Basic Cathodic Protection Basic Cathodic Protection Measurement and Measurement and

Monitoring- IR Monitoring- IR ConsiderationsConsiderations

Western Regional Gas Conference 2008Western Regional Gas Conference 2008Tempe, ArizonaTempe, Arizona

By Paul Sedlet, Accurate Corrosion Control, Inc.By Paul Sedlet, Accurate Corrosion Control, Inc.

Questions to be answeredQuestions to be answered

What IR needs to be considered?What IR needs to be considered?

Why consider it?Why consider it?

How can I consider it?How can I consider it?

What is IR?What is IR?

IRIR

Summary of This PresentationSummary of This Presentation

Criteria for Cathodic ProtectionCriteria for Cathodic Protection NACE SP-0169 NACE SP-0169 NACE TM-0497NACE TM-0497

Components of a Pipe-to-Soil MeasurementComponents of a Pipe-to-Soil MeasurementConsideration of Soil IRConsideration of Soil IRConsideration of Metal IRConsideration of Metal IR

Practical ExamplesPractical Examples

NACENACENACE SP-0169, Item No. 21001NACE SP-0169, Item No. 21001 Control of External Corrosion on Underground Control of External Corrosion on Underground

or Submerged Metallic Piping Systems. or Submerged Metallic Piping Systems. Specifically Section 6.2.2.1.Specifically Section 6.2.2.1.

NACE TM-0497, Item No. 21231NACE TM-0497, Item No. 21231 Measurement Techniques Related to Criteria Measurement Techniques Related to Criteria

for Cathodic Protection on Underground or for Cathodic Protection on Underground or Submerged Metallic Piping Systems. Submerged Metallic Piping Systems. Specifically Section 7.1.Specifically Section 7.1.

NACE SP 0169, NACE SP 0169, ForwardForward

NACE SP 0169 Criteria, NACE SP 0169 Criteria,

NACE TM 0497NACE TM 0497

NACE TM 0497NACE TM 0497

What is a Pipe-to-soil Potential What is a Pipe-to-soil Potential Measurement?Measurement?

Instruments necessary to take a P/SInstruments necessary to take a P/S Voltmeter with high internal impedanceVoltmeter with high internal impedance Reference electrodeReference electrode

Components of the MeasurementComponents of the Measurement Reference to electrolyte interfaceReference to electrolyte interface Pipe to electrolyte interfacePipe to electrolyte interface Soil IRSoil IR Metal IRMetal IR

Pipe-to-Soil PotentialPipe-to-Soil Potential

Ref Cell

Voltmeter

Test Wire

Test Box Ref to Soil Interface

Soil to Pipe Interface

Metal IR

Soil IR

Mixed Potential

Voltage Drops Other Than Across the Voltage Drops Other Than Across the Structure to Electrolyte BoundaryStructure to Electrolyte Boundary

Instrumentation and Internal ImpedanceInstrumentation and Internal Impedance

Reference to electrolyte boundaryReference to electrolyte boundary

Soil IRSoil IR

Metal IRMetal IR

Consideration of Soil IRConsideration of Soil IR

Polarization and Soil IRPolarization and Soil IR

PolarizationPolarization What is a “polarized” potentialWhat is a “polarized” potential How do you measure “polarization”How do you measure “polarization”

Soil IR is the voltage drop that results from Soil IR is the voltage drop that results from current flow in the soil portion of the circuitcurrent flow in the soil portion of the circuit

Methods to consider soil IRMethods to consider soil IR Instant OffInstant Off IR CouponIR Coupon

Waveform Example- Instant OffWaveform Example- Instant Off

Pipe to Soil PotenialEds_411_0_3_1_fl_41 xing toms eds.xls

1.19

1.24

1.29

1.34

1.39

1.44

1.49

1.54

1.59

1.64

1.69

1.74

1.79

1.84

1.89

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Time (sec)

