Note: Descriptions are shown in the official language in which they were submitted.
TTR2 3 CAq'E~ODlC~STOlRAG_ T~N~ BC)TTO~
_ This invention relates to a method ~or cathodically
protecting a new tank bottom by utilizing the old tank
bottom as the impressed current anode. The invention
also pertains to a tank on which the ~oregoing method
has been practiced.
BACKG~OUND OF THIS INVENTION
The corrosion process as it relates to buried,
partially buried, and submerged metallic structures is a
naturally occurring phenomenon. The principles of this
process and the benefits o~ cathodic protection in
controlling this type of corrosion are widely known.
Corrosion of steel and other metals is governed by
electrochemical principles. In ordar for corrosion to
occur, three prerequisites are necessaryo
1) There must be an anode (point of electrical
current discharge) and a cathode (point where
electrical current is received) with an
electrical potential difference between the
two.
2) The anode and cathode must be electrically
connected through a metallic return path.
3) The anode and the cathode must be in contact
with an electrically ionized solution
(electrolyte).
When these conditions are satisfied, ener~y in the
form of direct current will pass from the negative area
to the positive area through the electrolyte
(conventional current flow).
It is clear that galvanic corrosion relates very
closely to the natural reaction which occurs in a
battery. With the flow of electricity from the anode to
the cathode, positively charged atoms of metal detach
from the surface of the anode and enter into the
electrolyte. Conversely, the area known as the cathode
is protected from the loss of metal. The amount of `
metal lost varies with the type of material and in the
case of iron and steel it occurs at the rate of 20
pounds per ampere year.
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There has been considerable and increasing concern
over the integrity of underground storage tanks as well
as storage tanks resting on the ground, because of the
corrosive reaction ~etween the metal and the
electrolyte.
Cathodic protection is a proven method of
controlling corrosion of buri~d, partially buried and
submerged metallic structuresO In theory, a properly
designed, installed and operated cathodic protection
system will eliminate the anodic regions which exist on
a metal by passing direct current to the metal surface.
The direct current is discharged from electrodes
installed in the electrolyte adjacent to or near the
structure being protected. In this manner, corrosion
will be arrested when the direct current is of
sufficient magnitude and is adequately distributed over
the entire surface to be protected.
There are two basic types of cathodic protection
systems which can be utilized to arrest corrosion. One
of these is referred to as a galvanic or sacrificial
anode system. The galvanic anode system i~ based upon
the natural potential difference which exists between
the structure being protected and the auxiliary `
electrode known as an anode, which is installed in tha
electrolyte. As a result of the potential difference, a
battery e~fect is created, and just as in the case o~
galvanic corrosion, electrical current will ~low from
ths anode through the electrolyte to the structure and
then return to the anode through a metallic connection
between the anode and the structure. Materials which
are commonly used for galvanic anode systems are made of
magnesium, zinc and aluminum.
Another method of cathodic protection utilizes
anodes in conjunction with an external Direct Current
(D.C.) power source. This type of cathodic protection
system is knawn as an impressed current system. The
anodes are installed in soil or water and are connected
to the positiv~ terminal of a D.C. power source such as
a rectifier. The structure to be protected is connected ~
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to the negative terminal of the source and electrical
current is forced to flow from ths positi~e terminal to
the anodes through the electrolyte to the structure.
This type of cathodic protection system utilizes anode
materials made of high silicon chromium cast iron,
graphite, l~ad-silver alloy, and to a certain extent,
platinized niobium and platinized titanium.
The application of cathodic protection ~or tank
bottoms has been accomplished in a variety of approaches
with varying degrees of suc~ess. The use of sacrificial
anodes is typically limited to those applications where
the tanks ar~ of small diameter construction and where
they are effectively isolated from other underground
metallic structure~ such that the current requirements
to achieve protection are minimal. Impressed current
system designs have b~en utilized where current
requirements are msre substantial. Several different
types of anode in~tallations intended to distribute`the
protective current to the tank are possible. For
example, in one configuration, horizontal or vertical
anodes are installed around the periphery of the storage
tank. In another system, horizontal or vertical anodes
are installed under the tank bottom. Alternatively,
anodes can be installed in deep well configurations
located adjacent to the tank.
