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CorrosionCathProtBasicDesignSATX

This document provides an overview and agenda for a presentation on modern cathodic protection for piping. It discusses corrosion basics, criteria for cathodic protection, design considerations for galvanic anode systems, and types of cathodic protection systems. The document uses examples to demonstrate how to design a cathodic protection system using galvanic anodes to protect an underground coated steel pipe over 20 years. It compares zinc, standard magnesium, and high-potential magnesium anodes in terms of current output, life expectancy, and cost.

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2/13/2013 1 Modern Cathodic Protection for Piping Stanley Worcester, P.E. NACE International Certified Cathodic Protection Specialist February 20, 2013 worcesterstan@stanleygroup.com 303 925-8307 1.Corrosion Basics 2.Cathodic Protection (CP) Criteria 3.Cathodic Protection Design (galvanic anodes) 4.Types of Cathodic Protection Systems 5.Protected Structures 6.Structure Isolation 7.Design Considerations References Questions Agenda
2/13/2013 2 Is it a religion? (catholic?) Is it a security system? (protection?) Is it magic? What is Cathodic Protection (CP)? It is an electrochemical reaction that mitigates corrosion. How does it work with a natural voltage less than a “AA” battery? Anode, Cathode, Electrolyte, Metallic Path – Cable Connection “+”(Pipe) (Cable) “-” (Mg, Zn) Corrosion Protection (DC)
2/13/2013 3 What is CP? Electrochemical Reaction Electrical portion– electron flow (later) Chemical portion– removal of electrons (oxidation-Anodic) and consumption of electrons (reduction reaction-Cathodic) Anodic Cathodic Cathodic CP - Electrical Electrical – conventional current (electron) flow from a positive voltage to a negative voltage. Ohm’s Law: I=V/R, V=IR, E=IR, R=V/I I is current (Amperes) R is Resistance (Ohms, Ω) V or E is Voltage or potential (Volts) What is current? What is resistance? What is IR drop? Electrons leaving a power supply are trying to get to “Ground” (the earth)? True or False? Voltage drop False
2/13/2013 4 Electrical Question - Does current take the path of least resistance? Answer – No, current follows all paths. 1 Ohm 1,000 Ohms ↓ ↓ ↓↓ Anodic Cathodic
2/13/2013 5 New Pipe Corrosion 1. Iron – ductile, soft 2. Carbon - Strong, hard 3. Manganese 4. Silicon 5. Copper Elements in Carbon Steel Pipe Corrosion rate of buried steel is approximately 6 mils/yr. or 0.006”/yr. Since 1 mil = 0.001 inches. Assume pipe will fail when 1/4 of the wall thickness is gone, a 0.295” pipe wall thickness will fail in 12 years without any cathodic protection.
2/13/2013 6 Galvanic Corrosion Metallic Path – Dissimilar Metals Galvanic Corrosion Metallic Path – Dissimilar Soils
2/13/2013 7 Cathodic Protection (CP) Cathodic protection is considered to be effective when active corrosion is transferred from the metal structure surface to the installed anode. Effectiveness of transference can be determined by electrical measurements. Industry-accepted criteria for effective protection using these measurements are fully described in various NACE International publications including the Standard Practice “SP0169-2007 Control of External Corrosion on Underground or Submerged Metallic Piping Systems." Does protection reach this side of the pipe? Does Size Matter? Is one anode sufficient?
2/13/2013 8 Typical Packaged Galvanic Anode Backfill Material 50 Ohm-cm Cathodic Protection (CP) Anode Shape Difference Same weight Low resistance Large current Short life Short & Wide L o n g & T h i n Same weight High resistance Small current Long life
2/13/2013 9 Cathodic Protection Criteria Known as the -0.850 Volt, pipe-to-soil potential 1. This is a measured voltage between the steel structure & a copper-copper sulfate reference electrode contacting the electrolyte near the structure. If voltage difference is more negative than -0.850 Volts, then the structure is considered protected with consideration of voltage drop. If the value is more positive than -0.850 Volts, the structure is either unprotected or only partially protected. 2. A polarized potential with interruption of protection current (instant off) of -0.850V. 3. 100mV of cathodic polarization. The formation or decay of polarization is often measured in impressed current systems. Underground Permanent Reference Electrode?
2/13/2013 10 Typical CP Design Questions - Example What structure do I want to protect? External surface of underground coated, carbon steel 12” water pipe, 4’ depth, 100’ long. What is the surface area? 12” or 1 ft. Diam, 100 Ft long. PI x D X L = A (Surface Area) PI (π) = 3.14 D=1 ft. L=100 ft. A = 314 square ft. Typical CP Design Questions - Example Is the soil corrosive? Get soil resistivity (ρ)measurement results and have laboratory chemical analysis performed. ρ = 2,000 Ohm-cm, Chlorides = 100ppm, Sulfates = 100ppm, pH = 7. What else is in the soil? Know the area, rural, residential, congested, other structures, rocky, clay, sand, water table, etc….. What type of CP? Use galvanic anode design?
