CATHODIC PROTECTION OF TANKS BOTTOM BY MEANS OF LINEAR GRID ANODES. CURRENT AND POTENTIAL DISTRIBUTION

Above ground tanks for storage of liquid hydrocarbon are often erected with secondary containment membrane installed below the tank bottom to prevent soil contamination in case of leakage. The cathodic protection is mainly based on impressed current systems with distributed anodes installed in the space between the tank bottom and file membrane. Among available anodes, the most commonly used are the titanium grid or ribbon activated with noble metal oxides. The configuration of the grid or ribbon anode system confined in the closed space between bottom and membrane creates specific issues concerning the electrochemical reactions occurring at anode and cathode, the ohmic drops in the anode system and the potential and current distribution at the cathode. The paper reviews file specific aspects of this application of cathodic protection and provides results Of a number of numeric simulations performed to predict the distribution of current and potential. Distributions were modelled through 2D and 3D FEM analysis in order to evaluate effects of anodes spacing, power supply voltage, soil resistivity and ohmic drop in anodes and distributor, being fixed anode to secondary containment membrane distance (figure 1). Non linear boundary condition Were applied at cathode side and a coefficient was introduced to take into a count about coating defects. Constant anode potential was considered. The effect of soil resistivity oil potential and current distribution over transversal direction to anode is shown in figures 2 and 3. Increasing soil resistivity makes polarization terms negligible and ohmic drop becomes more important. Current distribution approach primary distribution as soil resistivity reaches 200-2000 Ohm.m. Even distributions are achieved for anodes spacing less than 2 times distance of anode to tank bottom (figure 4). The results were interpolated for obtaining relationships linking feeding voltage, protection current density, maximum current density, anode spacing, soil electrical resistivity, current Output, useful for cathodic protection design. The effect of ohmic drop into distributor and anode strip was evaluated by means of 3D fem model (Table 1). Ohmic drop mainly affect potential distribution at the cathode in low resistive soils. In soil with resistivity exceeding 200-2000 Ohm.m only produces very low variation oil cathode potential if ohmic drop remains less than 5% of feeding voltage. An empirical relationship for dimensioning spacing for both distributor and anode was proposed for estimating ohmic drop into distributor and