AC Voltage- Pipeline Safety and Corrosion MEA 2015
WHAT ARE THE CONCERNS ASSOCIATED WITH AC VOLTAGES ON PIPELINES? AC concerns Induced AC Faults Lightning Capacitive coupling Safety Code Induced AC Corrosion Induced AC Mitigation 2 INSERT TITLE OF PRESENTATION HERE
AC CONCERNS 3 INSERT TITLE OF PRESENTATION HERE
TYPICAL HIGH VOLTAGE AC LINE CONSTRUCTION Note: * Phases * Shield wires 4 INSERT TITLE OF PRESENTATION HERE
TYPICAL HIGH VOLTAGE AC LINE CONSTRUCTION 5 INSERT TITLE OF PRESENTATION HERE
INDUCED AC I 1 I 2 6 INSERT TITLE OF PRESENTATION HERE
FAULTS & LIGHTNING Faults A fault occurs when a path from phase to ground is introduced such that the full current available in the circuit flows to ground. This is a particular concern for lines on steel towers should the fault occur between a phase and the tower High voltage transmission lines typically do not have a neutral to carry full fault current Lightning Lighting can strike a phase or shield wire and be introduced into the ground through a tower or ground rod 7 INSERT TITLE OF PRESENTATION HERE
SAFETY (1) 8 INSERT TITLE OF PRESENTATION HERE
AC VOLTAGE AND SAFETY 9 INSERT TITLE OF PRESENTATION HERE
SAFETY Dry soils, dry shoes, and dry gloves can alter tolerable touch voltage levels. Voltage that might go undetected under dry conditions may give a nasty shock on a wet day. 10 INSERT TITLE OF PRESENTATION HERE
ADDITIONAL SHOCK RISKS Workers can still accidentally contact the pipeline, even after it s in the trench. Cathodic protection tests leads can give a shock. The same applies for other aboveground appurtenances such as valves, casing vents, fences, etc. When cutting pipe, a worker doesn t feel the current, so he believes he is safe. As soon as he separates the pipe the current may run through his body. He could be shocked and seriously injured. Adequate bonding across the point to be cut will eliminate the hazard, bond before starting the cut. Working aboveground pipes that are not electrically continuous, such as isolated flanges, joints, unions, or couplings. Putting his hands across the isolator could a worker s body a path for any current present on the pipeline 11 INSERT TITLE OF PRESENTATION HERE
LIGHTNING 12 INSERT TITLE OF PRESENTATION HERE
LIGHTNING 13 INSERT TITLE OF PRESENTATION HERE
GROUND FAULT 14 INSERT TITLE OF PRESENTATION HERE
GROUND FAULT OR INDUCED AC 15 INSERT TITLE OF PRESENTATION HERE
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SAFETY SUMMARY During construction, aboveground sections can be made safe with a simple temporary grounding and bonding. Measurements should be recorded prior to performing any work to ensure everyone s safety. Communications and measurements are required along the spread during construction because conditions may change as the installation progresses. Warning signs should be posted and RED ZONES clearly designated, including at electrical power system crossings. Both NACE SP0177 and CAN/CSA-C22.3 No.6-M91 recognize 15 V as a potential shock hazard. Check the weather forecast prior to beginning work. Work should be stopped when lightning activity is present. 17 INSERT TITLE OF PRESENTATION HERE
CAPACITIVE COUPLING When underground metallic pipelines are in close proximity to HVAC transmission lines, there are three ways in which HVAC can influence pipelines. [4] Capacitive coupling occurs between two conductors that are separated by a dielectric.. The power lines are one conductor, the air is the dielectric, and the pipeline is the other conductor. The electrical charge from the power line conductors is transferred into the pipeline over time. Once a pipeline is buried, the impacts of capacitive coupling to the pipeline are typically negligible. When the pipeline is isolated above ground during construction, hazardous charges can accumulate on the pipeline. 18 INSERT TITLE OF PRESENTATION HERE
RESISTIVE COUPLING (FAULTS OR LIGHTNING) Resistive coupling between the power line and pipeline occurs when the power line transmits an electrical charge directly into the earth at grounded structures. This is short duration occurrence that is not typical of proper system operation, but it may occur during lightning strikes and electrical transmission fault scenarios. When this charge is transmitted into the soil near a pipeline, the pipeline can provide a lower resistance path. The current pickup and return locations for this charge can result in coating damage and rapid metal loss. 19 INSERT TITLE OF PRESENTATION HERE