2/13/2013 11 Soil Resistivity Measurements Do I need CP? Soil Laboratory Analysis Do I need CP?
2/13/2013 12 It’s the Law Cathodic protection is a mandated requirement of federal & state regulations governing underground transmission pipeline, gas distribution systems, hazardous liquids & underground petroleum tanks. These requirements include installation, monitoring, & maintenance of cathodic protection systems*. *See CFR References Typical CP Design Questions - Example How long do I want the anodes to protect the structure? Client wants 20 years. (CP System Life) Which galvanic anode material is best? Zinc anodes are typically installed in water or soil with resistivity's less than 2,000 Ohm-cm. Std Magnesium is typically 1000-5,000 Ohm-cm Hi-Potential Magnesium is typically 2000 Ohm-cm and higher. What is the pipe coating efficiency? 90-98% for field applied coatings 95-99.9% for new factory coatings
2/13/2013 13 Typical CP Design Questions Does a pipe coating alone provide enough protection? Is there a perfect coating? How long will it last? Is polyethylene encasement of ductile iron water pipe enough? 48# Mag Anode
2/13/2013 14 Typical CP Design Questions - Example Where do I install galvanic anodes? In permanently moist soil (min 5 ft. deep) Deeper than bottom of pipe Protected from future construction 10’ away from pipe Vertical or horizontal? How do I connect anodes to pipe? Exothermic weld lead wire to pipe directly? Terminate anode lead wire in test station and bond to pipe lead wire.
2/13/2013 15 Support lead wire and provide slack wire for settlement 48# Mag Anode
2/13/2013 16 One Galvanic Anode – Direct Connection What if copper cable is exposed? Test Station Connection – Anode Bank
2/13/2013 17 Cathodic Protection (CP) Design Guidelines Assume a minimum of 1 mA per square foot required to protect bare structure. Assume a minimum of 2 mA per square foot of exposed surface area required to protect a coated structure. Current requirements (density) increase with low soil resistivity, high temperature, moisture, water flow, dissimilar metals, & chloride content. Typical CP Design Questions What current density do I need? Reference Army Technical Manual TM 5-811-7 1985
2/13/2013 18 CP Design - Example Current Requirement for coated pipe 95% coated or 5% of 314 sq. ft. of surface area is bare. Bare surface area is 314 x 0.05 =15.7 sq. ft. 15.7 sq. ft. x 2mA/sq. ft = 31.4 mA. Driving Potential of anodes Zinc = -1.10 V – 0.85 (pipe)= 0.25V Std Mg = -1.55 V – 0.85 (pipe) = 0.70V Hi-Pot Mg = 1.75 V – 0.85 (pipe) = 0.90V Maximum anode resistance R=V / I Zinc = .25/.0314 = 8.0 Ohms Std Mg = .70/.0314 = 22.3 Ohms Hi-Pot Mg = .90/.0314 = 28.7 Ohms CP Design - Example Find Total Resistance RT of CP system. Rv = Anode resistance of vertical anode Rw = Lead wire resistance of 25’ of #12AWG Rp = Pipe resistance 100’ of 12” diameter Rc = Coating resistance – Specified by supplier – (negligible on large structures and this example) RT = Rv + Rw + Rp + Rc RT = Rv + 0* RT = Rv *Since only the anode resistance is significant in this example.
2/13/2013 19 CP Design - Example Find lead wire resistance Lead wire resistance of 25’ of #12AWG, HMWPE Look up cable table in NEC, Rw = 1.93 Ohms / 1000 ft. or 0.00193 Ohms per ft. or .0483 Ohms for 25’ (negligible) Find pipe resistance Pipe resistance 100’ of 12” diameter In Peabody Rp = 5.82 microhms / ft. or 0.00000582 Ohms / ft. (negligible) CP Design - Example Comparison of 3 anode types Zinc anode 18 lbs 5”x38” Packaged Std H-1 Mg 17 lbs 6.5”x19” Packaged Hi Potential Mg 17 lbs 6”x29” Packaged Compare these similar weight anodes for current output and life expectancy. Do you ever install just one anode?
2/13/2013 20 CP Design - Example Zinc anode 18 lbs, 5” x 38” Packaged Dwight’s Equation for vertical anode resistance to earth - feet Dwight’s Equation for multiple vertical anodes in parallel Ohms 8.0 Ohms max. CP Design - Example Zinc anode 18 lbs, 5” x 38” Packaged The resistance to earth for one anode is 10.23 Ohms and for two anodes in parallel is 5.40 Ohms. One anode maximum current output is I = V / R I = 0.25V / 10.23 Ohms = .024 Amps or 24 mA, which is less than the 31.4 mA required. Two anodes in parallel maximum current output is I = V / R I = 0.25V / 5.4 Ohms = .046 Amps or 46 mA max or 31.4 / 2 = 15.7 mA each which is acceptable One zinc anode could not produce enough current to cathodically protect the coated pipe.
