Anode current output is calculated using Ohm's Law:
V = IR
I = current in amperes
V = potential differential in volts
R = anode resistance in ohms
The voltage difference is the protected potential of -0.85 volts subtracted from the open circuit potential of the anode. For high potential anodes, the difference is -0.9 volts. For H-1 alloy, the difference is -0.55 volts.
Resistance to earth can be calculated by Dwight's Equation:
R = resistance in ohms
= soil resistivity in ohm-cm
L = anode length in feet
d = anode diameter in feet
Tefankjian, in his article "Application and Maintenance of Control Facilities" offers a simple process for determining anode current output with consideration of coating, potential, and multiple anode installation. The process involves calculation of anode current for a single high-potential anode installed 10' away from a bare pipeline using the formula:
i_{m} = current output in mA
P = soil resistivity in ohn-cm
f = size correction factor - Table 1
Y = potential correction factor - Table 2
Step 1: For a well-coated pipeline, the constant of 150,000 should be reduced 20% to 120,000.
Step 2: Select anode size correction factor from Table 1.
Anode Weight (pounds) | Standard Anodes | Factor (f) |
---|---|---|
3D3 | (packaged) | 0.53 |
5D3 | (packaged) | 0.60 |
9D3 | (packaged) | 0.71 |
17D3 | (packaged) | 1.00 |
20D2 | (packaged) | 1.60 |
32D3 | (packaged) | 1.06 |
48D5 | (packaged) | 1.09 |
Step 3: Select potential correction factor from Table 2.
P / S | Magnesium |
---|---|
-0.70 | 1.14 |
-0.80 | 1.07 |
-0.85 | 1.00 |
-0.90 | 0.93 |
-1.00 | 0.79 |
-1.10 | 0.64 |
-1.20 | 0.50 |
Step 5: Adjust for multiple anode installation by application of adjustment factors in Table 3.
No. of Anodes in Parallel |
ADJUSTING FACTORS Anode Spacing in Feet |
|||
---|---|---|---|---|
5' | 10' | 15' | 20' | |
2 | 1.839 | 1.920 | 1.946 | 1.964 |
3 | 2.455 | 2.705 | 2.795 | 2.848 |
4 | 3.036 | 3.455 | 3.625 | 3.714 |
5 | 3.589 | 4.188 | 4.429 | 4.563 |
6 | 4.125 | 4.902 | 5.223 | 5.411 |
7 | 4.652 | 5.598 | 6.000 | 6.232 |
8 | 5.152 | 6.277 | 6.768 | 7.036 |
9 | 5.670 | 6.964 | 7.536 | 7.875 |
10 | 6.161 | 7.643 | 8.304 | 8.679 |
As Tefankjian states in his article, the above process should be used only as a guide to estimate current output.