Cathodic Protection Equipment and Measurementcorrosionshortcourse.com/2017 slides/2017 UCSC...

Cathodic Protection Equipment and Measurement

Basic Corrosion Course 2017

February 21-23, 2017 Luke Harris – Integrity Solutions 1

Period #6

Overview

• Safety • Pipe-to-Soil Potentials • CP Current Flows • Resistance Measurements • Rectifier Readings • Surveys • Soil Resistivity

2017 Underground Corrosion Short Course CP Measurements Part 1– Period #6

Safety

• Traffic • Slips, Trips, and Falls • Shock Hazards • Chemicals

Pipe-to-Soil Potential

• What is a pipe-to-soil potential? • What equipment is required? • Why do we take them? • Regulations

Pipe-To-Soil Measurements

• Used to determine the effectiveness of CP • Locate stray current • Check for shorts. Electrical isolation, electrical

continuity • Taken with CP

– ON – Off – Cycling – Native State

Components to take a P/S Measurement

• Volt Meter • Lead Wires • Reference Electrodes • Electrolyte • Pipeline or Structure

Voltmeters

• Available in various types • Digital

• Numerical read-out • Analog (D’ Arsonval movement)

• Moving needle • Computerized Data Logger

• Computer chip stores the readings • Can collect other data

• GPS • Must be high input resistance (10 megaohms or more) • Durable/Rugged • Temperature Stable

Common Voltmeters used for CP

• Digital

• Analog

• Computerized Data Logger

Lead Wires

• Connections Secure – To the Structure – Low Resistance – Tight Connections

• Loose connections – High Resistance – Unstable or incorrect reading

• Contact point – Clean – Electrically continuous

Reference Electrodes (Half-Cells)

• Two Common Types – Copper Sulfate – Silver-Silver Chloride

• Contaminant free • Saturated solution • Solution should be clear, not milky or cloudy • Reference electrodes are also called half-cells:

– Reference cell forms one-half of an electrical cell – pipeline being the other half of the cell

Copper Sulfate Reference Electrode

• Come in various sizes and types • Most commonly used for pipeline work • Temperature sensitive • Do not leave in direct sunlight for long periods of time

or subjected to high temperatures – cover it from direct sunlight – overheating will cause a voltage shift in the potential readings – may cause it to leak or become unsaturated

Half-Cells • 2A Pencil

• 8A Flat Tip

• 6A Flat Tip

• 6B Pointed Tip

• 8B Pointed Tip

• 3A Flat Bottom

• W7 Waterproof connector

• DBA Direct burial

Placement of Electrodes • Important for accurate readings • Directly over the pipeline

– The closer the better • Good electrolyte contact • Do not place the electrode over an anode

– get readings in between anodes whenever possible – don’t take readings directly at test stations that have an

anode connected through them – do not place the electrode near the anode when testing

service lines that have anodes attached to them – get the reading somewhat remote from the anode

• readings over an anode will yield an erroneous reading because of the voltage (IR) drop

• Close to the pipeline as possible without touching it

Good Electrolyte Contact • Good contact between the half-cell and the electrolyte • Wetting the electrolyte with water can help improve contact

– A wet sponge can also be used – The “Fat Boy” electrode

• Issues with taking readings through concrete or asphalt surfaces • Get the tip through the top layers of the soil for better contact.

– Good contact; potential readings will settle down • Gravel or stones

– may have to scrape away a couple of inches to get to the soil underneath

– may need to install a tube through the stone to get to the soil • Landscaping may have plastic for drainage or architectural effects

with mulch or stone – puncture the plastic and get the electrode down to the soil or

use a tube to get through the plastic

Sign Convention

• Pipeline connected to the voltmeter (-) and your half-cell is connected to the voltmeter (+), the value would read +VDC

• Pipeline connected to the voltmeter (+) and your half cell is connected to the voltmeter (-) the value would be -0.850 VDC.

• Either case is correct • Must track the polarity • Safety note: Connect reference cell first and place it on

the ground before connecting to the test lead.

Why Do We Take P/S Potentials • Regulations

– Department of Transportation (DOT) – State

• Monitoring CP levels • Criteria

– Two most common • Pipe-to-Soil Potential of at least -0.85 Volts With Respect

to a Copper-Copper Sulfate Reference Electrode – Measured with Cathodic Protection Applied – Measured with Cathodic Protection Off

• 100mV (0.1) Volts Polarization – IR Drop

Effect of Voltage (IR) Drop • Ohm’s Law defines the relationship among the three electrical

quantities, voltage (E), current (I) and Resistance (R). Ohm’s Law states that voltage equals current times resistance, or E=IxR. Voltage is frequently referred to as simply IR.

• A voltage drop occurs whenever a current passes through a resistance

• The earth has resistance – Place the reference cell close to the pipe

• The voltage at the location of the electrode will be more negative than that at the location of the pipe – an erroneous reading, more negative than that the voltage at

the pipeline – cathodic protection criteria require that the IR drop be

“considered” in the reading; taking steps to obtain the most accurate, IR drop free data

-.850V Criterion

• Commonly used criterion • Applicable to steel and cast/ductile iron • Useful on coated pipelines because of the

relatively small current required to protect them • Bare pipelines may require excessive current to

polarize them up to this potential • There are two versions of this criterion.

