MIL-HDBK-1004/10
b) Area of bare pipe to be cathodically protected based on 80
percent coating efficiency:
A = 21,482 x 0.2
A = 4,296 ft
c)
Maximum protective current required based on 4 mA/ft of bare
metal:
I = 4,296 x 4
I = 17,184 ma or 17.2 A
d)
Maximum weight of anode material for 15 years life:
(1)
Graphite anodes are used.
(2)
Average deterioration rate of graphite is 2.0 mA/ft
(3)
Maximum weight of anode material required:
W = YSI/E
where
Y
=
15 years
S
=
2.0 mA/ft
I
=
17.2 A
E
=
0.50 efficiency
W
=
15 x 2.0 x 17.2/0.5 pounds
W
=
1,032 pounds
9.8.3
Groundbed Design
a) Anode size, 3-inch by 60-inch (backfilled 10-inch by 84-inch)
weighing 25 pounds per anode unit.
b)
Resistance of a single anode to earth:
R
=
PK/L
v
where
P = 1,000 ohm-cm
L = 7.0 feet (backfilled size)
K = 0.0167, L/d = 8.4 (refer to para. 6.2.1.4 a)
R = 1,000 x 0.0167/7.0
v
R
= 2.39 ohms
v
194
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