MIL-HDBK-1004/10
where
P
=
4,000 ohm-cm
D
=
5 feet
d
=
2 inches or 0.166 feet
R
=
12 ohms
L
=
(0.012 x 4,000 log 5/0.166)12
L
=
(48 x log 30.1)/12
L
=
(48 x 1,479)/12
L
=
5.92 feet
Number of units = 5.92/0.75 = 7.9 or 8 units
In order to get proper current distribution in the riser pipe, the anode units
should not be placed too far apart. It is generally considered that each
anode unit protects a length along the riser pipe equal to 1-1/2 times the
spacing of the anode from the riser pipe wall.
Riser height = 115 feet
Spacing (center of anode to tank wall) = 2.5 feet
Length of riser protected by one anode = 1.5 x 2.5 = 3.75 feet
Number of units required = 115/3.75 = 30.7 or 31 units.
To satisfy the maximum anode discharge current for a G-2 anode:
3.62 A/0.1 amp = 36
Therefore, 36 anodes are needed instead of 31 or 8.
(3)
Anode resistance based on the use of 36 anode units:
R = (0.012P log D/d)/L
where
P
=
4,000 ohm-cm
D
=
5 feet
d
=
2 inches or 0.166 feet
L
=
36 x 9 inches = 324 inches or 27 feet
R
=
(0.012 x 4,000 log 5/0.166)/27
R
=
48 log 30.1/27
R
=
2.63 ohms
L/d ratio for the riser anode string is 324/2 or 162; thus no
fringe factor correction is applied.
t)
Voltage drop in riser anode circuit:
(1) Electrical conductor to riser anodes. Wire size No. 2
AWG, 0.159 ohms/1,000 feet (refer to Table 10), estimated length 200 feet:
R = 200/1,000 x 0.159 = 0.032 ohm
169
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