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Agenda • • • • • In class training over PowerPoint Open discussion and review Lunch Hands on and field testing Follow up back in class room setting Definition of Stray Current Interference The National Association of Corrosion Engineers (NACE) recommended practice on cathodic protection underground structures provides several insights to the definition and evaluation of interference. Stray current is defined as “current through paths other than the intended circuit” or “the deterioration of a material, usually a metal that results from a reaction with its environment”. Interference Objectives • Recognize • Test • Mitigate Recognize • Pipe to soil potentials indication of possible interference situation • Areas of high voltage gradients or polarization • Structural effect of stray current pickup and discharge Test • Basic survey measurements techniques • What are the minimum requirements? • Selection of testing equipment and materials • IR and Polarization measurements Mitigate • Why mitigate? • Difficulties in interpreting data? • Selection and method of mitigation Fundamental Review • Corrosion diminishes the integrity of the pipeline, increases the probability of pipeline failures and loss of system reliability. • Failure to properly analyze and arrest corrosion may result in loss of life and property, jeopardize pipeline integrity and corporate image, not to mention the economic forfeiture of pipeline system revenues. Fundamental Review Continued This training session encompasses interference stray current, effect on pipeline polarization, recognition and role of voltage gradients, importance of the consideration of voltage drops, and understanding when stray current interference may result in corrosion. Basic Faradays’ Law Rates of Metal Loss Fe (Iron) / STEEL Al (Aluminum) Cu (Copper) Pb (Lead) Mg (Magnesium) Zn (Zinc) 20LBs /Amp/ Year 6.5 45.7 74.5 8.8 23.6 Given: 1 amp of current discharging from a pipeline for 1 year. Metal loss: Approximately 20 lbs. EQUIVALENT METAL LOSS Equivalent Length Pipe Diameter/W.T. Pipe Weight/Foot of Pipe Loss 4” = 4.500” O.D. x 0.188 W.T 8.66 lbs/ft. 2.31 ft. 6” = 6.625” O.D. x 0.280 W.T. 18.97 lbs/ft. 1.05 ft. 10” = 10.750” O.D. x 0.188 W.T. 21.21 lbs/ft. 0.94 ft. 16” = 16.000” O.D. x 0.250 W.T. 42.05 lbs/ft. 5.70 in. 20” = 20.000” O.D. x 0.250 W.T. 52.73 lbs/ft. 4.55 in. Stray Current cases can result in high Amp discharges, in which results to rapid corrosion or metal loss Remember the basic law of corrosion - For corrosion to occur all the components of a basic corrosion cell must be present. Conventional Current Flow Metallic connection Conventional current Flow - is the flow of current from positive to negative in a electrical circuit. Electron Flow Electrolyte Current Flow - needs to return to it’s original power source. Cathode Positive Anode Negative Electromotive Force is the potential difference between the two structure. Conventional Current Flow – Rectified System Anode bed Rectifier Unit Current Flow (+) Positive (+) Negative (-) Current Flow Structure - Pipeline (-) Recognize Stray Current • Foreign structures nearby – other pipelines – buried tanks or petroleum facilities • Increase leakage in area • Readings increase more negative or positive (Annual monitoring, Bi-monthly inspections) • Corrosion focus on a pinpoint location onto structure • Coating disbondment near foreign line Types of Interference There are two types of interference 1. Dynamic 2. Static Dynamic Interference • Dynamic interference is recognized by pipe-to-soil measurements that fluctuate as a result of the source of stray current. These currents are continually varying in amplitude and/or continually, changing their electrolytic paths. • Light rail systems. • DC mining activities. • DC welding on the pipeline. Two areas of discharge Dynamic Interference Example Two Interference bonds connected Bond between pipeline and DC substation Reverse Switch or Diodes Used • The rail system potential builds up positive • In prevention of shorting out company system with the return interference bond, in which bring the pipe line structure positive with the rail system, diodes are used Static Interference • Static interference is a steady, continuous stray current source, such as, an impressed cathodic protection rectifier. • This session deals primarily with the definition, recognition, testing, and mitigation of static interference stray current. Conventional Current