Cathodic Protection Design Calculation

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Venture Way, Grantham, Lincs, NG31 7XS, UK. Tel: +44 (0) 1476 590666 Fax: +44 (0) 1476 570605

CATHODIC PROTECTION

TETCO

GADARIF STRATEGIC DEPOT PROJECT

EXTERNAL TANK BASE SYSTEM

AND

INTERNAL TANK SYSTEMS

DETAILED DESIGN CALCULATIONS

Document Number: 16227-CP-03-DD

0 28/03/2012 Issued for client approval DM SF DM

Rev. Date Description Prepared Checked Approved

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1.2.1 INTERNATIONAL STANDARDS ... 3

2 CORROSION CONTROL METHODS – EXTERNAL CP ... 4

2.1 GENERAL ... 4

2.2 COATING SYSTEMS ... 4

2.3 CATHODIC PROTECTION ... 4

2.3.1 DESIGN LIFE ... 4

2.3.2 COATING BREAKDOWN AND DESIGN CURRENT DENSITIES ... 4

2.3.3 CURRENT DRAIN ... 5

2.3.4 CURRENT DEMAND ... 5

2.3.5 CP PROTECTION POTENTIAL RANGES ... 5

3 DESIGN CONSIDERATIONS – EXTERNAL CP ... 6

3.1 DESIGN PARAMETERS ... 6

3.2 CATHODIC PROTECTION PHILOSOPHY ... 6

4 DESIGN CALCULATIONS – EXTERNAL CP ... 8

4.1 CURRENT REQUIREMENTS ... 8

4.2 MMO ANODE RIBBON CALCULATIONS ... 9

4.3 RIBBON ANODE BED RESISTANCE ... 11

4.4 Ti CONDUCTOR BAR CALCULATIONS ... 13

4.5 EQUIVALENT CIRCUIT RESISTANCE ... 14

4.5.1 GROUNDBED VOLT DROP CALCULATION ... 15

4.5.2 MMO RIBBON VOLT DROP ... 15

4.5.3 CONDUCTOR BAR VOLT DROP ... 16

4.5.1 CABLE VOLT DROP ... 17

4.6 CALCULATION SUMMARY ... 18

5 SUMMARY OF INSTALLATION – EXTERNAL CP ... 19

6 CORROSION CONTROL METHODS – INTERNAL CP... 20

6.1 GENERAL ... 20

6.2 COATING SYSTEMS ... 20

6.3 CATHODIC PROTECTION ... 20

6.3.1 DESIGN LIFE ... 20

6.3.2 COATING BREAKDOWN AND DESIGN CURRENT DENSITIES ... 20

6.3.3 CURRENT DRAIN ... 21

6.3.4 CURRENT DEMAND ... 21

6.3.5 CP PROTECTION POTENTIAL RANGES ... 21

7 DESIGN CONSIDERATIONS – INTERNAL CP ... 22

7.1 DESIGN PARAMETERS ... 22

7.2 CATHODIC PROTECTION PHILOSOPHY ... 22

8 DESIGN CALCULATIONS – INTERNAL CP ... 23

8.1 CALCULATION SUMMARY ... 23

8.2 CURRENT REQUIREMENTS ... 24

8.3 NUMBER OF ANODES BY CURRENT OUTPUT ... 25

8.4 NUMBER OF ANODES BY WEIGHT ... 26

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Document Title Detailed Design Calculations Date 28/03/2012

1 INTRODUCTION

1.1 GENERAL

This document is the design philosophy and detailed calculations that shall be adopted to provide the permanent cathodic protection system installed to protect the new tanks at the Gadarif Strategic Depot project.

The requirements in this document apply to permanent corrosion protection by impressed current cathodic protection to the external surfaces of the tank base and by sacrificial anode current cathodic protection to the internal surfaces of the tank.

1.2 REFERENCE DOCUMENTS

This section lists the codes, standards and project documents / drawings, which are applicable to the detailed design.

