COE 107.02 Design Basics for Cathodic Protection Systems

Engi neerin g Ency clo pedia Saudi Aramco Desktop Standards

DESIGN BASICS FOR CATHODIC PROTECTION SYSTEMS

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Electrical File Reference: COE 107.02

For additional information on this subject, contact PEDD Coordinator on 862-1026

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Corrosion Design Basics for Cathodic Protection Systems

Section

Page

OBJECTIVES

........................................................................................................ 1

TERMINAL OBJECTIVE....................................................................................... 1 ENABLING OBJECTIVES .................................................................................... 1 INFORMATION

........................................................................................................ 3

INTRODUCTION.................................................................................................. 3 OPERATION AND APPLICATIONS OF GALVANIC ANODE SYSTEMS ............................................................................................................ 4 OPERATION OF GALVANIC ANODE SYSTEMS................................................ 4 Galvanic Anodes........................................................................................ 5 Function of Major Components of Galvanic Anode Systems..................... 9 APPLICATIONS OF GALVANIC ANODE SYSTEMS......................................... 15 Advantages and Disadvantages of Galvanic Anode Systems ................. 15 Buried Pipeline Applications .................................................................... 16 Vessel and Tank Interior Applications...................................................... 16 Marine Applications.................................................................................. 18 CALCULATING GALVANIC ANODE DRIVING VOLTAGE ................................ 21 Example 1........................................................................................................... 22 Calculating Circuit Resistances of Galvanic Anode Systems ............................. 23 Circuit Resistance, R .......................................................................................... 24 Structure-to-Electrolyte Resistance, RS ............................................................. 25 Lead Wire Resistance, RLW............................................................................... 25 Anode Bed Resistance, R ab.............................................................................. 25 Example 2........................................................................................................... 26 OPERATION AND APPLICATIONS OF IMPRESSED CURRENT SYSTEMS .......................................................................................................... 27 Operation of Impressed Current Systems .......................................................... 27 Direct Current Power Sources ................................................................. 28 Impressed Current Anodes ...................................................................... 38 Functions of Major Components of Impressed Current Systems............. 39 Advantages and Disadvantages of Impressed Current Systems............. 43

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Anode Beds ............................................................................................. 44 Buried Pipeline Applications .................................................................... 48 Onshore Well Casing Applications........................................................... 49 Vessel and Tank Interior Applications...................................................... 50 In-Plant Facility Applications .................................................................... 52 Marine Applications.................................................................................. 53 SELECTING IMPRESSED CURRENT ANODE BED SITES ............................. 55 Example 3........................................................................................................... 55 CALCULATING THE DRIVING VOLTAGE FOR AN IMPRESSED CURRENT DC POWER SOURCE ....................................................................................... 57 Example 4........................................................................................................... 57 CALCULATING CIRCUIT RESISTANCES OF IMPRESSED CURRENT SYSTEMS .......................................................................................................... 58 Structure-to-Electrolyte Resistance (Rs) ............................................................. 59 Cable Resistance (RLW) ..................................................................................... 59 Maximum Circuit Resistance .............................................................................. 60 Allowable Anode Bed Resistance....................................................................... 60 Example 5........................................................................................................... 61 WORK AIDS ...................................................................................................... 62 Work Aid 1A. Data Base for Calculating Galvanic Anode Driving Voltage......... 62 Work Aid 1B. Procedure for Calculating Galvanic Anode Driving Voltage......... 63 Work Aid 2. Formulas and Procedure for Calculating Circuit Resistances of Galvanic Anode Systems .............................................................. 64 FORMULAS........................................................................................................ 64 Circuit Resistance.................................................................................... 64 Structure-to-Electrolyte Resistance ......................................................... 64 Maximum Circuit Resistance ................................................................... 65 Galvanic Anode Driving Voltage .............................................................. 65 Allowable Anode Bed Resistance ............................................................ 65 Procedure ........................................................................................................... 66 Conductor Resistance Table .............................................................................. 67 Work Aid 3. Procedure for Selecting Impressed Current Anode Bed Sites ....... 69

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Procedure ........................................................................................................... 69 Work Aid 4. Procedure for Calculating the Driving voltage of DC Power Sources......................................................................................... 70 Procedure ........................................................................................................... 70 Work Aid 5. Formulas and Procedure for Calculating Circuit Resistances of Impressed Current Systems.......................................................... 72 FORMULAS........................................................................................................ 72 Driving voltage of an Impressed Current DC Power Source ................... 72 Circuit Resistance.................................................................................... 72 Structure-to-Electrolyte Resistance ......................................................... 72 Allowable Anode Bed Resistance ............................................................ 73 Procedure ........................................................................................................... 73 Conductor Resistance Table .............................................................................. 75 GLOSSARY ...................................................................................................... 79

