MIL-HDBK-1004/10
where
Y
=
15 years
S
=
8.8 lbs/A yr
I
=
0.858 A
E
=
0.50 efficiency
W
=
(15 x 8.8 x 0.858)/0.50 pounds
W
=
227 pounds
It should be noted that the 227 pounds are based on an output current of 0.86
A for the full design life of the cathodic protection system, 15 years.
Strictly speaking, this is not the true condition, because 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 0.03 mA/ft of pipe
area.
e) Current output of a single 17-pound standard packaged magnesium
anode to ground:
i=
Cfy/P
where
C = 120,000, constant for well-coated structures using
magnesium
f = 1.00, (refer to Table 3)
y = 1.00, (refer to Table 4)
P = 4,500 ohm-cm
i = 120,000 x 1.00 x 1.00/4,500
i = 26.7 mA
Because the structure is well coated, the anode spacing is
relatively great. Therefore, the "multiplying factor for magnesium anode
groups" (refer to Table 5) is not used.
f)
Number of anodes (n).
n = I/i
where
I
=
858 mA
i
=
26.7 mA
n
=
858/26.7
n
=
32.1 (use 32 anodes)
g)
Anode distribution:
(1)
Area of pipe protected by one anode:
A = 4,288/32
A = 134 ft/anode
186
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