A Cathodic Protection system can provide effective corrosion control against external corrosion threats to above-ground storage tanks; be it related to tank construction materials, coating degradation over operational life span or environmental corrosion caused by tank foundation, soil etc. Traditionally, several different types of anode installation schemes were practiced for current distribution to the tank bottom. These were ‘Horizontal or vertical’ anode installation distributed around the tank periphery or angular drilled anode installation to extend the anodes toward center of the tank bottom. Deep-well anode systems with multiple anodes in a single long bore-hole at relatively remote location were also used to provide common cathodic protection system for multiple tanks in tank farm area. These conventional anode-beds were easy to install, monitor and maintained. For safety and environmental reasons in new storage tank construction, an impermeable plastic membrane is now required to be laid under the tank to contain any corrosion leak if it happens. The use of a membrane beneath the tank bottom as secondary containment and as a means of leak detection thwarts any attempt of conventional anode-bed outside the tank periphery to be effective. The anode-bed and references electrodes or other monitoring systems are therefore installed within the space available between the membrane and the tank bottom during construction of the tank, as retro-fitting of anodes during operational service life would not work because of the inaccessibility below the tank bottom. A robust design of the cathodic protection system for a tank bottom is therefore imperative to ensure intended design life.
This paper briefly discusses the changing perspectives of the cathodic protection system from conventional anode-beds to a grid system showing the detail design approach adopted and highlights the implications of miss-design based on a practical example of a newly constructed 100 meter dia crude oil storage tank with 40 years design life if relevant design considerations are not addressed.
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