Hydrocarbon Processing - June 2021 - GP-28

TURBOMACHINERY
Ultimately, all rotor dynamic models
should be verified on the test stand prior to
operation in the field. Referring to a rotor
dynamics analysis report during the evaluation
of a mechanical run test is an important
part of making sure that a machine has
been manufactured and assembled according
to design. More importantly, validation
of the model on the test stand can provide
assurance that the machine will operate as
expected in field conditions.
As previously mentioned, the typical
turboexpander should not present a significant
rotor dynamic challenge for the
OEM. These machines typically operate
below significant shaft critical speeds, and
minimal vibration can be expected given
well-balanced components. In addition,
instability is a rare issue with well-designed
and manufactured bearings and seals.
After a machine has been commissioned,
it is important for the operations
team to collect vibration and bearing temperature
data on a regular basis. These
long-term trends prove invaluable when
troubleshooting a vibration issue.
The most common rotor dynamics
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issue observed in the field is high synchronous
vibration. Wearing components
over time can contribute to rotor imbalance,
steadily increasing vibration levels.
Another common cause of imbalance
increase over time is the formation of hydrates
or other solids in expander wheels
in cryogenic natural gas processing applications.
Sudden, high levels of vibration
can be attributed to major failures in
either rotating components and/or bearings,
such as blade loss in an impeller.
Other, more serious and difficult issues,
such as rotor instability, will usually present
themselves immediately during commissioning.
These types of problems can
be due to unforeseen (and unmodeled)
process loads or poorly modeled rotor dynamics
analysis. They are not easily solved
by improving component balancing and
usually require intensive efforts to redesign
or add components, which can cause
lengthy and expensive delays. For this reason,
it is critical to ensure that a proper rotor
dynamics analysis is performed during
the design phase to rule out any possibility
of rotor bearing instability. GP
LITERATURE CITED
1
Avetian, T., " Fundamentals of turboexpander
design and operation, " Gas Processing & LNG, May/
June 2020.
2
American Petroleum Institute, " API Standard 617:
Axial and centrifugal compressors and expander
compressors, " 8th
Services, Washington, D.C.
3
Avetian, T., L. E. Rodríguez and J. Park, " Addressing
high sub-synchronous vibrations
in a
turboexpander
equipped with active magnetic bearings, "
Proceedings of the 48th Turbomachinery
Symposium, 2019.
TADEH AVETIAN is Director of
Engineering at L.A. Turbine,
responsible for identifying and
defining research and development
projects, as well as directing
turboexpander design for new and
aftermarket equipment. Mr. Avetian
is a California-registered professional engineer with
BS and MS degrees in mechanical engineering from
California State Polytechnic University of Pomona.
LUIS E. RODRÍGUEZ is a Design
Engineer at L.A. Turbine,
responsible for the mechanical
design of turboexpanders. Prior to
L.A. Turbine, he worked for 10 yr at
Sulzer Turbo Services in La Porte,
Texas. Mr. Rodriguez is a Texasregistered
professional engineer with a BS degree
from Universidad Simón Bolívar in Venezuela and
an MS degree from Texas A&M University.
Ed., 2014, API Publishing
28 MAY/JUNE 2021 | GasProcessingNews.com

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Hydrocarbon Processing - June 2021

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