TM 5-811-7
a.
Design data.
(1) Average soil resistivity is 2000 ohm-centimeters.
(2) Effective coating resistance at 15 years is estimated at 2500 ohms per square foot.
(3) Pipe has a 6-inch outside diameter.
(4) Pipe length is 6800 feet.
(5) Design for 15-year life.
(6) Design for 2 milliamperes per square foot of bare pipe.
(7) Design for 90 percent coating efficiency based on experience.
(8) The pipeline must be isolated from the pumphouse with an insulating joint on the main line inside
the pumphouse.
(9) HSCBCI anodes must be used with carbonaceous backfill.
(10) The pipe will be coated with hot-applied coal-tar enamel and will be holiday-checked before
installation.
(11) Anode bed must not exceed 2 ohms.
(12) Electric power is available at 120/240 volts a.c. single phase from a nearby overhead distribution
system.
(13) Current requirement test indicates that 2.36 amperes are needed for adequate cathodic
protection.
b. Computations.
(1) Find the gas main's outside area:
Pipe size - 6 in.
Pipe length - 6800 ft
Pipe area - 6800 x B A = L B d = 6800 B 6
= 10,681 sq ft.
2
12
(2) Check the current requirement using equation 2-1:
I = (A)(I')(1.0 - CE)
I = 10681 sq ft (2 mA/sq ft)(1.0 - 0.9)
I = 2136 mA,
which agrees with the current requirement test in 13 above.
(3) Select an anode. From table 2-4, choose the 60-pound anode with a 2.8-square-foot surface area
(arbitrary selection).
(4) Calculate the number of anodes needed to meet the anode supplier's current density limitations; use
equation 2-9:
I
N'
(A1)(I1)
2360 mA
N'
,
(Recommended maximum
(28 sq ft/anode)(1000 mA/sq ft)
current density output for
high-silicon chromium-
bearing cast-iron anodes is
1000 mA/sq ft.)
N = 0.84 anode
(5) Calculate the number of anodes required to meet the design life requirements from equation 2-10:
(L)(I)
N'
(1000)(W)
(15 years)(2360mA)
N'
' 0.59 anode
(1000)(60 lb/anode)
D-2
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