-.850 with Cathodic Protection Applied

• Applicable to pipelines that have galvanic anodes connected directly to them –No way of disconnecting or interrupting

the protective current –Consider IR Drop

• IR Drop Free Coupon Test Station • Placement of Reference Cell

-.850 with Cathodic Protection Off • Pipe-to-soil potential measurements taken with the

cathodic protection current momentarily interrupted, or turned off – Usually easy to accomplish with impressed current

systems • Interrupters • Also be done when galvanic systems are

connected to the structure through a test station • When the cathodic protection is off, there is no current

flow in the earth and thus no IR drop (since voltage drop [E] = IxR [Ohm’s Law], if I=0, E must = 0) – These “off” readings are accurate measurements of

the actual structure-to-soil potential

100mV (.1V) Polarization

• Applicable to steel and cast/ductile iron as well as aluminum and copper

• Useful for steel structures that are poorly coated or bare and which would require an excessive amount of protective current to meet one of the -0.850V criteria – frequently used for well casings

• The 100mV (0.1V) polarization refers to the difference between – 1) the original, unprotected potential and the “off” potential

after cathodic protection has been installed or: – 2) the difference between the “instant off” potential and a

potential reached after a period of depolarization

Cathodic Protection Current Flow

• Direct Readings with an Ammeter – Inconvenient – Slow – Dangerous – Meter in Series with Circuit – Off Too Long – Skew Readings

Cathodic Protection Current Flow

• Using a Shunt – Accurate and Fast – Voltmeter Across Known Resistance – Ohms Law

• E=IR

Shunt Ratings

Shunt Styles

OHM’S LAW

• Mathematically it can be stated three ways:

(1) I = E/R (2) R = E/I (3) E = I x R or E = IR I = Current in Amperes

E = Voltage in volts R = Resistance in Ohms

The volt is the basic unit of electrical pressure which forces an electrical current (electrons) to flow through an electrical circuit.

1000 mV = 1 V

1 mV = 0.001 V

SYMBOL is either V or E

50 mV = _____V 2.5 V = ______mV

250 mV = ______V 10.0 V = _____mV

850 mV = ____ V 3.67 V = _____mV

10,000

OHM’S Law • Sample Calculations:

I V R 1. _____ 10 V 5 ohms 2. 3A _____ 2 ohms 3. _____ 10 mV 0.1 ohms 4. 1200 mA 12V _____

100 mA (.1 A)

10 ohms

Shunts Calculation

50 mV - 50 A Determine Amps/mV 1 mV = ____ A = ____ A / mV mV Shunt Resistance R = E = _____V = _____ Ω I A If Measure - 50 mV I = V = ____ mV = ________mV = _____ A R Ω Ω Additional samples provided at the end of the chapter.

0.001 50,000 50

Shunts Calculation

Determine Shunt Factor (SF)

SF X Measurement (mV) = Amps

50A / 50mV shunt = SF of _____

SF ____ X 4.2 mV = ____ A

50A / 25mV shunt = SF of _____

SF ____ X 4.2 mV = ____ A

0.5 2.1

Shunts Calculation

V / R = I

Measurement (V) / R (0.1Ω, 0.01Ω, or 0.001Ω) = I (A)

Measurement of 32.1 mV = _____V / 0.1Ω = ____ A

.0321 .321

.0321 3.21

.0321 32.1

Rectifier Readings

• AC Input – Voltage at Disconnect or Behind Breaker – Current by Clamp-On Ammeter – Power =(3600x Kh x N) / T

• AC Throughput – Voltage Across Main Lugs of Taps

• DC Output – Voltage Across the Output Lugs – Current: Voltage Across the Shunt

AC Input Voltage

• Safety note: Rectifiers are general charged with AC with 120 volts or more.

• Readings taken at: – Electrical disconnect – Behind main breaker – These are taken ahead of protective devices

AC Throughput

• Measure AC voltage into the stacks • Across the main lugs of the taps • Across input lugs of the stacks

DC Output

• DC voltage across the output lugs • Current through the shunt • Meters on the unit

Surveys

• Locating • GPS • Close Interval Survey (CIS) • Direct Current Voltage Gradient (DCVG) • Alternating Current Voltage Gradient (ACVG)

Conductive Locating

Transmitter Receiver

Inductive Locating

Transmitter Receiver

Close Interval Survey

Close Interval Survey Data

Direct Current Voltage Gradient

Alternating Current Voltage Gradient (ACVG)

Soil Resistivity

P1 P2 C1 C2

Resistivity Test Instrument

Review

• Pipe-to-soil potentials • CP Current Flows • Resistance Measurement • Rectifier Reads • Surveys • Soil Resistivity • Safety

Luke Harris 989-436-3894

luke.harris@isfieldservices.com

Cathodic Protection Equipment and Measurementcorrosionshortcourse.com/2017 slides/2017 UCSC...

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