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مدیر وبلاگ : pc7a
TABLE OF CONTENT
2. CODES AND STANDARDS
3. AMBIENT CONDITION
5. SYSTEM REQUIREMENT
6. DESIGN BASIS
7. DESIGN LIFE
8. SOIL INVESTIGATION
9. DESIGN CONDITION
This document covers the basic requirements and calculations for the design,
fabrication, installation test and inspection of a complete impressed current cathodic
protection system, to be constructed for protection of external surfaces of the buried
steel pipes in SOUTH PARS GAS FIELD DEVELOPMENT project.
2. CODES AND STANDARDS
The systems specified shall be manufactured, installed, tested and commissioned in
strict accordance with all applicable requirements of the following standards or
2.1 Iranian Petroleum Standards (IPS)
2.2 National Association of Corrosion Engineers (NACE)
2.3 American Society for Testing and Materials (ASTM)
2.4 International Electro technical Commission (IEC)
2.5 National Electric Manufacturers Association (NEMA)
2.6 American National Standards Institute, Inc. (ANSI)
2.7 National Electric Code (NEC)
3. AMBIENT CONDITION
All the materials and equipments shall be designed for the following conditions:
Maximum ambient temperature 480 C
Minimum ambient temperature 50 C
Level from sea app. 20 meters
Climate tropical and humidity
Technical definition have been used on this project can be summarized and
explain as follows.
Anode: The electrode of an electrochemical cell at which oxidation (corrosion)
occurs. Antonym: cathode.
Backfill: Material placed in a hole to fill the space around anodes, vent pipe, and
buried components of a cathodic protection system. Anodes can be prepackaged
with backfill material for ease of installation.
Cathode: The electrode of an electrochemical cell at which a reduction reaction
occurs. Antonym: anode.
Cathodic protection: A technique to reduce corrosion of a metal surface by
making the entire surface the cathode of an electrochemical cell.
Continuity bond: A metallic connection that provides electrical continuity.
Corrosion: The deterioration of a material, usually a metal, that results from a
reaction with its environment.
Current density: The current per unit area flowing to or from a metallic surface.
Deep ground bed: One or more anodes installed vertically at a nominal depth of
15 m (50 ft) or more below the earth’s surface in a single drilled hole for the
purpose of supplying cathodic protection.
Differential aeration cell: An electrochemical cell the electromotive force of
which is due to a difference in air (oxygen) concentration at one electrode as
compared with that at another electrode of the same material
Electrical isolation: The condition of being electrically separated from other
metallic structures or the environment.
Electrochemical cell: An electrochemical system consisting of an anode and a
cathode immersed in an electrolyte so as to create an electrical circuit. The anode
and cathode may be separate metals or dissimilar areas on the same metal. The
cell includes the external circuit which permits the flow of electrons from the
anode toward the cathode.
Electrode potential: The potential of an electrode as measured against a
reference electrode. (The electrode potential does not include any resistance
losses in potential in either the electrolyte or the external circuit. It represents the
reversible work required to move a unit charge from the electrode surface through
the electrolyte to the reference electrode.)
Electrolyte: A chemical substance containing ions that migrate in an electric
field. For the purposes of this recommended practice, electrolyte refers to the soil
or liquid adjacent to and in contact with the bottom of an aboveground petroleum
storage tank, including the moisture and other chemicals contained therein.
Foreign structure: Any metallic structure that is not intended as a part of a
system under cathodic protection.
Ground bed: Consists of one or more anodes installed below the earth’s surface
for the purpose of supplying cathodic protection.
Impressed current: An electric current supplied by a device employing a power
source that is external to the electrode system, (An example is direct current for
Insulating coating system: All components of the protective coating, the sum of
which provides effective electrical insulation of the coated structure.
IR drop: The voltage generated across a resistance by an electrical current in
accordance with Ohm’s Law: E = I x R. For the purpose of this recommended
practice, the most significant IR drop is the portion of a structure-to-soil potential
caused by a high resistance electrolyte between the structure and the reference
electrode or by current flow from the anodes to the tank bottom.
