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TM 58117
Ra = RT  Rc,
(eq 25)
choices to find the one with minimal lifecycle 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.
b. Impressed current cathodic protection system
groundbed resistance increases). To calculate the
design. Thirteen steps are required when designing
number of anodes required, equation 26 is used:
impressed current cathodic protection systems.
(0.0052)(D)
8L
Appendix D gives examples of impressed current
&1],
N'
[1n
(eq 26)
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
resistivity in ohms, Ra is the maximum allowable
design calculations and location of anode ground
groundbed resistance in ohms (as computed in eq
bed.
25), 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 22.
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 27:
24 gives common anode sizes and specifications.
The anodes used most often are made of high
silicon chromiumbearing castiron (HSCBCI).
(L) (I)
N'
,
(eq 27)
When impressed currenttype 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 26 or 27 will be used as
graphite particles. The backfill serves three basic
the number of anodes needed for the system.
functions: (a) it decreases the anodetoearth
(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 28:
provides a uniform environment around the anode,
minimizing deleterious localized attack. The car
AT
bonaceous backfill, however, cannot be expected to
A'
,
(eq 28)
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 longterm
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 ohmcentimeters. The ceramic anode
(8) Calculate lifecycle cost for proposed de
consumption rate is 0.0035 ounce per ampereyear
sign. NACE Standard RP02 should be used to
compared to 1 pound per ampereyear for HSCBCI
calculate the system's lifecycle cost. The design
anodes. Appendix E gives the design and specifi
process should be done for several different anode
cations for the ceramic anode.
25


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