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ETL 1110-9-10(FR)
5 Jan 91
4)
Calculate the required length of the backfill column.
Spacing between anodes depends primarily on the
resistivity of both the backfill column (coke breeze).
If low resistivity calcined fluid petroleum coke is
used, than the spacing between rod anode (1/8-in. and
1/4-in. diameter) should not be greater than twice the
individual rod length.
Based on previous design
experience, a spacing between anode rods of 11 ft is
selected.
The minimum length of the backfill column is then
calculated as follows:
6 anodes at 6 ft per anode =
36 ft
Spacing between anodes: 5 at 11 ft =
55 ft
Space above and below anode string* =
20 ft
Total length
111 ft
* Generally, the coke breeze column extends from
10 ft below the bottom anode to 10 ft above the top
anode as shown in figures 2-7 and 2-8.
5)
Calculate
the
backfill column-to-earth resistance
(RA).
This can be done from equation 1-4. Because several
attempts may have to be made to obtain the required
groundbed resistance, the process is facilitated by
using the curves in figures 2-9A through 2-9C. Typical
hole diameters are 6 or 8 in.; a 6-in. diameter hole
has been selected for this groundbed. Figure 2-9C will
be used for this design.
Figure 2-9C shows that, for a 111-ft long coke breeze
backfill column, the resistance to earth per 1000 ohm-
cm is 0.31 ohm.
In 1500 ohm-cm soil, the resistance
is 0.31 x 1500/1000 = 0.47 ohm (groundbed resistance in
1000 ohm-cm soil, times actual soil resistivity in ohm-
cm per 1000).
This is well below the design
requirement of 0.75 ohm.
6)
Determine total circuit resistance (RT) from equation
1-3 :
RT =
RA + RW + RC (Because the cathodic
protection system utilizes a single
deep anode groundbed RA = RN)
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