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List of Figures Figure 1. Typical Galvanic Anodes in Soil (arrows show the direction of current flow).. 5 Figure 2. Typical 45.5 kg (100 lb) Magnesium Galvanic Anode ..................................... 6 Figure 3. Typical 13.6 kg (30 lb) Zinc Anode.................................................................. 7 Figure 4. Aluminum Anodes for Offshore Structures...................................................... 8 Figure 5. Magnesium anode......................................................................................... 10 Figure 6. Lead Wire...................................................................................................... 11 Figure 7. Graphic Summary of the Thermite Welding Procedure................................. 12 Figure 8. A 5-Terminal Junction Box, Standard Drawing AA-036274........................... 13 Figure 9. One-Pin Test Station Details, Standard Drawing AA-036907........................ 14 Figure 10. Magnesium Anodes at a Road Crossing, Standard Drawing AA-036352... 16 Figure 11. Galvanic Anodes in the Water Section of a LPPT ....................................... 17 Figure 12. Galvanic Anodes in a Water Storage Tank, Standard Drawing AA-036354 ................................................................................................. 18 Figure 13. Marine Aluminum Alloy Galvanic Anodes, Standard Drawing AA-036348 ................................................................................................. 19 Figure 14. Galvalum III Bracelet anode on a Subsea Pipeline, Standard Drawing AA-036335 ................................................................................... 20 Figure 15. Representation of the Driving voltage of a Galvanic Anode ........................ 21 Figure 16. Representation of a Galvanic Anode System as an Equivalent Circuit ...... 23 Figure 17 Typical Rectifier Impressed Current System ............................................... 27 Figure 18. Single-Phase Transformer .......................................................................... 28 Figure 19. Silicon Diodes ............................................................................................. 29 Figure 20. A Silicon Diode in an AC Circuit .................................................................. 30 Figure 21. Operation of a Single-Phase Bridge Rectifier.............................................. 31 Figure 22. Schematic of a Three-Phase Bridge Rectifier ............................................. 32 Figure 23. Schematic of a Typical Single-Phase Rectifier............................................ 33 Figure 24. Air-Cooled and Oil-Cooled Rectifier Enclosures.......................................... 34 Figure 25. Solar Module System .................................................................................. 37

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Figure 26. Dual Vertical Anodes in Coke Breeze Backfill and Vertical Anode in Subkha, Standard Drawing AA-036346.................................................................... 39 Figure 27. Center-Tapped Anode................................................................................. 41 Figure 28. 12-Terminal Junction Box, Standard Drawing AA-036275 .......................... 42 Figure 29. Area of Influence of a Close Anode (top view) ............................................ 44 Figure 30. Two Areas of Influence Caused by a Remote Anode Bed .......................... 46 Figure 31. Typical Deep Anode Bed, Standard Drawing AA-036385 ........................... 47 Figure 32. Anode Bed of 10 Horizontal Anodes, Standard Drawing AA-036346 ................................................................................................. 48 Figure 33. Surface Anode Bed Cathodically Protecting a Well Casing......................... 49 Figure 34. Impressed Current AnodesInside a Water Tank, Standard Drawing AA-036353 ................................................................................... 51 Figure 35. Impressed Current Anodes Protecting theExterior Bottom of a Storage Tank, Standard Drawing AA-036355 .......................................................... 53 Figure 36. Impressed Current System on an Offshore Platform, Standard Drawing AA-036348 ................................................................................... 54 Figure 37. Soil Resistivity Survey along a 6 km-Section of Pipeline............................. 56 Figure 38. Representation of a Buried Impressed Current System asan Equivalent Circuit ......................................................................................................... 58

List o f Tables Table 1. Practical Galvanic Series.................................................................................. 4 Table 2. Impressed Current Anodes............................................................................. 38 Table 3. Saudi Aramco’s Required Potentials for Various Structures........................... 62 Table 4. Cable Requirements for Various Cathodic Protection Applications ................ 67 Table 5. Correction Factors for Other Temperatures .................................................. 68 Table 6. Ratings of Rectifiers Used by Saudi Aramco.................................................. 71 Table 7. Cable Requirements for Various Cathodic Protection Applications ................ 75 Table 8. Correction Factors for Other Temperatures ................................................... 77

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OBJECTIVES TERMINAL OBJ ECTIVE This module will introduce the participant to the two general types of cathodic protection systems: galvanic and impressed current. Upon completion of this module the participant will be able to select and apply the appropriate design criteria from the appropriate cathodic protection Engineering Standard.

ENABLING OBJECTIVES In order to accomplish the Terminal Objective, the Participant will be able to: •

•

•

•

•

•

•

Learn about the operation and applications of galvanic anode systems. Calculate the driving voltage of galvanic anodes, using anode material specifications. Calculate the circuit resistance of galvanic anode systems, using data from conductor resistance table. Learn about the operation and applications of impressed current systems. Select favorable sites for impressed current anode beds using soil resistivity survey data. Calculate the circuit resistance of impressed current system. Calculate the correct driving voltage for the dc power source, using the system current requirement and circuit resistance parameters.

Note: Definitions of words in italics are contained in the Glossary.

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INFORMATION INTRODUCTION Module 107.01 described how cathodic protection supplies electrons (electric current) to a metal to reduce the corrosion rate. The module also provided procedures to calculate the amount of current needed to cathodically protect various structures. In this module, we will discuss two cathodic protection systems that provide electric current to protect structures—galvanic anode systems and impressed current systems. We will discuss the operation and applications of each system. The discussion will include detailed information about their components. To determine design criteria for galvanic anode systems and impressed current systems, we will represent them as equivalent electrical circuits.

