit for a natural gas
application was designed and installed
in the early 1960s. Over the years, many
technological advances in design and
manufacturing have allowed these machines
to contribute to improvements in
the efficiency of gas processing plants.
Today, turboexpanders are standard in
the natural gas industry for liquefaction
and dewpoint control (FIG. 1). They are
also used in the petrochemical industry
for ethylene plants, air separation, refrigeration
and power generation. Their relatively
simple construction and hermetically
sealed design generally lead to high
reliabilities, with some plants operating
the same units for many decades. Today,
more than 5,000 turboexpanders are in
operation worldwide.
Like a steam or gas turbine, a turboexpander
is a rotating machine with an
expansion turbine that converts the energy
contained in a gas into mechanical
work.1
The work produced by the turbine
is absorbed by a " loading " element that is
mounted on the same shaft as the turbine.
The loading element can be a dyno (oil
brake), an electric generator or a centrifugal
compressor stage. For the latter two,
turboexpanders afford the opportunity to
utilize energy that otherwise would not
be available with a Joule-Thomson (J-T)
FIG. 1. Typical turboexpander design.
valve. For instance, a compressor stage
can be used as a pressure booster to meet a
need in the process that otherwise would
have necessitated a separate compressor
driven by an electric motor or an engine.
In the authors' experience, the more
plant designers and end users understand
about what is inside the " black box " of a
turboexpander, the more successful they
are when making critical decisions about
their equipment. By being familiar with
turboexpander rotor dynamic design, new
machine designers and operators involved
in a turboexpander project will be better
equipped to fulfill their roles. The goal of
rotor dynamic analysis is to ensure that catastrophic
failure of a turboexpander does
not occur due to vibration instabilities.
This article is meant to aid end users as
they engage with turboexpanders, either at
the procurement or operational stage, and
offers an overview of turboexpander rotor
dynamics, suggestions for what to look for
from original equipment manufacturers
(OEMs), and a case study describing the
typical data presented in a turboexpander
rotor dynamic design document.
FIG. 2. Simplified rotor schematic.
The purpose of rotor dynamics. The
field of rotor dynamics is concerned with
the motions and associated forces of rotating
shafts. In the study of rotor dynamics,
emphasis is placed on five degrees of freedom,
or types of motion, spanning lateral
and torsional. Lateral motion is perpendicular
to the shaft's rotation axis (the
X-Y plane shown in FIG. 2). This includes
translations in the X and Y directions and
rotations about the X and Y axes. Torsional
motion is rotation about the shaft's
rotation axis-the Z axis. Axial motion in
the Z direction is typically neglected for
Gas Processing & LNG | MAY/JUNE 2021 23
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