Cathodic protection design calculation

In cathodic protection design, it is essential to be able to estimate the anode output and its lifetime. From these estimates, anode spacing over the structure to be protected can be calculated (i.e., the number of anodes required to protect the structure). 

Figure 16-10 shows a plan of a bridge structure for supplying the Krakatau steel works in Indonesia [20]. The tubular piles are between 25 and 30 m in length. In designing the cathodic protection, a surface area of 4 x 10 m was calculated. The piles... 

Obviously, the total weight of the anode material must equal or be greater than the total weight, IF, calculated above. Similarly each anode must be of sufficient size to supply current for the design life of the cathodic protection system. The anodes must also deliver sufficient current to meet the requirements of the structure at the beginning and end of the system life. That is, if current demand increases (as a result of coating breakdown, for example) the output from the anodes should meet the current demands of the structure. 

The basic design of sacrificial CP system includes calculation of cathodic protection circuit resistance, potential difference between the anode and structure, anode output, number of anodes, and the anode life expectancy. A schematic of the cathodic protection test is given in Fig. 15.11. To estimate current requirements, a test is needed to determine the current i ) necessary to provide adequate protection for the pipeline. This can be done by applying current using a temporary test setup and adjusting the current from the rectifier until the cathodic protection criteria is reached. 

The advantage of calcium nitrite is that it can be added to the mix and has no serious effects on the design, construction and performance of the structure other than its effect as a set accelerator. Mix design may require adjusting to include a retarder. It.s disadvantage is that there must be enough to stop corrosion and it is consumed with its exposure to chlorides. It is therefore important to calculate the chloride expo.sure for the life of the structure and add sufficient inhibitor. It does not inhibit the application of cathodic protection or chloride extraction in later life of the structure if necessary... 

The described property of spherical electrodes permits to use equation (12.41) for the design of cathodic protection systems for structures buried in soil. In this case, the anode is usually much smaller than the structure it protects and often has an approximately spherical shape. Instead of having to calculate the ohmic resistance of the entire system, a more complicated task, it may be enough to estimate the ohmic resistance in the vicinity of the anode by using equation (12.49). A number of expressions for the ohmic resistance of buried anodes of other geometries are available in the literature [34]. 

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