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OPERATION AND APPLICATIONS OF GA LVANIC ANODE SYSTEMS Opera tion of Galvanic Anod e Systems Galvanic anode systems are based on the principle of the galvanic corrosion cell. A galvanic corrosion cell is two dissimilar metals connected together in a common electrolyte. Corrosion current flows from the metal with the more negative potential to the metal with the least negative potential. The metal with the least negative potential is protected from corrosion. For example, the Practical Galvanic Series in Table 1 below shows the potentials of metals in soil with respect to a Cu-CuSO4 reference electrode. If two metals in the series form a galvanic couple, the metal nearest the top will be anodic to any metal below it.

Table 1. Practic al Galvanic Se ries PRACTICAL GALVANIC SERIES IN NEUTRAL SOIL Metal

Normal Electrode Potential, vol ts vs . Cu-CuSO 4

Magnesium alloy (contains Al, Mn)

-1.70*

Magnesium alloy (contains Al, Zn, Mn)

-1.55

Zinc

-1.10*

Aluminum alloy (Contains ln, Zn)

-1.10*

Commercially pure aluminum

-0.80

Mild steel

-0.50 to -0.80

Cast iron

-0.50

Brass, bronze, or copper

-0.20

High silicon cast iron

-0.20

Mill scale on steel

-0.20

Carbon, coke, graphite

+0.30

More anodic

More cathodic

* Minimum allowable potential in accordance with 17-SAMSS-006

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When galvanic anodes are connected to a buried structure such as the steel pipeline, as illustrated inFigure 1, a galvanic corrosion cell develops. Electric current flows from the anodes, through the electrolyte, and to the pipeline. The pipeline becomes cathodically protected. To complete the circuit, current returns to the anodes through a lead wire.

Junction box Lead wire

Lead wire

Galvanic anode in chemical backfill

Figure 1 . Typical G alvanic Anodes in S oil (arrows show the dir ection of current fl ow)

The components in a typical underground galvanic anode system include anodes, chemical backfill, lead wire, and a junction box. We will now describe these components in more detail. Galvanic Anodes Galvanic anodes corrode and discharge current to protect the structure. When galvanic anodes corrode, all of their energy is not used to provide protective current. Local corrosion cells on the anode surface also use energy to produce corrosion current. The energy used by these local corrosion cells is not used to protect the structure. The ratio of the anode weight used to produce useful current to the total anode weight multiplied by 100 is called the anode efficiency. Efficiency is not mentioned in

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the Saudi Aramco Engineering Standards because it has been incorporated in the consumption rate value. A galvanic anode provides a given amount of electrical energy based on its composition and efficiency. Each anode material has a theoretical energy contentgiven in ampere-hours per kg. An ampere-hour is any combination of amperage and time that equals 1.0 ampere flowing for 1 hour. For example, both 0.5 ampere flowingoffor hours and 2.0 Amp flowing forStandard 0.5 hour are the equivalent 12 ampere-hour. The Engineering specifies the consumption rate, which is the reciprocal of the theoretical energy. The three most common galvanic anode materials are magnesium, zinc, and aluminum. The typical characteristics of these anodes are discussed below. Magnesium Anodes - Magnesium is the most widely used material for buried galvanic anodes. Saudi Aramco normally uses magnesium anodes on pipelines at road and fence crossings and at mainline valves. A typical 45.5 kg (100 lb) magnesium anode is shown inFigure 2. Lead wire Potting compound Silver solder connection

Magnesium alloy .

Galvanized steel core

Figur e 2. Typical 45.5 kg (100 lb) Magnesium Galvani c Ano de

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Two types of magnesium anodes are available—standard alloy and high-potential alloy. Both have a consumption rate of 7.71 Kg/A-yr. The open circuit potentials are –1.55 volts and –1.70 volts respectively vs. Cu-CuSO4. We use high-potential magnesium anodes almost exclusively. Zinc Anodes - Zinc anodes are most often used in soil resistivities below 700 ohm-cm or in vessel interiors. Occasionally they are used in soils up to 2,500 ohm-cm. Pure zinc has a consumption rate of 11.79 kg/A-yr and an open circuit potential of -1.10 volts versus a Cu-CuSO 4 reference electrode. Zinc galvanic anodes for soil applications have long slender shapes to achieve low resistance to earth Figure ( 3). Their shape also provides practical current output despite their low driving voltages. Zinc anodes are not subject to significant polarization when they are used in suitable backfill. CAUTION: Use high temperature zinc (HTZ) anodes rated for high temperature service in electrolytes that exceed 50°C.

Lead wire Silver solder connection (insulated with rubber and tape)

.) n i 0 6 ( m c 2 5 1

Zinc

Galvanized steel core

Figur e 3. Typical 13. 6 kg (30 lb) Zinc A nod e

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Alum in um Anod es - Aluminum anodes are used in offshore applications or for protecting vessel and tank interiors. There are generally three types of aluminum anodes as follows •

Heat-treated aluminum zinc-tin alloy

•

Aluminum-zinc-mercury alloy

•

Aluminum-zinc-indium alloy

All of these alloys have a consumption rate of 3.7 kg/A-yr and an open circuit potential of –1.1 volt versus Cu-CuSO 4. Aluminum galvanic anodes are manufactured so they can attach directly to an offshore structure. Three types of core arrangements are shown inFigure 4.

