5 Jan 91
Calculate the required length of the backfill column.
Spacing between anodes depends primarily on the
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
The minimum length of the backfill column is then
calculated as follows:
6 anodes at 6 ft per anode =
Spacing between anodes: 5 at 11 ft =
Space above and below anode string* =
* 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.
backfill column-to-earth resistance
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.
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.
Determine total circuit resistance (RT) from equation
RA + RW + RC (Because the cathodic
protection system utilizes a single
deep anode groundbed RA = RN)