The design of galvanic anode ground beds involves procedures similar to those used for impressed current ground beds. Design charts, however, differ somewhat with anode
TO PIPELINE 2 TO PIPELINE
MAGNESIUM OR ZINC ANODE
L
L = LENGTH OF SPECIFIED ANODE
Figure 7.6 Typical galvanic anode installation.
dimensions and backfill used with them. Galvanic anodes (see Chapter 9) use special backfill having resistivities in the order of 50 ohm-cm. Calculations for design charts in this chapter, however, are based on a conservative figure of 300 ohm-cm. Using proce-dures described previously earlier in this chapter under Impressed Current Ground Beds, similar charts may be developed based on lower resistivity backfill mixtures.Figure 7.6 illustrates two typical types of galvanic anode installation.
The following is a calculation procedure similar to those used for impressed current anodes. The resistance to earth of a 17-lb packaged anode (at the left ofFigure 7.6) is approximately 7.17 ohms in 1000-ohm-cm soil. Resistance to earth of the longer anodes (at the right of the figure) will range from 3.48 ohms for a 1.4-in× 1.4-in cross section anode to 3.38 ohms for a 2-in× 2-in cross-section anode in 1000-ohm-cm soil. As stated above, these figures are based on 300 ohm-cm chemical backfill resistivity. Comparable figures using 50-ohm-cm backfill (which reduces the internal resistance between anode and outer edge of backfill column) would be 6.36, 2.94 and 2.92 ohms respectively.
For vertical galvanic anodes in parallel, design curves are provided inFigure 7.7. The curves shown are for 17-lb magnesium anodes at 15-ft spacing and for 5-ft long zinc or magnesium anodes at 15-ft spacing. Similar curves for other spacing may be calculated by procedures described earlier.
Seventeen-pound anodes, being short, need not be installed horizontally except in specific circumstances. Longer anodes may require horizontal installation at locations where soil conditions are not favorable for the more usual vertical configuration. Figure 7.8 includes a design curve that may be used as a guide in determining the parallel resistance of 5-ft horizontal anodes centered in a 6-ft. length of 8-in× 8-in cross section clay-gypsum backfill in a 4-ft deep trench with 15-ft spacing, center to center, between anodes.
APPROX. INTERNAL ANODE RESISTANCE IN 300 OHM-CM BACKFILL 17-LB MAG ANODE : 0.96 OHM
Figure 7.7 Typical design charts for vertical galvanic anodes.
0 5 10
Figure 7.8 Typical design chart for horizontal galvanic anodes.
0 50 100 150
INTERNAL RESISTANCE IN OHMS
RESISTANCE TO EARTH OF BACKFILL LAYER
INTERNAL RESISTANCE
Figure 7.9 Typical design chart for horizontal continuous galvanic anode.
Typically little can be gained by installing individual galvanic anodes in a continuous horizontal layer of backfill as described for impressed current ground bed installations.
Longitudinal resistance of the higher resistivity backfill tends to prevent good current distribution along such a bed. If, however, continuous strip anodes are used (available in both zinc and magnesium), this type of backfill construction becomes practical. The design curve inFigure 7.9may be used as a guide for continuous anode strips centered in a continuous horizontal layer of clay-gypsum backfill, 8-in× 8-in cross section, in a 4-ft deep trench.
As an example of galvanic anode ground bed design, assume a requirement involving the installation of vertical 2×2×60-in magnesium anodes (seeFigure 7.6) in 800-ohm-cm soil to furnish 0.5 A of protective current with the pipeline polarized to−1.0 V to CSE.
Assume that anodes will be installed on 15-ft centers, that the nearest anode will be 20 ft from the pipeline, and that No. 8 copper wire will be used for the header cable. Assume that effective resistance between pipeline and earth at the installation site is 0.4 ohm.
The driving voltage available to force current from the magnesium anodes through the circuit resistance will be the polarized open circuit potential of the magnesium used less the polarized pipeline potential. Assume magnesium open circuit potential of stan-dard magnesium (Chapter 9) at−1.55 V to copper sulfate. The driving potential in this case, then, will be 1.55−1.00−0.10 (anode polarization) = 0.45 V. Maximum permissible current resistance to provide 0.5 A output will be 0.45 V/0.5 A = 0.90 ohm.
From the 0.90 ohm circuit resistance, subtract the effective pipe-to-earth resistance (0.4 ohm in this case) and the resistance of No. 8 wire from pipe to first anode (0.02 ohm used in this case to allow for 35 ft which will permit slack and test point connections).
The total circuit resistance is 0.90 − (0.4 + 0.02) = 0.48 ohm for anode-to-earth resistance plus effective header cable resistance. Using the design curve of(Figure 7.7), convert the 0.48 ohm to a 1000 ohm-cm soil resistivity base.
0.48 × (1000/800) = 0.60 ohm.
Using the curve for 60 in anodes at 15-ft spacing fromFigure 7.7, seven anodes in parallel would be selected for the first attempt. Assuming 7 anodes, the resistance would be 0.55 ohm (from the curve)×(800/1000) (to convert to the 800 ohm-cm soil resistivity at the installation site)+ (0.60/7) (the internal anode resistance) + 0.030 ohm (the resistance of 45 ft of No. 8 wire, half the header cable length). This equals 0.556 ohm, which is too high. By increasing the number of anodes by trial and error, it is found that 9 anodes will be the minimum number that will meet the requirements of the example. The resistance, by the above procedure, will be (0.45×(800/1000))+(0.60/9)+0.039 = 0.466 ohm which is within the 0.48 ohm requirement. The above calculations were made using the higher resistivity chemical backfill. Using the lower resistivity (50 ohm-cm) backfill, the internal anode resistance would be lower and the anode bed resistance lower.
A similar approach to that detailed above would be used for design problems in which other galvanic anode design charts included herein are employed to arrive at a desired value of anode bed resistance. The useful life to be expected must be considered also. See Chapter 9 for data on selection of galvanic anode materials, sizes, and life calculations.