MIL-HDBK-1004/10
d) Minimum weight of anode material:
W = YSI/E
where
Y
=
15 years
S
=
8.8 lbs/A yr
E
=
0.50 efficiency
I
=
0.73 A
W
=
15 x 8.8 x 0.73/0.50
W
=
193 pounds
It should be noted that the 193 pounds arrived at above is based on
an output current of 0.73 A for the full designed life of the cathodic
protection system, 15 years. Strictly speaking, this is not the case because
the current output following a new installation is much less due to the high
coating efficiency. The average current requirement at first may be as low as
0.015 mA/ft of pipe.
e) Current output of a single 9-pound packaged, "high-potential"
magnesium anode:
i = Cfy/P
where
C = 120,000, constant for magnesium on well-coated
structures
f = 0.71, anode size factor (refer to Table 3)
y = 1.00, structure potential factor (refer to Table 4)
P = 5,000 ohm-cm, soil resistivity
i = 120,000 x 0.71 x 1.00/5,000
i = 17.0 mA
Because the structure is well coated, the anode spacing is
relatively great. Therefore, the multiplying factor for magnesium anode
groups is not used.
f)
Number of anodes (n):
n = I/i
where
I
=
726 mA, total current required
i
=
17.0 mA, current output of one anode
n
=
726/17
n
=
42.7 (use 43 anodes)
198
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