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Technical Information
INTROUCTION
Design of an efficient and effective cathodic
protection scheme can be extremely complex. The tables and formulae
listed below are provided for the customer's guidance, to enable
them to carry out budget design and cost calculations at early
project stage, and then to present sufficient information for ETC to
design the final scheme. Many factors need to be considered, and
information additional to that listed below, may be required. ETC's
engineers are available to carry out the detailed site survey and
design work that are necessary. Data required to produce a cathodic
protection proposal:
1. Geographical location of job site and climatic conditions if
considering solar powered system, give latitude and longtitude.
2. An estimate of all submerged,
wetted and buried areas of the structure to be protected.
3. The configuration, layout and dimensions of all structural steel
work to ensure adequate distribution of anodes.
4. Nature, types, thickness and
extent of coating applied to steel work.
5. Estimate of percentage of
mechanical breakdown of coating during and after installation, the
age and condition of the structure.
6. Period of time for which
protection is required.
7. Location of electrical power supply, indicating voltage,
frequency, phase and whether equipment is required for use in
hazardous areas.
8. Customer preference in terms of anode material and type of
protection equipment if any.
9. Design limitations in terms of
size and weight of equipment, due to fabricators or site contractors
lifting capabilities.
10. Special easement problems and
/or requirements. Anode ground beds can be located upto 200m from
structure.
11. Degree of specialist
involvement, e.g supervision only, turnkey project, etc.
12. In case of pipelines; material,
length, wall thickness and size, together with information regarding
electrical continuity, major road crossings (cased/ uncased),
overhead fly line crossings.
13. For pipeline running offshore,
weigery ht and thickness of the concrete weight coating and depth
immersed in sea-bed.
14. For buried installation, nature
and variation of soil, presence of anaerobic areas.
15. Water or soil resistivity and
temperature range. In the case of cross country pipeline,
measurement at 1-2 km intervals are required.
16. Details of any foreign structure, pipelines or services close to
protected structure which may be affected by interference.
17. Details of any sufficient
steelwork attached to the structure. or branch lines, valves,
washouts, earthing, etc, which must be protected, or isolated from
the system.
18. Size, thickness and pressure rating of all main and branch
lines, for determining details of insulated joints.
CALCULATION OF REQUIRED CURRENT
The first step in the design is to calculate the surface area of the
structure exposed to an electrolyte. The total current required can
be then determined by multiplying this with an estimated current
density for protection. The precise current density cannot be
predicted as it is highly dependent upon the local environment, and
can change with the season or coating deterioration. The following
table provides a guide to the relative current requirement under
different conditions, where widerly varying soil resisitivities are
encountered such as in the case of cross country pipeline, these
variations need to be taken into account.
Guidance on current densities for
Cathodic Protection of bare mild steel:
Environment
Estimated Resistivity
Current Density
(ohm- cm)
(mA/m2)
Seawater
15-25
100-200
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Saline mud
100
25
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Very aggressive Soil/Water
< 100
25
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Aggressive Soil/ Water
10000
10
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Moderately Aggressive soil/water 1000
10
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Mildly Aggressive soil/ water
50000
2-5
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Highly Aerated Seawater
25
250
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Steel in Concrete
From
0.5 -15
3000
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The total current (Amperes) is given by I =
A x C
1000
where A = Surface area to be protected, m2
C
= Current density mA /m2
Notes:
1. In the case of a coated structure, it is necessary to estimate
the percentage coating breakdown over the life of the structure.
2. On a structure which has already suffered corrosion, it may be
necessary to allow as much as 30% additional area to take into
account the state of structure.
3. The three galvanic anodes commonly used are aluminium, magnesium
and zinc.
CHOOSING A SYSTEM
The cathodic protection design process for some structure can be
very complex, requiring many iterations before arriving at an
acceptable design, an example for structure is outlined in the below
flow chart
SEAWATER APPLICATION
These can be broadly classified as:
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Jetties/Sheet piling
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Subsea pipelines
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Inplant facilities
(storage tanks, pipelines, condenser water boxes)
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Screening equipment and pumps ( N.B. Ships and platforms are dealt
within our marine and offshore sections)
For structure requiring low currents, constraints on use of
impressed current and for spot requirements sacrificial anodes are
most economical. ETC manufactures complete range of Zinc, Magnesium
and Aluminium anode which are the three galvanic anodes commonly
used for protection of steel structures, sufficient potential
difference must exist between the galvanic anode and the structure
to overcome the circuit resistance and supply adequate current
to achieve polarization of the structure.
LAND APPLICATIONS
Land based applications of cathodic
protection, cover a wide range, including:
Pipelines
Vessels
Tanks (base and internal)
Refinery Pipework
Well Casings
Rebar in concrete
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Internal Cathodic
Protection of A/G Storage Tanks at RELIANCE Jamnagar
Refinery |
ICCP
System for Mounted LPG Bullets at IOCL Dumad |
SACRIFICIAL GALVANIC SYSTEM
Higher resisitivities are generally encountered on land as compared
with marine conditions. Onshore resisitivities can vary from 20 to
over 100,000 cm. In highly corrosive environment with low
resistivities, Zinc and Aluminium anodes are able to provide
protective current economically. In resistivities up to
approximately 3000 ohm cm, the higher available driving voltage from
Magnnesium anode means that they are capable of supplying more
current than Zinc and Aluminium. Consequently they are used almost
exclusively under these conditions where sacrificial anodes are
specified. Magnesium anodes are normally supplied as standard
potential anodes, or (HP) High potential, which in some situations
can allow the use of fewer anodes with reduced installation cost.
All anodes have a central cast-in, perforated zinc-coated steel tube
of special design which ensure close bonding between magnesium alloy
and the insert. The core extends into a recess in the form of a
smaller diameter tube into which the lead wire is brazed, forming an
extra strong joint. After the brazing operation, the recess is
filled with an insulating and sealing material. Extruded magnesium
ribbon in an alloy equivalent to Galvomag is available in long
length for use in high resisitivity soils and for special
application, such as the annular spacing between carrier pipe and
causing pipes.
Aluminium anodes generally require chloride ions in the electrolyte
to function properly, thus use of aluminium anodes are generally
limited to marine and storage tanks internal protection. High purity
Zinc anodes can be used in low resisitivity soil and fresh water
applications. ETC also manufactures pre-packed zinc anodes use as
grounding cell or for structure
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