POWER November 2021 - 30

LUBRICATION
Combating Varnish in Turbines:
A Two-Step Solution
As operational conditions continue to evolve and change with power generation,
so does the degree of complexity within those systems, becoming a common pain
point for plant managers that requires rigorous attention to detail. A preventative
approach produces drastic savings.
Zach Sutton
T
he most critical piece in a power
plant? The turbines. A key factor
impacting efficient operations?
Varnish buildup due to oil degradation,
which can reduce operational efficiency
or even grind machinery to a halt, often
costing operations tens of thousands of
dollars at minimum-with some operations
facing maintenance, replacement,
and downtime costs ranging in the millions.
How can plant operators ensure
varnish buildup isn't just something they
have to monitor, but a problem that can
now be solved completely?
The Problem with Varnish Buildup
Varnish is a term encompassing various
types of deposits in oil systems. The
range of deposits, which have troubled
plant managers for years, run from hard
deposits to soft and sticky. To this point,
there have been limited solutions.
Typically, varnish forms in stagnant areas
and is triggered by thermal events,
such as excessive heat, which begins
the process of oil degradation leading to
the formation of impurities. While impurities
within oil are not uncommon, they
accumulate, forming suspended submicron
particles. Over time, these particles
become electrically attracted to metal
surfaces, resulting in varnish.
It is important to note varnish does
not appear in the oil, but rather on seals
and metal surfaces in contact with the oil
(Figure 1). In addition to thermal-induced
impurities, cross-contamination of oils
with incompatible additives is another
process resulting in varnish build up.
How are plant managers currently trying
to solve their varnish buildup issues?
Past Solutions for Varnish Buildup
Not Enough
A common strategy to combat varnish
buildup is filtering and high-velocity
flushing, which removes weld slag and
particulates from the system but typi30
cally
does not remove deposited varnish.
With traditionally available options, filtering
and flushing has only been moderately
effective, leading to maintenance
downtime, increasing costs, lost capacity,
and potential penalties from failing to
deliver output.
For example, a baseload plant turbine
operating at 24 hours a day could see
an average of $288,000 in lost sell price
to the grid. A merchant plant turbine
operating eight hours a day could see
upwards of $500,000 in lost sell price
to the grid, in addition to an estimated
$40,000 cost per turbine per year in
varnish-combating maintenance costs.
(Results may vary depending on equipment
type, operating conditions, and
utility costs.) Altogether, varnish build
up can amount to hundreds of thousands
or even millions of dollars of cost
and lost revenue for plants. Some recent
solutions have been making headway in
solving this ongoing problem.
In discussions with plant managers,
we consistently heard several of
the same issues arise. In cases where
lower lubricant flashpoints were encountered
with low volatility components,
plant managers had to reduce
operating temperatures. Still, there was
a higher fire and explosion risk during
cleaning or from residual cleaner in the
system. Filter plugging was another
pain point resulting from the release of
a large amount of varnish, potentially
exacerbated by incompatibilities with
in-service fluids. To mitigate this, costly
external filtration and frequent filter
changes are necessary, which lead to
operational constraints, including unexpected
downtime. Even during careful
maintenance, residual cleaning chemicals
can cause foaming and emulsion
issues or may react with oil additives,
leading to spillage and air/water being
pulled into the system, which can degrade
oil quality and performance.
In recent years, the industry has seen
www.powermag.com
1. Steam and gas turbines are among the
most critical pieces of equipment at power
plants. Reducing varnish building up their oil
systems will help them operate more reliably.
Courtesy: Chevron
an increased adoption of polyalkylene glycol
(PAG-based) turbine oils, some claiming
to be varnish free. However, a series
of 10 industry-standard ASTM tests highlighted
some interesting findings.
Conducted by Dr. David Wooton, an
independent lubrication and technical
consultant, the tests compared a PAGbased
formulation and three different
mineral oils of both off-the-shelf and inservice
samples. Dr. Wooton performed
a number of experiments using D2272,
the most widely accepted bench test for
oxidation, a primary cause of oil degradation
and varnish build up.
His results showed similar fail-tocritical
varnish levels between PAGbased
and mineral oils. Using the ASTM
D7843 MPC test, the industry standard
for varnish production, results showed
a lower propensity to develop varnish
than expected, but the buildup was
still considered critical. Ultimately, PAG
fluid wasn't " varnish proof, " but instead
performed similarly in terms of oxidation
and varnish formation. Therefore, a
change would be recommended when
the fluid reaches 25% of new oil, regardless
of base formulation.
As we've noted, more permanent varnish
buildup solutions are hard to come
by, but Chevron recently created an innovative
new product for plant managers.
POWER | November 2021
http://www.powermag.com
POWER November 2021

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