Anode installation

The first anode installation for the cathodic protection of gas pipelines in New Orleans consisted of a 5-m-long horizontal cast-iron tube. Later old tramway lines were used. Since in downtown New Orleans there was no suitable place to install impressed current anodes and to avoid detrimental effects on other pipelines, Kuhn recommended the use of deep anodes which were first installed in 1952 at a depth... 

The backfill is either poured into the borehole, or the anodes are enclosed in sacks of permeable material filled with backfill. Such anodes are sunk into the borehole and backfilled with water and fine soil. Anodes installed in this way deliver their maximum current after only a few days. 

Recommended Extended anode installation Mostly used for Aggressive soils and Soils Deep C/i... 

Fig. 9-9 Measured grounding resistances of anode installations [the straight line corresponds to Eq. (9-5) with from Fig. (9-3)].

The wattage is directly proportional to the grounding resistance of the whole anode bed and therefore to the specific soil resistivity. Equation (9-5) gives the grounding resistance of the anode installation which either consists of n horizontal or vertical single anodes or of anodes with a horizontal continuous coke bed of total length I = ns. The total cost function is given by [1] ... 

The current output of galvanic anodes depends on the specific soil resistivity in the installation area and can only be used in low-resistivity soils for pipelines with a low protection current requirement because of the low driving voltage. Impressed current anode installations can be used in soils with higher specific soil resistivities and where large protection currents are needed because of their variable output voltage. 

The information in Section 9-1 covers the anode installation. Housing, layout and circuitry of the transformer-rectifier unit are described in Chapter 8, and type and possible choices of anode materials in Chapter 7. 

The grounding resistance of the three anodes with the stated dimensions of the coke backfill, a soil resistivity of 75 D m and an interference factor, F = 1.2, was calculated from Eq. (24-35) as about 14 Q. After the anode installation was in operation, measurements of the grounding resistance gave a value of about 12 Q. 

Figure 12-2 shows as an example the arrangement of the anode installation for the local cathodic protection of pipelines in a power station. The cooling water pipelines have a nominal diameter of DN 2000 and 2500 and a covering of earth up to 6 m. The fire-fighting pipelines have a nominal diameter of DN 100 and a covering of 1 m. All the pipelines have a bitumen coating. 

Cathodic protection with impressed current anodes is used predominantly with cables or steel casing in which the cable is inserted, outside built-up areas where it is possible to build large anode installations without damaging interference with other lines. In densely populated areas, protection with impressed current anodes is often only possible with deep anodes, with surface anodes or locally at individual problem points (local cathodic protection, see Chapter 12). 

Anode installation near surface 100-m cable trench, including cable and laying (58 DM m" ) DM 7600... 

Fig. 10.11 Schematic diagram showing an impressed-currem anode installed in a groundbed remote from a pipeline. The positive current produces a field gradient in the soil, but the gradient tends to zero close to the line producing a uniform lateral electrical potential on the soil...

Manually Controlled System A manually controlled system comprises one or more transformer-rectifiers each with its associated control panels which supply the d.c. to the various anodes installed in the water box spaces. Each transformer-rectifier is provided with its own control panel where each anode is provided with a fuse, shunt and variable resistor. These enable the current to each anode to be adjusted as required. Reference cells should be provided in order to monitor the cathodic protection system. In the case of a major power station, one transformer-rectifier and associated control panel should be provided for separate protection of screens, circulating water pumps and for each main condenser and associated equipment. 

Impressed current systems are normally based upon anodes of silicon iron, platinised titanium or platinised niobium. The method of anode installation is usually by suspension. The anode configuration and number must be such as to ensure uniform current distribution. Considerable use is made of wire-type platinised-titanium, and niobium anodes which offer minimal weight and relative ease of mounting/suspension. 

Fig. 10.33 Total cost in relation to number of anodes installed...

Corrosion of the outermost rebar may be mitigated by installing CP however, a conventional CP system with anodes installed on the concrete surfaces likely will not be effective because of a dry environment at the anode-concrete interface that does not facilitate current flow caused by electrochemical osmosis. Because the interior of the tunnel is not exposed to rain or splashing sea water, the moisture available would come from the air. Water from inside the concrete would not reach the surface anodes because the surface cracks were sealed. 

Anode installation should be done according to the manufacturer s instructions. The anode must not come in direct electrical contact with the reinforcing steel. There must be no metallic fixings into the anode. 

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