ac voltage- pipeline safety and corrosion

AC Voltage- Pipeline Safety and

Corrosion

MEA

2015

2 INSERT TITLE OF PRESENTATION HERE

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


AC CONCERNS


TYPICAL HIGH VOLTAGE AC LINE CONSTRUCTION

Note:

* Phases

* Shield wires


TYPICAL HIGH VOLTAGE AC LINE CONSTRUCTION


INDUCED AC

I2

I1


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


SAFETY

(1)


AC VOLTAGE AND SAFETY


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.


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


LIGHTNING


GROUND FAULT


GROUND FAULT OR INDUCED AC


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.


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.

• .


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.


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


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


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.


• Discharge of AC at the pipe to soil interface can result in accelerated

corrosion that is detrimental to the integrity of the pipeline


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.


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.


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]


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.


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


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


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


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.


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.


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.


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.


SOURCES

• (1) Some Safety Considerations for Pipelines Near Overhead Power

Lines, NACE, 2005


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