2/13/2013 21 CP Design - Example Std Mg anode 17 lbs, 6.5”x19” Packaged Dwight’s Equation for vertical anode resistance to earth - feet Dwight’s Equation for multiple vertical anodes in parallel Ohms 22.3 Ohms max. CP Design - Example Std Mg anode 17 lbs, 6.5”x19” Packaged The resistance to earth for one anode is 14.18 Ohms and for two anodes in parallel is 7.48 Ohms. One anode maximum current output is I = V / R I = 0.70V / 14.18 Ohms = .049 Amps or 49 mA, which is more than the 31.4 mA required and therefore acceptable. Two anodes in parallel maximum current output is I = V / R I = 0.70V / 7.48 Ohms = .095 Amps or 95 mA, which is more than the 31.4 mA required and therefore acceptable. One or two Std Mg anodes could produce enough current to cathodically protect the coated pipe.
2/13/2013 22 CP Design - Example Hi-Pot Mg anode 17 lbs, 6” x 29” Packaged Dwight’s Equation for vertical anode resistance to earth - feet Dwight’s Equation for multiple vertical anodes in parallel Ohms 28.7 Ohms max. CP Design - Example Hi-Pot Mg anode 17 lbs, 6” x 29” Packaged The resistance to earth for one anode is 11.44 Ohms and for two anodes in parallel is 6.0 Ohms. One anode maximum current output is I = V / R I = 0.90V / 11.44 Ohms = .079 Amps or 79 mA, which is more than the 31.4 mA required and therefore acceptable. Two anodes in parallel maximum current output is I = V / R I = 0.90V / 6.0 Ohms = 0.150 Amps or 150 mA, which is more than the 31.4 mA required and therefore acceptable. One or two High Pot Mg anodes could produce enough current to cathodically protect the coated pipe.
2/13/2013 23 CP Design - Example Zinc anode Life expectancy for coated structure L (1) = 0.0424 x 18 x 0.90 x 0.85 = 24.3 yrs. .024 (one anode) L (2) = 0.0424 x 18 x 0.90 x 0.85 = 37.2 yrs. .0157 (each anode) One anode is not sufficient for pipe surface area and two anodes are acceptable at 37.2 yrs. each. CP Design - Example Std Mg anode Life expectancy for coated structure L (min) = 0.116 x 17 x 0.50 x 0.85 = 17.8 yrs. .047 (max one anode) L (1) = 0.116 x 17 x 0.50 x 0.85 = 26.7 yrs. .0314 (1 anode) L (2) = 0.116 x 17 x 0.50 x 0.85 = 53.4 yrs. 2 anodes .0157 (each anode) One anode is sufficient for pipe surface area and capable of 47 mA discharge but only 31mA is required for protection.
2/13/2013 24 CP Design - Example Hi-Pot Mg anode Life expectancy for coated structure L (min) = 0.116 x 17 x 0.50 x 0.85 = 10.6 yrs. .079 (max one anode) L (1) = 0.116 x 17 x 0.50 x 0.85 = 26.7 yrs. .0314 (1 anode) L (2) = 0.116 x 17 x 0.50 x 0.85 = 53.4 yrs. 2 anodes .0157 (each anode) One anode is sufficient for pipe surface area and capable of 79 mA discharge but only 31mA is required for protection. CP Design - Example Summary of 3 anode type Comparison Which Anode is best?
2/13/2013 25 CP Design – Example 20 Yrs. & 31.4 mA $100 $110 $170 Previous CP Design Example Assumptions Only pipe design, only external design No dissimilar metals, no aggressive soils Only vertical anode installation not horizontal Welded (electrically continuous) pipe not gasketed bell and spigot joint pipe. No ROW restrictions No valves, taps or modifications in pipeline No shielding by structures, no isolation issues No backfill damage or future construction damage No line crossings, no stray current No rectifiers, no casings No concrete encasement….
2/13/2013 26 Types of Cathodic Protection Systems Galvanic or sacrificial anode system. (magnesium, zinc, or aluminum anodes) Impressed current system. (rectifier with graphite, mixed metal oxide, silicon iron or several other anode materials) Galvanic Advantages: flexibility in application, anodes can be installed (evenly distributed) in a variety of applications & configurations. No outside power is required, minimal maintenance required for these systems to function. Disadvantages: limited protection current available, limited life, rapid corrosion (consumption), require replacement on a regular basis, Design life of a pipeline system anode is ten to twenty years.