1.2.1

INTERNATIONAL STANDARDS

All CP equipment shall be designed, manufactured, tested and supplied in accordance with applicable codes of practice, British Standards, NACE and other applicable standards listed below:

Document Number Title

BS 7361:Part1 Cathodic Protection, Code of Practice for Land and Marine Applications

NACE-RP-0193:1993 External Cathodic Protection of On-Grade Metallic Storage Tank Bottom

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2 CORROSION CONTROL METHODS – EXTERNAL CP

2.1 GENERAL

The corrosion protection system selected shall be based on a high integrity coating system in combination with a cathodic protection system.

2.2 COATING SYSTEMS

The tank base plates shall be coated in line with the project specification.

2.3 CATHODIC PROTECTION

The permanent CP system for the tank will be an impressed current cathodic protection (ICCP) system. The design criteria for the cathodic protection system are outlined in the sections below:

2.3.1

DESIGN LIFE

The design life shall be 25 years for permanent CP system.

2.3.2

COATING BREAKDOWN AND DESIGN CURRENT DENSITIES

The current density below is taken from CPCL experience and international specifications:

Structure Surface Minimum Current Density (mA/m2)

Tank Bottom 20

For the protection of structure with elevated operating temperatures the minimum design current densities given above shall be increased by 25% per 10 °C rise in temperature above 30 °C. All tanks will be at an operating temperature at 48 oC therefore, a design current density of 31.25 mA/m2 is considered suitable.

For the purposes of the design the coating breakdown has been assumed to be a maximum of 50 %:

Structure Surface Coating Breakdown (%)

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2.3.3

CURRENT DRAIN

The tank does not need to be electrically isolated from any foreign structures using isolation joints / flanges and polarisation cells as this is a close anode system. Where possible the tank should be electrically isolated from earthing / grounding systems and any re-bar used in construction.

2.3.4

CURRENT DEMAND

The current demand for each tank is calculated based on the surface area and the applicable final current density for steel as given in section 2.3.2.

2.3.5

CP PROTECTION POTENTIAL RANGES

The effectiveness of the cathodic protection system should be determined by potential shift. The following “instant off” or IR free potentials should apply in the case of all tank bases.

The protection criteria for items in contact with soil are in line with international specifications and in summary:

• Steel in Soil -0.850 to -1.200 Volts with respect to a Cu/CuSO4 reference

electrode.

• 100 mV polarisation shift.

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3 DESIGN CONSIDERATIONS – EXTERNAL CP

3.1 DESIGN PARAMETERS

Structure to be protected : Two Gas Oil Tanks Two Gasoline Tanks One Fire Water Tank

Diameter : 25 m

17.5 m 17.5 m

Coating : 50 % Coating Breakdown

Design Life : 25 Years – Permanent CP

Design Current Density : 31.25 mA/m²

CP Protection Criteria, ECU : -0.85 V (IR Free)

Soil Resistivity : 100 Ohm.m

3.2 CATHODIC PROTECTION PHILOSOPHY

The cathodic protection system for all external surfaces will be based on an impressed current cathodic protection (ICCP) system and the design life for the permanent impressed current system shall be 25 years.

All cable will be XLPE/PVC where running underground. Structure to cable connections will be by bolted connections. All junction boxes will be GRP and have a minimum of IP67 protection and be ATEX certified for use in hazardous areas.

The permanent cathodic protection system for the tank base will be based by a “close” anode grid arrangement with a ribbon type anode. Temporary protection is not required as the permanent system can be energised upon completion of the tank.

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The tank will be electrically continuous from the remainder of any piping and earthing but as this is “close” anode system and there will be a containment layer – hence will not be affected by any current drains.

DC Power supply shall be transformer rectifiers to CPCL specifications. The current required is less than 5 A for the 17.5 m tanks and 10 A for the 25 m tanks and it is recommended that a 5 A or 10 A CP station is installed for each tank.

