This work is focused on the simulation of the type of ICCP system in which a grid of anode ribbons and distribution bars, buried below the base of a tank, is supplied with power from a transformer rectifier unit. Current flows from the anode ribbons through the wet sand in which they are embedded, to the base of the tank, and then back to the TRU.
The ICCP system is represented mathematically as a circuit including the TRU, distribution cables connecting it to a number of distribution bars, an array of anode ribbons welded to the distribution bars, and the return cable connecting the tank base to the return of the TRU. The electrical circuit equations are solved to determine current flow and electrical potential throughout the grid of anode ribbons and distribution bars. Current flow from the surfaces of the anode ribbons into the surrounding electrolyte, and from the electrolyte into the surface of the tank base, is described using polarization curves, which encapsulate the non-linear relationship between current density and potential difference across the metal / adjacent electrolyte junction. Current flowing through the electrolyte is determined by solving the Laplacian equation, using the boundary element method. The entire process is non-linear, and is solved iteratively.
The results of the mathematical modelling include current density and protection potentials on all parts of the tank base, as well as power loss, current and potential throughout the circuit. Using the boundary element method means that potential and current density can be accurately calculated anywhere inside the sand, so providing information about the potential at any reference electrode position.
Although modelling a fully functioning system allows assessment of whether or not a particular ICCP design will work, it is valuable in addition to be able to determine the effects of damage or degradation of system components. Consideration is made in the paper of the effects of poor welds and broken connections for a particular ICCP system, with the aims firstly of determining whether the system can perform properly despite the damage, and secondly of evaluating effects of remedial modifications to the system. Investigation is made into the TRU output required to provide some minimum level of protection everywhere on the tank base, and corresponding reference electrode readings are established. Finally, the paper compares performance of a number of different ICCP system designs applied to protection of a tank, and attempts to select the “best” design based on considerations like uniformity of potential, and greatest degree of over-protection.
Keywords: ICCP; modelling; circuit; tank-base; design
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