Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF DECONTAMINATING PCB TRANSFORMERS
FIELD OF THE INVENTION
The present invention relates to a method of
decontaminating PCB-containing transformers, more
particularly to such a method involving a cleaning process of
internal surfaces using a solvent Wash process.
BACKGROUND OF THE INVENTION
Polychlorinated biphenyls (PCB) are synthetic
chemical compounds consisting of chlorine, carbon and
hydrogen. First synthesized in l881, PCBs are relatively fire
resistant, very stable, do not conduct electricity and have a
very low volatility at normal temperatures. These and other
properties have made them desirable components in a wide
range of industrial and consumer products. Some of these
same properties make PCB environmentally hazardous,
especially their extreme resistance to chemical and
biological breakdown by natural processes in the environment.
The use of PCB as the insulating fluid in
transformers and other electrical products such as
fluorescent light ballasts was discontinued in l978. Up to
this time many transformers contained PCB as the insulating
liquid replacing mineral oil in applications where a
transformer failure with the resulting fire could prove
disastrous. Most PCB-containing transformers are located in
office buildings.
PCBs are now listed as a toxic substance under the
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Canadian Environmental Protection Act (CEPA) and its use in
new products, and its release into the environment have been
prohibited under the Chlorobiphenyl Regulations of CEPA.
Liability considerations are the chief reason for
the early phase out of PCB equipments which are otherwise in
serviceable condition and have adequate capacity for the
operating loads. The risk of accidental release or fire is
very small. However, there are substantial costs associated
with the spill cleanup or building cleanup following fires
originating from a non-PCB source but involving PCB
equipment. The vapourization of PCB by high heat generates
dioxins and dibenzofurans, which are identified carcinogens.
This risk is one of the reasons that many organizations plan
for the early retirement of their PCB transformation
equipment.
In addition to the total destruction and
replacement of PCB-containing transformers, the
decontamination of operational PCB transformers is normally
carried out by either a series retrofill or in-situ processor
method. Both these methods have major flaws in that they
either generate large amounts of contaminated transformer
fluid as in the series retrofill method or require long
processing times as with the in-situ processor method.
There is some doubt associated with the long term
benefits of decontaminating PCB transformers without removing
the core/coil assembly. This is supported by Environment
Canada (EC) who have recommended that even after a
transformer has been drained, retro-filled and the fluid
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decontaminated (with a time frame of 2 to 3 years), the
transformer should be tested for an additional three years if
the leaching fluid is left in and an additional ten years if
it is replaced with silicone fluid. During this time it will
remain on Environment Canada's list of PCB-containing
equipments and must be treated and labeled as a PCB
transformer. Processes that do not remove the core/coil
assembly must contend with the problem of leach back. Leach
back occurs because of the large amount of porous material
such as insulation and wood used in the manufacture of
transformer core/coil assemblies. This material by necessity
is extremely dry when installed in the transformer. The
estimated amount of transformer fluid absorbed by the wood
and insulation is between 3~ and 5~ of the total amount used
to fill the transformer. For a 2000kVa transformer which
would hold approximately 1500 litres, the amount absorbed
would be between 45 and 75 litres. To contaminate 1500
litres of transformer oil to over 50 ppm would only take
approximately 60 grams of PCB. This illustrates the
difficulty in decontaminating a PCB transformer Without
removing the core and coil. The PCB in the core and coil
will leach back into the transformer fluid for years. Even
after the leach back has slowed to an acceptable level, the
PCB contained in the wood has been shown to remain over the
acceptable limit of 50 ppm indefinitely and at the end of the
transformer life will have to be removed and stored for
eventual destruction as PCB waste.
This situation will require that upon the end of
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the transformer life it will have to be disassembled and the
interior components tested for PCB content. As it has been
determined by Environment Canada that there will be
components that contain PCB over the 50 ppm, these will have
to be removed and sent to a licensed PCB destruction
facility.
A prior art search conducted at the Canadian Patent
Office revealed the following patents that disclose methods
of decontaminating PCB transformers: U.S. Patent Nos.
4,483,717 (Olmsted et al), 4,699,667 (Walsh of Westinghouse),
4,950,837 (Horneck et al of General Electric) and 4,983,222
(Green et al of Union Carbide). They do not appear very
satisfactory.
There remains a need for a method of transformer
decontamination that avoids the problems associated with
known methods and gives consistent reliable results.