Steel core

TypAe

TypBe

TypCe

Figure 4. Alumi num Anodes for Offshore S truct ures

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Function of Major Components of Galvanic Anode Systems Anode Chemical Backfill - Anode chemical backfill is the special material that surrounds the buried anode. A typical backfill mixture for magnesium anodes is 75% hydrated gypsum, 20% bentonite clay, and 5% sodium sulfate. Clays in the backfill absorb water from the soil and keep the anode moist for maximum current output. Chemical backfill also has low resistivity which reduces the anode to earth resistance. When backfill has a lower resistivity than the surrounding soil, the effective anode dimensions are increased to the dimensions of the backfill. If an anode is buried in soil without backfill, variations in the soil’s composition may start local corrosion cells on the anode surface. For example, chloride ions in soil increase the corrosion of magnesium anodes and lower their efficiency. Bicarbonates and carbonates in soil may react with magnesium and zinc anodes to form surface films with high electrical resistance. Surface films cause the anodes to “go passive” and cease to produce enough current to protect the structure. Galvanic anodes are frequently pre-packaged in backfill material and buried directly in the soil. Figure 5 is a cutaway view of a pre-packaged 27.3 kg (60 lb) magnesium anode.

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Figur e 5. Magnesium a nod e

Conductors Cafor bles - A conductor is a metal wire that provides easyand flow electric current. Copper is the most common material used in standard electrical applications. An insulated conductor is surrounded with a high resistance polymeric material. These insulators provide electrical and mechanical protection.Figure 6 shows an insulated conductor, or lead wire, with two extra protective layers—a jacket and stranded metallic braid.

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Figur e 6. Lead Wire

Saudi Aramco uses cables and lead wires to: •

•

•

Connect galvanic anodes to the structure Connect impressed current rectifier output negative terminal to the structure, and the positive terminal to the anode bed Connect the negative lead from the structure to the test station

The type of metal and its size determines the amount of current a conductor can carry. Cables and conductors are available in different types and sizes. The National Electric Code (NEC) specifies the number and size of conductors in a cable. The number and size should be enough to dissipate heat and prevent damage during installation or withdrawal. Cable types and sizes are specified on standard Saudi Aramco engineering drawings. Cables are usually thermite welded to structures. Proper thermite welding eliminates the expense of welding.Figure 7 summarizes the thermite welding procedure used by Saudi Aramco.

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2

1

Starting powder Weld metal

Mold

Tap hole Weld cavity

Remove 4" X 4" section of coating

Pour weld metal and starting powder in mold

4

3

Flint igniter gun

Lead wire

Place wire and mold on clean pipe surface

Place cable and mold clean surface Ignite the powder and on hold moldpipe for 1/2 minute

6

5

After cooling, tap lightly to test weld

Repair the coating

Figure 7 . Graphic Summary of t he Thermite We ldin g Procedure

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Junction Boxes - The anode lead wires go to a junction box as shown in Figure 8. A shunt resistor is inserted in each anode lead wire inside the junction box. A common shunt resistance is 0.001 ohms. This allows the current output of each anode to be measured by determining the voltage drop across the shunt. For example, the current output of an anode with a voltage drop of 0.75 millivolts across a 0.001 ohm shunt is 0.00075 volt/0.001 ohm = 0.75 Amp.

50A/50mV Shunt No. 8 anode lead wire

Bus bar

No. 8 lead wire to pipeline

Figur e 8. A 5-Term inal Jun cti on Box , Standard D rawin g AA-036274

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Test Stations - A test station is a test point for measuring pipeto-soil potential. It contains a lead wire which is thermite welded to the pipeline. We require potential test stations at each kilometer marker of a pipeline, insulated cased crossing, major road crossing, and other locations as needed. Figure 9 shows a typical one-pin test station for a buried galvanic anode system. The pipe-to-soil potential is measured using a voltmeter and a Cu-CuSO4 reference electrode.

0.80

-

Voltmeter No. 8 AWG wire to pipeline

connection

+

Figur e 9. One-Pin Test Station Details, Standard Drawing A A-036907

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App licat ions of Gal vani c A node Sys tems Ad vantages and Disadva ntages of Galvanic Anode Systems Galvanic anode systems are used when current requirements are low. The main advantages of galvanic anode systems are as follows •

An external power source is not required.

•

Installation costs are low.

•

Maintenance costs are low.

•

Sacrificial anodes seldom cause interference problems with other structures.

The main disadvantages are as follows: •

The driving voltage is limited.

•

The current output from individual anodes is low and limited.

•

Sacrificial anodes are effective in a limited range of soil resistivities.

The following information discusses various applications of galvanic anode systems.

