from the oil, it must be periodically purified. The
following are some of the most common meth-
Gravity Purification. Gravity purification is sim-
ply the separation, or the settling of
contaminants that are heavier than the oil.
Gravity separation occurs while oil is in storage,
but is usually not considered an adequate
means of purification for most applications.
Other purification methods should also be used
In addition to gravity separation.
Centrifugal Purification. Centrifugal purification
is gravity separation accelerated by the centrif-
ugal forces developed by rotating the oil at high
speed. Centrifugal purification is an effective
means of removing water and most solid con-
taminants from the oil. The rate of purification
depends on the viscosity of the oil'and the size
of the contaminants.
Mechanical Filtration. Mechanical filtration re-
moves contaminants by forcing the oil through
a filter medium with holes smaller than the con-
taminants. Mechanical filters with a fine filter
medium can remove particles as small as 1
micron, but filtration under five microns is not
recommended as many of the oil's additives
will be removed. A typical mechanical filter for
turbine oil would use a 6 to 10 micron filter. The
filter medium of a mechanical filter will require
periodic replacement as the contaminants col-
lect on the medium's surface.
Coalescence Purification. A coalescing filter
system utilizes special cartridges to combine
small, dispersed water droplets into larger ones.
The larger water drops are retained within a
separator screen and fall to the bottom of the
filter while the dry oil passes through the screen.
A coalescing filter will also remove solid con-
taminants by the mechanical filtration principle.
Vacuum Dehydration. A vacuum dehydration
system removes water from oil through the
application of heat and vacuum. The
contaminated oil is exposed to a vacuum and Is
heated to temperatures of approximately 100 to
140° F. The water is removed as a vapor. Care
must be taken so that some of the desirable, low
vapor pressure components or additives are not
removed by the heat or vacuum.
Adsorption Purification. Adsorption or surface
attraction purification uses an active type media,
such as fullers earth, to remove oil oxidation
products by their attraction or adherence to the
large internal surfaces of the media.
sorption purification will also remove most
of an oils additives es well and should not
be used for turbine oil purification.
4.5 Oil Operating Temperature
A recommended range for the oil operating
temperature for a particular application Is usu-
ally specified by the equipment manufacturer.
Exceeding this range may reduce the oil's vis-
cosity to the point it can no longer provide
adequate lubrication. Subjecting oil to high
temperatures also increases the oxidation rate.
For every 18° F (10° C) above 150° F (66° C),
an oil's oxidation rate doubles which means the
oil's life is essentially cut in half. This Is espe-
cially critical to turbine oil in hydroelectric
generating units where the oil is expected to last
for years. Ideally the oil should operate between
120 to 140° F (50 to 60° C). If the oil operates
consistently above this range, some problem,
such as misalignment or tight bearings, may
exist and should be corrected. If It is necessary
to operate at higher temperatures, the oil's
neutralization number should be checked more
frequently. An Increase in the neutralization
number indicates the oxidation inhibitors have
been used up and the oil is beginning to oxidize.
The lubricant manufacturer should be contacted
for recommendations regarding the continued
use of the oil.
(FIST 2-4 11/90)