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Cathodic Protection Co. Limited Project Ref: 12/P16227
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
Rev.
Date
Description
Prepared
No.
SF
DM
Checked Approved
ORIGINATOR
CLIENT
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
TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
TABLE OF CONTENTS 1 1.1 1.2
1.2.1 2 2.1 2.2 2.3
2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 3 3.1 3.2 4 4.1 4.2 4.3 4.4 4.5
4.5.1 4.5.2 4.5.3 4.5.1 4.6 5 6 6.1 6.2 6.3
6.3.1 6.3.2 6.3.3 6.3.4 6.3.5 7 7.1 7.2 8 8.1 8.2 8.3 8.4 9
Document Title Document No CPCL Project No
INTRODUCTION........................................................................................................................ 3 GENERAL .................................................................................................................................. 3 REFERENCE DOCUMENTS..................................................................................................... 3
INTERNATIONAL STANDARDS ........................................................................ 3 CORROSION CONTROL METHODS – EXTERNAL CP ......................................................... 4 GENERAL .................................................................................................................................. 4 COATING SYSTEMS ................................................................................................................ 4 CATHODIC PROTECTION ....................................................................................................... 4
DESIGN LIFE ..................................................................................................... 4 COATING BREAKDOWN AND DESIGN CURRENT DENSITIES ...................... 4 CURRENT DRAIN .............................................................................................. 5 CURRENT DEMAND ......................................................................................... 5 CP PROTECTION POTENTIAL RANGES ......................................................... 5 DESIGN CONSIDERATIONS – EXTERNAL CP ...................................................................... 6 DESIGN PARAMETERS ........................................................................................................... 6 CATHODIC PROTECTION PHILOSOPHY ............................................................................... 6 DESIGN CALCULATIONS – EXTERNAL CP .......................................................................... 8 CURRENT REQUIREMENTS ................................................................................................... 8 MMO ANODE RIBBON CALCULATIONS ............................................................................... 9 RIBBON ANODE BED RESISTANCE .................................................................................... 11 Ti CONDUCTOR BAR CALCULATIONS ............................................................................... 13 EQUIVALENT CIRCUIT RESISTANCE .................................................................................. 14
GROUNDBED VOLT DROP CALCULATION ................................................... 15 MMO RIBBON VOLT DROP ............................................................................ 15 CONDUCTOR BAR VOLT DROP .................................................................... 16 CABLE VOLT DROP ........................................................................................ 17 CALCULATION SUMMARY ................................................................................................... 18 SUMMARY OF INSTALLATION – EXTERNAL CP ............................................................... 19 CORROSION CONTROL METHODS – INTERNAL CP......................................................... 20 GENERAL ................................................................................................................................ 20 COATING SYSTEMS .............................................................................................................. 20 CATHODIC PROTECTION ..................................................................................................... 20
DESIGN LIFE ................................................................................................... 20 COATING BREAKDOWN AND DESIGN CURRENT DENSITIES .................... 20 CURRENT DRAIN ............................................................................................ 21 CURRENT DEMAND ....................................................................................... 21 CP PROTECTION POTENTIAL RANGES ....................................................... 21 DESIGN CONSIDERATIONS – INTERNAL CP ..................................................................... 22 DESIGN PARAMETERS ......................................................................................................... 22 CATHODIC PROTECTION PHILOSOPHY ............................................................................. 22 DESIGN CALCULATIONS – INTERNAL CP ......................................................................... 23 CALCULATION SUMMARY ................................................................................................... 23 CURRENT REQUIREMENTS ................................................................................................. 24 NUMBER OF ANODES BY CURRENT OUTPUT .................................................................. 25 NUMBER OF ANODES BY WEIGHT ..................................................................................... 26 SUMMARY OF INSTALLATION – INTERNAL CP................................................................. 27
Detailed Design Calculations 16227-CP-03-DD 12/P16227
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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 Title Document No CPCL Project No
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
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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 Tank Bottom
2
Minimum Current Density (mA/m ) 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 o 30 °C. All tanks will be at an operating temperature at 48 C therefore, a design current 2 density of 31.25 mA/m is considered suitable. For the purposes of the design the coating breakdown has been assumed to be a maximum of 50 %: Structure Surface Tank Bottom
Document Title Document No CPCL Project No
Detailed Design Calculations 16227-CP-03-DD 12/P16227
Coating Breakdown (%) 50
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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.
Document Title Document No CPCL Project No
•
100 mV polarisation shift.
•
All above potentials are IR free or “OFF” potentials.
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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.
Document Title Document No CPCL Project No
Detailed Design Calculations 16227-CP-03-DD 12/P16227
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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 o
at two points 180 from each other. Permanent reference electrodes and slotted monitoring tube shall be installed under the tank to allow effective monitoring.
