cathodic protection electrical measurement and monitoring ......direct current voltage gradient...

Corrosion Protection

Presenter: Dr. Zalilah Sharer

© 2016 Gas Technology Centre

Universiti Teknologi Malaysia

27 February 2016

Cathodic Protection – Electrical Measurement and Monitoring

Part 3

Potential Measurement Equipment

1. Reference Electrodes • Cu/CuSO4

• Ag/AgCl

• Zinc


2. Voltmeter (multimeter) • high impedance ( 1 – 100 MΩ)

• BS 7361 specified minimum 10 MΩ


3. Recorders And Data Logging Equipment - when need continuous monitoring for eg DC traction effects, telluric effect or other earth currents - can incorporate with computer

How to measure potential?

1. Making electrical contact with the structures ---> test points 2. Placing of reference electrode ---> iR drop As close as possible to structures to be measured; but not in-

contact. Potential is shown by where the reference electrode is placed; NOT where the contact to structure is made !!

3. The use of coupon 4. Permanent reference electrode

How to measure potential?

BEWARE WHEN USING DIVER

ESPECIALLY FOR ICCP

ELECTRICAL SHOCK !!

How to measure direct current in CP circuit

Generally, insert ammeter in series in the circuit Ammeter , but with some errors due to added resistance

But, ammeter is o.k. for sacrificial anode system, but better with shunt (eg. 50mV drop scale)

Measure CP Current - shunt


For ICCP

current measurement needed at ……

T/R output and anodes

higher current, direct ammeter in series

not suitable

can used shunt + voltmeter (multimeter)

50A 75mV shunt = 0.0015 ohm ---> 1.5mV=1A


When more than 10A ---> can use “clamp-on meter” less than 10A, clamp-on meter has poor sensitivity thus less accurate

Special CP Surveys

1. Close Interval Potential Survey (CIPS)

provides overall protection level

normal test post locations are too general

record & experience shown failures due to

corrosion at away from test posts

can incorporate instantaneous off method

during survey

Close Interval Potential Survey (CIPS)

Close Interval Potential Survey (CIPS) for Cathodic Protection

Special CP Surveys

2. Pearson Survey to locate defects in buried pipeline coatings by a.c. signal between pipeline and remote earth 3. Current Attenuation Surveys to locate defects in buried pipeline coatings by monitor actual current flow in the line

Special CP Surveys

4. Direct Current Voltage Gradient (DCVG) to locate buried pipeline coating damage by detecting iR drop at the defect areas more accurate and fast

Special CP Surveys

Direct Current Voltage Gradient (DCVG)

Direct Current Voltage Gradient Survey (DCVG) for Coating Defect Detection

Coating defect with Inadequate CP

Coating defect ready for repair

Coating defects detected by DCVG

Pipeline Current Mapper

Basic Theory: EM 1

Basic Theory: EM 3

Basic PCM Locating

Current Measurements

Current Measurements

When a fault is encountered the current quickly drops. A fault will result from coating damage, contacts with other services, etc. The loss of PCM+ current will be virtually proportional to the amount of CP current being used at the fault. NOTE: Linear current loss may naturally occur along the pipeline due to the

age of the pipe and coating conditions.

Data Interpretation

Other Measurement

Soil resistivity Meggar earth meter 4-pin Wenner Method Insulation checker checking insulation at IF, MIJ GPS Global positioning cordinates (useful for cross country CIPS & DCVG) Pipe Locators

How to measure the soil electrolyte resistivity

Most popular method ----> Wenner 4-pin method Driving 4 steel pins into earth in a straight line, equal spaced Pin spacing = depth

Soil resistivity = average resistivitiy at that particular depth

Four-pin Wenner Method


On resistance meter…..

C1, C2 = current spikes

P1, P2 = potential spikes

Result on meter = resistance (ohm)

Resistivity ……..

(ohm-cm) = 2 π a R 100

a = pin spacing in meter R= resistance

(from meter)

Wenner Four Pin Method

Soil Resistivity Measurement


PRECAUTIONS : 1. Soil box sample ---> preserve normal moisture

content.

2. Measurement closely parallel to a bare underground pipeline ---> not accurate (usually very low values)

3. Ensure proper pin contact 4. Electrical shock


Soil Box ---> soil samples

Collect soil sample to fill up the soil box, then measure resistivity by resistance meter. Use same arrangement, i.e. C1,C2,P1,P2

Soil Resistivity Measurements

Important step in CP Current requirement differ from one soil

resistivity to another No precise method to determine the degree of

corrosivity caused by soil, only give rough guide to the corrosivity of soil

Most methods used is Four Pin Wenner’s Method

Four-pin Direct Current Method

Significance of Soil Resistivity

Soil resistivity gives indication of the corrosivity of soil. Some approximation are given below;

Soil resistivity (ohm.cm) Corrosion

0-1000 Severe corrosion

1000-3000 Severe corrosion

3000-10000 Moderate corrosion

Over 10000 Very mild corrosion

Average resistivities of some soils

Environment Resistivity (ohm.cm)

Brackish river water 1

Sea water 25

Town supply water 1000-1200

Alluvial soil 1000-2000

Clay 1000-5000

Gravel 10000-25000

Sand 25000-50000

Mid Point Contact Surveys

Tow Fish Trailing Wire Survey

Close Electrode Trailing Wire Survey

Two Electrode Survey (PipeScan)

Three Electrode Survey (common)

Meter – Care Taking (Precautions)

Do not mesure values beyond range of meter

For current & voltage measurement, always use highest range first

Avoid dropping meter

Pack carefully for travel

Store properly

Question?

cathodic protection electrical measurement and monitoring ......direct current voltage gradient...

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