Vo

lts

, N

eg

ati

ve

All On (Delay Between Cycles) Ed's 11ea. Rects OffToms 4ea. Rects Off Sallys 2ea. Rects OffQTs 3ea. Rects Offr Sams 1ea. Rect OffNot Used All 21ea. Rects Off (Instant Off)

Instant Off

Soil IR

Waveform Example- Polarized Waveform Example- Polarized PotentialPotential

Pipe to Soil PotenialEds_411_0_3_1_fl_41 xing toms eds.xls

1.19

1.24

1.29

1.34

1.39

1.44

1.49

1.54

1.59

1.64

1.69

1.74

1.79

1.84

1.89

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Time (sec)

Vo

lts

, N

eg

ati

ve

All On (Delay Between Cycles) Ed's 11ea. Rects OffToms 4ea. Rects Off Sallys 2ea. Rects OffQTs 3ea. Rects Offr Sams 1ea. Rect OffNot Used All 21ea. Rects Off (Instant Off)

Instant Off & Polarized Potential

Soil IR

Polarization Decay

Example- Soil IR ON-Off CISExample- Soil IR ON-Off CIS

Soil IR

Example High Soil IRExample High Soil IR

Using Test Coupons to Using Test Coupons to Consider Soil IRConsider Soil IR

NACE RP 0104-2004, The Use of Test NACE RP 0104-2004, The Use of Test Coupons for Cathodic Protection Coupons for Cathodic Protection Monitoring ApplicationsMonitoring Applications

Things to knowThings to know Use coupon of same material as pipe.Use coupon of same material as pipe. Size of coupon should be that of largest expected Size of coupon should be that of largest expected

coating holiday.coating holiday. Coupon needs to be installed and allowed to corrode Coupon needs to be installed and allowed to corrode

for months before use.for months before use. Should not be installed in a foreign voltage gradient.Should not be installed in a foreign voltage gradient.

Consideration of Metal IRConsideration of Metal IR

Copper Cable Resistance DataCopper Cable Resistance DataAWG Stranded Copper Wire Resistance and Voltage Drop

Size Diameter Resistance @ 77ºFResistancefor length(Ohms)

VoltageDrop

(Milli-volts)

AWGMetricmm2

Circularmils

inch mm ohm/1000' ohm/km

20 0.518 1,022 0.0369 0.938 10.3632 34.000 .3109 3109

18 0.823 1,624 0.0465 1.182 6.5227 21.400 .1957 1957

16 1.309 2,583 0.0587 1.491 4.0843 13.400 .1225 1225

14 2.081 4,107 0.0740 1.880 2.5756 8.450 .0773 773

12 3.309 6,530 0.0933 2.371 1.6215 5.320 .0486 486

10 5.261 10,383 0.1177 2.989 1.0180 3.340 .0305 305

8 8.366 16,510 0.1484 3.770 0.6401 2.100 .0192 192

6 13.302 26,251 0.1871 4.753 0.4023 1.320 .0121 121

4 21.151 41,741 0.2360 5.994 0.2533 0.831 .0076 76

2 33.631 66,371 0.2976 7.558 0.1594 0.523 .0048 48

1 42.408 83,693 0.3341 8.487 0.1265 0.415 .0038 38

0 53.475 105,535 0.3752 9.530 0.1003 0.329 .0030 30

ENTER Length in feet 30      Amps 10

0.3103.1090.1951.9570.1221.2250.0770.7730.0480.4860.0300.3050.0190.1920.0120.1210.0070.0760.0040.0480.0030.0380.0030.03309.144100.3103.1090.1951.9570.1221.2250.0770.7730.0480.4860.0300.3050.0190.1920.0120.1210.0070.0760.0040.0480.0030.0380.0030.03309.14410

Rectifier Negative Lug ExampleRectifier Negative Lug Example

Example of metal IR error from reading a Example of metal IR error from reading a P/S potential from a rectifier negative lug, P/S potential from a rectifier negative lug, using previous table.using previous table.

