Impressed current anode beds

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

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 installation costs for a single impressed current anode of high-silicon iron can be taken as Kj = DM 975 (S550). This involves about 5 m of cable trench between anodes so that the costs for horizontal or vertical anodes or for anodes in a common continuous coke bed are almost the same. To calculate the total costs, the annuity factor for a trouble-free service life of 20 years (a = 0.11, given in Fig. 22-2) should be used. For the cost of current, an industrial power tariff of 0.188 DM/kWh should be assumed for t = 8750 hours of use per year, and for the rectifier an efficiency of w = 0.5. The annual basic charge of about DM 152 for 0.5 kW gives about 0.0174 DM/kWh for the calculated hours of use, so that the total current cost comes to... 

Anode materials and their properties along with the recommended uses are noted in Table 1.31. For a number of anodes required for an impressed current ground bed, we calculate the weight required by the equation,... 

GROUND BED - Cathodic protection, an interconnected group of impressed-current anodes that absorbs the damage caused by generated electric current... 

Impressed current anodes must be corrosion resistant and otherwise durable in the environment in which they are used. They must have low consumption rates when coimected to a CP source. All materials used for impressed current anodes are cathodic (more noble) than steel. High silicon cast iron (Fe-0.95C-0.75Mn-14.5Si-4.5Cr) is used for onshore CP applications and in other locations where abrasion resistance and other mechanical damage considerations are important [1]. Graphite anodes are extensively used for onshore pipeline CP applications in which they can be buried in multiple-anode ground beds. However, graphite must be stored and handled carefully because of its brittle nature. 

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)...

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 following example shows how the design curve can be used to obtain the resistance of impressed current and galvanic anodes. Separate charts are constructed for impressed current ground-beds and horizontal ground-beds. With each chart the following design information is provided ... 

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. 

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

These are very commonly used for the impressed current system. A typical vertical anode installation is shown in Fig. 5.27. This type of ground-bed has a low resistivity which is an outstanding advantage. Almost all of the current flow is from the anode to the backfill by direct contact so the consumption of the material is mostly at the edge of the backfill. Vertical anodes are installed with a carbonaceous backfill. 

Design an impressed current system to protect a coated pipeline 4 mile long, 6(5/8)" OD in a soil of 2000 ohm-cm resistivity. Graphite anodes 3" X 5 are to be used. The back voltage between the pipeline and ground-bed is 3.0 V. 

Ground-bed Anodes (impressed current or galvanic) buried in a soil with special backfill and connected to the positive terminal of a current source. 

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