Due to the high surface soil resistivity anticipated it is recommended that a grid type anode is installed at a minimum depth of 300 mm and maximum depth of 550 mm below the tank base in a mesh arrangement with MMO coated ribbon running in one direction and Titanium conductor bar in the other. The maximum current output of the anode is based upon a soil resistivity of up to 100 Ohm.m which would be as anticipated in this method of installation and backfilling.

Impressed current anodes shall consist of a Mixed Metal Oxide (MMO) / Titanium ribbon anode with Titanium conductor bar to carry the current and connected via cable and a junction box to the transformer rectifier. The MMO ribbon is to be spot welded to the conductor bar at each intersection. Various power feed connectors are then connected to each of the conductor bars and to a cable which exits the tank to the junction box.

The negative cable will be connected to tank via a junction box from the transformer rectifier unit at two points 180o from each other.

Permanent reference electrodes and slotted monitoring tube shall be installed under the tank to allow effective monitoring.

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4 DESIGN CALCULATIONS – EXTERNAL CP

A grid of MMO / Ti ribbon and Ti conductor bar is the selected design for the installation of an anode below the bottom of the aboveground storage tank. The design of such a grid will be done by taking a simplified approach, which is based on the diameter of the tank, depth from the location of the anode bed, the ribbon anode width, and the cathodic protection current requirement for the given environment. The equations derived enabled the determination of the length of anodes and the anode bed resistance.

4.1 CURRENT REQUIREMENTS

The current required for the external tank bases to be protected is shown below. The external surface area of the structure is calculated using the following formula:

2 .r

SA

=

Π

Where for Gas Oil in first column then Gasoline and Firewater in second column:

Π

= Pi 3.142 3.142

r = Radius 12.5 8.75 m

SA = Surface Area (m²) 490.87 240.53 m²

The total current required can then be calculated using the following:

(

c

)

tot

SA

J

F

I

=

.

Where,

SA = Surface Area 490.87 240.53 m² J = Current Density 31.25 31.25 mA/m² Fc = Coating Breakdown Factor 50 50 %

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4.2 MMO ANODE RIBBON CALCULATIONS

The minimal total ribbon length, L, is determined by the current rating for the size and type of anode material selected:

a

tot

I

L

_min

=

Where,

Itot = Total Current 7.67 3.76 A

Ia = Anode Current Output (A/linear m)

1

0.019 0.019 A

Lmin = Minimum Ribbon Length 410.0 201.0 m

The space between anodes should be adjusted to achieve uniform current distribution under the tanks. The MMO ribbon spacing is calculated by:

1 J

I

S

a

mmo

=

Where,

Ia = Anode Current Output (A/linear m) 12 12 mA

J1 = Applied Current Density 10 10 mA/m²

S1 = MMO Ribbon Spacing 1.20 1.20 m

MMO spacing restricted to 1.2 m for correct spacing.

1

The ribbon anode output is considered to be a maximum of 3 A / m2 then for standard ribbon this is equivalent to 0.035 A (35 mA) / linear m for a lifetime of 25 years and therefore any value lower is acceptable.

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The lengths of the MMO ribbon anodes are calculated below using the following formula: Initial Spacing = 0.5 x (MMO ribbon spacing m)

Subsequent Spacing = initial spacing + (MMO ribbon spacing m)

)

.(

.

4 ₁

₁

1 S

d

S

L

=

−

Where,

S1 = 0.5 x MMO Ribbon Spacing 0.6 0.6 m

D = Diameter of Tank 25 17.5 m

L1 = MMO Ribbon Length 7.65 6.39 m

The length of cord in each row is calculated from the above equation.

