(10) Calculate rectifier voltage. Equation 2-

16 is used to determine voltage output (Vrec) of the

ohms (L)

Rw '

,

(eq 2-12)

rectifier:

100 ft

where L is the structure's length in feet. Economics

Vrec ' (I)(RT)(150%),

(eq 2-16)

are important in choosing a cable, and may indeed

be the controlling factor. To determine the total

annual cable cost, Kelvin's Economic Law can be

used as shown in equation 2-13.

where I is total protection current in amperes, RT is

total circuit resistance, and 150 percent is a factor

(0.0876)(I 2)(R)(L)(P)

to allow for aging of the rectifier stacks.

' (0.15)(S)(L),

T'

(11) Select a rectifier. A rectifier must be

E

chosen based on the results of equation 2-16. Many

(eq 2-13)

rectifiers are available commercially; one that sat-

isfies the minimum requirements of (I) and (Vrec) in

where T is total annual cost in dollars per year, I is

equation 2-16 should be chosen. Besides the more

total protection current in amperes, R is cable

common rectifiers being marketed, a solar cathodic

resistance in ohms per 1000 feet, L is cable length

protection power supply (for d.c. power) may be

in feet, P is cost of electrical energy in farads per

considered for remote sites with no electrical

kilowatt-hour, E is the rectifier efficiency expressed

power. Three factors that should be considered

as percent, and S is the cable's initial cost in dollars

when specifying a solar cathodic protection power

per foot.

supply are:

--The cost of the solar cathodic protection

(c) Calculate structure-to-electrolyte resist-

power supply in dollars per watt of continuous

ance using equation 2-14:

power.

--The solar cathodic protection power sup-

ply's much higher initial cost compared to selenium

R

Rc '

,

(eq 2-14)

rectifiers operated by a.c. power.

N

--The additional maintenance required for a

where Rc is the structure-to-electrolyte resistance,

solar cathodic protection power supply, mainly to

R is the coating resistance in ohms per square feet,

keep the solar panels free of dirt deposits. Appen-

and N is the coated pipe area in square feet.

dix F discusses rectifier current interference.

(d) Calculate total circuit resistance. To

(12) Calculate system cost. As with the gal-

vanic cathodic protection system, the choice of

used:

anode for design calculation is arbitrary. When

several anodes have been used in the design calcu-

RT ' Ra % Rw % Rc,

lations, an economic analysis should be done as

(eq 2-15)

recommended in NACE Standard RP-02.

where variables are the same as for equations 2-11,

(13) Prepare plans and specifications.

242, and 2-14.

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