TM 5-811-7
a.
Design data.
(1) Average soil resistivity is 4500 ohm-centimeters.
(2) Design for 90 percent coating efficiency, based on experience.
(3) Design for 15-year life.
(4) Design for 2 milliamperes per square foot of bare pipe.
(5) Packaged-type magnesium anodes must be used.
(6) Insulating couplings are used on all service taps. Mains are electrically isolated from all other metal
structures in the area.
(7) All pipe has been precoated at the factory and wrapped with asbestos felt. The coating has been
tested over the trench for holidays and defects have been corrected. The coating is considered to be better
than 99.5 percent perfect when installed.
b. Computations.
(1) Find the total outside area of piping (table C-3).
(2) Find the area of bare pipe to be protected cathodically based on 90 percent coating efficiency:
A = 4288x0.1
A = 429 sq. ft.
(3) Find the maximum protective current required based on 2 milliamperes per square foot of bare
metal:
I = 2 x 429
I = 858 mA or 0.858 A.
Table C-3. Dimensions for finding outside area of pipe
Pipe size
Pipe lingth
Pipe area
Pipe area
(in.)
(ft)
(sq ft/lin ft)
(sq ft)
3
600
0.916
550
2
1500
0.622
933
1
1800
0.497
894
1
2400
0.344
826
3900
0.278
1084
Total area of pipe in square feet
4288
U.S. Air Force.
(4) Find the weight of anode material required based on maximum current requirement and 15-year
life. Use equation C-1:
YSI
W'
,
E
where Y = 15 years, S = 8.8 pounds per ampere-year, I = 0.858 ampere, and E = 0.50 efficiency. Thus,
(15 yr)(8.8 lb/A&yr)(0.858 A)
W'
,
0.50
W = 227 lb.
Note that the 227-pound value is based on an output current of 0.86 ampere for the cathodic protection
system's full design life, 15 years. Strictly speaking, this is not the true condition, because current output after
new installation is much less due to the high coating efficiency. The average current requirement at first may
be as low as 0.03 milliampere per square foot of pipe.
C-6
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