Fire Water Storage Tank Internal Cathodic Protection System Resistance Calculation Using Finite Element Analysis

For fire water storage tanks internal corrosion will occur due to internal surfaces are in contact with water. In oil and gas industry in addition of coating, cathodic protection is used to reduce or mitigate internal corrosion. According to dimensions of fire water tanks in petrochemical plants and refineries and high current density requirements, impressed current system used for tanks internal protection. Generally suspended MMO tubular anodes in string form used. These strings placed on circle with some offset from tank wall for uniform distribution of current and potential. Due to close distance between anodes to tank shell and bottom plates, using conventional formulas that assume reference potential at infinite distance will introduce error in calculated resistance, also anodes distribution along string is not addressed in conventional methods. Anode system resistance affect transformer-rectifier size in impressed current system, for tank internal cathodic protection this matter is important due to effect on nozzles quantity and location for suspending anode strings that affect tank construction. In this paper finite element analysis by COMSOL software and mathematical derivation used for calculation of anode system resistance and results compared by conventional methods. According to the results it is observed that error in calculated anode system resistance by conventional methods is more than 17% in comparison with finite element analysis results.