Horizontal anodes

Fig. 9-3 Grounding resistance of anodes in a continuous coke bed with a covering of earth t = 1 m and a diameter d = 0.3 m for a specific soil resistivity of p = 10 Q m. Horizontal anodes from Eq. (24-23), see line 9 in Table 24-1 vertical anodes R ...

Fig. 9-5 Voltage cone of horizontal anodes with the z axis at right angle to the anode = voltage anode/pipeline = voltage of ground at Z against remote ground.

Fig. 9-6 Voltage cone of a horizontal anode in the direction of the anode axis, x t4 = voltage anode/pipeline t/x = voltage of ground at X against remote ground.

The current requirement of the protected object basically determines the design of the anode bed. For example, for a pipeline requiring 10 A with horizontal anodes laid in soil with p = 45 H m, according to Fig. 9-14, eight anodes are necessary. The grounding resistance of one anode amounts to Rq = 14 H. From Fig. 9-8, the grounding resistance of the anode bed with an interference factor F= 1.34 for 8 anodes spaced at 5 m comes to R = 2.34 Q.. 

Additional individual anodes must be installed at points on the protected object where a sufficiently negative pipe/soil potential cannot be achieved. Since usually only the voltage cone is of interest, the place of installation does not depend on the specific soil resistivity. Coke backfill is not necessary, and the place of installation is determined by the local circumstances. Individual horizontal anodes are conveniently installed parallel to the pipeline at the depth of the pipe axis. The voltage, length and distance of the anodes from the protected object are chosen according to Section 9.1 so that criterion No. 6 or No. 7 in Table 3-3 is fulfilled. 

Fig. 12-2 Local cathodic protection in a power station. deep anodes O horizontal anodes Potential readings Ccu-cuso4 volts (A) free corrosion potential before commissioning the cathodic protection (B) 4 months after switching on...

The amalgam cells consist of slightly inclined steel troughs, over the bottoms of which flow a thin mercury layer, which absorbs the sodium and acts as the cathode. Horizontal anodes adjustable in height at which chlorine is produced are incorporated into the lid of the cells. The chlorine is drawn off upwards through gas extraction slits. 

Fig. 28 Results of measurements, using laser-Doppler velocimetry, by Shekhar and Evans on a water model for studying gas driven electrolyte flow beneath anodes. Flow beneath a flat horizontal anode (inner rectangle), as seen from above, is depicted. The flow is seen to be very slow [44, 54].

Anode Resistance The anode resistance has a significant role in determining the amount of the anode material to be used [84]. The anodes use in the sacrificial cathodic protection system is of three types vertical anodes, horizontal anodes, and grouped anodes. A typical graphite anode vertical and horizontal installation is shown in Figs. 15.12 and 15.13. The resistance of the vertical anode to earth is calculated using the Dwight formula [87]. 

If horizontal anodes are used, the resistance to earth is given by a modified Dwight formula [57,87]. 

Figure 5.28 Typical horizontal anode installation for impressed current cathodic protection system...

Figure 5.49 shows a typical horizontal anode design chart where the number of anodes is shown on the abscissa and the resistance of the anode at different spacings on the ordinate axis. The following basic information is required for the construction of a design chart ... 

Figure 5.49 Typical horizontal anode design chart for impressed current ground beds. (From TEXACO Cathodic Protection - Design and application school, Texaco Houston Research Center, Training Manual. Reproduced by kind permission of Cheveron, USA)...

The above procedure can be used to determine the anode to earth resistance. Design charts can be constructed for vertical anodes and horizontal anode ground-beds for both impressed and galvanic anode systems. The development of design curves is discussed in Section 5.31.2. 

H. B. D Wights equation can also be used to determine the resistance to earth for a single horizontal anode. 

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