SUN~IP~RY OF THE INVENTION
Broadly stating, the present invention provides a
method of decontaminating a PCB transformer to a level below
50 ppm when the transformer is re-filled with a new non-PCB
fluid, which comprises: (a) removing a transformer core/coil
assembly from the PCB transformer from which PCB has been
drained off; (b) cleaning a11 interior surfaces and remaining
components of the transformer by application of a cleaning
solvent; and (c) installing a new transformer core/coil
assembly.
In a preferred embodiment, the process may also
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include between steps (b) and (c), (d) flushing interior
surfaces of the cooling fins with a replacement non-PCB
fluid.
According to the process of this invention, a PCB-
filled transformer can be reclaimed as a PCB-free transformer
by the removal of those components that cannot be adequately
cleaned and the solvent washing of a11 interior surfaces and
components. The remaining product will be permanently less
than 50 ppm PCB.
DESCRIPTION OF PREFERRED EMBODIMENTS
The existing PCBs are drained from the transformer
and may be put in storage for eventual destruction. The PCB
draining step is not an essential step of the present
invention, since a transformer from which PCBs have already
been drained off may be received for processing. The
core/coil assembly is removed and may be placed in storage
for eventual destruction. The physical construction of the
core and coils prevents us from cleaning this portion of the
transformer below the levels required to have the transformer
declared non-PCB with no danger of leach back.
To a11 interior surfaces and remaining components
of the transformer, a cleaning solvent is applied. A
convenient manner of application is to wash and wipe clean
them at least once, preferably 2 to 5 times and especially
preferably 3 times, with a solvent. Varsol* solvent was used
in testing this method, however other solvents may be as
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effective. A preferred solvent is a non-volatile (e.g., a
boiling point of at least about 100~C) hydrocarbon solvent,
especially an aliphatic hydrocarbon (such as naphtha) having
a boiling point of from about 150 to about 200~C. Varsol* DX
3139 solvent is an aliphatic hydrocarbon (naphtha) solvent
having a boiling point of from 159 to 196~C marketed by
Imperial Oil. Other useful solvents include halogenated
hydrocarbon solvents such as perchloroethylene. Water
containing a detergent may also possibly be used. The
cleaning step may be performed manually or mechanically. The
volume of the solvent used in the washing step is not
critical. By conducting simple experiments, a person skilled
in the art would be able to determine appropriate
commercially feasible amounts of the solvent and a most
appropriate number of washes. The Environment Canada surface
contamination criteria are only applicable for material that
is going to be declared waste and will have to be stored or
transported to a destruction facility. This invention is
based on calculations of the total surface area of the inside
of a test transformer showing that there are 85 m2 of surface
area that will be contaminated with PCB. As the transformer
will remain in service, the contamination levels are only
relevant in the context of how much can remain, and be
combined with the new non-PCB fluid that will be added after
the transformer is rebuilt. The resulting fluid must have a
PCB contamination level of less than 50 ppm to have the
transformer declared non-PCB. The data showing the results of
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the cleaning of the transformer cooling fins at a pilot
project located at Sault Ste Marie, Ontario, show levels in
the range of 200 to 300 ~g of PCB's/100cm2, adequate to
retrofill the transformer without danger of contamination by
residual PCBs.
After the transformer cooling fins are drained of
a11 PCB fluid and the bottom portion emptied using a suction
pump equipped with a wand, the cooling fin headers may be
plugged and the fins may be filled with the solvent. In one
preferred embodiment, the solvent is then circulated at least
once, preferably 2 to 5 times, more preferably 3 separate
times for an appropriate time, say 15 minutes to one hour,
preferably about 30 minutes, each time with a pump connected
to the top and bottom of the cooling fin or bank of fins.
The volume of the solvent used is not critical. It is most
convenient to use the same volume of the solvent as the
interior volume of the fins each time. For example, in the
case of a 2000kVa transformer, about 50 liters of the solvent
may be most appropriate for each bank of the fins. Between
each solvent circulation cycle, the fluid is drained and the
bottoms of a11 fins are emptied using a suction pump. After
the surface cleaning process, any PCB left in the bottom
corners of the fins may be removed by using a suction pump.
It was found during preliminary investigations and tests,
that it is especially preferable to remove any PCB residue
left in the bottom corners by a suction pump. The
effectiveness of the cleaning methods was verified by
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measurements of the Varsol* solvent contamination after
flushing the fins, typically the measurements after three
rinses showed PCB levels under 300 ppm which indicate
remaining PCB would not contaminate the retrofill fluid over
the allowable limit.