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Buried Pipeline Ap pl ic ati on s Saudi Aramco uses galvanic anodes to protect mainline valves, road and camel crossings, and short sections of pipelines that are not part of an impressed current system.Figure 10 shows how pre-packaged, 27.3 kg (60 lb) magnesium anodes are often used to protect pipelines under roads or camel crossings. In Subkha, bare 45.5 kg (100 lb) magnesium anodes are used. For high soil resistivities, magnesium anodes cannot push current for long distances. Junction box

Pipe

Pre-packaged magnesium anodes

Figur e 10. Magnesium Ano des at a Road Cross ing , Standard Drawing AA-036352

Vessel and Tank Interior Applications Produced brine can cause severe corrosion problems inside oil field production vessels such as free water knock outs, desalters, and separators. Cathodic protection can increase the service life of these vessels. Current density requirements range from 3 mA/m2 for coated vessels to 30 mA/m2 for uncoated vessels (check current Engineering Standard). An “see”. anode, however, can only protect the surfaces that it can Consequently, the number of anodes required is usually

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determined more by vessel geometry and current distribution than by current requirements. We use HTZ anodes to protect water-wet areas inside production and process vessels.Figure 11 shows a lowpressure production treater (LPPT), which contains anodes in its water section. The anodes are attached to the vessel wall with brackets. The brackets also hold the anodes above any sludge that may settle on the bottom. We areinternals. testing the application of impressed current anodes for vessel

Oil Water

Anodes

Figur e 11. Galvanic Ano des in the Wa ter Secti on of a LPP T

Saudi Aramco mainly uses magnesium and aluminum galvanic anode strings to protect the interior of water tanks Figure ( 12). The lead wires from each string are connected to the exterior of the tank via a junction box. Each junction box contains a 0.01ohm shunt, which is used to measure the current output of the anode string. Magnesium anodes are not used if the water resistivity is less than 500 ohm-cms. Aluminum anodes are not used if the water resistivity is more than 1000ohm-cm. Mercuryactivated aluminum and zinc anodes are not used in potable water tanks because of health concerns. Firewater tanks in remote areas are generally regarded as potable because of local practice. The Engineering Standard does not require tank protection if the water resistivity will not drop below 2000 ohmcm during the life of the tank.

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Junction box 0.01 ohm shunt

Weld Anode Lead wire

Cable Polypropylene rope

Figur e 12. Galvanic Ano des in a Water Stor age Tank, Standard Drawing AA-036354

Marine Applications Saudi Aramco cathodically protects offshore platforms, subsea pipelines, breasting dolphins, and loading and mooring buoys. Galvanic systems are used on most marine structures. Marine galvanic anodes are usually indium doped aluminum alloys or zinc-tin doped aluminum alloys.

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The objective is to quickly polarize offshore platforms to a minimum of -0.90 volt versus a (Ag/AgCl) reference electrode. This has two advantages. First, little corrosion occurs. Second, chemical reactions at the cathode form a protective carbonate scale. Scale reduces current requirements and allows current to reach metal surfaces further from the galvanic anode.Figure 13 shows aluminum alloy anodes on an offshore platform. Offshore platforms have surfacetoareas and require anodes. The anodes arelarge positioned completely protectmany the structure and parts of the immersed section of the well casing.

Aluminum alloy anode

AA-035348

Figure 1 3. Marine Aluminu m Alloy Galvanic Anodes, Standard Drawing AA-036348

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There are two ways to cathodically protect a subsea pipeline. It can be electrically connected to a platform and share the platform’s CP system, or it can be electrically isolated from the platform and have its own CP system. The Engineering Standard calls for connecting subsea pipelines to platforms so that they become part of the platforms’ CP system. The purpose of this connection is to eliminate possible interference effects from impressed current systems. We also install Galvalum III bracelet type anodes at intervals along subsea pipelines. This provides even current distribution along the pipeline and reduces the current requirements from the platform’s CP system.Figure 14 shows a Galvalum III bracelet anode on a subsea pipeline. Normally, bracelet anodes are preinstalled on joints of pipe onshore. When they are consumed, a new anode is connected to the frame of the old bracelet anode.

Figur e 14. Galvalum III Bracelet anode on a S ubs ea Pipelin e, Standard Drawing AA-036335

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CALCULATING GALVANIC ANODE DRIVING VOLTAGE Many galvanic anodes may be required to generate the amount of current needed to protect a portion of a structure. According to Ohm’s Law, the amount of current generated by the galvanic anode system is determined by the following formula: I = ED/R Where ED = the potential difference between the anode and structure (the driving voltage). R = the circuit resistance of the system The potential difference between an anode and a structure is calculated as follows: ED = EO – EP Where EO = the open circuit potential ofthe anode material. EP = the protected potential of the structure.

The data and procedure used to calculate driving voltage are provided in Work Aid 1.

Driving potential

ED Pre-packaged magnesium anode

Figure 1 5. Representation of th e Drivi ng vol tage of a G alvanic Anode

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Example 1 The following example will demonstrate how to calculate the driving voltage of a high potential magnesium galvanic anode that will protect a section of pipeline under a road crossing. ED = EO- EP ED = 1.7 V - 1.20 V = 0.5 V versus Cu-CuSO4 For design calculations, we will use the absolute value, 0.5 V.

