1. Basic Cathodic Protection Measurement and Monitoring- IR Considerations Western Regional Gas Conference 2008 Tempe, Arizona By Paul Sedlet, Accurate Corrosion Control, Inc.

2. Questions to be answered
What IR needs to be considered?
Why consider it?
How can I consider it?

3. What is IR? IR

4. Summary of This Presentation
Criteria for Cathodic Protection
NACE SP-0169
NACE TM-0497
Components of a Pipe-to-Soil Measurement
Consideration of Soil IR
Consideration of Metal IR
Practical Examples

5. NACE NACE SP-0169, Item No. 21001 NACE TM-0497, Item No. 21231
Control of External Corrosion on Underground or Submerged Metallic Piping Systems. Specifically Section
NACE TM-0497, Item No
Measurement Techniques Related to Criteria for Cathodic Protection on Underground or Submerged Metallic Piping Systems. Specifically Section 7.1.

6. NACE SP 0169, Forward

7. NACE SP 0169 Criteria,

8. NACE TM 0497

9. NACE TM 0497

10. What is a Pipe-to-soil Potential Measurement?
Instruments necessary to take a P/S
Voltmeter with high internal impedance
Reference electrode
Components of the Measurement
Reference to electrolyte interface
Pipe to electrolyte interface
Soil IR
Metal IR

11. Pipe-to-Soil Potential
Voltmeter
Ref Cell
Test Box
Ref to Soil Interface
Mixed Potential
Soil IR
Test Wire
Soil to Pipe Interface
Metal IR

12. Voltage Drops Other Than Across the Structure to Electrolyte Boundary
Instrumentation and Internal Impedance
Reference to electrolyte boundary
Soil IR
Metal IR

13. Consideration of Soil IR

14. Polarization and Soil IR
What is a “polarized” potential
How do you measure “polarization”
Soil IR is the voltage drop that results from current flow in the soil portion of the circuit
Methods to consider soil IR
Instant Off
IR Coupon

15. Waveform Example- Instant Off
Soil IR
Instant Off

16. Waveform Example- Polarized Potential
Soil IR
Instant Off & Polarized Potential
Polarization Decay

17. Example- Soil IR ON-Off CIS

18. Example High Soil IR

19. Using Test Coupons to Consider Soil IR
NACE RP , The Use of Test Coupons for Cathodic Protection Monitoring Applications
Things to know
Use coupon of same material as pipe.
Size of coupon should be that of largest expected coating holiday.
Coupon needs to be installed and allowed to corrode for months before use.
Should not be installed in a foreign voltage gradient.

20. Consideration of Metal IR

21. Copper Cable Resistance Data
AWG Stranded Copper Wire Resistance and Voltage Drop
Size
Diameter
77ºF
Resistance for length (Ohms)
Voltage Drop (Milli-
volts)
AWG
Metric mm2
Circular mils
inch mm
ohm/1000'
ohm/km 20
0.518
1,022
0.0369
0.938
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
53.475
105,535
0.3752
9.530
0.1003
0.329
.0030 30
ENTER Length in feet 30      Amps 10

22. Rectifier Negative Lug Example
Example of metal IR error from reading a P/S potential from a rectifier negative lug, using previous table.
Rectifier DC amperage output = 10 A
Negative cable is 30 feet long and #4 copper
Then metal IR error is 76 mV

23. Standard Steel Pipe Size Data (Courtesy of Mesa Products Website)
Pipe Size inches
Outside Diameter inches
Wall Thickness inches
Weight per Foot pounds
Nom DC Resistance micro-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
54.60
5.29 16
16.00
62.60
4.61 18
18.00
70.60
4.09 20
20.00
78.60
3.68 22
22.00
86.60
3.34 24
24.00
94.60
3.06 26
26.00
102.60
2.82 28
28.00
110.60
2.62 30
30.00
118.70
2.44 32
32.00
126.60
2.15 34
34.00
134.60 36
36.00
142.60
2.03

24. Consideration of Metal IR in CIS
Metal IR is the error that occurs in the pipe-to-electrolyte measurement due to current flow in the pipeline or in a cable.
Metal IR can be measured between test points as the voltage drop between the test points and relates approximately to near ground (NG) and far ground (FG) potentials.
Metal IR error can either add or subtract from the actual potential, depending upon the resultant current flow between test points

25. Gas Distribution CIS Example
If conducting a CIS on a 2” diameter gas line, with a rectifier, output 10 amps, what is the possible metal IR influence?
From the previous table, 2” pipe is 79.2 micro-ohms per foot. At the end of a 1000 foot run going towards or away from the rectifier, then:

26. Gas Distribution Continued
x 1000 = ohms x 10 amps = .792 volts or 792 mV.
This means that your “On” P/S could be off by nearly 800mV in this example!

27. Example: Metal IR Influence (error) in CIS Data

28. Example- Method of Metal IR Compensation
The above graph is the last portion of a 3300 foot CIS run. The run ends at a rectifier.
Measured “On” metal IR drop is 800mV, far ground lead positive.
“On” NG is -1600mV, FG is -800mV
Correction method, assuming even current pickup on the pipeline
800mV/3300ft = -.24mV per foot applied cumulatively along the run.

29. Metal IR Compensated CIS Data