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.

(1) Find the total outside area of liquid fuel pipes serving the hydrant refueling area (table C-1).

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*

(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

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