2/13/2013 27 Impressed Current Advantages: unlimited current opportunities & longer life, installed where the structure to be protected is large, requiring higher levels of current. Easily adjusted to suite needs and changes. Disadvantages: requirement for an outside power source & higher maintenance requirements. Outside power might come from sources such as commercial AC converted to DC through a rectifier, thermoelectric generator, or solar panels. A significantly higher monitoring & maintenance effort is required by comparison to sacrificial anode systems. Higher capital cost and maintenance costs. Stray currents. Shielded structures. Impressed Current What if anode cable is exposed? What if “+” and “-” are reversed?
2/13/2013 28 Impressed Current Rectifier Impressed Current External power source that pushes DC current through long-lasting anodes. Typical source of power is AC power converted to DC by a rectifier. Designed for long life at adjustable high current output. This requires selection of materials with very low corrosion (consumption) rates, including treated graphite, high silicon cast iron, mixed metal oxide, & to a lesser extent, platinum. Anodes are normally installed in grouped configurations in the electrolyte. These groupings at one location (both horizontal & vertical) in underground application are called groundbeds.
2/13/2013 29 Deep Groundbed Deep Groundbed Drilling - Rural
2/13/2013 30 Deep Groundbed Drilling - City Deep Groundbed Drilling - City
2/13/2013 31 High Silicon Cast Iron Anodes Anode Centralizers
2/13/2013 32 Impressed Current Deep Groundbeds Advantages: Located in congested areas, less real estate on small footprint, lower resistance, better current distribution, reduced shielding and stray current, less damage caused by construction, no seasonal variation, anodes can be replaced in future. Disadvantages: A higher installation cost, higher anode inspection or replacement cost, care to prevent ground water intermixing or contamination.
2/13/2013 33 Structure Junction Box – Rectifier Negative Anode Junction Box – Rectifier Positive
2/13/2013 34 Cathodically Protected Structures Underground Pipelines: Both sacrificial & impressed current systems are used. Federal & state regulations require cathodic protection for most petroleum or gas pipeline systems. Underground Storage Tanks (USTs): required by EPA to either have functional cathodic protection systems or to be of a non-corrosive material. Aboveground Storage Tank (ASTs): bottoms can be protected from soil-side corrosion with cathodic protection. Unique problems involved with tank applications include difficulty of distributing current uniformly over tank bottom & monitoring effectiveness of systems. Any metal connected to protected structure. (This requires additional current). Above-Ground Storage Tank
2/13/2013 35 Structure Isolation Protected structures need to be isolated from other structures by separation, coatings, or insulated materials. If isolation is not possible, cathodic protection anodes can be placed in close proximity to structure to ensure protection is achieved. Insulated Flange Kit
2/13/2013 36 Testing Design Considerations Limitations of cathodic protection must be recognized during the design process. Cathodic protection will be effective only on metal surfaces in continual contact with the electrolyte. Above-ground structures will not be protected. The distribution of current to desired areas becomes difficult in congested, shielded or remote areas. Examples include multiple pipeline right-of-ways, shorted castings and storage tank bottoms.
2/13/2013 37 Design Considerations Design Considerations
2/13/2013 38 Design Considerations Design Considerations
2/13/2013 39 Stray Current –Parallel Pipelines Stray Current - Crossing
2/13/2013 40 Testing Soil resistivity measurements. Current requirement tests. Test stations every 500-2000 feet. Insulated flange kit isolation tests. Permanent reference electrode tests. Rectifier voltage adjustments, structure current resistance, anode currents, current interruption for instant off readings. Test Equipment
2/13/2013 41 Portable Reference Electrodes Underground Reference Electrode
2/13/2013 42 Above Ground Test Station Below Grade Test Station
2/13/2013 43 Below Grade Test Stations Below Grade Test Stations
2/13/2013 44 Test Station Terminations Test Station
2/13/2013 45 Test Station problems Test Station problems
2/13/2013 46 1.Corrosion Basics 2.Cathodic Protection Criteria 3.Cathodic Protection Design (galvanic anodes) 4.Types of Cathodic Protection Systems 5.Protected Structures 6.Structure Isolation 7.Design Considerations Summary References “Control of Pipeline Corrosion” A.W. Peabody. NACE Std SP0169-2007 Control of External Corrosion on Underground or Submerged Metallic Piping Systems. Code of Federal Regulations CFR 49.192-2012 Transportation of Natural and other Gas by Pipeline. Code of Federal Regulations CFR 49.195-2012 Transportation of Hazardous Liquids by Pipeline. “Galvanic Corrosion” Harvey P. Hack. “NACE Corrosion Engineer’s Reference Book” R. S. Treseder. ASTM G57 Soil resistivity using the Wenner 4-pin method.
2/13/2013 47 References ARMY Technical Manual TM 5-811-7, 1995 Electrical Design, Cathodic Protection. USACE Engineering Manual EM 1110-2-2704, 2004 Cathodic Protection systems for Civil Works Structures. NACE Std RP0193-2001 External Cathodic Protection of On- Grade Carbon Steel Storage Tank Bottoms. “NACE Std RP0572-2001 Design, Installation, Operation, and Maintenance of Impressed Current Deep Ground beds. NFPA 70-2011 National Electric Code (NEC) IEEE Std 81. IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System. Questions?