Each of the foregoing systems has advantages and
disadvantages, and the selection of each system is
dependent upon design and economic ~actors as well as
operational requirements. ;~
Paramount with selection o~ system type is the
method of verification that corrosion control has indeed -
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been achieved. Past experience indicates that standard
monitoring procedures may not be adequate in analyzing
the degree o~ protection being af~orded to a storage ~ `
tank resting on the ground. Experience has shown the
~ollowing~
l. That structure-to-earth potential measurements
obtained at the perimeter of the tank do not
indicate actual cathodic protection l~v81s at
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the cPnter, particularly on larger diameter
tanks.
2. That potential measurements at the center o~
the tank and at other areas under the tank are
necessary if tank bottom cathodic protaction
surveys are to be complete.
3. That the liquid level in a tank is a
significant factor in determining the cathodic
protection status of the tank bottom at the
center.
4. That certain types of cathodic protection
designs may not be effective in providing
adequate cathodic protection to the tank
bottoms in certain areas.
5. That an improved design for a distributed
groundbed utilizing slant or angle drilling
techniques to locate anodes under the tank
bottom can improve the current distribution to
the center by reducing the tendency for most
of the current from conventional distributed
beds to flow to the tank at the perimeter.
The standard method of determining the
effectiveness of cathodic protection on any structure is
the structure-to-electrolyte potential measurement.
These measurements are performed utilizing a high
impedance voltmeter and a stable re~erence electrode :
contacting the electrolyte.
The natural potential o~ a carbon steel structure
in contact with an electrolyte is usually on the order
of -0.600 volts when measured with respect to a copper~
copper sulfate reference electrode (CU/CuSO4). Values
which are more positive are typically indicative o~
steel structures which have undergone corrosion while
values more negative are indicative of new~ well coated
structures or a structure which is under the influence
of cathodic protection.
A structure is considered to be effectively
protected when a potential measurement o~ -0.850 volts
(Cu/CuSO4) is obtained (one of several accepted
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criteria). For a true representation of electrical
potential measurements, the reference electrode is to be
placed as close as practicable to the struc~ure under
investigation (either directly over or adjacent to).
5 GENERAL DESCRIPTION OF THIS INVENTION
In view of the problems and difficulties mentioned
in the preceding section, it is an object of one aspect
of this invention to provide an inexpensive and reliable
method for replacing a corroded tank bottom with a new
tank bottom, and using the corroded tank bottom as an
anode for the cathodic protection of the new tank
bottom, utilizing the impressed current technique.
It is an ob~ect of another aspect of this invention
to provide a tank having a new bottom, with the bottom
being cathodically protected through the use of the old
tank bottom as an anode.
More particularly, this invention provides a method
for cathodically protecting a replacement bottom in a
storage tank made of a metallic material and of which
the original bottom has become corroded, the replacement
bottom being installed in spaced rPlation above the
original botto~ with fill material between the two
bottoms, the method comprising the steps:
a) removing a closed loop of said metallic
material from the original bottom, the closed loop being
close to the perimeter of the original bottom, thus
electrically isolating the tank from the remainder of
the original bottom within the removed closed loop,
b) electrically connecting an insulated cable to
said remainder of the original bottom,
c) applying an electrically insulative medium to
less than all of the inside surface of said remainder of
the original bottom, in an unbroken pattern which
includes the location where the cable is connected to
said remainder of the original bottom, thereby to create
shielded portions of said remainder of the original
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bottom and unshielded portions of said reminder of the
original bottom, and thus to protect said shielded
portions of said remainder of the original bottom under
said medium from being anodically sacrificed so that
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such shielded portions can function to carry current to
said unshielded portions of said remainder of the
original bottom, and
d) impressing a direct currsnt between said
insulated cable and the tank, the insulated cable being
positive.
