TM 5-811-7
2500 ohms/sq ft
Rc '
7264 sq ft
Rc ' 0.345 ohm.
(d) Find maximum allowable groundbed resistance using equation 2-2:
RT ' Ra % Rw % Rc
1.23 ohm = Ra + 0.345 ohm (assume Rw is negligible)
0.89 ohm = Ra
(e) Calculate number of anodes from equation 2-6:
(0.0052)(D)
8L
[1n &1],
N'
(Ra)(L)
d
(0.0052)(500 ohm&cm)
(8)(1.42 ft)
N'
&1]
(values for L
[1n
(0.89 ohm)(1.42 ft)
(0.5 ft)
and D from
supplier.)
N = 44 anodes.
(4) Calculate number of anodes based on system's life expectancy and using equation 2-7:
(L)(I)
N'
,
49.3 (W)
(25 yr)(726 mA)
N'
,
49.3 (9 lb/anode)
N = 41 anodes.
(5) Select number of anodes. Since 44 anodes are required to meet maximum allowable groundbed
resistance (e above), that will be the number used.
(6) Select groundbed layout. Determine the area to be covered by each anode using equation 2-8:
AT
A'
N
7264 sq ft
A'
44 anodes
A = 164 sq/ft anode.
(7) Find anode spacing (table C-2).
Table C-2. Requirements for anode spacing
Number of
Pipe area
Pipe
Anode
length (ft)
spacing (ft)
Pipe section
(sq ft)
anodes
Laterals
4994
30
2880
96
Headers
1860
12
1172
98
Supply and
return lines
410
2
180
90
Reproduced from J.R. Myers and MA. Aimone, Corrosion Control for Underground Steel Pipelines: A Treatise on Cathodic
Protection. J.R. Myers and Associates, Franklin, OH. Used with permission.
C-3
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