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CALCULATING CIRCUIT RESISTANCES OF GALVANIC ANODE SYSTEMS The anode current output is also a function of the resistance in the galvanic anode system. If we represent a system as an equivalent electrical circuit, we can determine the resistance in the circuit. Figure 16 is a representation of a galvanic anode system as an equivalent electrical circuit. The equivalent circuit includes the driving voltage of the anode material, E D, and the resistances of the circuit elements. For example, there is resistance in the anode lead wires. There is also resistance between the structure and the soil and resistance between the anode(s) and the soil.

Figur e 16. Representati on of a Galvanic Anod e System as an E quivalent Circui t

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We can calculate the total circuit resistance using the formula R = RS + RLW + Rab Where R =

circuit resistance

R =

the resistance between the structure and the electrolyte

S

RLW = the resistance in the lead wire Rab = the anode bed resistance The following information describes the circuit resistance elements above.

Circuit Resist ance, R The amount of current that flows from the anode bed is determined by the resistance of the system, or circuit resistance. For design purposes, the circuit resistance, R, must not exceed the maximum circuit resistance, Rmax. The maximum circuit resistance is the anode driving voltage, ED, divided by the required current. The relationship between the circuit resistance and the maximum circuit resistance is as follows R ≤ Rmax = ED/I Where ED = the driving voltage of the galvanic anode material (volts) I

= the current required to protect the structure (Amp)

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Structu re-to-E lectrol yte Re sist ance, R S The resistance to earth of the structure can be determined from current requirement test data, but we rarely conduct any current requirement tests in Saudi Aramco. For new structures (except well casings), it can be neglected. This resistance mainly depends on the quality of the coating. The better the coating, the higher the structure-to-electrolyte resistance. If the test was done by a Contractor, then you can calculate the structure-toelectrolyte resistance using the formula: RS= (Von - Voff) /Ion Where Von = the structure-to-electrolyte potential withthe current on Voff = the structure-to-electrolyte potential with the current off Ion = current applied to give the potential Von

Lead Wire Resistanc e, R LW You can calculate lead wire resistance, RLW, by multiplying the length of the conductor (m) by its characteristic resistance (ohm/meter). A resistance table for copper conductors is provided in Work Aid 2 (from NEC). For a single anode that is close to a structure (less than 5 meters), the cable resistance will be so small that it can usually be ignored.

Ano de Bed Resi st ance, R ab The anode bed resistance is the resistance of all the anodes to earth and depends on the soil resistivity, the dimensions of the anodes or backfill, and the orientation of the anodes. These design factors will be covered in detail in Module 107.03. For design purposes, the anode bed resistance, R ab, must not exceed the allowable anode bed circuit resistance, Raab. Raab

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Rmax - (Rs + RLW)

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Example 2 Calculate the allowable anode bed resistance of ten (10) high potential magnesium anodes that will protect 75 meters of 36" diameter pipe. Assume that the current requirement for the pipe of 300 mA was measured in the field, as was the structure resistance (0.83 ohms). Assume that 15 meters of No. 8 AWG lead wire is used from the pipe to the junction box,. First, calculate the lead wire resistance: RLW = [15 m + (10%)(15 m)] [2.15 x 10-3 ohm/m] = 0.035 ohms The maximum circuit resistance that will allow the required current is calculated as follows: Rmax = ED/I = [1.7 V -1.0 V)]/0.300 A) = 2.33 ohms Therefore, the allowable anode bed resistance is Raab

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Rmax - (Rs + RLW) = 2.33 - (0.83 + 0.035) = 1.465 ohm

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OPERATION AND APPLICATIONS OF IMPRESSED CURRENT SYSTEMS Opera tion of Impressed Current Systems When current requirements are high, the Engineering Standard requires impressed current (IC) systems. The operation of a typical IC system is shown inFigure 17. An electrical grid supplies high-voltage alternating current to a rectifier. The rectifier reduces the voltage of the alternating current and converts it to a pulsating direct current. The direct current goes from the positive terminal of the rectifier to a junction box. At the junction box, the current is distributed to an anode bed of impressed current anodes. The anodes drive, or impress, the current into the earth. The current migrates through the earth and protects the structure. The current returns to the negative terminal of the rectifier via a cable, which is connected to the structure.

1

Stepdown Transformer

Cable returns current to rectifier

Electrical grid delivers high-voltage alternating current

Rectifi er reduces vol tage and converts alternating current to 2 a pulsating direct current Junction box 3 distributes current to the anode ground bed

6

5

4

Structure Collects Current

Ano des d is trib ute current through the soil to the structure

Figure 17 Typical Rectifier Impressed Current System

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The following information describes the operation of impressed current system components in more detail.

Direct Current Power Sources We use three types of direct current (dc) power sources— rectifiers, solar power systems, and engine generators. Rectifiers - Electrical transmission systems supply high-voltage single phase or three-phase alternating current (ac). Rectifiers step down the voltage and convert the alternating current to direct current. A rectifier contains a transformer and rectifying elements. The transformer reduces the voltage. A representation of a singlephase transformer is shown inFigure 18.

