Cathodic Protection Design Calculation

Cathodic Protection Co. Limited

Project Ref: 12/P16227

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

E-mail: [email protected] Website: www.cathodic.co.uk Registered Office: Minalloy House, Regent Street, Sheffield S1 3NJ, UK Vat No. 116 8408 71

Registered in England No. 478098

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

No. ORIGINATOR CLIENT

TETCO – Gadarif Strategic Depot Project

Tank External Base and Internal Cathodic Protection

Detailed Design Calculations

Document Title Detailed Design Calculations Date 28/03/2012

Document No 16227-CP-03-DD Revision 0

CPCL Project No 12/P16227 of 27

TABLE OF CONTENTS

1 INTRODUCTION........................................................................................................................ 3

1.1 GENERAL .................................................................................................................................. 3

1.2 REFERENCE DOCUMENTS ..................................................................................................... 3

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

9 SUMMARY OF INSTALLATION – INTERNAL CP ................................................................. 27







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

EDS-508 Revision A Cathodic Protection Specifications







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 (%)

Tank Bottom 50







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.

• All above potentials are IR free or “OFF” potentials.







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.







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.







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.rSA 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:

( )ctot FJSAI ..=

Where,

SA = Surface Area 490.87 240.53 m²

J = Current Density 31.25 31.25 mA/m²

Fc = Coating Breakdown Factor 50 50 %

Itot = Total Current 7.67 3.76 A







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

IL =

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:

1J

IS 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 / m

2 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.







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)

).(.4111

SdSL −=

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







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:

)24

(ln2

2

−=hw

L

LR

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:







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







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)

).(.4222

SdSL −=

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







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:

TRlossesvdvdvdvd VVVVVV ≤++++

)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

See below for calculations of each component







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:

totGBvd IRV .)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:

LRIV vd ..)2( =

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


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

).5.0/(/21

SLII 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 Data from MMO anode manufacture







4.5.3 CONDUCTOR BAR VOLT DROP

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

LRIV vd ..)3( =

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


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

case).

).5.0/(/32

LLII 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

3 Data from conductor bar manufacture







4.5.1 CABLE VOLT DROP

Cable volt drops are calculated using the following formula:

4321)4( VVVVV 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:

( )cabcabcabvd ILRV ..1000/=

Where,

RCab = Resistance of Cable4 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 From cable manufacturers







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

VTR = Transformer Rectifier Voltage 24 24 V







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 mm

2

Powerfeed Cable Length 150 m 100 m

Feeder Cable Size 35 mm2 35 mm

2

Total Feeder Length (including 20 % spare for routing)

200 200

Number of Reference Electrodes 5 5







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

Firewater 10







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.

• All above potentials are IR free or “OFF” potentials.







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.







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

At = Anode Number Required 40 24 34 No







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:

hdrSA ...2 ππ +=

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:

( )cf FJSAI ..=

Where for tank,

SA Surface Area 596.41 m²

J Current Density 156.25 mA

Fc Coating Breakdown Factor 10 %

If = Final Current 8.90 A







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

aR

VI =

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

fn

I

IA =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

)14

(ln2

−=f

f

f

fr

l

lR

π

ρ







8.4 NUMBER OF ANODES BY WEIGHT

The total anode weight is calculated using the following equation:

UFZ

tIM 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

tn

M

MA =2

Where for tank,

Mt = Total Anode Mass 917.6 kg

Ma = Individual Anode Mass 25.2 kg

An2 = Anode Number 37 No.







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:


An2 = Anode Number (Mass) 37 No.

An2 = Anode Number (Distribution) 40 No.

At = Total Anode Number 40 No.

Gasoline Tanks:





Firewater Tank:





Cathodic Protection Design Calculation

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