Flow Current returns through the soil + Current Discharge – Corrosion Current Discharge – Corrosion Static interference caused by a cathodic protection system Conventional Current Flow Current Discharge – Corrosion Foreign Line Crossing and Stray Current Pickup and Discharge Conventional Current Flow Current Discharge – Corrosion Testing non crossing foreign pipeline for stray current interference Current Discharge – Corrosion Voltage Gradients • Remember the rectifier transformers, – The voltage gradient is build around the primary coils, energize a iron ore, which the secondary coils picks up the voltage potential and current flows through the circuit – Current still returns back – Stray current works in the same principal – When ever pipe passes through one of these voltage gradients from a foreign line, it picks up current Ion Flow • Remember basic corrosion, electrons flow the opposite direction as current • At the cathode area (pick up), electrons will flow and build up to a negative potential • At the anode area (discharge location), loss of electrons builds up to a positive potential Conventional Current Flow • Conventional current will flow, from positive to negative, – From the foreign ground bed system to the companies pipeline pickup area – Through the pipeline to the anode – From the anode (discharge area) to the foreign structure through the electrolyte Interference Consideration Factors Impacting Corrosion Severity • Separation and routing of facilities • The location of the interfering current source • Magnitude and density of the current • Coating quality • Absence of external coating on the structures involved • Presence and location of mechanical joints of high electrical resistance Interference Detection • When stray current interference is detected – Time is of the essence – Leakage can occur with in days or weeks Testing Effect of Interference Stray Current Pickup and Discharge Laws of Electricity and Interference Current • Current will always take the path of least resistance. • Stray current will always return to the source. Effect of Interference Stray Current Pickup and Discharge Current Discharge May Only Reduce Polarization. • Where the stray current discharges, a detrimental affect will occur. • Pipe-to soil readings will be less negative. • May result in possible corrosion • Whether actual structural damage of corrosion occurs depends upon the existing level of cathodic polarization. • If there is adequate polarization, current discharge may not cause metal loss. • Potential shifts less negative are not necessarily indicative of interference corrosion Electronic or Ionic When the stray current discharges, one of two reactions will occur. 1. If adequate polarization exists, the current discharge will result in an electronic exchange electrochemically. – – No corrosion occurs. Reduces the cathodic polarization. Electronic or Ionic 2. When there is a lack of cathodic polarization present, the discharge of current will result in corrosion damage. Foreign Stray Current Affect on Polarization • Stray current pickup increases polarization, this is represented by the higher,more negative, pipe-to-soil readings • Stray current discharge decreases polarization, this is represented by a more positive or less negative pipe-to-soil readings IR drop information needed to use 100 mV shift Criteria or -.850 V polarization criteria Pick Up and Discharge Areas • Need to identify areas of Pick up and Discharge – Pick up area, • More negative – Discharge area, • More Positive • Determine locations by CIS – Interrupting foreign structure – Data logger is the best tool to use Connect Interrupter in series with the structure or ground cable. In this case, we used the structure cable. Interrupter MCM used to find peak and valleys of reads. Sincorder 2 for the CIS Discharge Area • Indicated in CIS as the most positive potential reading • The area considered anodic • The area that will corrode – Faraday's law = 1amp = 20lb’s per yr • Most likely found at the point of crossing or the maximum exposure to the foreign line • The location for the bond to be established Graph of Stray Current Pickup and Discharge on Bare Pipeline Graph of Stray Current Pickup and Discharge on Coated Pipeline Pipe-to-Soil Readings Through Foreign Influence and Resultant Depression in Potentials Rules of Thumb - Interference Testing 1. Current will always take the path of least resistance. 2. Current must always return to its source. 