Further, this invention provides a method of
replacing a corroded bottom of a tank made of a metallic
material, and of cathodically protecting the replacement
bottom, comprising the steps:
a) removing a closed loop of said metallic
material from the original bottom, the closed loop being
close to the perimeter of the original bottom, thus
electrically isolating the tank from the remainder of
the original bottom within the removed closed loop,
b) electxically connecting an insulated cable to
said remainder of the original bottom,
c) applying an electrically insulative medium to ~:
less than all of the inside surface of said remainder of
20 the original bottom, in an unbroken pattern which ~ `
includes the location where the cable is connected to
said remainder of the original bottom, thereby to creatP
shielded portions of said remainder of the original ~:
bottom and unshielded portions of said reminder of the
25 original bottom, and thus to protect said shielded
portions of said remainder of the orignal bottom under
said medium from being anodically sacrificed so that
such portions can function to carry current to the
unshielded portions of said remainder o~ the original
30 bottom,
d) placing fill material above the original
bottom,
e) installing a new bottom of a metallic material .
above the fill material, the new bottom being in
35 electrical communication with the rest of the original
tank, and
f) impressing a direct current between the
insulated cable and the rest of the original tank, the
insulated cable being positive.
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Finally, this invention provides a metal storage
tank made of a metallic material and having an original
bottom and a replacement bottom above the original
bottom, the replacement bottom being in electrical
communication with the tank, fill material between the
two bottoms, the original bottom being intarrupted by
virtue of a removed closed loop of material, the closed
loop being close to the perimeter o~ the original bottom
~ and defining a remainder of the original bottom which is
electrically isolated from the rest of the tank, an
insulated cable electrically connected to said remainder
of the original bottom, an electrically insulative
medium applied to less than all of the inside surface of
:~ said remainder of the original bottom in an unbroken
. 15 pattern which includes the location where the cable is
~ connected to said remainder of the original bottom,
thereby to create shielded portions of said remainder o~
the original bottom and unshielded portions of said
remainder of the original bottom, and thus to protect ~:
said shielded portions of said remainder of the original
bottom under said medium from being anodically
sacrificed so that such shielded portions can function
to carry current to said unshielded portions of said
remainder of the original bottom, and means for
impressing direct current between said insulated cable
. and the tank with the insulated cable being positive.
GENERAL DESCRIPTION OF THE DRAWINGS
One embodiment of this invention is illustrated in
: the accompanyin~ drawings, in which like numerals denote
like parts throughout the several views, and in which:
. Figure 1 is a vertical sectional view through the
bottom portion of a tank to which a new tank bottom has
been applied in accordance with this invention: and
Figure 2 is a plan view of the original t~nk
bottom, showing its preparation to act as an impressed
current anode for the cathodic protection of the new
tank bottom.
D~,TAILED DESCRIPTION OF THE DRAWINGS
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Attention is first directed to Figure 1, which
shows a portion of an existing tank side wall at 10, the
tank having had an original tank bottom 12 with a
periphery 22 adjoining the side wall 10 (ignoring the
broken lines at 20). The tank is made of an electrically
conductive material, such as a metal. In accordance
with one aspect of the method o~ this invention, a new
-~ tank bottom 14 is installed above and in spaced relation
- from the original tank bottom 12, with sand or other
granular fill material 16 between the two bottoms.
However, before the fill material 16 and the new tank
bottom 14 are put into place, certain operations are
carried out on the existing or original tank bottom 12
T as will now be explained with reference to Figure 2.
