Current output is controlled by three factors:
- Soil resistivity – Current output increases as soil resistivity decreases. Generally magnesium anodes are installed in relatively low resistivity soils. Economic application decreases significantly in soil resistivities exceeding 5,000 ohm-cm. Practically, magnesium anodes are not effective above 10,000 ohm-cm.
- Anode surface area – Current output is proportional to surface area. As the surface area increases, current output increases. Increased surface area is usually achieved by increasing the length of the anode.
- Alloy potential – High potential anodes have open circuit potentials approximately 20-25% higher than H-1 alloy. (-1.75 volts versus -1.40 volts). This creates a higher anode current and results in a lower total anode requirement.
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:
im = 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)|
Step 3: Select potential correction factor from Table 2.
|P / S||Magnesium|
Step 4: Calculate anode current output with formula.
Step 5: Adjust for multiple anode installation by application of adjustment factors in Table 3.
Anode Spacing in Feet
As Tefankjian states in his article, the above process should be used only as a guide to estimate current output.