TM 5-811-7
(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.
2-7
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