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Design of Sacrificial Anodes

1. Process Calculation

The calculation content of sacrificial anode design includes anode grounding resistance, anode output current, required number of anodes, and anode service life. Since the current generated by a single sacrificial anode is limited, and the underground pipeline has resistance, which causes the current to diverge, the protection range of a single sacrificial anode is local. For the entire pipeline, multiple sacrificial anodes must be connected in parallel to achieve protection collectively.

(1) Calculation of Anode Grounding Resistance

Grounding resistance is the resistance formed at the interface between the sacrificial anode and the soil. The magnitude of this resistance directly affects the current output of the sacrificial anode and is a key parameter for the protection area and degree of sacrificial anode protection.

For single horizontal and vertical cylindrical sacrificial anodes, when L_a (length of the anode backfill layer) is much larger than d (equivalent diameter of the anode), and t (distance from the anode center to the ground) is much larger than L/4 (L is the anode length), the anode grounding resistance can be calculated using Equations (5-1) to (5-3).

For a single vertical cylindrical sacrificial anode without backfill, the grounding resistance is:

(5-1)

For a single vertical cylindrical sacrificial anode with backfill, the grounding resistance is:

(5-2)

For a single horizontal cylindrical sacrificial anode with backfill, the grounding resistance is:

(5-3)

In the equations, R_v is the grounding resistance of the vertical anode, Ω; R_H is the grounding resistance of the horizontal anode, Ω; ρ is the soil resistivity, Ω·m; ρ_a is the backfill material resistivity, Ω·m; L is the anode length, m; L_a is the length of the anode backfill layer, m; d is the equivalent diameter of the anode, m; D is the diameter of the backfill layer, m; t is the distance from the anode center to the ground, m.

The total grounding resistance of multiple anodes in parallel is larger than the theoretical value due to the interaction between anodes. It can be corrected based on the spacing between anodes:

(5-4)

In the equation, R_total is the total grounding resistance of the anode group, Ω; R_v is the grounding resistance of a single anode, Ω; N is the number of parallel anodes; η is the correction coefficient, refer to Figure 5-2.

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