Laminated steel

Primary winding To load To primary ac power source Secondary winding

Magnetic flux

Figur e 18. Singl e-Phase Transfor mer

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The voltage in the secondary winding can be adjusted using connection points called “taps”. Changing the tap connections changes the output voltage of the rectifier. Taps are used to make coarse, (medium) and fine output voltage adjustments. Alternating current from the secondary winding fluctuates between positive and negative values.Figure 19is a diagram of a semi-conductor silicon diode. Diodes have forward breakdown voltages from 0.2 to 0.8 volts and reverse breakdown voltages in the hundreds of volts. This allows current to flow smoothly in one direction but prevents current flow in the opposite direction. The arrowhead shows the direction in which positive current can easily flow.

Forward polarity

Reverse polarity Figur e 19. Silic on Diodes

If a diode is connected in an ac circuit as shown inFigure 20, the diode allows only the positive fluctuations to pass to the load. The wave never goes negative so it is called pulsating dc.

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Positive pulses passed

Diode

Output

Input ac power source

Load

R L

1 cycle

Negative pulses blocked

Figure 2 0. A Silicon Diode in a n AC Circui t

The diode in the previous figure only allowed half of the initial ac energy to reach the load. This is called half-wave rectification. The single-phase bridge rectifier inFigure 21 provides full-wave rectification. The frequency of pulses across the load is called the ripple frequency.

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+

A

INPUT

T1

1

D3

D1

ac power source

2 4 D2

5

-

D4

T2-

RL

3

+

OUTPUT

T1

B

5 ac power source

INPUT

D3

D1

4 2 1

D2

-

D4

RL

+

T2

3

+

C

Figur e 21. Operation of a Singl e-Phase Bri dge Rectif ier

The three-phase bridge inFigure 22 A is the most common rectifier circuit when three-phase power is available. Each phase of a three-phase alternating current is spaced 120 electrical degrees apart. Therefore, the voltage of each secondary winding reaches its peak at a different time.

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A

A

t u p in c a

EAC

E

IAC

IDC

EDC

B

B

C

D

F

One cycle

dc wave Figur e 22. Schematic of a Three-Phase Bri dge Rectif ier

The three-phase rectifier circuit contains three bridges. This results in a more constant direct current output from the rectifier (Figure 22).

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Error! Refe rence source not fo und. shows other useful components inside rectifiers. Lightning arrestors are installed on the input and output. They protect sensitive components, especially diodes, from high voltage surges caused by lightning. Circuit breakers are placed on the ac power inlet for overload protection and to allow a person to turn the unit on and off.

Lighting arrester Circuit breaker 115V

2 3 1 4

230V

4 5

Secondary tap change

1 3 2

+

AC rectifier stack Volt-ammeter

Shunt Meter switch Lightning - arrester

+ DC output

Figur e 23. Schematic of a Typical Singl e-Phase Recti fier

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The transformer and rectifier elements generate heat inside a rectifier cabinet. This heat must be dissipated for the rectifier to work properly. Two methods are used to cool rectifiers—aircooling and oil immersion. In air-cooled rectifiers, the transformer windings and diode heat sinks are surrounded by ambient air. Heat is removed by natural convection of the surrounding air through holes and louvers in the metal housing (Figure 24A). Heat also radiates from different parts of the rectifier. Air-cooled rectifiers are usually mounted on a pole. They are cheaper than similarly rated oilcooled units.

A

B

Air-cooled rectifier enclosure

DC positive DC negative AC input

Oil drain

Ground rod

Oil-cooled rectifier enclosure Figur e 24. Air -Cool ed and Oil-Cooled R ectif ier Encl osu res

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Oil-immersed rectifiers (Figure 24B) are used if corrosive or explosive vapors are present. Oil-immersed rectifiers are often required for dusty areas, marine environments, and plant locations. Mineral oil transfers heat from interior elements to the exterior surface of the rectifier. Temperature differences cause oil to circulate in the cabinet by natural convection. The warm oil expands and becomes lighter. It rises to the top and releases excess heat. As the oil cools, it becomes heavier and sinks to the bottom of the cabinet. Then the heat exchange cycle begins again. Oil also insulates the transformer windings from each other and from the core material.

Oil Cooled Rectifier

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Control Enclosure

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Solar Power Systems – We use solar power systems in areas where electric power is not available, or AC power lines cannot be easily extended to provide AC power. Examples of these are remote locations on cross country pipelines, or isolated well casings. A typical solar power system is shown in Figure 25. It includes a photovoltaic solar array, a battery bank, and a charge regulator. The solar panel array consists of banks of solar cells that convert the sunlight to direct current. The electrical output of a single solar cell depends on the intensity of the sunlight and the exposed area of the silicon-boron layer. The solar cells are connected in series to obtain the proper voltage, or they are connected in parallel to obtain the needed current.