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CorrosionCathProtBasicDesignSATX

  • 1.
    2/13/2013 1 Modern Cathodic Protection forPiping Stanley Worcester, P.E. NACE International Certified Cathodic Protection Specialist February 20, 2013 worcesterstan@stanleygroup.com 303 925-8307 1.Corrosion Basics 2.Cathodic Protection (CP) Criteria 3.Cathodic Protection Design (galvanic anodes) 4.Types of Cathodic Protection Systems 5.Protected Structures 6.Structure Isolation 7.Design Considerations References Questions Agenda
  • 2.
    2/13/2013 2 Is it areligion? (catholic?) Is it a security system? (protection?) Is it magic? What is Cathodic Protection (CP)? It is an electrochemical reaction that mitigates corrosion. How does it work with a natural voltage less than a “AA” battery? Anode, Cathode, Electrolyte, Metallic Path – Cable Connection “+”(Pipe) (Cable) “-” (Mg, Zn) Corrosion Protection (DC)
  • 3.
    2/13/2013 3 What is CP? ElectrochemicalReaction Electrical portion– electron flow (later) Chemical portion– removal of electrons (oxidation-Anodic) and consumption of electrons (reduction reaction-Cathodic) Anodic Cathodic Cathodic CP - Electrical Electrical – conventional current (electron) flow from a positive voltage to a negative voltage. Ohm’s Law: I=V/R, V=IR, E=IR, R=V/I I is current (Amperes) R is Resistance (Ohms, Ω) V or E is Voltage or potential (Volts) What is current? What is resistance? What is IR drop? Electrons leaving a power supply are trying to get to “Ground” (the earth)? True or False? Voltage drop False
  • 4.
    2/13/2013 4 Electrical Question - Doescurrent take the path of least resistance? Answer – No, current follows all paths. 1 Ohm 1,000 Ohms ↓ ↓ ↓↓ Anodic Cathodic
  • 5.
    2/13/2013 5 New Pipe Corrosion 1.Iron – ductile, soft 2. Carbon - Strong, hard 3. Manganese 4. Silicon 5. Copper Elements in Carbon Steel Pipe Corrosion rate of buried steel is approximately 6 mils/yr. or 0.006”/yr. Since 1 mil = 0.001 inches. Assume pipe will fail when 1/4 of the wall thickness is gone, a 0.295” pipe wall thickness will fail in 12 years without any cathodic protection.
  • 6.
    2/13/2013 6 Galvanic Corrosion Metallic Path– Dissimilar Metals Galvanic Corrosion Metallic Path – Dissimilar Soils
  • 7.
    2/13/2013 7 Cathodic Protection (CP) Cathodicprotection is considered to be effective when active corrosion is transferred from the metal structure surface to the installed anode. Effectiveness of transference can be determined by electrical measurements. Industry-accepted criteria for effective protection using these measurements are fully described in various NACE International publications including the Standard Practice “SP0169-2007 Control of External Corrosion on Underground or Submerged Metallic Piping Systems." Does protection reach this side of the pipe? Does Size Matter? Is one anode sufficient?
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    2/13/2013 8 Typical Packaged GalvanicAnode Backfill Material 50 Ohm-cm Cathodic Protection (CP) Anode Shape Difference Same weight Low resistance Large current Short life Short & Wide L o n g & T h i n Same weight High resistance Small current Long life
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    2/13/2013 9 Cathodic Protection Criteria Knownas the -0.850 Volt, pipe-to-soil potential 1. This is a measured voltage between the steel structure & a copper-copper sulfate reference electrode contacting the electrolyte near the structure. If voltage difference is more negative than -0.850 Volts, then the structure is considered protected with consideration of voltage drop. If the value is more positive than -0.850 Volts, the structure is either unprotected or only partially protected. 2. A polarized potential with interruption of protection current (instant off) of -0.850V. 3. 100mV of cathodic polarization. The formation or decay of polarization is often measured in impressed current systems. Underground Permanent Reference Electrode?
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    2/13/2013 10 Typical CP DesignQuestions - Example What structure do I want to protect? External surface of underground coated, carbon steel 12” water pipe, 4’ depth, 100’ long. What is the surface area? 12” or 1 ft. Diam, 100 Ft long. PI x D X L = A (Surface Area) PI (π) = 3.14 D=1 ft. L=100 ft. A = 314 square ft. Typical CP Design Questions - Example Is the soil corrosive? Get soil resistivity (ρ)measurement results and have laboratory chemical analysis performed. ρ = 2,000 Ohm-cm, Chlorides = 100ppm, Sulfates = 100ppm, pH = 7. What else is in the soil? Know the area, rural, residential, congested, other structures, rocky, clay, sand, water table, etc….. What type of CP? Use galvanic anode design?