INDUCTIVE COUPLING Inductive coupling occurs as a result of the electromagnetic field (EMF) that is created around the electric conductors in the HVAC system. Each conductor creates an EMF with a direction and magnitude that are related to the direction and magnitude of the alternating current (AC) flow in the conductor. If the pipeline is the area of influence for the EMF, the EMF will induce an alternating current on the pipeline. Inductive coupling is primarily of concern on electric power lines with voltage ratings of 69 kv or higher, however severe phase imbalances on electric lines with lower voltage ratings can result in significant AC interference on a pipeline 20 INSERT TITLE OF PRESENTATION HERE
THE IMPACT OF HVAC ON PIPELINES AC interference can impact all types of metallic pipeline including petroleum liquids, natural gas, water, and wastewater.. The industry has been aware of this issue for decades but in recent years, both the frequency and the magnitude of occurrences seems to be increasing. Induced AC interference appears to b eon the rise with the increased emphasis on collocation of pipelines and HVAC power transmission lines coupled with increasing transmission currents Improved pipeline coating quality on new pipelines 21 INSERT TITLE OF PRESENTATION HERE
FACTORS INFLUENCING INDUCED AC The amount of AC that is induced onto the pipeline is influenced by several factors. The information listed in Table 1 is not meant to be all inclusive of the influencing factors, and is only presented as a general representation of system influence. Table 1: Impact of System Properties on Induced AC Voltage Property Change Impact to the Magnitude of Induced AC on the Pipeline Soil Resistivity Increases Increases* Pipeline Coating Resistance Increases Increases Pipeline Outside Diameter Decreases Increases HVAC Current Load Increases Increases Distance between the Tower and Pipeline Decreases Increases Length of Collocation Increases Increases *Soil resistivity will have the opposite relationship with AC density. 22 INSERT TITLE OF PRESENTATION HERE
Discharge of AC at the pipe to soil interface can result in accelerated corrosion that is detrimental to the integrity of the pipeline 23 INSERT TITLE OF PRESENTATION HERE
COATING DEFECT SIZE IMPACT ON CORROSION RATE The size of the coating defect is critical. While large coating defects are of concern related to the application of cathodic protection and remediation of typical galvanic corrosion, the opposite is of concern related to AC corrosion. Large defects can behave more as a grounding effect. Small defects, generally estimated to be 1 cm 2, are the greatest risk as the AC discharge density is focused and more likely to cause accelerated corrosion. This is especially true in lower resistivity soils. 24 INSERT TITLE OF PRESENTATION HERE
AC CORROSION STANDARDS Current Density 0 to 20 A/m 2 (0 to 2 ma/cm 2 ) 20 to 100 A/m 2 (2 to 10 ma/cm 2 ) Greater than 100 A/m 2 (Greater than 10 ma/cm 2 ) Table 2: Current Density Likelihood of AC Corrosion Low or Unlikely Medium or Unpredictable High or Anticipated NACE has established TG 430 with the task of publishing criteria for AC corrosion. At the time of this writing, this task group has not published any criteria but has given consideration to the information included in the NACE State of the Art Publication 35110, EN- 15820:2013, and PRCI Member Study PR-405-113604. These documents generally discus AC density criteria in the range of 10 A/m 2 to 30 A/m 2 with consideration for excursion above these criteria if specific DC density criterion is maintained. 25 INSERT TITLE OF PRESENTATION HERE
CALCULATING AC CURRENT DENSITY The calculation for estimating the AC density at a holiday is listed below. where: i AC = 8V AC ρπd Equation 1 i AC = AC density at a coating holiday in amps per square meter [A/m 2 ] V AC = AC voltage of the pipeline to remove earth in volts [V] ρ = soil resistivity in ohm meters [Ω-m] d = diameter of circular holiday having an area equal to that of the actual holiday in meters [m] 26 INSERT TITLE OF PRESENTATION HERE
Measuring Current Density with Coupon Test Stations A coupon test station typically consists of two coupons. One coupon is usually referred to as the protected or CP coupon, and the other commonly called the native coupon. T The protected coupon is electrically connected to the pipeline, while the native coupon is not. Since the coupon size is known, measuring the AC discharge through the protected coupon is an effective way of determining the AC density. Coupon test stations are designed to represent the environments the pipeline are in. Therefore, they should be installed close to the pipeline, and in the same soil environment as the pipeline. 27 INSERT TITLE OF PRESENTATION HERE