3. Get the “big picture” of all metallic structures and possible stray current sources, and. Interference Testing Rules of Thumb 4. Follow the data if practical by finding the corresponding stray current discharge point when a stray current pickup is found. 5. Simplest test is to measure the metallic voltage shifts. The greatest voltage shift Beware of Interference Testing Difficulties • Limited access to the pipelines due to blacktop or concrete requires drilling to obtain measurements • Polarization testing is complex and time consuming • Polarization testing may require substantial number of current interrupters that are synchronizable Beware of Interference Testing Difficulties If you have more then one rectifier, you need to have synchronizable current interrupters. Time programmable Master – Slave GPS Testing Criterion • It is necessary prior to conducting any field-testing to gain agreement on what criterion will be utilized to test, evaluate, interpret, and mitigate any stray current problems that may be identified. • Prior to conducting field tests all parties should agree to the standard remediation requirements. Testing Criterion • Columbia’s acceptable criteria – 50 mV voltage shift, with foreign system interrupted, more positive – .850- V CSE Criteria, with foreign CP system operating • Both criteria's must be met Interference Testing Outline Summary Data Needed to be Obtained: – A survey of the pipelines with the existing current from groundbeds in the area (On Potential Reading of the foreign structure) – A survey of each pipelines as if no foreign pipelines were present with only the companies current (Off Potential Reading of foreign structure) – A survey of each pipeline depolarized to obtain a static potential Interference Testing Outline Summary How to Accomplish This? • Conduct an ON/OFF survey of Columbia’s pipeline with the existing bonds broken and all known foreign influencing rectifiers interrupted (Columbia’s CP system operating) • Perform an ON/OFF survey of Columbia’s system with only Columbia’s rectifiers interrupted. The foreign companies are to be turned off at least 12 hours prior to each survey • Obtain potentials after all rectifiers have been turned off for at least 48 hours Interference Testing Outline Summary Why an ON/OFF Survey? • An ON survey alone does not give insight into the actual condition of pipe regarding its actual cathodic protection • The actual cathodic protection is demonstrated by measuring the chemical activity at the pipeline surface that regards corrosion • On potentials have included in the measurement – IR through the soil – IR in the pipe – Chemical activity representing polarization – Native potential of the steel Interference Testing Outline Summary Why an ON/OFF Survey? Continued • Instant OFF potentials only include the static potential of the steel and the chemical polarization. By simultaneously shutting off the current, the IR through the soil and steel of the pipe is eliminated • The actual chemical polarization of the pipeline is determined after static potentials are obtained Instant OFF – Static = chemical Polarization Interference Testing Outline Summary Determine the Acceptable Amount of Interference: • Must have at least 100 mV of polarization in the “as existing” condition. This results in a protected pipeline with no metal loss • Loss of polarization between the individual company does not mean metal loss as long as at least 100 mV of chemical polarization exists. The companies affected must agree upon the acceptable level of polarization loss or gain due to interference Interference Testing Outline Summary Determine the Acceptable Amount of Interference: • If the potentials of the pipeline is above the .850- V CSE criteria with the foreign line CP operating, this indicates adequate polarization on Columbia’s pipeline to prevent corrosion • However, due to possible miss-interrupted readings due to soil conditions with seasonal effects, -50 mV shift is used as minimum accepted criteria with the foreign system interrupted Interference Testing Outline Best Practice • Interrupt the foreign structure • Perform CIS over Columbia’s structure • Set interrupter for 500 milliseconds “Off” and 1 second “ON” • Log survey on data logger • Identify Low points & High points on the “ON” cycle • Identify & measure Voltage shift to the most positive direction (maximum exposure area) • Mark locations Mitigation Mitigation of Stray Current 1. “Design and installation of electrical bonds of proper resistance between the affected structures. 