TM 5-811-7
a.
Design data.
(1) Average soil resistivity is 5000 ohm-centimeters.
(2) Effective coating resistance at 25 years will be 2500 ohms per square foot, as suggested by the
manufacturer.
(3) Design for 90 percent coating efficiency, based on experience.
(4) Design for 25-year life.
(5) Design for 1 milliampere per square foot of bare pipe after polarization (corrosion history of area
indicates this value is adequate).
(6) Magnesium packaged-type anodes must be used (soil resistivity is greater than 2000 ohm-
centimeters).
(7) System is insulated well enough from foreign structures.
(8) All piping is mill-coated with hot-applied coal-tar enamel and wrapped with asbestos felt. Coating
has been tested over the trench for holidays and defects have been corrected. Coating is assumed better than
99.5 percent perfect at installation.
b. Computations (fig C-1).
(1) Find the total outside area of liquid fuel pipes serving the hydrant refueling area (table C-1).
Table C-1. Outside area of liquid fuel pipes
Pipe size
Pipe length
(in.)
(ft)
Pipe area (sq ft/ft)
3 (defueling header)
2 x 293 =
586
586 x 0.916 = 537
6 (defueling return)
90
90 x 1.734 = 156
8 (refueling header)
2 293 = 586
586 x 2.258 = 1323
10 (supply line)
90
90 x 2.82 254
6 (hydrant laterals)
3 x 960 = 2880
2880 x 1.734 = 4994
Total area of POL pipe in square feet
=7264
Reproduced from J.R. Myers and M.A. Aimone, Corrosion Control for Underground Steel Pipelines: A Treatise on Cathodic
Protection, J.R. Myers and Associates, Franklin, OH. Used with permission.
(2) Some experience has shown that steel in this type soil can be cathodically protected with
approximately 1 milliampere per square foot of uncoated surface. Thus, find the required current based on
this value and using equation 2-1:
I = (A)(I')(1.0 - CE)
= (7264 sq ft)(1.0 mA/sq ft)(1.0 - 0.8)
= 726 mA.
(3) Calculate the number of anodes needed based on maximum groundbed resistance limitations.
(a) Select a 9-pound anode, 3.5 by 3.5 by 13 inches, from table 2.4. Driving potential as provided
by the manufacturer is 0.9 volt.
(b) Calculate total circuit resistance using equation 2-3:
)E
RT '
,
I
0.9
RT '
1.23 ohms.
0.726
(c) Calculate structure-to-electrolyte resistance from equation 2-4:
R
Rc '
N
C-2
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