Row Number

Spacing Length Spacing Length

(m) (m) (m) (m) 1 0.60 7.62 0.58 6.28 2 1.79 12.88 1.75 10.50 3 2.98 16.19 2.92 13.04 4 4.17 18.63 4.08 14.80 5 5.36 20.52 5.25 16.04 6 6.55 21.98 6.42 16.87 7 7.74 23.11 7.58 17.34 8 8.93 23.96 8.75 17.50 9 10.12 24.54 9.92 17.34 10 11.31 24.89 11.08 16.87 11 12.50 25.00 12.25 16.04 12 13.69 24.89 13.42 14.80 13 14.88 24.54 14.58 13.04 14 16.07 23.96 15.75 10.50 15 17.26 23.11 16.92 6.28 16 18.45 21.98 17 19.64 20.52 18 20.83 18.63 19 22.02 16.19 20 23.21 12.88 21 24.40 7.62 Total 413.65 207.26

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4.3 RIBBON ANODE BED RESISTANCE

The total resistance between the tank bottom and the bed of grid ribbon anodes without taking into account the mutual interference between the anodes is:

)

2

4 (ln

2

−

=

hw

L

R

π

ρ

Where

ρ

= Soil Resistivity 100 100 Ohm.m

L = Length of Anode 7.62 6.28 m h = Distance between Anode and Tank 0.5 0.5 m w = Ribbon Anode Equivalent Diameter 0.005 0.005 m

R = Resistance 20.78 24.23 Ohm

The resistance of each cord is calculated from the above equation and the total resistance calculated by summing the resistance of each cord. Thus the total anode resistance is calculated:

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Row Number Spacing (m) Resistance (ohm) Spacing (m) Resistance (ohm) 1 0.60 20.78 0.58 24.23 2 1.79 13.59 1.75 16.05 3 2.98 11.26 2.92 13.45 4 4.17 10.03 4.08 12.13 5 5.36 9.26 5.25 11.35 6 6.55 8.74 6.42 10.89 7 7.74 8.38 7.58 10.64 8 8.93 8.13 8.75 10.56 9 10.12 7.97 9.92 10.64 10 11.31 7.88 11.08 10.89 11 12.50 7.85 12.25 11.35 12 13.69 7.88 13.42 12.13 13 14.88 7.97 14.58 13.45 14 16.07 8.13 15.75 16.05 15 17.26 8.38 16.92 24.23 16 18.45 8.74 17 19.64 9.26 18 20.83 10.03 19 22.02 11.26 20 23.21 13.59 21 24.40 20.78 Total (Ohm) 0.458 0.860

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4.4 Ti CONDUCTOR BAR CALCULATIONS

The total length of titanium conductor bar is based upon an assumption that conductor bar separation should not exceed 4.0 m.

The lengths of the individual Ti Conductor bar chords are calculated as below using the following formula:

Initial Spacing = 0.5 x (Conductor bar spacing m)

Subsequent Spacing = initial spacing + (Conductor bar spacing m)

)

.(

.

4 ₂

₂

2 S

d

S

L

=

−

Where,

S2 = 0.5 x Conductor Bar Spacing 1.75 1.75 m

d = Diameter of Tank 25 17.5 m

L2 = Initial Conductor Bar Length 12.88 10.5 m

The length of cord in each row is calculated from the above equation.

Row Number Spacing (m) Length (m) Spacing (m) Length (m)

1 1.79 12.88 1.75 10.5 2 5.36 20.52 5.25 16.04 3 8.93 23.96 8.75 17.5 4 12.50 25 12.25 16.04 5 16.07 23.96 15.75 10.5 6 19.64 20.52 7 23.21 12.88 Total (m) 86.6 70.6

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4.5 EQUIVALENT CIRCUIT RESISTANCE

This section proves through calculation that there is sufficient driving voltage in the CP circuit to enable the system to operate at its design current after the groundbed voltage, cable volt drops and other losses have been taken into consideration.

The total system volt drop is the sum of the following components: • Groundbed volt drop

• MMO Ribbon volt drop • Conductor bar volt drop • Cable volt drop

• Back EMF and other losses

For the CP to be effective this total must be equal to or less than the voltage of the Transformer Rectifier.

