PCBs AND ASSOCIATED AROMATICS 917
A method which is often used for the PCB decontamina-
tion of mineral oil transformers is to drain the fluid into drums
for disposal, flush and drain the transformer with a flushing
oil, and then refill the unit with uncontaminated mineral oil.
The volume of flushing oil needed to reduce the PCB con-
centration of the retrofilled transformer to less than 50 ppm
PCB depends upon the initial concentration of the oil and the
amount of residual oil left after each flushing operation.
The PCB concentration after a drain and flush sequence
can be predicted according to the following equation:CCR P
PssP
10(^1001)
1
100
=⋅ +
⎡
⎣
⎢
⎤
⎦
⎥⋅−
⎡
⎣⎢
⎤
⎦⎥
(18)
where
C 1 PCB concentration in oil after draining and refilling.
C 0 PCB concentration in oil before draining and refilling.
R Percent of fluid remaining in the transformer after
draining.
P s Percent of PCB in solid materials before draining.
P 1 Percent of PCB in oil before draining.
The mathematics assumes that the oil in the tank is
homogeneous and therefore represents a ‘best-case’situation.
In a transformer, however, the volume of oil trapped in the
interstices of the windings are not likely to be removed by
a simple drain and flush procedure. Then, when the unit is
put back into service, the trapped, highly PCB contaminated,
original oil, gradually convects into the retrofilled bulk oil to
give an apparent leaching effect. If the trapped, original oil
had a PCB concentration of 2000 ppm PCB and becomes
diluted by a factor of 40 by convection, then the final con-
centration of the retrofilled fluid will be 50 ppm PCB.
The higher the PCB concentration in the mineral oil, the
more likely it will become necessary to remove trapped oil
in order to achieve a retrofilled concentration of less than
50 ppm PCB. The areas which must be cleaned in particular
are the core/coil assembly and other regions which contain
flow restrictions.
It has been mentioned above that a vapor cleaning system
has been proven successful in removing surface PCB con-
tamination from areas previously considered inaccessible.
The method has the advantage that a minimum quantity of
PCB contaminated fluid is derived for disposal in contrast
to the drain and flush method which produces several times
the volume of the transformer tank. Also, as the level of
PCB contamination in the mineral oil increases, the vapor
cleaning method become increasingly necessary and even-
tually becomes the only viable, cost-effective procedure.
THE VIABILITY OF PCB REPLACEMENT FLUIDS
The need to reduce or eliminate PCBs from the dielectric
fluid of transformers is partly because of the generation of
compounds of concern in a fire incident. However, it is also
important to consider that the original reason for the use of
transformer askarels was because they were intended to pro-
vide a measure of fire safety.
Fire hazard factors for replacement askarel transformer
fluids are neither well defined nor easily estimated, but
include:
- the ignition sources available:
- the availability of fire protection systems:
- the ease of ignition of the transformer:
- the close environment of the transformer:
- the effects of burning liquid on the surroundings:
- the depletion of oxygen by burning liquid:
- the smoke evolution from burning liquid:
- the toxicity of combustion products:
- the temperature at which the liquid is used.
The test methodology used to describe a system as com-
plicated as a real transformer fire is complex, and while the
factors which affect fire hazards can be listed in broad terms,
their interdependent description in an overall assessment is
difficult using the rudimentary test methods presently avail-
able. More importantly, the interpretation of such results
to provide a quantitative measure of the overall fire hazard
should be treated with caution.
Ideally, a fire hazard assessment should be predicated
on incidence studies in actual usage. Such an assessment is
difficult for “less flammable” fluids, i.e., fluids with a fire
point above 300°C, because of the lack of statistics on fires
in which these fluids are involved.
Section 450-23 of the National Electrical Code speci-
fied in the past that the controlling provision for fluids used
in indoor transformer applications should be that the insu-
lating liquid have a 300°C fire point. The required tempera-
ture has some validity in that it is known that flaming will
not persist in a wood slab not subjected to supplemental
heating unless the average temperature within the slab is
greater than about 320°C.
In the absence of primary current limiting fusing, the
intensity and duration of a high energy arc are limited by
the recovery capability of the liquid. When there is sufficient
energy to produce an explosion, liquid aerosol and gaseous
decomposition products are expelled from the transformer
as a hot plume. The magnitude and probability of potential
loss will be partially dependent upon whether the plume
ignites. Subsequently, the effect of the transformer failure
will depend upon the ignitability of the fluid and the material
surrounding the transformer.
A report was issued by the National Electrical
Manufacturers Association (NEMA) in 1980 concerned with
“Research on Fire Safety Test Methods and Performance
Criteria for Transformers Containing PCB Replacement
Fluids”. The scenario used by NEMA to describe an eventful
failure is as follows:1) Incipient fault induction period.
2) Growth of incipient fault to large arc.
3) Failure of electrical protection devices to remove
the transformer from the line before tank rupture.C016_003_r03.indd 917C016_003_r03.indd 917 11/18/2005 1:12:41 PM11/18/2005 1:12:41 PM