Remote Monitoring of Test Stations In areas where AC interference is a concern, continuous remote monitoring of test stations is recommended. Because HVAC transmission loads can vary throughout the day, week, and seasons as supply and demand fluctuate, it is appropriate for the coupon test station monitoring the effects of the AC interference to be remotely monitored. There are many industry products available to remotely monitor coupon test stations. These products are generally battery powered, with batteries that are designed to last multiple years. These remote monitors can be configured to take AC density and AC voltage readings, as well as many other cathodic protection related measurements, at coupon test stations. Typically, the measurement frequency can be adjusted to be suitable for various applications 28 INSERT TITLE OF PRESENTATION HERE
Communication with the Power Companies To perform AC interference calculations and mitigation system design, information is required from the company operating the power lines suspected of causing AC interference on the pipeline. Typically, the pipeline company submits an inquiry form to the power company requesting design characteristics of the power line system. Typical items of interest are listed below. Plan and profile drawings for the power line systems. Electronic system maps that can be used in conjunction with electronic pipeline maps to improve work process efficiency. Design ratings such as maximum and emergency load ratings Design materials conductor types, insulator types, and shield wire types Design geometries such as tower configurations and phase arrangements Fault currents 29 INSERT TITLE OF PRESENTATION HERE
Establishing Design Parameters There is no current industry standard to establish design parameters for AC mitigation but design considerations include the following: Power line current load limit; average, peak, or emergency loads may be used. Circuit configurations; many power lines contain two circuits that can be operated independently of one another. The effect of an individual circuit in operation may be worse than if both circuits are in operation. The opposite is also true. Fault current; the magnitude and duration of a fault can vary with depending on the type of fault. AC voltage criterion; the limit is often set at 15 V AC, but some situation may require lower criterion. AC current density criterion; the limit for the maximum allowable current density will need to be established 30 INSERT TITLE OF PRESENTATION HERE
AC Interference Modeling Computer modeling is required to predict AC interference on a pipeline. In addition to the information obtained from the power company, pipeline and environmental properties are needed. The typical pipeline information includes: Pipeline diameter and wall thickness Coating and coating quality Location relative to the power lines Location of any electrical isolation joints. 31 INSERT TITLE OF PRESENTATION HERE
AC Mitigation System Using the computer model, a mitigation system can be designed to reduce the AC interference to levels below the established criterion at the established design conditions. AC mitigation systems are typically comprised of a combination of grounding systems designed to provide a parallel path to ground for the induced AC on the pipeline. Gradient control mats can also be used in specific locations to mitigate step and touch hazards. If gradient control mats are used at stations that require grounding for other purposes, a common ground must be established between the gradient mat system and the station grounding systems. 32 INSERT TITLE OF PRESENTATION HERE
AC CORROSION SUMMARY An increase in collocation of power lines and pipelines combined with increased AC system loads may increase the amount of AC interference on pipelines. This interference can become hazardous if the step and touch voltages exceed established criterion, AC discharge caused by AC interference results in accelerated corrosion of the pipeline. The industry accepted method for measuring AC corrosion risk is AC current density, which can be measured with coupon test stations. 33 INSERT TITLE OF PRESENTATION HERE
AC CORROSION SUMMARY In situations where AC interference is suspected, industry has access to technology, such as remote monitored coupon test stations, processes, such as computer modeling, that can assess the hazards associated with AC interference. In areas where pipelines are found to have voltages or current densities above the established criterion, it is advisable to consider application of such technologies and processes to install effective AC mitigation systems. 34 INSERT TITLE OF PRESENTATION HERE
SOURCES (1) Some Safety Considerations for Pipelines Near Overhead Power Lines, NACE, 2005 35 INSERT TITLE OF PRESENTATION HERE
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