Efficient Plant January 2020 - 18

feature | lubrication strategies

+

Friction coefficient upon
initial valve actuation

byproducts formed upon initial oxidation act
analysis on hydraulic systems should be done
as a catalyst for further oxidation. As such,
monthly, it should be supplemented two to
once oxidation starts to occur, it tends to
four times a year with an MPC (membrane
VARNISH: THE SILENT
increase fairly rapidly unless remedial action
patch colorimetry) test. This test indicates the
MACHINE KILLER
is taken immediately.
"varnish potential" of a fluid by measuring the
Learn more about how varnish
Thermal failure doesn't require oxygen.
fluid color and the weight of small (0.45 μm
forms, its impact on machinery,
It results from heating alone. For most oils,
and greater) particles suspended in the oil.
and how to deal with it by
temperatures need to exceed 400 F (200 C)
Based on this, an MPC number, on a scale of 0
listening to our webinar on the
for thermal degradation to occur, resulting
to 100, is assigned. For critical hydraulics, the
subject. Go to efficientplantmag.
in "burnt on" deposits. In most lubrication
MPC number should always be less than 20,
com/2019varnishwebinar to learn
and hydraulic systems, thermal stress occurs
but lower is always better.
more about this silent killer.
through direct contact with a
Once varnish has been identified,
hot metal surface or by adiabatic
rapid removal is paramount. While
Figure 1: Impact of varnish formation
compression. Adiabatic comthere are several ways to do this,
on servo-valve operation
pression occurs any time the
the most effective, fastest, and most
0.5
oil becomes aerated and passes
broadly applicable method is to use
0.45
from a low-pressure zone to a
specialized varnish-removal car0.4
high-pressure zone such as in a
tridges. The cartridges force the oil
Oxidized oil
0.35
New
oil
hydraulic pump.
through special cellulose media made
0.3
Static discharge occurs
with long-strand fibers. As the oil
0.25
because most oils are fairly
flows through the fibers and capillar0.2
non-conductive, which is why oil
ies within the media, chemisorption
0.15
has been used for many years to
and physisorption occurs, stripping
0.1
prevent arcing in transformers.
varnish and varnish precursors from
In lubrication systems, as the
the oil. This approach is preferred
0.05
oil flows through pumps, pipes,
over varnish-control methodologies
0
0
5
10
15
20
25
valves, and filters, static electricity
that use electrostatics because celluServo-valve dwell time (min.)
can build up. Because the oil has
lose is not affected as much by water
The
graph
shows
that
it
takes
twice
as
much
force
to
move
a
servo
valve
that's
very low conductivity, this static
and can be used at high temperatures
been idle for 10 min. or more in degraded fluid than it does in new fluid.
charge is retained by the oil rathwhere precursors are more soluble.
er than discharged to machine
Unlike particles and moisture, vardwell time in "new" fluid versus a degraded
surfaces. As the voltage increases, dielectric
nish is often a "hidden" contaminant that lurks
(oxidized) hydraulic fluid.
breakdown can occur causing arcing from the
insidiously in the background, waiting to creAs the data shows, it takes twice as much
oil to a metal component within the system,
ate problems. While varnish doesn't always
force to move a servo valve that's been idle
much like a lightning strike. The result is a very
occur in hydraulics and lube-oil systems, if
for 10 min. or more in degraded fluid than it
high temperature along the arcing pathway
lubricating oil or hydraulic fluid has been in
does in new fluid. Depending on the applicathat thermally stresses the oil and causes
service for too long or you are experiencing an
tion, this extra force can cause the solenoid to
deposits to form. In some instances, special
unacceptable number of valve failures, even if
work harder, increasing the likelihood of an
filter media can be deployed within the oil-filthe fluid tests clean, take a sample and perform
electrical failure. Sluggish response times, often
tration system to help dissipate static charge
an MPC test. You may be glad you did. EP
referred to as valve stiction, can also have a
and prevent voltage buildup within the oil.
negative impact on precision-control circuits
Mark Barnes, PhD, CMRP, is Senior Vice
EFFECTS OF VARNISH
in critical hydraulic applications.
President at Des-Case Corp., Goodlettsville, TN
Varnish affects not just the fluid and compo(descase.com). He has 21 years of experience in
CONTROLLING VARNISH
nents but system function as well. For example,
lubrication management, oil analysis, and
The secret to controlling varnish is to first
Figure 1 shows the force required to move
contamination control and has published more
know you have a problem. While routine oil
a hydraulic valve after a defined amount of
than 150 technical articles and white papers.

18

| EFFICIENTPLANTMAG.COM

JANUARY 2020


http://www.descase.com http://www.EFFICIENTPLANTMAG.COM

Efficient Plant January 2020

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