Anode Beds

With impressed current stations on privately owned sites, it is advisable to conclude contracts with the property owner in order to guarantee access to the installation at all times, to prevent damage to the cables and to exclude construction or modifications in the area of the anode beds. 

Increased anode resistance Increase rectifier voltage, check anode bed... 

Voltage cones also occur where the protection current enters through defects in the pipe coating (see Sections 3.6.2 and 24.3.4). Figure 9-1 shows schematically the variation of the voltage cone of an anode bed and a cathodically protected pipeline that results from the raising and lowering of potential. 

The header cable between anode bed and rectifier must be particularly well insulated. For this reason cables with double plastic sheathing of type NYY-O are used. The cable sheath must not be damaged during installation because the copper core at the defects will be anodically attacked in a very short time and the connection to the rectifier broken. Damage to the cable sheath is not so serious if a multicored cable is used. Usually not all the core insulation is damaged so that the operation of the anode bed is not interrupted. In addition, measurement of resistance and detection of defects is easier. 

The constant, C, depends on the dimensions of the anode bed. It follows from line 4 of Table 24-1 for a horizontal ground in the half space... 

For installations with continuous coke backfill, the anodes can be installed at double the spacing of the anode bed extension. The lower the ratio p /p (i e., the higher the specific soil resistivity), the further apart the anodes can be placed. 

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

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... 

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] ... 

It must be emphasized that areas with low soil resistivity should be chosen for siting the anode beds. 

Fig. 9-14 Economic number of anodes for impressed current anode beds.

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.. 

For installations with several storage tanks and a protection current of several tens of an mA, uniform protection current distribution should be the goal, so that the current injection occurs via a number of anodes distributed over the site or via a more distant anode bed. Dividing up the protection current over several anodes avoids large local anodic voltage cones and therefore effects on neighboring installations. 

Since in oilfields or reservoirs borehole casings are frequently sited close to one another, they can be affected by the voltage cone of the anode bed and the cathodic voltage cone of other casings. Thus, in the case of large oilfields and reservoirs, it is appropriate to protect all wells cathodically. 

The construction of the anode bed is determined by the details of the field or reservoir. The anode bed should have a spacing of >100 m to give an even potential distribution at the well casing. The distance from other well casings not... 

The anodic and cathodic current densities are inversely proportional to the length of the anode bed or the well casing. Since the length of the casing is usually greater by a factor of 10 than the length of the anode bed, the current density of the... 

Installation of an anode bed with 4 FeSi anodes in coke backfill, cable and connecting sleeves, including earthworks DM 6000... 

The effective costs of a cathodic protection station depend very much on the local conditions, particularly on the costs of the power supply and the extent of the anode bed. Table 22-2 contains the costs of a protection station with four silicon iron anodes. 

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