Efficient Plant January 2020 - 17

feature | lubrication strategies

Varnish
PARTICLE CONTAMINATION is the
number one cause of lubrication-related failures in rotating and reciprocating equipment.
In fact, most industry experts agree that as
many as 80% of hydraulic-system failures and
50% of bearing failures are caused directly or
indirectly by particle contamination. What is
less understood is the role that varnish plays in
hydraulic- and lubrication-system reliability.
Varnish is a ubiquitous term used to
describe the formation of deposits within a
lubricated system. While that term might imply a single process at work, there are 20 to 30
lubricant degradation pathways. Each pathway
involves complex tribo-chemical interactions
and reactions between the lubricant base oil
and additives, and system components such as
pumps, valves, piping, coolers, and filters.
Varnish is like bad cholesterol in the human
body. Once too many deposits form in a manufacturing system, performance is impaired,
resulting in reduced oil flow, sluggish operation, loss of system control, bearing deposits,
and pump failure.

VARNISH FORMATION
The three most common ways that varnish
forms are oxidation, thermal failure, and static
discharge. In each case the physical and chemical processes at play are very different. As
such, the byproducts of lubricant degradation
are also different. However, what is common
is that each results in the formation of soluble
and/or insoluble deposits inside the hydraulic
or lubrication system as the lubricant degrades.
Oxidation is the most common degradation
pathway. Oil oxidation involves oxygen react-

JANUARY 2020

Mark Barnes, PhD, CMRP
Des-Case Corp.

Operating conditions that cause lubricant
degradation can result in damaging
deposits on machinery surfaces.

Images show sludge and varnish build up on a journal bearing (left) and a gear case. Clearly this buildup
will affect performance.

Four images show that even an aged fluid that had not be changed for many years, and with an initial MPC
of 63.4, can be re-conditioned in a fairly short period of time, reducing the MPC value to 1.6.

ing with organic and organometallic components of the base oil and/or additives. Initially,
oxidation generates intermediate compounds
such as aldehydes, ketones, and peroxides.
These byproducts are very reactive, particularly
in the presence of metal catalysts within the
lube oil or hydraulic system. As these chain
reactions occur, smaller molecules chemically
combine to form larger molecules. Eventually
the molecules fall out of solution and deposit
on surfaces.
Like any process in which a substance is
dissolved in a liquid, the solubility of oxidation

byproducts and other varnish-forming degradation pathways is temperature dependent.
As such, varnish formation tends to occur in
cooler areas within the system or where the oil
remains static for extended periods of time.
Like most chemical reactions, oxidation
rates increase with increasing temperatures.
In fact, it is generally recognized that once oil
temperatures reach/exceed the 130 to 140 F
(55 to 60 C) range, the oxidation rate increases
by a factor of two for every 18 F (10 C)
increase in temperature. Oil oxidation is also
an autocatalytic reaction, meaning the
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Efficient Plant January 2020

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