When the cleaning procedure is complete, the
residual solvent is desired to be removed from the fin
surfaces. This may be accomplished preferably by flushing
the interior surfaces of the fins with a small amount of a
proposed retrofill fluid. When the proposed retrofill fluid
is thick (i.e., viscous) at room temperature, it is desirable
to heat it to a certain temperature (for example about 60~C)
to facilitate a good flushing action. The fluid is then
removed from the transformer interior and the bottom of the
fins is emptied, for example, by using a suction pump.
when the cleaning is completed, the assembly can be
rebuilt by installing a new core/coil assembly by anybody
regularly engaged in this work. When the transformer has
been cleaned, rebuilt, tested and processed to ensure the new
fluid does not contain any impurities, it can be filled
with any approved non-PCB dielectric fluid and energized.
The safe handling of PCBs is of the highest
importance. From the careful removal of the tank top and the
replacement of the interior components to ensuring adequate
ventilation of the tank interior and surrounding space
cleaning and rebuilding methods must ensure that all safety
precautions are observed.
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An advantage of this method is that the time
required is relatively short (for example several hours to a
few days to process one transformer). Another advantage is
that no particular sophisticated machine or equipment is
needed. A further advantage is that a vaporization of a
solvent is not normally required, hence the process is safe
to both workers and environment.
Test
The validity of this method of rebuilding a PCB
transformer was demonstrated through a pilot project
conducted at the Confederation Heights central heating plant,
in Ottawa, Ontario, Canada.
The methodology used included:
(1) opening the transformer and doing lab tests to
determine the levels of PCB on all interior surfaces;
(2) disassembling the core and coils and taking
swab tests of a11 interior surfaces of the copper windings
and the steel core laminations in an attempt to decontaminate
the steel or copper for reuse;
(3) cleaning a11 interior surfaces with a variety
of solvents, cleaners and abrasives, followed by lab tests to
determine the success of the various PCB removal methods;
(4) blocking the cooling fins and circulating a
solvent through them with a pump;
(5) removing the cooling fins and cutting them open
to determined if the interior surfaces were below the level
that would allow one to refill the transformer and have the
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insulating fluid remain below the required 50 ppm PCB level.
The testing method was as follows: swab tests were
taken using the Environment Canada recommended wipe test
sampling methodology for transformer metal surfaces. The
tests were performed by a certified test lab.
The result obtained were as follows: swab tests
taken from various locations in the tank interior showed PCB
levels of between 878 and 2246 dug of PCB's/100 cm2. After
washing with Varsol* solvent, the readings dropped to between
143 and 18.5 ~g of PCB's/100 cm2. The cooling fins were
rinsed by circulating Varsol* solvent through them. To
verify the results of this technique, the fins had to be
removed from the transformer and cut open.
The following table shows the contamination found
on the interior transformer parts:
AREA TESTED SOLVENT RESULTS
cover none 878 ~ug/100Cm2
cover sandblast 24 ~,g/100Cm2
upper tank none 1878 ~g/100Cm2
middle tank none 2246 ~g/100Cm2
inside cover 3 washes Varsol* 79 ~,g/100Cm2
solvent
cover plate 3 washes Varsol* l8.5 ~,g/100Cm2
solvent
cover paint remover 74.7 ~,g/100Cm2
inside cover grinding & paint 37 ~,g/100Cm2
remover
cooling fin Varsol* solvent 143 ~,g/100Cm2
rinse, 2 hours
cooling fin none 792 ~.g/100Cm2
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AREA TESTED SOLVENT RESULTS
copper wire Varsol* solvent 1.4 ppm,
(150 ~g/100Cm2)
The results indicate as follows. The levels of PCB
surface contamination which were able to be attained by
washing the interior of the transformer will make it possible
to replace the core and coil with new core and coil, refill
the transformer with a non-flammable insulating fluid and
have the resulting transformer below the required 50 ppm.
This level would not be subject to leach back as happens with
other decontamination processes.
Because of the physical construction of the core
and coils, this portion of the transformer could not be
cleaned below the levels of 2.5 ~.g of PCB's/100cm2. While
there is no Federal surface contamination criterion these
levels have been designated by EC as a permissible
contamination criterion therefore neither the core nor coil
could be reused or recycled. They will be packaged and
placed into storage for eventual destruction.
It should be noted that a variety of modifications
may be made without departing from the essence of the
invention expressed in the main claim of this application and
a11 such modifications are within the scope of the invention.
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