The close anode ground bed cathodic protection (CP) system design for the Aboveground Storage Tank (AST) bottoms uses the area of influence surrounding each anode ground bed, in contributory area design, to provide adequate CP. The area of influence surrounding each anode ground bed, mainly depends on the adopted CP criteria, and is greatly affected by the changing electrolyte resistivities of the AST tank pads and the distance between the anode and the structure. Accommodating for changing electrical properties is a standard requirement for CP designs and the spacing of anodes is adjusted to get proper current distribution coverage. In case of a deficient CP design, where the design parameters are not cautiously considered or when the field parameters change considerably from the design assumptions, CP will be inadequate.
In this paper, the CP current distribution with changing resistivities and the area of influence required to meet effective CP criteria, is studied. The results indicate that the tank pad electrolyte resistivity plays a significant role in achieving uniform CP current distribution. The paper also explores the use of Vapor Corrosion Inhibitor (VCI) and its effect on electrolyte resistivity and the resulting CP current distribution.
The design guidelines for cathodic protection (CP) current distribution in an anode ribbon grid system for AST bottom plate protection uses two factors. The first is the distance between the anode and the bottom plate, which is a known factor, and the second factor is the expected anode throw from each anode, to determine the anode coverage length.1 The coverage length from each anode is the contributory area of the AST plate receiving cathodic protection current. The cathodic current densities across electrolyte/structure interface will be higher on top of the anode and will be lower at the point furthest away from the anode in the contributory area configuration. The polarization levels required for effective cathodic protection mainly depend on the cathodic current densities occurring across the electrolyte/structure interface. When the CP current is applied, primary current distribution happens by relative resistivities of the current paths and secondary current distribution happens with the buildup of cathodic reaction products. In an anode grid system, the adjacently spaced anodes are designed to provide adequate current densities at the point furthest away from the anode location and meet the required polarization criteria. However, the presence of contaminants in the tank pad will further affect the pad electrolyte resistivities and the localized polarization levels.