Figur e 25. Solar Modu le System

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We use rechargeable lead-acid storage batteries, which are similar to automobile batteries. The batteries in each module can supply 1200 ampere-hours of current to the anode bed. This is sufficient for five days without recharging. Batteries supply current to the impressed current anodes at night, during shamals, and on cloudy days. While the batteries are being charged, the solar panels apply current to the anodes. Engine Generators - We also use engine driven generators to provide power to impressed current anode beds. Remote areas of the East-West Pipeline and the Q-Q Pipeline are protected with engine driven impressed current systems. Impressed Current An od es Saudi Aramco uses various types of impressed current anodes. These anodes are discussed below. Table 2. Impress ed Current Anodes Anode

Current Density mA/cm2

Consumption Rate

High Silicon Chromium* Cast Iron

0.7

0.45 kg/A-yr

Scrap Steel*

0.4

9.1 kg/A-yr

Mixed Metal Oxide** Composite

60

0.0005 g/A-y

Platinized Niobium**

40

0.0086 g/A-y

* As detailed in SAES-X-400, Section 4.6 ** As detailed in SAES-X-300, Section 4.6

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Functions of Major Components of Impressed Current Systems Carbonaceous Backfi ll - Except for scrap steel, impressed current anodes in soil are usually surrounded with carbonaceous backfill. The carbonaceous backfill is usually calcined petroleum coke. It is sometimes called coke breeze. It is composed of 99.77% carbon. In Subkha soil, coke breeze does notdistribution. improve anode output, does facilitate uniform current Figure 26 is abut diagram of dual vertical anodes in coke breeze backfill and a vertical anode in Subkha.

Figur e 26. Dual Verti cal Anodes in Coke Bree ze Backf ill and Verti cal An ode in Sub kha, Standard Drawing AA-036346

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Coke breeze serves two purposes. 1.

It increases the effective size of the anode and lowers the anode-to-ground resistance.

2.

It extends the life of the anode. (In a wet environment, it was found that most of the current transmitted from the anode surface to the coke is ionic (88%). Materials Performance, July 1989, p.14-21)

Coke breeze consumption depends on good electrical contact between the anode and the backfill. The backfill must be packed solidly around the anode. Cables and Lead Wires - Impressed current systems contain cables and lead wires. Cables electrically connect the following: •

•

The positive terminal of the dc power source to the junction box The junction box to the anode header cable or to additional junction boxes

•

The structure to the negative terminal of the dc power source

•

The structure to other protected or unprotected structures

(bonding) Lead wires electrically connect the following •

•

Individual anodes to the junction box or header cable Pipelines to test stations

Anode lead wires and header cables have a positive potential with respect to the soil. If there are imperfections in their insulation, they will discharge current and be severed by corrosion. This will cut off current from all or part of the anode bed. All cables and wires should be surrounded with high quality insulation that has a minimum 600-volt rating. Saudi Aramco holiday tests all (+) cables before burial with 18,000 VDC holiday detector. The anode lead wire is mechanically connected to the impressed anode. Insulating areFigure used to protect the current connection from moisturematerials penetration. 27 shows a typical center-tapped anode. Center-connections

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reduce the accelerated consumption of anode material usually seen at the ends of anode with end-type connections.

#6 stranded copper wire

Lead wire

Epoxy sealant

Brass stud

Figure 27 . Center-Ta pped Anod e Junction Boxes - A single cable goes from the positive terminal of the dc power source to a junction box as shown in Figure 28. The junction box is connected to the individual anode lead wires. A shunt is inserted in each anode lead wire inside the junction box. We commonly use 0.001-ohm (50mV-50A) shunts. This allows the current output of each anode to be measured by determining the voltage drop across the shunt. For example, the current output of an anode with a voltage drop of 10 millivolts across a 0.001 ohm shunt is 0.01volt/0.001 ohm = 10 Amp.

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0.001 ohm shunt

Bus bar No. 8 AWG lead wires from anodes

Positive cable to rectifier

Figur e 28. 12-Termi nal Junc tio n Box, Standard Dra win g AA-036275

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Ad vantages and Disadva ntages of Impressed Current Systems Impressed current systems have the following advantages: •

Greater driving voltages

•

Higher current outputs

•

•

Adjustable current output Constant current

Impressed current systems have the following disadvantages: •

Higher equipment and installation costs

•

Higher maintenance costs

•

•

Possible interference problems with foreign structures Frequent monitoring

Saudi Aramco uses impressed current systems for the following: •

•

•

•

Buried pipelines Offshore pipelines within the area of influence of offshore platforms Offshore structures, if power is available Piers

•

External storage tank bottoms

•

Interiors of water tanks

•

•

•

Well casings Seawater intake systems Ship hulls (if galvanic anode systems are not applicable)

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An od e Beds There are basically two types of anode beds—close and remote. The terms close and remote relate to thearea of influence in the electrolyte around the anodes. The area of influence is the area in which cathodic protection is achieved. Close anode beds are used to cathodically protect limited areas of metal structures (e.g., congested pipe in plants where metallic isolation cannot be achieved or is not allowed). A single close anode provides protection by making the earth positive with respect to the structure. Figure 29 shows a close anode next to a buried pipeline. The anode is located so that a small area of the structure is in the anode’s area of influence.

Figur e 29. Area of Influ ence of a Close Anode (top view)

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For a close anode, the amount of potential shift (and the length of pipeline that can be protected) is a function of the voltage impressed on the structure by the anode. The shaded area shows the area of influence in which the pipe-to-soil potential exceeds -1.2 volts versus Cu-CuSO4. Close anode beds are also called distributed anode beds. Distributed anode beds are installed as surface anodes (