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Cathodic Protection Of Plant Areas
CATHODIC PROTECTION OF PLANT AREAS
i
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CATHODIC PROTECTION OF PLANT AREAS
For the design and installation of cathodic protection systems for buried and submerged structures for
plant facilities, the following structures would normally be considered to be included in a cp scheme:
a)
b)
c)
d)
e)
f)
Pressurized steel hydrocarbon pipelines
Bottom or soil side of above ground storage tanks
Buried storage tanks
Sea walls and associated anchors
Buried portions of process, gas, water, firewater or liquids pipelines
Buried or the buried portions of steel valves, hydrants, monitors and fittings
Because of the complexity of Plant Facilities, two basic principles may be employed:
a)
Isolated systems where every buried structure is isolated where it comes above ground giving
rise to a large number of insulated flanges. This in turn leads to maintenance and reliability
problems because it only requires one bolt in an insulated flange to be short-circuited for the
whole system to fail. It is also very difficult to find the offending bolt.
b)
Blanket system where, except between on plant and off plant facilities, no isolation is provided.
This means that all buried metal work on the site is protected including the earthing systems
leading to considerably larger systems.
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Cathodic Protection Of Plant Areas
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Plant facilities usually require a considerable amount of current and are therefore cathodically
protected with impressed-current systems unless soil resistivities are very low. Short buried sections of
piping, small isolated coated structures, etc., may be protected with galvanic anodes.
The impressed current CP system can utilize distributed anodes, local or remote surface anode beds,
local or remote deep anodes, or a combination of the above. When choosing and/or combining anode
bed types, the conceptual and the final designs must be co-ordinated with other plant operators to
ensure all existing facilities are known together with any planned extensions or new facilities. The
following is a typical example of an isolated system with a remote surface groundbed.
Structures protected or influenced by CP systems must be electrically continuous. Bonds are installed
(where required) to ensure electrical continuity.
The CP system design will probably use distributed impressed current anodes for CP systems
connected to buried facilities in congested areas, such as buried hydrocarbon pipelines inside the plant
fence, tank bottoms, ring walls, structures connected to plant ground grids and buried steel reinforced
concrete structures. Anode placement, separation distance, current output, and soil resistivity need to
be considered when calculating the soil voltage gradient necessary to achieve the minimum required
structure-to-soil potential. Additional test stations and soil access test holes may be installed for
monitoring cathodic protection levels of these facilities.
For isolated structures, typical magnesium anodes with connection boxes would be required for the
following structures.
a)
b)
c)
d)
e)
Buried valves: 2 No 27 kg magnesium anodes
Pipeline thrust anchors: 4- No 27 kg magnesium anodes
Buried gas accumulators: 4- No 27 kg magnesium anodes
Electrically isolated sleeve casings: minimum, 2- No 27 kg magnesium anodes each
end
Thrust bored crossings: minimum, 2- No 27 kg magnesium anodes each end
Current Density Criteria
Typical minimum current densities detailed in Table I. However, local experience may change
these depending upon the application
Table I - Minimum Allowable Surface Current Densities
Structure Surface
Uncoated Copper
Uncoated Steel
Tape or P-2 Wrap
Coal Tar Epoxy
Fusion Bonded Epoxy (FBE)
Polyethylene (PE)
Current Density (mA/sq. m)
40.00
20.00
1.25
0.75
0.10
0.10
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Anodes
Bulk Plants: The distributed anodes shall be equally spaced around the tank perimeter and
shall be installed a minimum of 1/4 the tank diameter from the tank wall and a maximum of one
tank diameter from the tank wall and a maximum spacing of 30 meters (center-to-center).
Tank Farms and Process Areas: The distributed anodes shall be equally spaced around the
tank perimeter and installed a minimum of 1/4 the tank diameter from the tank wall and at a
maximum spacing of 20 meters (center-to-center).
Distributed Anodes for Piping
Distributed impressed current anode installations are required where a remote anode bed will
not provide satisfactory current distribution or will not be practical. Distributed anodes are
normally used for buried pipelines, or piping installed in congested areas, at fence crossings, at
full-thrust anchors, at GOSP manifolds or flow line corridors, and where operational needs
dictate.
Anode spacing normally varies from 15 to 30 meters, with a 15 meter spacing or less in very
congested areas, and up to a 30 meter spacing in less congested areas.
The anode distance from buried pipelines or piping will normally be a minimum of 1 meter to a
maximum of 5 meters.
Anodes to be installed deeper than 15 meters require Hydrology Division, Geological
Department approval for drilling depth.
Design of cathodic protection system
The design of the power supplies and groundbeds are effectively the same as for buried
pipelines utilizing the same formulae
Monitoring Facilities
A test station for measuring pipe-to-soil potential shall be provided at; insulated cased
crossings, paved road crossings, thrust-bored road crossings, negative connections, buried
bond connections, and other locations as required by operational needs.
On inaccessible buried structures and pipelines, one pin or flush mount test stations shall be
installed with a permanent zinc reference electrode every 100 meters (or less) or one per
structure, or as required by the proponent.
In concrete and asphalt (paved) areas, soil access test boxes shall be installed:
-
one at the beginning and one at the end of each pipeline segment
-
at the midpoint over buried pipeline segments shorter than 30 meters
-
at maximum 15 meter intervals over buried pipelines longer than 30 meters,
and less than 100 meters
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at maximum 30 meter intervals on buried pipelines longer than 100 meters
-
around the periphery and within one (1) meter of tank bottoms, at the mid-point
between anodes, with at least one access box per quadrant.
All newly constructed tanks shall have reference electrodes installed beneath the bottom plates
according to Table II.
Table II - Reference Electrodes Beneath the Tank Bottom Plates
Tank
Diameter (M)
No. of
Electrodes
Location of Electrodes
Less than 20
2
Center and midway between center
and edge
20 - 39
3
Center and equally spaced on radius
line between center and edge
40 - 79
4
Center and one each, equally spaced
on 120 deg radius lines between center
and edge
80 - 99
7
Center and two each, equally spaced
on 120 deg radius lines between center
and edge
Greater than 100
9
Center and two each, equally spaced
on 90 deg radius lines between center
and edge
Bonding
All buried structures influenced by cathodic protection systems shall be electrically continuous.
Bonds shall be installed to ensure electrical continuity and to prevent interference. All bond
terminations between structures shall be made above-grade in an approved electrical
enclosure.
The minimum bond conductor size shall be 16 sq. mm (6 AWG).
Electrical Isolation
Insulating flanges, insulating joints or spools, or other electrical isolating devices are not allowed
except to limit protective current flow between plant facilities and off-plot pipelines.
Isolating devices shall be located in non-hazardous locations.
Isolating devices shall not be installed in any buried or submerged portions of a pipeline.
All isolating devices shall be provided with an above-grade bond box.