Chemical Engineering July 2015 - 59

Dynamic pressure transducers that
detect the surge and other flow instabilities
in the air axial-compressor are
an important element for some gas
turbine packages. The stability in the
combustion process could also be
monitored by the same method. The
application of dynamic pressure transducers
in the combustion sections,
especially in modern, low-NOx combustors,
could ensure that each combustor
section is burning evenly without
any issue, problem or instability.
Lubrication oil for gas turbine
The lubrication oils for gas turbine
are subject to a wide range of harsh
conditions, such as extreme heat,
high contamination, inadvertent mixing
with different substances and
more. These effects can degrade the
integrity of the oil base stock and deplete
any additives, causing irreversible
molecular changes and hence
changes in the lubrication oils.
Most gas turbine trains use a relatively
low-viscosity oil (compared to
that used by gear units and reciprocating
machines), for example, ISO
viscosity grade (VG) 46.
A lubrication oil with optimum viscosity
reduces the power waste for
operation because frictional power
involved in bearings and other lubricated
parts would be reduced. The
usual expectation by users is relatively
low makeup oil for the gas turbine
lubrication oil (say on average
below 5-10% oil added per year).
This low makeup requirement is a
factor that encourages high-quality,
long-life lubricant applications
for a gas turbine. Usually, the oil in
a gas turbine train (without a gear
unit), if selected properly and maintained
correctly, does not need to be
drained and replaced with much frequency
and thus could last for a relatively
long time. It is hard to note an
expected life, but it could be a matter
of years. The gas turbine oils should
be well-maintained (such as avoiding
possible contamination by water and
other fluids, using good seals, and so
on) to extend their service life and simultaneously
provide the maximum
machinery performance.
Generally, for gas turbines operating
at high temperatures, oxidation
of lubrication oil could be an important
issue. High temperature directly
affects the oxidation. Heat also reduces
the oil life. For high-temperature
applications, the oxidation rate
is usually doubled for every 10°C increase
in the oil temperature.
Lubrication oils can also fail because
of contamination. A good
solution is to use the correct sealing
system to eliminate the potential for
gas leakage to the oil.
Any additives used in a gas-turbine
lubrication oil should extensively be
verified and tested. The oil and additives
must be carefully formulated
in a tightly controlled process. The
key to an excellent lubrication oil is
to retain the desired properties. Successful
(long-term satisfactory operation)
references are important. In
other words, it is important to check
if proposed lubrication oil were used
in similar gas turbines in more or less
the same conditions successfully.
ysis using a bichromatic microscope.
The particles are then examined and
classified by size, shape, concentration
and metallurgy. The information
carried by the wear particles is valuable
for the identification of the wear
mode and mechanism.
Analytical ferrography can be particularly
effective in the detection of
soft contaminants and in the identification
of their nature. This can be
a powerful technique to identify the
oil-related issues of the machinery,
root-cause analysis, the morphology
and characteristics of the insoluble
particles, and the progressive mechanism
of varnish formation. While the
ferrography test procedure is lengthy
and requires highly skilled analysts,
the benefits can outweigh the costs.
This is a recommended test if any
Integration is the key for gas-turbine packaging.
The lubrication oil in most modern
aero-derivative gas turbines and advanced
high-efficiency gas turbines
could be in contact with metal surfaces
above 200°C. These high temperatures,
and the possibility of cyclical
operation, can result in significant
thermal and oxidative effects on the
oil. For these applications, sophisticated
synthetic oils are the only available
options. Mineral oils used in oldfashioned
gas turbines are not usually
suitable for modern gas turbines.
Instead of degradation occurring in
an orderly, predictable fashion, many
lubricants that are used in modern
gas turbines fail rapidly. And some of
the standard oil-analysis tests offer
little indications as the gas turbine lubricant
starts to degrade. Thus, it is
critical to study previous lubrication
oil behaviors in similar gas turbines
and operating situations.
" Ferrography " is a technique that
provides valuable information about
wear evolution in machinery through
analysis of a representative lubrication
oil sample. In an analytical ferrography
study, the solid debris
suspended in a lubricant sample
is separated. The solids are then
passed across a bipolar magnetic
field. After that, a solvent " wash "
cycle removes any lubricant remaining
on the substrate, resulting in a
" ferrogram " where the particles are
all arranged by size and permanently
attached to the slide for optical analChemiCal
engineering www.Chemengonline.Com july 2015
abnormal wear is observed.
Fourier transform infrared (FTIR)
analysis can be useful to measure
organic molecular components,
monitor additive depletion and identify
organic degradation byproducts
(oxidation). FTIR is the preferred
method of IR spectroscopy. In IR
spectroscopy, IR radiation is passed
through a sample. Some of the IR
radiation is absorbed by the sample
and some of it is passed through
(transmitted). The resulting spectrum
represents the molecular absorption
and transmission, creating a molecular
fingerprint of the sample. Like a
fingerprint, no two unique molecular
structures produce the same infrared
spectrum.
Author
Amin Almasi is a rotating-equipment
consultant in Australia
(Email: amin.almasi@ymail.com).
He previously worked at Worley
Parsons Services Pty Ltd. (Brisbane,
Australia), Technicas Reunidas
(Madrid, Spain) and Fluor
Corp. (various offices). He holds a
chartered professional engineer
license from Engineers Australia
(MIEAust CPEng - Mechanical), a chartered engineer
certificate from IMechE (CEng MIMechE), RPEQ (registered
professional engineer in Queensland) and he also
holds M.S. and B.S. degrees in mechanical engineering.
He specializes in rotating machines, including centrifugal,
screw and reciprocating compressors, gas and
steam turbines, pumps, condition monitoring and reliability.
Almasi is an active member of Engineers Australia,
IMechE, ASME, Vibration Institute, SPE, IEEE, and
IDGTE. He has authored more than 60 papers and articles
dealing with rotating machines.
59
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Edited by Suzanne Shelley
http://www.Chemengonline.Com
Chemical Engineering July 2015

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