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TECHCON 2007 PANEL DISCUSSION
“New, re-refined or
refurbished? Options for mineral oil”
Is there any
such thing as new mineral oil? Take the carcasses of a few
million dinosaurs, a few hundred hectares of trees and
sub-vegetation, bury them under hundreds of meters of mud and
sludge for a thousand years or so and you have ‘new’ oil. Put
it though a refining process and it is readily accepted by
engineers everywhere.
Once that same
oil has been in service for 20 or 30 years then passed through
a passive microscopic filtration process to remove all
contaminants and many of the same engineers refuse to consider
its use. Why? Most common answer is, “we don’t want second
hand oil in our equipment”. Surely that is an emotive response
more suited to the ‘arts sector’ than practical, fact based
engineering types?
In this brief
presentation I hope I can help dismiss some of the myths, cut
through some of the sales talk and hopefully begin an
objective thinking process in most of you.
Firstly;
What
is Oil Refurbishment?
Refurbishment of
oil has been carried out using adsorption by Fullers Earth for
decades and is often still used as the final stage in oil
refining or re-refining. Adsorption is the tendency of a
liquid, gas or small particle to cling to the surface of
another substance by physical rather than chemical means.
Fullers Earth is
a hydrated magnesium and aluminum silicate with a unique
crystalline structure. Once activated through high
temperature, this clay possesses up to 13 hectares of surface
area per kilo. Most of the contaminants found in serviced oil
are polar in nature and are therefore easily adsorbed by the
Fullers Earth.
When coupled
with fine particulate filters (<0.5 micron) plus a high
vacuum degasser/dryer system, virtually all oxidation
by-products can be removed and the oil returned to original,
new oil specifications. The Refurbishment process also removes
corrosive sulfur and metals from the oil.
Once the natural
inhibitors consumed in the oxidation process have been
replaced by a synthetic anti-oxidant, usually 2,6
Di-tert-butyl-4-Methylphenol (also called DBPC and BHT), the
refurbished oil is often more stable than new oil.
Table 1
Oil Analysis of
Refurbishment by Fluidex Fullers Earth treatment process
400kV Transformer, National
Grid Company,UK
|
Test |
Before Process |
After
Refurbishment |
After 1 Year
Operation |
After 2 Years
Operation |
|
Moisture ppm |
23 |
8 |
10 |
11 |
|
Acidity mgKOH/gm |
0.20 |
<0.01 |
0.01 |
0.02 |
|
Dielectric kV |
35 |
76 |
71 |
69 |
|
Sludge content % |
0.02 |
<0.01 |
<0.01 |
<0.01 |
|
Resistivity at 90C |
2.5 |
226 |
184 |
160 |
|
DDF at 90C |
0.095 |
0.005 |
0.006 |
0.009 |
|
Oxidation Stability
Total Acid mg
KOH/g sludge% by mass |
0.48
2.29 |
0.16
1.23 |
0.18
1.30 |
0.19
1.32 |
|
Viscosity at 40C |
11.9 |
11.8 |
11.8 |
11.6 |
|
Interfacial Tension |
25 |
40 |
38 |
36 |
|
Aromatic Carbon |
10 |
10 |
|
|
|
Paraffinic Carbon |
48 |
48 |
|
|
|
Napthenic Carbon |
42 |
42 |
|
|
|
Sulphur
Content % |
0.333 |
0.320 |
0.321 |
|
|
Corrosive Sulphur |
Positive |
negative |
negative |
negative |
|
Phosphorus ppm |
11 |
ND |
ND |
ND |
|
Zinc ppm |
3 |
ND |
ND |
ND |
Does
Refurbished oil meet the same specifications as new oil?
Table 2
|
Test Parameter |
Test Method |
Refurbished Oil
ESI Bulk Tank |
New Oil Specification |
Pass/Fail |
|
Moisture
(ppm) |
D1533 |
10 |
<30 |
Pass |
|
Dielectric BV
(kV) |
IEC156 |
85 |
>60 |
Pass |
|
Acid Number
(mgKOH/gm) |
D974 |
0.01 |
<0.03 |
Pass |
|
Interfacial
Tension (mN/m) |
D971 |
42.7 |
>40 |
Pass |
|
Dielectric
Dissipation Factor (% at 25C) |
IEC247 |
0.011 |
0.05 |
Pass |
Does
Refurbished Oil last in service?