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Detailed Design Calculations 16227-CP-03-DD 12/P16227
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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:
SA = Π.r 2 Where for Gas Oil in first column then Gasoline and Firewater in second column:
Π
=
Pi
r
=
Radius
SA
=
Surface Area (m²)
3.142
3.142
12.5
8.75
490.87
240.53
m m²
The total current required can then be calculated using the following:
Itot = (SA.J .Fc ) Where,
Document Title Document No CPCL Project No
SA
=
Surface Area
J
=
Current Density
Fc
=
Coating Breakdown Factor
Itot
=
Total Current
Detailed Design Calculations 16227-CP-03-DD 12/P16227
490.87
240.53
31.25
31.25
50
50
%
7.67
3.76
A
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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:
Lmin =
I tot Ia
Where, Itot
=
Total Current
Ia
=
Anode Current Output (A/linear m)
Lmin
=
Minimum Ribbon Length
1
7.67
3.76
A
0.019
0.019
A
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:
S mmo =
Ia J1
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
2
The ribbon anode output is considered to be a maximum of 3 A / m 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. Document Title Document No CPCL Project No
Detailed Design Calculations 16227-CP-03-DD 12/P16227
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
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)
L1 = 4.S1 .(d − S1 ) 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 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Total
Document Title Document No CPCL Project No
Detailed Design Calculations 16227-CP-03-DD 12/P16227
Spacing (m) 0.60 1.79 2.98 4.17 5.36 6.55 7.74 8.93 10.12 11.31 12.50 13.69 14.88 16.07 17.26 18.45 19.64 20.83 22.02 23.21 24.40
Length (m) 7.62 12.88 16.19 18.63 20.52 21.98 23.11 23.96 24.54 24.89 25.00 24.89 24.54 23.96 23.11 21.98 20.52 18.63 16.19 12.88 7.62 413.65
Spacing (m) 0.58 1.75 2.92 4.08 5.25 6.42 7.58 8.75 9.92 11.08 12.25 13.42 14.58 15.75 16.92
Length (m) 6.28 10.50 13.04 14.80 16.04 16.87 17.34 17.50 17.34 16.87 16.04 14.80 13.04 10.50 6.28
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:
ρ 4 L2 R= (ln − 2) 2πL hw 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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Detailed Design Calculations 16227-CP-03-DD 12/P16227
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
Row
Spacing
Resistance
Spacing
Resistance
Number
(m)
(ohm)
(m)
(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)
Document Title Document No CPCL Project No
Detailed Design Calculations 16227-CP-03-DD 12/P16227
0.458
0.860
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
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)
L2 = 4.S 2 .(d − S 2 ) Where, S2
=
0.5 x Conductor Bar Spacing
d
=
Diameter of Tank
L2
=
Initial Conductor Bar Length
1.75
1.75
m
25
17.5
m
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)
Document Title Document No CPCL Project No
Detailed Design Calculations 16227-CP-03-DD 12/P16227
86.6
70.6
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
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:
V (vd1)+V (vd 2)+V (vd 3)+V (vd 4)+Vlosses ≤ VTR
Where, V(vd1)
=
Groundbed Volt Drop
V(vd2)
=
V(vd3)
4.6
4.3
V
MMO Ribbon Volt Drop
0.005
0.005
V
=
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
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Detailed Design Calculations 16227-CP-03-DD 12/P16227
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
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:
V ( vd1) = RGB. Itot Where,
4.5.2
RGB
=
Groundbed Resistance to Earth
Itot
=
V(vd1)
=
0.458
0.860
Ohm
Total Current
10
5
A
Groundbed Volt Drop
4.6
4.3
V
MMO RIBBON VOLT DROP
The MMO ribbon volt drop is calculated using the following formula:
V ( vd 2 ) = I .R.L Where, I
=
MMO Ribbon Load / m
R
=
Resistance of MMO Ribbon
L
=
Length of MMO Ribbon
V(vd2)
=
MMO Ribbon Volt Drop
2
0.021
0.02
A
0.138
0.138
1.79
1.75
m
0.005
0.005
V
Ohm/m
Where the load is the current required for half the distance between two conductor bars.
I = I tot / L1 /(0.5.S 2 )
2
Where, Itot
=
Total Current
L1
=
Total Length of MMO Ribbon
S2
=
Spacing of Conductor Bar
I
=
MMO Ribbon Load / m
10
5
A
413.65
207.26
m
3.5
3.5
m
0.014
0.014
A
Data from MMO anode manufacture
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Detailed Design Calculations 16227-CP-03-DD 12/P16227
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4.5.3
CONDUCTOR BAR VOLT DROP
The Conductor Bar volt drop is calculated using the following formula:
V ( vd 3) = I .R.L Where, I
=
Load
R
=
Resistance of Conductor Bar
L
=
V(vd2)
=
0.223
0.152
A
0.069
0.069
Ohm/m
Length of Conductor Bar
12.50
9.75
m
MMO Ribbon Volt Drop
0.192
0.092
V
3
Where the load is the current required for half the length of the longest conductor bar (worst case).
