Fuels & Lubricants Magazine

mm 2 /s, authors [4] attempted to extrapolate
the coefficient α to other oil
viscosities as well. The results are on
Pictures 4 and 5. Picture 4 shows the
correlation between viscosity-pressure
coefficient α and kinematic
viscosity of these oils at 200 MPa.
Picture 5 shows the same correlation
at 600 MPa. Both charts are valid for
the temperature range 25 – 80 °C.
Figure 4: Pressure coefficient at 200 MPa
N - naphtenic oil
SI - silicone oil
M - paraffinic mineral oil
PAO - polyalfaolefine oil
ES - ester oil
PG - polyalkyleneglycol oil
MPa are PG, Ester or PAO synthetic
oils. Paraffinic mineral oil is worse
and naphtenic is the worst.
All these oils were measured the
same way as paraffinic mineral oil
in Figure 3, but as there are not any
data about solidification, it is issued
only brief description based of high
viscosity measurement.
The best was polyglycol oil with
KV40°C = 100 mm 2 /s. Its viscosity
400 000 mPa.s was reached at 500
MPa for oil temperature 26.8 °C.
The pressure 800 MPa increased
the viscosity only to 20 000 mPa.s at
70.2 °C. The worst was silicone oil
with KV 40 °C = 120 mm 2 /s, which
reached viscosity 400 000 mPa.s
already at half pressure than PG oil
– at 250 MPa (and at 26.8 °C). The
higher temperature = 70.1 °C improves
its behaviour to get viscosity
400 000 mPa.s at 430 MPa.
Following the Picture 3 it is possible
to estimate the curve of paraffinic
mineral oil with KV40 °C = 9.62
mm 2 /s. The shape and slope of this
curve will be similar as the curve belongs
to the temperature 37 °C, but
the starting point is shifted down to
10 1 mPa.s, it can be calculated more
exactly as 8.40 mPa.s. The solidification
point deducted from the chart is
at 520 MPa.
Figure 5: The same chart for 600 Mpa
Literature Data Discussion
The lowest viscosity-pressure
coefficient at normal conditions (see
Picture 4) shows very low viscosity
silicone oil, but there are doubts
about its very high pressure behavior,
especially when higher viscosity
silicone oils have really enormous
steep slope of viscosity-pressure
relationship. This is strange, because
generally all other oils show
the higher is viscosity index, the
lower is coefficient α. It is confirmed
by Stepina [2] as well. Maybe the
structure based on Si-O exhibit
different relationship. Nevertheless
the lubricating properties of low
viscosity silicone oil or even silicone
solvent are poor and that is why it is
impossible to recommend it for this
expensive apparatus.
Following the Picture 5 the best
candidates for the range of temperatures
25 °C – 80 °C and pressure 600
Data Comparison and
Evaluation
The temperature of actually used
transformer oil was 30 °C, kinematic
viscosity was 13.7 mm 2 /s (12.0
mPa.s) at that temperature. Solidification
Point from the chart (Figure 3)
is at 490 MPa, if it would be paraffinic
oil. See Figure 6. As the transformer
oil is naphtenic origin, then lower
pressure can be expected to cause
solidification. 420 – 460 MPa is expected
pressure which cause solidification
of transformer oil at 30 °C.
This value is in a very good agreement
with experiment. The pressure
interval between 420 – 500 MPa is
caused by increasing temperature
Fuels&Lubricants No. 3 OCTOBER 2018 7