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    2/13/2013 11 Soil Resistivity Measurements DoI need CP? Soil Laboratory Analysis Do I need CP?
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    2/13/2013 12 It’s the Law Cathodicprotection is a mandated requirement of federal & state regulations governing underground transmission pipeline, gas distribution systems, hazardous liquids & underground petroleum tanks. These requirements include installation, monitoring, & maintenance of cathodic protection systems*. *See CFR References Typical CP Design Questions - Example How long do I want the anodes to protect the structure? Client wants 20 years. (CP System Life) Which galvanic anode material is best? Zinc anodes are typically installed in water or soil with resistivity's less than 2,000 Ohm-cm. Std Magnesium is typically 1000-5,000 Ohm-cm Hi-Potential Magnesium is typically 2000 Ohm-cm and higher. What is the pipe coating efficiency? 90-98% for field applied coatings 95-99.9% for new factory coatings
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    2/13/2013 13 Typical CP DesignQuestions Does a pipe coating alone provide enough protection? Is there a perfect coating? How long will it last? Is polyethylene encasement of ductile iron water pipe enough? 48# Mag Anode
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    2/13/2013 14 Typical CP DesignQuestions - Example Where do I install galvanic anodes? In permanently moist soil (min 5 ft. deep) Deeper than bottom of pipe Protected from future construction 10’ away from pipe Vertical or horizontal? How do I connect anodes to pipe? Exothermic weld lead wire to pipe directly? Terminate anode lead wire in test station and bond to pipe lead wire.
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    2/13/2013 15 Support lead wireand provide slack wire for settlement 48# Mag Anode
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    2/13/2013 16 One Galvanic Anode– Direct Connection What if copper cable is exposed? Test Station Connection – Anode Bank
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    2/13/2013 17 Cathodic Protection (CP)Design Guidelines Assume a minimum of 1 mA per square foot required to protect bare structure. Assume a minimum of 2 mA per square foot of exposed surface area required to protect a coated structure. Current requirements (density) increase with low soil resistivity, high temperature, moisture, water flow, dissimilar metals, & chloride content. Typical CP Design Questions What current density do I need? Reference Army Technical Manual TM 5-811-7 1985
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    2/13/2013 18 CP Design -Example Current Requirement for coated pipe 95% coated or 5% of 314 sq. ft. of surface area is bare. Bare surface area is 314 x 0.05 =15.7 sq. ft. 15.7 sq. ft. x 2mA/sq. ft = 31.4 mA. Driving Potential of anodes Zinc = -1.10 V – 0.85 (pipe)= 0.25V Std Mg = -1.55 V – 0.85 (pipe) = 0.70V Hi-Pot Mg = 1.75 V – 0.85 (pipe) = 0.90V Maximum anode resistance R=V / I Zinc = .25/.0314 = 8.0 Ohms Std Mg = .70/.0314 = 22.3 Ohms Hi-Pot Mg = .90/.0314 = 28.7 Ohms CP Design - Example Find Total Resistance RT of CP system. Rv = Anode resistance of vertical anode Rw = Lead wire resistance of 25’ of #12AWG Rp = Pipe resistance 100’ of 12” diameter Rc = Coating resistance – Specified by supplier – (negligible on large structures and this example) RT = Rv + Rw + Rp + Rc RT = Rv + 0* RT = Rv *Since only the anode resistance is significant in this example.
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    2/13/2013 19 CP Design -Example Find lead wire resistance Lead wire resistance of 25’ of #12AWG, HMWPE Look up cable table in NEC, Rw = 1.93 Ohms / 1000 ft. or 0.00193 Ohms per ft. or .0483 Ohms for 25’ (negligible) Find pipe resistance Pipe resistance 100’ of 12” diameter In Peabody Rp = 5.82 microhms / ft. or 0.00000582 Ohms / ft. (negligible) CP Design - Example Comparison of 3 anode types Zinc anode 18 lbs 5”x38” Packaged Std H-1 Mg 17 lbs 6.5”x19” Packaged Hi Potential Mg 17 lbs 6”x29” Packaged Compare these similar weight anodes for current output and life expectancy. Do you ever install just one anode?
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    2/13/2013 20 CP Design -Example Zinc anode 18 lbs, 5” x 38” Packaged Dwight’s Equation for vertical anode resistance to earth - feet Dwight’s Equation for multiple vertical anodes in parallel Ohms 8.0 Ohms max. CP Design - Example Zinc anode 18 lbs, 5” x 38” Packaged The resistance to earth for one anode is 10.23 Ohms and for two anodes in parallel is 5.40 Ohms. One anode maximum current output is I = V / R I = 0.25V / 10.23 Ohms = .024 Amps or 24 mA, which is less than the 31.4 mA required. Two anodes in parallel maximum current output is I = V / R I = 0.25V / 5.4 Ohms = .046 Amps or 46 mA max or 31.4 / 2 = 15.7 mA each which is acceptable One zinc anode could not produce enough current to cathodically protect the coated pipe.
