TM 5-811-7
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.
b. Impressed current cathodic protection system
groundbed resistance increases). To calculate the
design. Thirteen steps are required when designing
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.
2-5
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