Efficient Plant March 2022 - 20

feature | lubrication solutions
proper breather and seal management, as
well as pre-fi ltering new oil prior to use, can
help extend oil-drain intervals. Real-time
offl ine fi ltration to supplement inline (full
fl ow and return line) fi lters can easily double
or triple oil life in many circumstances.
Having optimized lubricant selection,
controlled oil temperature to a reasonable
range, and implemented contamination-control
measures, the fi nal step to
eff ective condition-based oil changes is to
use oil analysis to trend lubricant health. In
doing so, we need to make sure that we are
measuring the correct oil properties. Simply
waiting for an oil to fall out of grade due
to an increase or decrease in viscosity is a
surefi re way to machine failure. Oil viscosity
is one of the last " oil health " properties to
change and typically only occurs during the
last 5% to 10% of an oil's life when sludge,
varnish, acids, and other deposits have
already started to form.
Fig. 2 , p. 21 shows the typical oxidative
degradation pathway of a hydrocarbon
oil. Th e fi rst thing to notice is that the oil
oxidation is not linear. Th is is because oil
oxidation is an autocatalytic reaction, i.e.,
once oxidation starts to occur, the by-products
that are formed catalyze the reaction,
increasing the eff ective oxidation rate.
To eff ectively change oil, based on condition,
a series of " oil health " tests should be
conducted every two to three months and
the results compared to a baseline sample
of new oil. Th e importance of having a
baseline cannot be overstated. Oil formulations
vary from time-to-time and from
brand to brand. As such, a sample of new
oil in use should be submitted to the lab as
a " new oil reference " that is compared with
the physical and chemical properties of the
in-service oil.
TESTING OIL HEALTH
For industrial fl uids, the most common oilhealth
tests to perform include:
20 | EFFICIENTPLANTMAG.COM
Fig. 1: Oil life expectancy
(years)
12
10
8
6
4
2
120 130
140
150 160
170
180 190
Bulk oil operating temperature (F)
A good rule of thumb is to consider that oil life is cut in half for every 18 F (10 C) increase in
temperature above 140 F (60 C).
S Viscosity: While viscosity, by itself, should
never be used to time an oil change, any shiſt
in viscosity can indicate the addition of the
wrong oil or contamination.
S Elemental analysis: Most oil-analysis labs
report elements such as zinc (Zn), phosphorus
(P), and calcium (Ca), which are used in
certain oil additives, depending on oil type
and formulation. Trending additives elements
to measure oil life should be done judiciously
since these elements tend to remain in the oil,
even when the additive is chemically depleted.
S Acid number: As a hydrocarbon oil
degrades, the byproducts, such as aldehydes,
ketones, and peroxides, can chemically combine
to form organic acids. Leſt unchecked,
these acids can start to cause corrosion and
serve as a catalyst to further oxidation. Acid
number can be used to determine the degree
of base oil and/or additive degradation.
S FTIR (oxidation and nitration): FTIR can
provide a " molecular fi ngerprint " of an oil.
Measuring FTIR oxidation and nitration rates
can help determine the presence (or absence)
of degradation byproducts due to thermal or
oxidative failure.
S RULER test: Commonly used for turbine
and hydraulic fl uids, this test uses cyclic voltammetry
to directly measure the remaining
useful life of the oil, based on antioxidant
levels. Th e test is very sensitive to having the
correct new oil reference.
S Rotating Pressure Vessel Oxidation Test
(RPVOT): Used almost exclusively for turbine
oils, this uses " oxygen uptake " to test an
oil's susceptibility to oxidation. Th e RPVOT is
becoming less favored.
To schedule a condition-based oil
change, all the above tests, perhaps with
the exclusion of RPVOT, should be trended
quarterly. Th e objective is to try to ascertain
where the in-service falls on the oxidation
curve depicted in Fig. 2. Condemning limits
should be established based on industry
standards and oil manufacturer recommendations.
Of particular value are the RULER
results because the antioxidant additives
serve to sacrifi cially protect the hydroMARCH
2022
200

http://www.EFFICIENTPLANTMAG.COM

Efficient Plant March 2022

Table of Contents for the Digital Edition of Efficient Plant March 2022