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    2/13/2013 21 CP Design -Example Std Mg anode 17 lbs, 6.5”x19” Packaged Dwight’s Equation for vertical anode resistance to earth - feet Dwight’s Equation for multiple vertical anodes in parallel Ohms 22.3 Ohms max. CP Design - Example Std Mg anode 17 lbs, 6.5”x19” Packaged The resistance to earth for one anode is 14.18 Ohms and for two anodes in parallel is 7.48 Ohms. One anode maximum current output is I = V / R I = 0.70V / 14.18 Ohms = .049 Amps or 49 mA, which is more than the 31.4 mA required and therefore acceptable. Two anodes in parallel maximum current output is I = V / R I = 0.70V / 7.48 Ohms = .095 Amps or 95 mA, which is more than the 31.4 mA required and therefore acceptable. One or two Std Mg anodes could produce enough current to cathodically protect the coated pipe.
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    2/13/2013 22 CP Design -Example Hi-Pot Mg anode 17 lbs, 6” x 29” Packaged Dwight’s Equation for vertical anode resistance to earth - feet Dwight’s Equation for multiple vertical anodes in parallel Ohms 28.7 Ohms max. CP Design - Example Hi-Pot Mg anode 17 lbs, 6” x 29” Packaged The resistance to earth for one anode is 11.44 Ohms and for two anodes in parallel is 6.0 Ohms. One anode maximum current output is I = V / R I = 0.90V / 11.44 Ohms = .079 Amps or 79 mA, which is more than the 31.4 mA required and therefore acceptable. Two anodes in parallel maximum current output is I = V / R I = 0.90V / 6.0 Ohms = 0.150 Amps or 150 mA, which is more than the 31.4 mA required and therefore acceptable. One or two High Pot Mg anodes could produce enough current to cathodically protect the coated pipe.
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    2/13/2013 23 CP Design -Example Zinc anode Life expectancy for coated structure L (1) = 0.0424 x 18 x 0.90 x 0.85 = 24.3 yrs. .024 (one anode) L (2) = 0.0424 x 18 x 0.90 x 0.85 = 37.2 yrs. .0157 (each anode) One anode is not sufficient for pipe surface area and two anodes are acceptable at 37.2 yrs. each. CP Design - Example Std Mg anode Life expectancy for coated structure L (min) = 0.116 x 17 x 0.50 x 0.85 = 17.8 yrs. .047 (max one anode) L (1) = 0.116 x 17 x 0.50 x 0.85 = 26.7 yrs. .0314 (1 anode) L (2) = 0.116 x 17 x 0.50 x 0.85 = 53.4 yrs. 2 anodes .0157 (each anode) One anode is sufficient for pipe surface area and capable of 47 mA discharge but only 31mA is required for protection.
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    2/13/2013 24 CP Design -Example Hi-Pot Mg anode Life expectancy for coated structure L (min) = 0.116 x 17 x 0.50 x 0.85 = 10.6 yrs. .079 (max one anode) L (1) = 0.116 x 17 x 0.50 x 0.85 = 26.7 yrs. .0314 (1 anode) L (2) = 0.116 x 17 x 0.50 x 0.85 = 53.4 yrs. 2 anodes .0157 (each anode) One anode is sufficient for pipe surface area and capable of 79 mA discharge but only 31mA is required for protection. CP Design - Example Summary of 3 anode type Comparison Which Anode is best?
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    2/13/2013 25 CP Design –Example 20 Yrs. & 31.4 mA $100 $110 $170 Previous CP Design Example Assumptions Only pipe design, only external design No dissimilar metals, no aggressive soils Only vertical anode installation not horizontal Welded (electrically continuous) pipe not gasketed bell and spigot joint pipe. No ROW restrictions No valves, taps or modifications in pipeline No shielding by structures, no isolation issues No backfill damage or future construction damage No line crossings, no stray current No rectifiers, no casings No concrete encasement….
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    2/13/2013 26 Types of Cathodic ProtectionSystems Galvanic or sacrificial anode system. (magnesium, zinc, or aluminum anodes) Impressed current system. (rectifier with graphite, mixed metal oxide, silicon iron or several other anode materials) Galvanic Advantages: flexibility in application, anodes can be installed (evenly distributed) in a variety of applications & configurations. No outside power is required, minimal maintenance required for these systems to function. Disadvantages: limited protection current available, limited life, rapid corrosion (consumption), require replacement on a regular basis, Design life of a pipeline system anode is ten to twenty years.