Isolation: Electrical isolation.
Liner: A system or device, such as a membrane, installed beneath a storage
tank, in or on the tank dike, to contain any accidentally escaped product.
Oxidation: The loss of electrons by a constituent of a chemical reaction.
Polarization: The change from the open-circuit potential of an electrode resulting
from the passage of current. (In this recommended practice, it is considered to be
the change of potential of a metal surface resulting from the passage of current
directly to or from an electrode.)
Rectifier: A device for converting alternating current to direct current. Usually
includes a step-down AC transformer, a silicon or selenium stack (rectifying
elements), meters, and other accessories when used for cathodic protection
Reduction: The gain of electrons by a constituent of a chemical reaction.
Reference electrode: An electrode whose open-circuit potential is constant
under similar conditions of measurement.
Sacrificial anode: Another name commonly used for a galvanic anode.
Secondary containment: A device or system used to control the accidental
escape of a stored product so it may be properly recovered or removed from the
environment. For the purpose of this recommended practice, secondary containment
refers to an impermeable membrane.
Shallow anode ground bed: A group of cathodic protection anodes installed
individually, spaced uniformly, and typically buried less than 20 feet below grade.
Stationary: Something that is permanently installed on the ground or on a
Stray current: Current flowing through paths other than the intended circuit.
Stray current corrosion: Corrosion resulting from direct current flow through
paths other than the intended circuit.
Structure-to-electrolyte voltage (also structure-to-soil potential or pipe-tosoil
potential): The voltage difference between a metallic structure and the
electrolyte which is measured with a reference electrode in contact with the electrolyte.
Structure-to-structure voltage (also structure-to structure potential): The
difference in voltage between metallic structures in a common electrolyte.
Test station: A small enclosed box-like housing and the usual termination point
of one or more test leads.
Voltage: An electromotive force, or a difference in electrode potentials expressed
in volts, also known as a potential.
5. SYSTEM REQUIRMENT
5.1 Complete soil investigation carried out as per item 8 of this document in order to
select and design the suitable cathodic protection system.
5.2 All underground coated steel pipes, including to two pipes 32" (length 7300 &
6. DESIGN BASIS
6.1 The cathodic protection is considered an impressed current system with manual
6.2 The cathodic protection system is designed so that the potential at any point of
the protected pipes are -0.85 to -1.3 volt with reference to copper / copper sulfate
6.3 All points not protected with impressed current system and those which have
been over protected, shall be improved by means of sacrificial anodes after
commissioning of system. Zinc anodes are suitable for increase of protection
potential and zinc earthing cell is used for reduction of protection potential of over
6.4 Test point is provided to measure the potential of protected pipes at suitable
6.5 The power to transformer shall be 400 volts, three phases, 50 Hz and will be
supplied from nearby substation.
6.6 The pipes shall be electrically isolated from other foreign structures/pipes and
earthing systems by suitable insulating material
7. DESIGN LIFE
The design life of impressed current system is considered 25 years.
8. SOIL INVESTIGATION
8.1 Soil Resistivity
There are a number of methods for measuring soil resistivity, the most common is
the Wenner 4-pin method.
The equipment required for field resistivity measurements consists of a hand-driven
earth-tester (vibroground equipment), four metal electrodes, and the necessary
wiring to make the connections. Terminals shall be of good quality to ensure that
low-resistance contact is made at the electrodes and the meter.
The Wenner four-electrode method requires that four metal electrodes be placed
with equal separation in a straight line in the surface of the soil to a depth not
exceeding 5% of the minimum separation of the electrodes. Watering in moderation
around the electrodes is permissible to ensure adequate contact with the soil. The
electrode separation should be selected with consideration of the soil strata of
interest. The resulting resistivity measurement represents the average resistivity of a
hemisphere of soil of a radius equal to the electrode separation.
A voltage is impressed between the outer electrodes, causing current to flow, and
the voltage drop between the inner electrodes is measured. Alternatively, the
resistance can be measured directly. The resistivity is then:
ρ (Ωcm) = 2πaR , (a in cm)
a = electrode separation, (cm)
R = resistance, ( Ω )
Using dimensional analysis, the correct unit for resistivity is ohm-centimeter.