I = I tot / L2 /(0.5.L3 ) Where,
3
Itot
=
Total Current
L2
=
Total Length of conductor Bar
L3
=
Length of Longest conductor Bar
I
=
Conductor Bar Load / m
10
5
A
139.7
70.6
m
25
17.5
m
0.006
0.008
A
Data from conductor bar manufacture
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Detailed Design Calculations 16227-CP-03-DD 12/P16227
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4.5.1
CABLE VOLT DROP
Cable volt drops are calculated using the following formula:
V(vd 4) = V1 + V2 + V3 + V4 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:
Vvd = (Rcab / 1000).Lcab .I cab Where, RCab
=
Resistance of Cable
Lcab
=
Icab Vvd
4
0.524
0.524
Ohm/km
Length of Cable
200
200
m
=
Current in Cable
10
5
A
=
Cable Volt Drop
1.1
0.524
V
The volt drop of each cable is calculated from the above equation.
4
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
From cable manufacturers
Document Title Document No CPCL Project No
Detailed Design Calculations 16227-CP-03-DD 12/P16227
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4.6 CALCULATION SUMMARY Parameter
Document Title Document No CPCL Project No
25 m
17.5 m
Unit
SA
=
Surface Area (m²)
490.87
240.53
m²
Itot
=
Total Current
10
5
A
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
Detailed Design Calculations 16227-CP-03-DD 12/P16227
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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
2
3
Transformer Rectifier Voltage
24 V
24 V
Transformer Rectifier Current
10 A
5A
414 m
208 m
7
5
Permanent CP Station
MMO Ribbon Length Powerfeed Number Powerfeed Cable Size Powerfeed Cable Length Feeder Cable Size Total Feeder Length (including 20 % spare for routing) Number of Reference Electrodes
Document Title Document No CPCL Project No
Detailed Design Calculations 16227-CP-03-DD 12/P16227
16 mm
2
150 m 35 mm
2
16 mm
2
100 m 35 mm
200
200
5
5
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
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
2
Minimum Current Density (mA/m )
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. o
All tanks will be at an operating temperature at 48 C therefore, a design current density of 2 156.25 mA/m is considered suitable. The current density below is taken from Project Document:
Document Title Document No CPCL Project No
Structure Surface
Coating Breakdown (%)
Gasoline / Gas Oil
5
Firewater
10
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
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.
Document Title Document No CPCL Project No
•
100 mV polarisation shift.
•
All above potentials are IR free or “OFF” potentials.
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
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 C
CP Protection Criteria, Eag
:
-0.80 V (IR Free)
Water Resistivity
:
200 Ohm.cm or less
o
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.
Document Title Document No CPCL Project No
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
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
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
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:
SA = π .r 2 + π .d .h Where for tank,
π
=
Pi
R
=
Radius
12.5
m
D
=
Diameter
25.0
m
H
=
Height
1.0
m
SA
=
Surface Area (m²)
3.142
569.41
m²
The current required for GAS OIL TANK can then be calculated using the following:
If = (SA.J .Fc ) Where for tank, SA
Surface Area
596.41
m²
J
Current Density
156.25
mA
Fc
Coating Breakdown Factor
If
Document Title Document No CPCL Project No
=
Final Current
Detailed Design Calculations 16227-CP-03-DD 12/P16227
10
%
8.90
A
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
8.3 NUMBER OF ANODES BY CURRENT OUTPUT The anode resistance is calculated using the following equation:
Rf =
4l f ρ (ln − 1) 2πl f rf
Where,
ρ
=
Water Resistivity
lf
=
Final Anode Length (90%)
rf
=
Rf
=
2
Ohm.m
112.5
cm
Final Effective Anode Radius (50%)
49.2
cm
Final Anode Resistance to Earth
1.19
Ohm
Then, the maximum current output per anodes is calculated using Ohm’s Law:
Ia =
V Rf
Where, V
=
Anode Driving Potential
Rf
=
Ia
=
5
0.30
V
Final Anode Resistance to Earth
1.19
Ohm
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.
An1 =
If Ia
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 Document Title Document No CPCL Project No
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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations
8.4 NUMBER OF ANODES BY WEIGHT The total anode weight is calculated using the following equation:
Mt =
I m .t.8760 Z .UF
Where for the tank, Im
=
Current
t
=
Design Life
Z
=
Alloy Anode Amp/Hour Capacity
UF
=
Utilisation Factor
Mt
=
Total Anode Mass
8.90 25 2500
A Years A.hours/kg
85
%
917.6
kg
Finally, the minimum number of anodes is calculated from the total mass required divided by the mass of each individual anode.
An 2 =
Mt Ma
Where for tank,
Document Title Document No CPCL Project No
Mt
=
Total Anode Mass
Ma
=
An2
=
Detailed Design Calculations 16227-CP-03-DD 12/P16227
917.6
kg
Individual Anode Mass
25.2
kg
Anode Number
37
No.
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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.
An1
=
Anode Number (Current)
19
No.
An2
=
Anode Number (Mass)
19
No.
An2
=
Anode Number (Distribution)
24
No.
At
=
Total Anode Number
24
No.
An1
=
Anode Number (Current)
30
No.
An2
=
Anode Number (Mass)
29
No.
An2
=
Anode Number (Distribution)
34
No.
At
=
Total Anode Number
34
No.
Gasoline Tanks:
Firewater Tank:
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