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    2/13/2013 27 Impressed Current Advantages: unlimitedcurrent opportunities & longer life, installed where the structure to be protected is large, requiring higher levels of current. Easily adjusted to suite needs and changes. Disadvantages: requirement for an outside power source & higher maintenance requirements. Outside power might come from sources such as commercial AC converted to DC through a rectifier, thermoelectric generator, or solar panels. A significantly higher monitoring & maintenance effort is required by comparison to sacrificial anode systems. Higher capital cost and maintenance costs. Stray currents. Shielded structures. Impressed Current What if anode cable is exposed? What if “+” and “-” are reversed?
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    2/13/2013 28 Impressed Current Rectifier ImpressedCurrent External power source that pushes DC current through long-lasting anodes. Typical source of power is AC power converted to DC by a rectifier. Designed for long life at adjustable high current output. This requires selection of materials with very low corrosion (consumption) rates, including treated graphite, high silicon cast iron, mixed metal oxide, & to a lesser extent, platinum. Anodes are normally installed in grouped configurations in the electrolyte. These groupings at one location (both horizontal & vertical) in underground application are called groundbeds.
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    2/13/2013 30 Deep Groundbed Drilling- City Deep Groundbed Drilling - City
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    2/13/2013 31 High Silicon CastIron Anodes Anode Centralizers
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    2/13/2013 32 Impressed Current DeepGroundbeds Advantages: Located in congested areas, less real estate on small footprint, lower resistance, better current distribution, reduced shielding and stray current, less damage caused by construction, no seasonal variation, anodes can be replaced in future. Disadvantages: A higher installation cost, higher anode inspection or replacement cost, care to prevent ground water intermixing or contamination.
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    2/13/2013 33 Structure Junction Box– Rectifier Negative Anode Junction Box – Rectifier Positive
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    2/13/2013 34 Cathodically Protected Structures UndergroundPipelines: Both sacrificial & impressed current systems are used. Federal & state regulations require cathodic protection for most petroleum or gas pipeline systems. Underground Storage Tanks (USTs): required by EPA to either have functional cathodic protection systems or to be of a non-corrosive material. Aboveground Storage Tank (ASTs): bottoms can be protected from soil-side corrosion with cathodic protection. Unique problems involved with tank applications include difficulty of distributing current uniformly over tank bottom & monitoring effectiveness of systems. Any metal connected to protected structure. (This requires additional current). Above-Ground Storage Tank
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    2/13/2013 35 Structure Isolation Protected structuresneed to be isolated from other structures by separation, coatings, or insulated materials. If isolation is not possible, cathodic protection anodes can be placed in close proximity to structure to ensure protection is achieved. Insulated Flange Kit
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    2/13/2013 36 Testing Design Considerations Limitations ofcathodic protection must be recognized during the design process. Cathodic protection will be effective only on metal surfaces in continual contact with the electrolyte. Above-ground structures will not be protected. The distribution of current to desired areas becomes difficult in congested, shielded or remote areas. Examples include multiple pipeline right-of-ways, shorted castings and storage tank bottoms.
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    2/13/2013 39 Stray Current –ParallelPipelines Stray Current - Crossing
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    2/13/2013 40 Testing Soil resistivity measurements. Currentrequirement tests. Test stations every 500-2000 feet. Insulated flange kit isolation tests. Permanent reference electrode tests. Rectifier voltage adjustments, structure current resistance, anode currents, current interruption for instant off readings. Test Equipment
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    2/13/2013 42 Above Ground TestStation Below Grade Test Station
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    2/13/2013 43 Below Grade TestStations Below Grade Test Stations
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    2/13/2013 46 1.Corrosion Basics 2.Cathodic ProtectionCriteria 3.Cathodic Protection Design (galvanic anodes) 4.Types of Cathodic Protection Systems 5.Protected Structures 6.Structure Isolation 7.Design Considerations Summary References “Control of Pipeline Corrosion” A.W. Peabody. NACE Std SP0169-2007 Control of External Corrosion on Underground or Submerged Metallic Piping Systems. Code of Federal Regulations CFR 49.192-2012 Transportation of Natural and other Gas by Pipeline. Code of Federal Regulations CFR 49.195-2012 Transportation of Hazardous Liquids by Pipeline. “Galvanic Corrosion” Harvey P. Hack. “NACE Corrosion Engineer’s Reference Book” R. S. Treseder. ASTM G57 Soil resistivity using the Wenner 4-pin method.
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    2/13/2013 47 References ARMY Technical ManualTM 5-811-7, 1995 Electrical Design, Cathodic Protection. USACE Engineering Manual EM 1110-2-2704, 2004 Cathodic Protection systems for Civil Works Structures. NACE Std RP0193-2001 External Cathodic Protection of On- Grade Carbon Steel Storage Tank Bottoms. “NACE Std RP0572-2001 Design, Installation, Operation, and Maintenance of Impressed Current Deep Ground beds. NFPA 70-2011 National Electric Code (NEC) IEEE Std 81. IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System. Questions?