The depth of the electrodes should not exceed the value a/20.
-The anodes made of MMO are used in cathodic protection system with 25×1000
mm in size, these anodes with an insulated conductor either singly or in groups
connect to the positive terminal of a transformer rectifier.
9.2 Transformer Rectifiers
- Transformer Rectifiers is considered oil immersed, complete with double
wound transformer and selenium or silicone type rectifier.
- The transformer rectifier is to be equipped with oil level gauge, voltmeter and
ammeter output DC ranged to 120% of maximum nominal values, lifting lugs ,
sun shade, oil drain cock with blocking plug, schematic circuit diagram plate,
rating plate, and name plate.
- Suitable means of disconnection such as molded case circuit breaker is
provided with the transformer rectifier unit. It will be possible to switch off the
unit for inspection and/or maintenance. Provisions will be included to lock the
disconnection means in closed or open position.
- The transformer rectifier control box will, easily accessible, provided with a
hinged lockable door and a reinforced glass window for viewing the meters.
- The cables feeding transformer will be four core 600/ 1000 volts copper
conductor, with PVC or XLPE insulation and black PVC over sheath.
- The cables shall meet the requirements of IPS-M-TP-750 Part 7.
- Cables, connecting rectifier units to anodes or to protected objects will be
single core copper conductor PVC or XLPE insulated with PVC over sheath.
- The cross section of the cables is considered:
Negative and positive output of transformer/rectifier: 35 sq. mm
Test points: 16 sq. mm
Cable bonding: 16 sq. mm
9.4 Test Point Station
Test point stations will be provided for monitoring the performance of the cathodic
Protection system. They will be located in a safe area with hinged shutter and
degree of IP 54.
9.5 Thermit Weld Powder
- Negative drain or bonding cable connections to steel surfaces is welded by means
of "Thermit weld" method.
- Thermit weld powder shall be in accordance with IPS-M-TP-750 Part 14
- All materials and equipment will be factory tested according to IEC 60146 and
witnessed by client representative.
- After complete installation of cathodic protection system, necessary tests shall be
carried out in order to make sure that the pipe line, tanks are catholically protected.
-The contractor shall supply the equipment and procedures required for all the
necessary tests and measurements.
Client and/or appointed inspectors reserve the right for inspection at any stage of
manufacturing, testing or preparation for shipment.
Preparation for shipment will be in accordance with Manufacturer's standards,
unless otherwise noted on the Request for Quotation and/or Purchase Order. The
Manufacturer will be solely responsible for the adequacy of the Preparation for
9.9 SPARE PARTS
Together with the supply of all equipment, a complete set of spare parts for
commissioning will be supplied for every and each equipment, and also
recommended spare parts list for two years of operation shall be included.
All the spare parts shall comply with the same standards and specifications of the
The complete cathodic protection system will be guaranteed, and the supplier shall
replace any damaged equipment resulting from faulty design, defective materials,
and/or poor workmanship.
The supplier will also replace any equipment failed under the following conditions:
1) Failure under start-up and commissioning tests.
2) Failure under normal usage for a minimum of 12 months after being placed in the
specified service, or 18 months after shipment whichever occur earlier.
10. DESIGN CONDITIONS
10.1 Current Density
Current density to be applied in this document is summarized on table 1.
10.2 Method of cathodic protection
The following factors influence the selection of a cathodic protection system:
a. Size and length of pipes and/or to be protected.
b. Current required.
c. Soil conditions such as resistivity.
d. Possibility of cathodic protection interference on adjacent structures.
e. Future developments and extensions to the storage system.
f. Cost of cathodic protection equipment, installation, operation, and
Within the consideration of high protective current requirements and soil resistivity
value in inter phases pipeline area, cathodic protection system to be provided will
be impressed current type and the ground beds arrangement will be design to
distribute protection current uniformly to the whole of the underside of U/G piping.