Rectifier DC amperage output = 10 ARectifier DC amperage output = 10 A

Negative cable is 30 feet long and #4 Negative cable is 30 feet long and #4 coppercopper

Then metal IR error is 76 mVThen metal IR error is 76 mV

Standard Steel Pipe Size DataStandard Steel Pipe Size Data (Courtesy of Mesa Products Website)(Courtesy of Mesa Products Website)

Pipe Size inches Outside Diameterinches

Wall Thicknessinches

Weight per Footpounds

Nom DC Resistancemicro-ohms/foot

2 2.375 0.154 3.65 79.20

4 4.500 0.237 10.80 26.80

6 6.625 0.28 19.00 15.20

8 8.625 0.322 28.60 10.10

10 10.750 0.365 40.50 7.13

12 12.750 0.375 49.60 5.82

14 14.00 0.375 54.60 5.29

16 16.00 0.375 62.60 4.61

18 18.00 0.375 70.60 4.09

20 20.00 0.375 78.60 3.68

22 22.00 0.375 86.60 3.34

24 24.00 0.375 94.60 3.06

26 26.00 0.375 102.60 2.82

28 28.00 0.375 110.60 2.62

30 30.00 0.375 118.70 2.44

32 32.00 0.375 126.60 2.15

34 34.00 0.375 134.60 2.15

36 36.00 0.375 142.60 2.03

Consideration of Metal IR in CISConsideration of Metal IR in CIS

Metal IR is the error that occurs in the pipe-to-Metal IR is the error that occurs in the pipe-to-electrolyte measurement due to current flow in electrolyte measurement due to current flow in the pipeline or in a cable.the pipeline or in a cable.

Metal IR can be measured between test points Metal IR can be measured between test points as the voltage drop between the test points and as the voltage drop between the test points and relates approximately to near ground (NG) and relates approximately to near ground (NG) and far ground (FG) potentials. far ground (FG) potentials.

Metal IR error can either add or subtract from Metal IR error can either add or subtract from the actual potential, depending upon the the actual potential, depending upon the resultant current flow between test pointsresultant current flow between test points

Gas Distribution CIS ExampleGas Distribution CIS Example

If conducting a CIS on a 2” diameter If conducting a CIS on a 2” diameter gas line, with a rectifier, output 10 gas line, with a rectifier, output 10 amps, what is the possible metal IR amps, what is the possible metal IR influence?influence?

From the previous table, 2” pipe is From the previous table, 2” pipe is 79.2 micro-ohms per foot. At the end 79.2 micro-ohms per foot. At the end of a 1000 foot run going towards or of a 1000 foot run going towards or away from the rectifier, then: away from the rectifier, then:

Gas Distribution ContinuedGas Distribution Continued

.0000792 x 1000 = .0792 ohms x 10 .0000792 x 1000 = .0792 ohms x 10 amps = .792 volts or 792 mV.amps = .792 volts or 792 mV.

This means that your “On” P/S could This means that your “On” P/S could be off by nearly 800mV in this be off by nearly 800mV in this example! example!

Example: Metal IR Influence (error) Example: Metal IR Influence (error) in CIS Datain CIS Data

Example- Method of Metal IR Example- Method of Metal IR CompensationCompensation

The above graph is the last portion of a 3300 The above graph is the last portion of a 3300 foot CIS run. The run ends at a rectifier.foot CIS run. The run ends at a rectifier.

Measured “On” metal IR drop is 800mV, far Measured “On” metal IR drop is 800mV, far ground lead positive.ground lead positive.

““On” NG is -1600mV, FG is -800mVOn” NG is -1600mV, FG is -800mV

Correction method, assuming even current Correction method, assuming even current pickup on the pipeline pickup on the pipeline

800mV/3300ft = -.24mV per foot applied 800mV/3300ft = -.24mV per foot applied cumulatively along the run.cumulatively along the run.

Metal IR Compensated CIS DataMetal IR Compensated CIS Data