The first step is to remove a closed loop 20 of the
conductive material from the original bottom 12, the
closed loop being close to the perimeter 22 o~ the
original bottom. This operation electrically isolates`
the tank from the remainder of the original bottom
within the removed closed loop 20. In both figures, the
said remainder of the original tank bottom is identi~ied
¦ by the numeral 24, to distin~uish from the original
entire bottom 12. The remainder 24 extends only to the
edge of the closed loop 20, whereas the original bottom
12 extends to the perimeter 22.
An insulated cable 26 is electrically connected at
points 28 to the remaindar 24 of the original bottom 12.
This can be done by welding, silver-soldering, or any
other appropriate step.
Next, an electrically insulative medium in strip
form, such as polyethylene tape, is applied to the
inside surface (the upper surface in Figure 1) of the
remainder 24 of the original tank bottom, in an unbroken
pattern which includes all locations 28 where the cable
26 is connected to the remainder 24 of the original tank
bottom. The unbroken pattern illustrated in Figure 2 by
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~-` way of example is a cross formation having four arms 30
extending at 90 intervals from a central cross location
-` 32. The connection location 28 for the cable 26 is at
the end of one of the arms 30. This operation shields
those portions of the remainder 24 of the bottom which
:~ are located under the insulative medium from being
, anodically sacrificed during the cathodic
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protection. This means that these protected portions
can function as current headers, thus carrying
; electrical current to the unshielded portions of the
` remainder 24.
S Following the steps just outlined, sand or other
granular fill material 16 is placed abo~e the original
- tank bottom, as seen in Figure 1, whereupon a new tank
bottom 14 is welded into place against the original tank
side wall.
If desired, one or more reference electrodes 34 can
be positioned within the sand or other granular fill
- material 16, for monitoring purposes. The reference
electrodes 34 are connected by insulated cables 36 to a
monitoring apparatus (not shown) of conventional kind.
The cable 36 can pass through the old wall below the new
tank bottom 14.
The final step in cathodically protecting the
replacement or new tank bottom 14 is to impress a dirèct
current between the insulated cable 26 and the remainder
24 of the tank 10, with the insulated cable 26 being
positive.
The tank 10 illustrated in the figures is a
~ cylindrical tank with a circular hori~ontal section.
;~ This means that both of the bottoms 12 and 14 are
-~ 25 substantially circular. In Figure 2, the removed closed
loop 20 is a circular annulus located close to the
periphery 22 of the original bottom 12. It will, o
course, be appreciated that the shape of the removed
closed loop 20 will vary depending upon the cross-
sectional shape of the tank in question, and that the
expression ~closed loop" does not necessarily connote ~;
circularity.
It will thus he appreciated that the invention
described above precludes the need for and expense of
conventional cathodic protection anode materials by
physically modifying an existing tank bottom and using
it as the impressed current anode. The power supply
utilized can be a simpl~ constant voltage type, or the
automatically controlled type, both of which are well
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known. The negative of the power supply may be
connected to the existing tank shell or the new tank
bottom by an insulated cable (not illustrated) at any
convenient location. As described above, when the power
supply is energized, the remaining existing tank bottom
24 functions as an impressed current anode, distributing
protective current to the new tank bottom. It is
important that the removed closed loop 2~ of material be
wide enough to prevent preferential protective current
discharge at the edge of the tank bottom anode (the
remainder ~4 of the original tank bottom). The minimum
radial width of the removed closed loop 20 will vary
depending upon the surrounding earth and the nature of
the fill material 16, however as a general rule~ the
radial dimension of the removed closed loop 20 should
not be less than the vertical separation between the
original bottom 12 and the new bottom 16.
The system described herein has the advantage of
improved current distribution and substantially lower
cssts, by comparison with conventional or known methods.
Furthermore, much of the cathodic protection fabrication
can be done by the tank fabricator, rather than
requiring specialized cathodic protection personnel.
While one embodiment of this invention has been
illustrated in the accompanying drawings and described
hereinabove, it will be evident to those skilled in the
art that changes and modifications may be made therein,
~ without departing from the essence of this invention, as
;; set forth in the appended claims.
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