This can be expressed in the following formula:

TR losses vd vd vd vd

V

+



≤

     ) 4 ( ) 3 ( ) 2 ( ) 1 ( Where,

V(vd1) = Groundbed Volt Drop 4.6 4.3 V

V(vd2) = MMO Ribbon Volt Drop 0.005 0.005 V

V(vd3) = Conductor Bar Volt Drop 0.192 0.092 V

V(vd4) = Cable Volt Drop 5.45 2.73 V

Vlosses = Back EMF and Structure Losses 4.0 4.0 V

VTotal = Total Voltage 14.23 11.13 V

VTR = Transformer Rectifier Voltage 24 24 V

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4.5.1

GROUNDBED VOLT DROP CALCULATION

The groundbed volt drop is a combination of the following components: • Groundbed to earth resistance

• Groundbed current

This can be expressed in the following formula:

tot GB

vd

R

I

V

( 1)

=

.

Where,

RGB = Groundbed Resistance to Earth 0.458 0.860 Ohm

Itot = Total Current 10 5 A

V(vd1) = Groundbed Volt Drop 4.6 4.3 V

4.5.2

MMO RIBBON VOLT DROP

The MMO ribbon volt drop is calculated using the following formula:

L

R

I

V

(vd2)

=

.

Where,

I = MMO Ribbon Load / m 0.021 0.02 A R = Resistance of MMO Ribbon2 0.138 0.138 Ohm/m L = Length of MMO Ribbon 1.79 1.75 m V(vd2) = MMO Ribbon Volt Drop 0.005 0.005 V

Where the load is the current required for half the distance between two conductor bars.

)

.

5 .

0 /(

/

L

₁

S

₂

I

=

_tot Where,

L1 = Total Length of MMO Ribbon 413.65 207.26 m

S2 = Spacing of Conductor Bar 3.5 3.5 m

I = MMO Ribbon Load / m 0.014 0.014 A

2

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4.5.3

CONDUCTOR BAR VOLT DROP

The Conductor Bar volt drop is calculated using the following formula:

L

R

I

V

(vd3)

=

.

Where, I = Load 0.223 0.152 A

R = Resistance of Conductor Bar3 0.069 0.069 Ohm/m L = Length of Conductor Bar 12.50 9.75 m V(vd2) = MMO Ribbon Volt Drop 0.192 0.092 V

Where the load is the current required for half the length of the longest conductor bar (worst case).

)

.

5 .

0 /(

/

L

₂

L

₃

I

=

_tot Where,

L2 = Total Length of conductor Bar 139.7 70.6 m

L3 = Length of Longest conductor Bar 25 17.5 m

I = Conductor Bar Load / m 0.006 0.008 A

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4.5.1

CABLE VOLT DROP

Cable volt drops are calculated using the following formula:

4 3 2 1 ) 4 (

V

_vd

=

+

Where,

V1 = Main Positive Cable Volt Drop 1.05 V

V2 = Main Negative Cable Volt Drop 1.05 V

V3 = Powerfeed Cable Volt Drop 2.3 V

V4 = Negative Cable Volt Drop 1.05 V

V(vd4) = Cable Volt Drop 5.45 V

Individual cable volt drops are calculated using the following formula:

(

_cab

)

_cab _cab

vd

R

L

I

V

=

/

1000

.

