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AC Voltage- Pipeline Safety and
Corrosion
MEA
2015
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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
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AC CONCERNS
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TYPICAL HIGH VOLTAGE AC LINE CONSTRUCTION
Note:
* Phases
* Shield wires
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TYPICAL HIGH VOLTAGE AC LINE CONSTRUCTION
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INDUCED AC
I2
I1
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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
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SAFETY
(1)
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AC VOLTAGE AND SAFETY
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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.
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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
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LIGHTNING
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LIGHTNING
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GROUND FAULT
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GROUND FAULT OR INDUCED AC
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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.
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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.
• .
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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.
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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
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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
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FACTORS INFLUENCING INDUCED AC
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
PipelineDecreases Increases
Length of Collocation Increases Increases
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.
*Soil resistivity will have the opposite relationship with AC density.
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• Discharge of AC at the pipe to soil interface can result in accelerated
corrosion that is detrimental to the integrity of the pipeline
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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 cm2,
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.
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AC CORROSION STANDARDS
Table 2: Current Density
Current Density Likelihood of AC Corrosion
0 to 20 A/m2
(0 to 2 mA/cm2)
Low or Unlikely
20 to 100 A/m2
(2 to 10 mA/cm2)
Medium or Unpredictable
Greater than 100
A/m2
(Greater than 10 mA/cm2)
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/m2 to 30 A/m2 with consideration
for excursion above these criteria if
specific DC density criterion is
maintained.
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CALCULATING AC CURRENT DENSITY
• The calculation for estimating the AC density at a holiday is listed
below.
𝑖𝐴𝐶 =8𝑉𝐴𝐶
𝜌𝜋𝑑Equation 1
• where:
• iAC = AC density at a coating holiday in amps per square meter
[A/m2]
• VAC = 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]
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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.
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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
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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
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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 VAC, but some situation
may require lower criterion.
– AC current density criterion; the limit for the maximum allowable current
density will need to be established
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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.
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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.
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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.
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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.
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SOURCES
• (1) Some Safety Considerations for Pipelines Near Overhead Power
Lines, NACE, 2005
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