Table 3
Case History – In-situ Oil
Refurbishment
1978 10MVA 33/6.6kV Wilson
Transformer, Serial no. 60580
|
Date |
Acidity
mgKOH/gm |
IFT
Dynes/cm |
DDF
% at 25C |
Inhibitor
% |
|
10/11/1992 |
0.180 |
19.4 |
|
|
|
13/10/1993 |
0.180 |
19.0 |
|
|
|
++ 29/10/1994 |
0.020 |
43.1 |
0.002 |
|
|
20/1/1996 |
0.020 |
42.2 |
0.080 |
0.49 |
|
28/8/1996 |
0.020 |
39.1 |
0.062 |
0.43 |
|
8/1/1998 |
0.020 |
39.9 |
0.082 |
0.48 |
|
** 7/12/1998 |
0.020 |
33.0 |
0.041 |
0.48 |
|
11/1/2000 |
0.030 |
32.6 |
0.080 |
0.32 |
|
** 18/1/2001 |
0.010 |
37.8 |
0.021 |
0.40 |
|
23/11/2002 |
0.010 |
37.0 |
0.027 |
0.40 |
|
21/1/2004 |
0.010 |
36.6 |
0.028 |
0.40 |
|
28/1/2005 |
0.020 |
36.7 |
0.053 |
0.42 |
|
30/1/2006 |
<0.010
|
37.2 |
0.035 |
0.38 |
|
9/2/2007 |
0.020 |
36.0 |
0.033 |
0.48 |
++ In-situ oil reclamation carried
out
* * Change in testing
facility
How
does in-situ oil refurbishment compare with retro-filling the
unit with new oil?
Table 4
Case History – Retro-fill
and In-situ Oil Refurbishment *
5MVA GE Transformer Serial
No.7935639
|
Date |
Treatment |
Acidity
mgKOH/gm |
IFT
Dynes/cm |
|
July
1966 |
|
0.40 |
15 |
|
July
1967 |
|
0.42 |
14.5 |
|
July
1967 |
Retro-filled
with new oil |
0.03 |
41 |
|
Sept
1968 |
|
0.12 |
24 |
|
Sep
1968 |
In-situ Oil
Refurbishment |
0.03 |
40 |
|
Mar
1970 |
|
0.045 |
36 |
|
Feb
1971 |
|
0.045 |
30 |
|
Mar
1972 |
|
0.05 |
32 |
|
Feb
1973 |
|
0.055 |
30 |
|
Feb
1974 |
|
0.06 |
32.5 |
|
Jan
1975 |
|
0.05 |
32 |
|
Jan
1976 |
|
0.055 |
32 |
|
Feb
1977 |
|
0.05 |
31.5 |
The oil test history detailed in
Table 4 illustrates a major difference in the actual in-field
performance following retro-filling and in-situ oil
refurbishment. There are two major factors are involved in the
poor performance of New Oil following
retro-filling.
- Retro-filling a
transformer will not remove sludge deposits from the core,
windings, radiators, and floor of the transformer. A
thorough flushing can only remove 10 to 15% of the sludge
deposits within a transformer. As soon as the transformer is
re-filled and energized, the sludge deposits begin to
contaminate the new oil and degradation occurs very
quickly.
- Even in sludge-free
transformers, the cellulose insulation and spacers retain
approximately 10% of the total oil volume within the unit.
This cannot be drained out and will begin to contaminate the
new oil as soon as the unit is re-filled.
In-situ Oil Refurbishment gives a vastly
superior performance mainly because during refurbishment, hot
clean oil is circulated through the transformer, between 6 and
16 passes, depending on the severity of the sludging. The
clean oil is returned to the top of the transformer above the
aniline point for transformer oil, which is the point at which
the oil will dissolve its own oxidation by-products. The
dissolved sludge is then drawn into the refurbishment plant,
via the main tank drain valve, and removed by the activated
Fullers Earth filter media. This not only removes surface
contamination but begins to clean deposits embedded in the
cellulose, particularly the outer insulation
layers.
Typically a
Fullers Earth Refurbishment plant will produce twice the daily
volume of refurbished oil in a ‘tank to tank’ situation (where
input and finished product oil specifications are similar)
compared to in-situ transformer work. This is due to the extra
time and adsorption required to dissolve and remove
contaminates from the within the transformer.
A decrease in
Transformer Top Oil Temperatures of 8 degreeC has been
observed in transformers following in-situ oil refurbishment
due to the removal of sludge deposits. I suggest this is
mainly due to the removal of sludge in the radiators. The
actual decrease in ‘Hot Spot Winding Temperature’ is likely to
be closer to twice this value as similar sludge deposits also
blanket the core and block cooling ducts. A reduction in
operating temperature of 8 degreeC will double the life
expectancy of a transformer.
Graph 1
Life Expectancy with
Variable Oxygen and Temperature
Lampe, Spicer and Carrander Study
-1977

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