Where,

RCab = Resistance of Cable

4

0.524 0.524 Ohm/km Lcab = Length of Cable 200 200 m

Icab = Current in Cable 10 5 A

Vvd = Cable Volt Drop 1.1 0.524 V

The volt drop of each cable is calculated from the above equation. RCab Lcab Icab Vvd

V1 0.524 100 10 1.05 V2 0.524 100 10 1.05 V3 1.15 100 10 2.3 V4 0.524 100 10 1.05 4

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4.6 CALCULATION SUMMARY

Parameter 25 m 17.5 m Unit

SA = Surface Area (m²) 490.87 240.53 m²

Lmin = Minimum Ribbon Length 414 208 m

L1 = Actual Ribbon Length 414 208 m

S1 = MMO Ribbon Spacing 1.2 1.2 m

L2 = Conductor Bar Length 139.7 70.6 m

S2 = Conductor Bar Spacing 3.6 3.5 m

VTotal = Total Voltage Drop 15 12 V

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5 SUMMARY OF INSTALLATION – EXTERNAL CP

In summary the CP system and groundbed will be sized as follows:

Item 25 m 17.5 m

Permanent CP Station 2 3

Transformer Rectifier Voltage 24 V 24 V

Transformer Rectifier Current 10 A 5 A

MMO Ribbon Length 414 m 208 m

Powerfeed Number 7 5

Powerfeed Cable Size 16 mm2 16 mm2

Powerfeed Cable Length 150 m 100 m

Feeder Cable Size 35 mm2 35 mm2

Total Feeder Length (including 20 %

spare for routing) 200 200

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6 CORROSION CONTROL METHODS – INTERNAL CP

6.1 GENERAL

The corrosion protection system selected shall be based on a high integrity coating system in combination with a cathodic protection system.

6.2 COATING SYSTEMS

The tank shall be coated internally in line with project specifications.

6.3 CATHODIC PROTECTION

The permanent CP system for the tanks will be a sacrificial anode cathodic protection (SACP) system. The design criteria for the cathodic protection system are outlined in the sections below:

6.3.1

DESIGN LIFE

The design life shall be 25 years for sacrificial anodes.

6.3.2

COATING BREAKDOWN AND DESIGN CURRENT DENSITIES

The current density below is taken from CPCL experience and international specifications:

Structure Surface Minimum Current Density (mA/m2)

Tank Internals 100

For the protection of structure with elevated operating temperatures the minimum design current densities given above shall be increased by 25% per 10 °C rise in temperature above 30 °C.

All tanks will be at an operating temperature at 48 oC therefore, a design current density of 156.25 mA/m2 is considered suitable.

The current density below is taken from Project Document:

Structure Surface Coating Breakdown (%)

Gasoline / Gas Oil 5

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6.3.3

CURRENT DRAIN

The tanks do not need to be electrically isolated from any foreign structures using isolation joints / flanges and polarisation cells as this is an internal system. Where possible the tank should be electrically isolated from earthing / grounding systems and any re-bar used in construction.

6.3.4

CURRENT DEMAND

The current demand for each tank is calculated based on the surface area and the applicable final current density for steel as given in section 6.3.2.

6.3.5

CP PROTECTION POTENTIAL RANGES

The effectiveness of the cathodic protection systems should be determined by potential shift. The following “instant off” or IR free potentials should apply in the case of all tank bases.

The protection criteria for items in contact with soil are in line with international specifications and in summary:

• Steel in Water -0.800 to -1.050 Volts with respect to a Ag/AgCl reference electrode.

• 100 mV polarisation shift.

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7 DESIGN CONSIDERATIONS – INTERNAL CP

7.1 DESIGN PARAMETERS

Structure to be protected and Diameter / Height considered for CP

: Gas Oil: 25 m / 1 m Gasoline: 17.5 m / 1 m Firewater: 17.5 m / 12.5 m

Coating : 10 % Coating Breakdown Factor 5 % for Firewater only

Design Life : 25 Years – Permanent CP

Current Density : 156.25 mA/m² at 30 oC

CP Protection Criteria, Eag : -0.80 V (IR Free)

Water Resistivity : 200 Ohm.cm or less

7.2 CATHODIC PROTECTION PHILOSOPHY

The cathodic protection system for the internal surfaces will be based on a sacrificial anode cathodic protection (SACP) system and the minimum design life for the permanent system shall be 25 years.

