Deep Anode Beds

Deep anodes are installed where the resistivity is high in the upper layers of soil and decreases with increasing depth. This type of installation is recommended for densely populated areas and for local cathodic protection (see Chapter 12) on account of the small space needed and the smaller voltage cone, which avoids interference with foreign structures. 

Deep anodes consist of parallel-connected single anodes which are set in boreholes 50 to 100 m deep with a diameter of 0.3 m. The boreholes can be produced by a variety of boring methods, but the air lift method has proved particularly suitable (see Fig. 9-10). The borehole is filled with water to ground level according to the principle of a mammoth pump. Compressed air is fed via a pipe to 

Centering equipment is used to ensure that the impressed current anode is centrally situated in the borehole. The anode with the centering device can be inserted in the borehole by use of, for example, plastic-insulated wire ropes (see Fig. 9-11). After each of the anodes is inserted, the free space is filled with No. IV coke up to the level of the next anode about 50 kg of coke are necessary per meter of anode bed. The wire rope is fixed to a support above the borehole and provides offloading to the anode cable. The anode cables are laid to a junction box so that the 

A significant rise in temperature AT is calculated in a volume of soil at a distance up to 3 /q from the anode, where /q is the anode radius. Factors are the thermal conductivity of the soil, k, length of the anode, L, the grounding resistance, Rq, and the current, I. The rise in temperature can be calculated from these parameters [10]. For deep anodes it amounts to  

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Fig. 9-10 Drilling for a deep anode bed with the air lift method.

The model shown in Figure 13 considers three parallel pipelines of approximately 70km in length sharing the same CP system which consists of 11 ICCP deep anodic beds delivering 10A each. The distance between pipelines is 10m in the local normal direction. The model considers two soil layers (0.02 S/m from ground level to -35m, and 0.005 from -35m to -100m). 

If the anodes are installed vertically in 3 to 6 m deep holes about 5 to 6 m away from the structural elements to be protected, the system is called a vertical distributed anode-bed system. When the anodes are installed in a cluster, away from the structures that are being protected, the system is called a remote anode-bed system. A deep anode-bed system is a variation of a remote anode-bed system in which anodes are located remotely from the structures being protected in a vertical plane. 

The versatile new fluoropolymer found use in other applications where tough and abrasion resistant properties were important. For example, corrosion engineers and cathodic protection service companies found that PVDF insulation solved many of the corrosion problems associated with underground anode-bed installations and down-well jacketing to protect the instrumentation cable in deep wells. 

Anode beds are sometimes installed deep below the surface (deeper than 30 m) (Fig. 13.15), a design that is particularly useful for installations where electrical interference problems are severe or where the resistivity of the soil near the surface is high. The deep installation causes the current flowlines to become parallel and thus provide a more uniform distribution of the protective current. 

These are used because of the near presence of (c) Deep Vertical Anodes the other structures and also because of the limitations of the soil depth. A typical installation is A deep ground-bed is defined as a ground-bed shown in Fig. 5.28. The anodes should always be in which the anodes are installed vertically in a placed in moist soil or in hygroscopic backfill so drilled hole at a depth of 50 ft or more. A typi-that it does not dry out during the dry season. cal deep anode installation is shown in Fig. 5.29. 

A deep well anode bed design is selected because... 

A common cathodic protection design is the use of bracelet anodes that originally were zinc but now, with improved alloys, are usually aluminum. Alternately, high-silicon cast iron anodes mounted on sleds, buried in the sea bed 250 feet from a given pipeline and midway between shore and the spar buoy ship connection have performed well. Anode return cables can be a maintenance problem unless properly secured to the pipelines and buried at least 5 feet into the sea bed, between the pipelines and the anode sled. Anode beds can also be installed in the beach itself, but they must be deep enough to be in the saltwater intrusion area. 

In total, three high-silicon iron anodes of 3 kg each were installed at points a, aj and as shown in Fig. 11-3. The anodes were bedded vertically in fine-grained coke in boreholes about 2.3 m deep and J = 0.2 m so that the length of the coke backfill was about 1 m. Each anode was connected by a separate cable to the anode bus bar of the transformer-rectifier to allow the current of individual anodes to be monitored. Three cathode cables 2x4 mm were installed for the return path of the protection current and attached on the tank end to the connecting clamps of the dome support. 

A number of methods may be used to reduce the interaction on neighbouring structures. In some circumstances it may be practicable to reduce the current output applied to the protected structure or to resite the ground-bed so that the anode effect on an unprotected pipe or cable is altered as required. The physical separation between the groundbed and nearby buried structures can be increased by installing anodes at the bottom of deep-driven shafts and substantial improvements can be made using this technique. 

Figure 13.15 Typical deep bed anode in normal soil strata.

As an example of the technique, we may consider the reduction of penta-chlorophenol (PCP). The reactor consisted of two compartments, each 10 cm long, 2 cm wide and 0.5 cm deep, the cathode being a packed bed of carbon fibre, which was separated from the anode by a cation-exchange membrane (Nation 426). In the batch-recycle mode, constant current (10 A) electrolysis of 1 dm of 50 mg dm PCP in 0.1 mol dm " Na2S04 plus 0.1 mol dm NaOH removed PCP to a level < 0.5 mgdm within 30 min. The overall current efficiency for complete dechlorination was approximately 1 %, the energy consumption being 36 kWhm"l... 

Figure 5.29 A heavy-wall deep impressed current anode ground-bed Casing method. (From Peabody, Control of Pipeline Corrosion, NACE. Reproduced by kind permission of NACE, Int, Texas, USA)...

Such anodes are used in areas where the resistivity of the soil is very high, such as in deserts. They are also suitable for areas where otherwise a large ground-bed electrically remote is required to keep the resistance to minimum. If the surrounding deep soil has a low resistivity, excellent distribution of current is obtained. The designs vary according to soil condition. 

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