INSTALLATION OF AN INSTRUMENTED CATHODIC PROTECTION SYSTEM ON A LARGE DIAMETER AST
(CP) systems installed in the shallow spaces between the two bottoms. This paper describes the installation of a highly instrumented CP system on a large above ground storage tank bottom and provides data demonstrating the effectiveness of the system.Corrosion on tank bottoms has been a problem for storage facilities that has led to many solutions. A common practice is to use secondary containment, or double bottom tanks as a means of ensuring that any leaks in the primary bottom would not lead to environmental contamination. These systems have been installed for several years, yet there has been little in the literature to document the effectiveness of the cathodic protection
When planning and installing cathodic protection on a double bottom tank, it is important to design and install sufficient monitoring capability into the system.
Reference electrodes must be placed exactly mid-way between anodes to minimize IR drop error in potential measurements. More precise potential measurements can be obtained by either interrupting the rectifier or using cathodic protection coupons placed very close to the reference electrodes.
(20 cm) below the primary bottom, a five foot (1 ½ m) inter-anode spacing was adequate to obtain cathodic protection out to the mid-point between anodes.In this particular installation where the anodes were placed eight inches
Polarization or depolarization takes time on any uncoated structures. The structure should be regularly monitored after the system is energized and applied current periodically adjusted in order to obtain the desired results.
For this application, it is best to use a constant current rectifier to have complete control over the applied current levels.
Anode life can be maximized and electrolyte dry out minimized by using the lowest level of current necessary to meet NACE criteria.