Several options are available for the protection, including;
a. Shallow anodes installed periphery of the pipe.
b. Use of a deep ground bed.
10.3 Anode Ground Bed
Where the resistivity is high in the upper layers of soil and decreases with
increasing depth and installation of cathodic protection system is going to protect
as wide distributed area such as inter phases pipeline deep ground beds could be
considered as the best solution.
This type of installation is recommended for densely populated areas and for local
cathodic protection on account of the small space needed and the smaller voltage,
which avoids interference with foreign structures.
Deep anodes consist of the string single anodes which are set in boreholes 30 to
60 m deep (which shall be subjected of calculation to meet required anode-soil
resistance and current output) with a diameter of 0.3 m.
10.4 Electrical Isolation & Bonding
Each buried pipe to be protected shall be electrically isolated from other metallic
structures that are not intended as a part of the cathodic protection system by
means of insolating joint or insulating kit.
The design of new plants shall provide an integrated cathodic protection system
for all buried metallic pipelines and where required for electrical grounding grids if
they are electrically connected to the buried metallic pipelines or other protected
11.1 Protective Current Requirements Calculations
Current requirements, I, shall be calculated from current density, i, and surface area
to be protected, s;
I = i . s
Initial coating efficiency is considered 95% and final coating efficiency after 25 years
is considered 87% (with 0.32% deterioration of coat in year).Required current for
protect of pipe is equal to required current to protect coated surface added to that
of bare surface.
c I = Current required to protect of coated surface in final
b I = Current required to protect of bare surface in final
f I = Required current to protect of pipe in final ( c I + b I )
p I = Protective current required with consideration of 1.5 safety factor for confronting
of stray current resulting from other cathodic protection systems
c I = 0.87 × S × 0.25 = 7.6 A
b I = 0.13 × S × 20 = 92 A
f I = c I + b I = 7.6 + 92 = 99.6 = 100 A , P I = 1.5 x I = 150 A
11.2 Number of Anodes Calculation
The number of anodes necessary to reach an efficient protection can be calculated
as following formulas:
No. of Anodes Required Based on Current Discharge
from anode manufacturer data
MD = Maximum discharge per anode (7.8 amps)
Current = Current required ( P I amps)
For this project is used of two CP station.
Each Station include a transformer rectifier & one deep well ground bed & other
Number of Anodes for each CP station = ( P I ÷ 2) ÷ 7.8 = 10 Nos.
11.3 Calculations for Power Supply
The transformer – rectifier shall be full wave bridge assemblies of silicon stacks with
manual output regulation equipment, oil immersed, weather proof & pad mounted
and protected by a sunshade cover with lightning protection on both input and
11.3.1. DC output voltage of transformer – rectifier
DC output voltage of transformer rectifier is determined by the summation of
voltage drops which is calculated with circuit resistance as follows:
t G a c d V = V +V +V +V
in which ;
t V = DC output voltage of transformer rectifier
G V = Voltage drops of ground bed
a V = Voltage drops of anodes (negligible)
Vc = Voltage drops of cables
d V = Decomposition voltage of anodes (2 v)
For calculation of G V and Vc we need to protective current ( P I ) and ground bed
resistance ( G R ), cables resistance ( c R ).
As Ohm law, we have:
c c V = I.R , G G V = I.R , = ( + ) + 2 t P c G V I R R
R = (0.005 L)[ln(8L d) −1] G ρ π
ρ = Coke breeze resistivity = 100 Ωcm
L = length of anode (m) = 30 m
d = diameter of anode (m) = 0.3 m
RG = 0.03 Ω
R L R c = ×
L = length of cables (m)
R = resistance of standard copper conductors per meter from table B.1 and B.2
= 1×35 R 0.56 Ω/Km in 400 c
In this design document, length of positive and negative cables totally in a cathodic
protection station are considered 20 m, therefore RC = 0.0112 Ω
RT = total resistance of chathodic protection circuit.
RT= 0.03 + 0.0112 = 0.0412 Ω
Vout = Iout × RT + 2
Vout = 75 × 0.0412 + 2 = 5 v
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