The permanent cathodic protection system for the internal of the tanks will be based on aluminium sacrificial anodes which shall be bolted to welded support brackets installed at various locations around the base of each tank.

Temporary protection is not required as the permanent system can be energised upon completion of the tank.

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8 DESIGN CALCULATIONS – INTERNAL CP

The design of the system is based on the diameter and height of the tank bottom, anode size and weight, and the cathodic protection current requirement for the given environment. The equations derived enabled the determination of the number of anodes and the anode resistances.

8.1 CALCULATION SUMMARY

The table below is an executive summary of the calculations in this section:

Tank Gas Oil Gasoline Firewater Unit

SA = Surface Area (m²) 570 296 928 m²

If = Current 8.90 4.62 7.25 A

Ia = Individual Anode Current 0.25 0.25 0.25 A

An1 = Anode Number for Current 36 19 29 No

Ma = Individual Anode Mass 25.2 25.2 25.2 kg

An2 = Anode Number for Mass 37 19 30 No

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8.2 CURRENT REQUIREMENTS

The current required for the internal tank surface to be protected is shown below:

The internal surface area of the structure (GAS OIL TANK ONLY) is calculated using the following formula:

h

d

r

SA

=

π

.

2 +

π

.

Where for tank,

π

= Pi 3.142

R = Radius 12.5 m

D = Diameter 25.0 m

H = Height 1.0 m

SA = Surface Area (m²) 569.41 m²

The current required for GAS OIL TANK can then be calculated using the following:

(

c

)

f

SA

J

F

I

=

.

Where for tank,

SA Surface Area 596.41 m²

J Current Density 156.25 mA

Fc Coating Breakdown Factor 10 %

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8.3 NUMBER OF ANODES BY CURRENT OUTPUT

The anode resistance is calculated using the following equation:

Where,

ρ

= Water Resistivity 2 Ohm.m

lf = Final Anode Length (90%) 112.5 cm

rf = Final Effective Anode Radius (50%) 49.2 cm

Rf = Final Anode Resistance to Earth 1.19 Ohm

Then, the maximum current output per anodes is calculated using Ohm’s Law:

f a

R

V

I =

Where,

V = Anode Driving Potential5 0.30 V Rf = Final Anode Resistance to Earth 1.19 Ohm

Ia = Individual Anode Current 0.25 A

The minimum number of anodes is calculated from the total current required divided by the maximum current output for each individual anode.

a f n

I

A =

1

Where for tank,

If = Final Current 8.90 A

Ia = Individual Anode Current 0.25 A

An1 = Anode Number (Current) 36 No.

5

Anode Driving Potential is the potentials difference between the protected potential and the open circuit anode potential

)

1

4 (ln

2 −

=

f f f f

r

l

R

π

ρ

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8.4 NUMBER OF ANODES BY WEIGHT

The total anode weight is calculated using the following equation:

UF

Z

t

I

M

m t

.

8760

.

=

Where for the tank,

Im = Current 8.90 A

t = Design Life 25 Years

Z = Alloy Anode Amp/Hour Capacity 2500 A.hours/kg

UF = Utilisation Factor 85 %

Mt = Total Anode Mass 917.6 kg

Finally, the minimum number of anodes is calculated from the total mass required divided by the mass of each individual anode.

a t n

M

A =

2

Where for tank,

Mt = Total Anode Mass 917.6 kg

Ma = Individual Anode Mass 25.2 kg

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Document Title Detailed Design Calculations Date 28/03/2012 9 SUMMARY OF INSTALLATION – INTERNAL CP

From the design calculations above the number of anodes required for the Permanent CP will be the greater of An1 and An2:

Gas Oil Tanks:

An1 = Anode Number (Current) 36 No.

An2 = Anode Number (Mass) 37 No.

An2 = Anode Number (Distribution) 40 No.

At = Total Anode Number 40 No.

Gasoline Tanks:

At = Total Anode Number 24 No.

Firewater Tank:

Cathodic Protection Design Calculation