Ra = RT - Rc,

(eq 2-5)

choices to find the one with minimal life-cycle cost.

(9) Prepare plans and specifications. When

which gives the maximum allowable groundbed

the design procedure has been done for several

resistance; this will dictate the minimum number of

different anodes and the final anode has been

anodes required (as number of anodes decreases,

chosen, plans and specifications can be completed.

groundbed resistance increases). To calculate the

number of anodes required, equation 2-6 is used:

impressed current cathodic protection systems.

(0.0052)(D)

8L

Appendix D gives examples of impressed current

&1],

N'

[1n

(eq 2-6)

cathodic protection designs.

(Ra)(L)

d

(1) Review soil resistivity. As with galvanic

where N is the number of anodes, is the soil

systems, this information will contribute to both

design calculations and location of anode ground-

groundbed resistance in ohms (as computed in eq

bed.

2-5), L is the length of the backfill column in feet

(2) Review current requirement test. The re-

(specified by supplier), and d is the diameter of the

quired current will be used throughout the design

backfill column in feet (specified by supplier).

calculations. The calculated current required to

(5) Calculate number of anodes for system's

protect 1 square foot of bare pipe should agree

life expectancy. Each cathodic protection system

with the values in table 2-2.

will be designed to protect a structure for a given

(3) Select anode. As with the galvanic sys-

number of years. To meet this lifetime requirement,

tem, the choice of anode is arbitrary at this time;

the number of anodes (N) must be calculated using

economy will determine which anode is best. Table

equation 2-7:

2-4 gives common anode sizes and specifications.

The anodes used most often are made of high-

silicon chromium-bearing cast-iron (HSCBCI).

(L) (I)

N'

,

(eq 2-7)

When impressed current-type cathodic protection

49.3 (W)

systems are used to mitigate corrosion on an

where L expected lifetime in years, W is weight (in

underground steel structure, the auxiliary anodes

pounds) of one anode, and I is the current density

often are surrounded by a carbonaceous backfill.

required to protect the structure (in milliamperes).

Backfill materials commonly used include coal coke

(6) Select number of anodes to be used. The

breeze, calcined petroleum coke breeze, and natural

greater value of equation 2-6 or 2-7 will be used as

graphite particles. The backfill serves three basic

the number of anodes needed for the system.

functions: (a) it decreases the anode-to-earth

(7) Select groundbed layout. When the re-

resistance by increasing the anode's effective size,

quired number of anodes has been calculated, the

(b) it extends the system's operational life by

area to be protected by each anode is calculated by

providing additional anode material, and (c) it

equation 2-8:

provides a uniform environment around the anode,

minimizing deleterious localized attack. The car-

AT

bonaceous backfill, however, cannot be expected to

A'

,

(eq 2-8)

increase the groundbed life expectancy unless it is

N

well compacted around the anodes. In addition to

where A is area to be protected by one anode, AT

HSCBCI anodes, the ceramic anode should be con-

is total surface area to be protected, and N is the

sidered as a possible alternative for long-term

total number of anodes to be used. For galvanic

cathodic protection of water storage tanks and

cathodic protection systems, the anodes should be

underground pipes in soils with resistivities less

spaced equally along the structure to be protected.

than 5000 ohm-centimeters. The ceramic anode

(8) Calculate life-cycle cost for proposed de-

consumption rate is 0.0035 ounce per ampere-year

sign. NACE Standard RP-02 should be used to

compared to 1 pound per ampere-year for HSCBCI

calculate the system's life-cycle cost. The design

anodes. Appendix E gives the design and specifi-

process should be done for several different anode

cations for the ceramic anode.

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