2. Cathodic protection current can be applied to the affected structure at those locations where the interfering current is being discharged. The source of cathodic protection may be galvanic or impressed current anodes. 3. Adjustment of the current output from the interfering cathodic protection rectifiers may resolve interference problems. Mitigation of Stray Current 4. Relocation of the groundbeds of cathodic protection rectifiers can reduce or eliminate the pickup of interference currents on nearby structures. 5. Rerouting of proposed pipelines may avoid sources of interference current. 6. Properly located isolating fittings in the affected structures may reduce interference problems. 7. Application of external coating to current pickup area(s) may reduce or resolve interference problems.” Mitigation of Stray Current • Applying coating to the pick up area, will provide a high resistant barrier for Columbia’s pipeline to pick up current from the foreign ground bed system Resolution of Interference Problems • Indications that interference or stray current problems have been resolved can be demonstrated by: Interrupt system (Foreign structure) – 500 milliseconds “ON” and 1 second “Off” • • • Perform CIS with data logger Indication of no voltage shift or less than 50mV Indication of no potential readings below .850- V CSE Bond at foreign pipeline crossing Setting a Resistant Bond – Best Practice • Attach two no. 8 and no. 12 wires onto both structures (Columbia and foreign structure) • Wire sizes may change due to design of higher expected ampere output • Mark the foreign structure wires for easy identification (normally with white or red tape) • Connect an high impedance volt meter to the companies no. 12 wire and place the CSE over the maximum exposure area Setting a Resistant Bond – Best Practice • Connect an amp meter in series with Columbia and the foreign structure to achieve the maximum current drain reading – Set meter at it’s highest setting to prevent blowing fuses • Connect a temporary bond rated for the ampere measured • Normal practice – set up a one ohm slide resister, with the setting half way (= .5 ohms) • Take potential reading at maximum exposure area before and after temporary connected Setting a Resistant Bond – Best Practice • If potential shift over structure goes from a depressed state to an impressed state, resistance is too low • If potential shift over structure is still in a depressed state, resistance is too high • Keep adjusting slide resistance to desire criteria is met by checking maximum exposure area BOND BOX ON / OFF SWITCH # 8 GAGE WIRE TO COMPANY LINE 2 # 12 GAGE WIRE TO COMPANY LINE ADJUSTABLE RESISTOR 8 GAGE WIRE TO FOREIGN LINE 2 # 12 GAGE WIRE TO FOREIGN LINE Galvanic anodes used to drain current Installation of Anodes • Method is not preferred – Due to large amount of current discharge normally consumes anode in rapid time frame, requiring regular replacement – Must connect the anode bed into the test station box for amp drain measurements – Decrease in amp drain measurements, may indicate depletion of anodes – Galvanic anodes used (Magnesium) Hydrogen Embrittlement • Pick area needs to be lowered below –2.00 V CSE due to possible coating disbondment of hydrogen build up and possible hydrogen embrittlement, in which results in pipe failure • Normal resolution to problem, after bond is set, high potentials exist, add more coating to increase resistance – Bond may need readjusting after completion of task Shield Installation • Not preferred, • Due to cost of excavation with material and labor • As like the anodes, will deplete over time and need to be replaced Example 1 • With our rectifier “on” the pipe-to-soil potential for our line is -0.990 • Foreign pipeline has a pipe-to-soil potential of -0.960 • Rectifier switched “off” • Our potential becomes more positive (-0.850) • Foreign pipeline becomes more negative (-0.980) OUR GROUNDBED FOREIGN LINE + OUR RECTIFIER _ STATION Foreign Line ON -0.960 V OFF -0.980 V V + 0.020 V OUR LINE Our Line ON -0.990 V OFF -0.850 V V -0.140 V Conclusion • Based on the recorded test data, our line is considered to be protected • The potential on the foreign line decreased (became more positive) when are rectifier was switched on • There is a possibility a holiday exists near the point of crossing • The reduction is not sufficient to indicate loss of protection, no corrective measures required Example 2 • With our rectifier “on” the pipe-to-soil potential for our line is –1.150 • Foreign pipeline has a pipe-to-soil potential of -0.580 • Rectifier switched “off” • Our potential becomes more positive (-1.040) • Foreign pipeline becomes more negative (-0.880) OUR GROUNDBED FOREIGN LINE + OUR RECTIFIER _ STATION Foreign Line ON - 0.580 OFF - 0.880 V + 0.300 OUR LINE Our Line ON - 1.150 OFF - 1.040 V - 0.110 Conclusion • Based on the recorded test data, our line is considered to be protected • The potential on the foreign line decreased (became more positive) when are rectifier was switched on • Need to set a resistance bond to bring the foreign pipeline on potential back to the off potential DOT • P/P • DOT Part 192.465 (c) Critical Bonds 6 times each calendar year, not to exceed 2 ½ months Non-Critical Bonds once each calendar year, not to exceed 15 months DOT • DOT Part 192.473 (a). Each operator whose pipeline system is subjected to stray currents shall have in effect a continuing program to minimize the detrimental effects of such currents. Columbia’s P/P • Critical bond is where the pipeline is conducting current through the bond and: The bond current is 0.5 Ampere or more Failure of the bond may result in a potential change of 100 mV or more below (less negative) the static potential of the pipeline Bi-monthly • All Critical bonds must be evaluated bimonthly or every two months not to exceed 15 days • Pipe to soil reading on company structure Annual Monitoring • Monitoring Bonds utilizing a diode or reverse current switch: A pipe-to-soil potential reading Bond current measurement Test to ensure the blocking device is operative Annual Monitoring • Monitoring All other bonds: Pipe-to-soil potential of all structures with the bond connected Pipe-to-soil potential of all structures with the bond disconnected Measurement of the bond current Typically five readings obtained Slide Resister Application Resister Wire Application The amount of Resistance is made by the Length of the Wire. Disconnect Bond Wire for Amp Drain Reading 1.36 Amp Current Drain Amp Drain Reading Make Connection in series with the Circuit Slide Resister Lighting Arrestor Direct Bond Connection, No Resistance Connection Made on Resister from Foreign Structure and Columbia. Disconnection to get Amp Drain must be made in series with the circuit. Shunt Resister Measuring Amp Drain by Measuring Voltage Drop Across the Shunt. Ratio is normally found on Shunt. .001V = 1A 7 milivolts = 7 Amps Make connection across Shunt. Polarity does not matter. Ratio for Shunt Shunts do not have to be disconnected in BI-monthly Quick Review - Basic Corrosion Cell 1. 2. 3. 4. Anode (more Neg.) Cathode (more Pos.) Metallic connection (pipeline surface, wire, any metal structure) A common electrolyte (water, soil, etc.) Basic Corrosion Cell To provide the driving voltage for current to flow in the corrosion cell, there must be a potential difference between the anode and the cathode. Take away any one of the four elements in the basic corrosion cell and it will stop the corrosion. Role of the Environment In the Corrosion Process, the Environment Plays a Major Role. • If soil resistivity is high, current flow is restricted, • Non-uniform environments restrict currents flow at the transition points, • If moisture is present, the corrosion reaction may accelerate, • As temperature rises the corrosion rate accelerate, • Other mechanisms, such as, differential aeration and soil pH will impact how and where the corrosion rates accelerate. Role of the Environment-pH • An understanding of pH is important in corrosion and CP work • For many metals, the rate of corrosion increases appreciably below a pH of about 4 • Between 4 and 8 corrosion rate is fairly independent of pH • Above 8, the environment becomes passive and the corrosion rate decrease • Cathodic polarization increases the pH at the pipe surface Role of the Environment-pH Neutral pH = 7 Acid pH < 7 Alkaline pH > 7 0 Acid 7 Neutral 14 Alkaline The pH scale is logarithmic, for each unit of pH the environment become ten times more acid or alkaline Corrosion Prevention Coatings. • Coating is the first line of defense in corrosion prevention. • Isolating the steel from the environment disrupts the basic corrosion cell. • Coating damage on the pipeline is called coating holidays. Figure 2 – Coal Tar Enamel Coating with Area of Coating Removed Corrosion Prevention Cathodic Protection. • Cathodic protection (CP) is a supplement in the prevention of corrosion. • CP is the application of a DC current. • The desired effect is to shift the anodic (corrosion) areas on the pipeline to cathodic (protected). Polarization • Cathodic protection current collects at the coating holidays. • This DC current forms cathodic “polarization”. • This creates a protective layer at the coating holiday, when sufficient DC current is available. • Polarization can be measured by conducting an instant off shift test. Pipe-to-soil Measurement Measurement of the CP effectiveness is accomplished by obtaining voltages, which are commonly called “pipe-to-Soil readings”. Readings measure the three components: 1. Chemical activity of the pipe or Polarization 1. Soil or electrolyte voltage (IR) drop 2. Metal voltage (IR) drop Pipe-to-soil Measurement • Each component adds voltage to the pipe-to-soil measurement. • Polarization adds protection to the pipeline. • Polarization plus native potential most accurately represent the level of CP protection achieved from the CP current. • Soil and metal voltage drop, add error to the pipe-to-soil readings. Possible Pipe-to-soil Measurement Errors • Increases to the pipe-to-soil measurement are a result of current (I) passing through the earth resistance (R). • Large soil/metallic voltage drop may add significant error to the pipe-to-soil reading. • Small soil/metallic voltage drop will have a minimal impact. • P/S reading with the current applied = [static potential + polarization] + [soil IR + metal IR]. Possible Pipe-to-soil Measurement Errors Current Interruption to Remove Voltage Drops • Interrupting the CP sources and measuring the instant off reading removes the IR voltage drops. • Failure to test for IR voltage drop error can give misleading data and result in misleading conclusions. • Polarized or Instant Off Potential = [Static Potential + Polarization] + [Soil and Metal I x R = 0] Recognize Interference Cathodic Protection Voltage Gradients • There are two locations that generate voltage gradients: 1. Anodic 1. Cathodic Typical Anodic Gradient Field Developed By An Isolated Anode Cathodic Gradient Surrounding Pipeline Recognize Interference • Metallic structure picks up stray current from voltage gradients. -Path of least resistance • Voltage gradient effect on pipe-to-soil measurements. -Failure to recognize will result in pipe-to-soil data interpretation error Foreign Voltage Gradients • When there are anodic voltage gradients present from foreign cathodic protection systems, data interpretation is more difficult • Not only are the soil/metal voltage drops present from the company’s CP system, there also from the foreign system • The presence of these voltage gradients dose not prove that stray current has been picked up or that interference corrosion will be present Foreign Voltage Gradients • • • • • • Current pick up is dependent upon: Pipeline cathodic polarization Coating resistance and quality Foreign current driving force The earth resistance Magnitude of concentration Current flow and path of least resistance Cathodic Protection Criteria • - 850 mV current applied criterion includes IR drop for pipe-to-soil readings • 100 mV shift criterion removes the IR drop from the pipe-to-soil readings Setting A Resistance Bond Best Practice 5. Measure the temporary bond current (IT) between the pipeline and its source of interference and observe its direction. At the same time, measure the change of the pipe-to-soil potential (ET) caused by the temporary bond current. Setting A Resistance Bond Best Practice 6. Measure the resistance (RT) of the temporary bond. 7. Determine the pipe-to-soil voltage change (E1) required to return the pipeline to the original or desired potential from step 2. 8. Use the values (IT), (ET), and (E1) to calculate the current required. Setting A Resistance Bond Best Practice IMAX = IT / ET x E1 IMAX is the current required to correct the stray corrosion at the point of maximum exposure. Setting A Resistance Bond Best Practice RMAX = IT x RT / IMAX RMAX = maximum resistance of the final drain wire IT = temporary drain current value in amperes RT = temporary drain resistance in ohms IMAX = bond current in amperes required to correct the problem Setting A Resistance Bond Best Practice • Select a cable size of the required length that will give a resistance of a little less that the calculated RMAX. • The final conditions at the maximum exposure point must be checked after drainage bond is installed to determine if the return potentials are satisfactory.