AWWA D104-97 pdf download.AUTOMATICALLY CONTROLLED, IMPRESSED-CURRENT CATHODIC PROTECTION FOR THE INTERIOR OF STEEL WATER TANKS
4.1.2 General design. The cathodic protection system provided under this standard shall achieve and maintain the criteria for protection of the specific tank (Sec. 4.1.1). The design of an impressed-current system shall include the evaluation of the tank information (Sec. III.A of the foreword) and criteria for protection (Sec. 4.1.1).
4.1.2.1 Output current capacity. Nominal current capacity of the rectifier shall be determined based on the estimated current density required for cathodic protection. The required current density is expressed in amps per unit area of total submerged bare surface. The current density shall be determined for each specific tank and depends on the conductivity of the water and the condition of the coatings.
4.1.2.2 Output voltage capacity. The output voltage capacity is a function of the protective current requirement and the total circuit resistance in accordance with Ohm’s law. The circuit resistance is composed of the anode-to-water resistance, the electrolyte resistance, the wiring resistance, and the coating resistance.
4.1.2.3 Anode configuration. The anode array shall provide and maintain a uniform distribution of protective current without exceeding the potential limits established in Sec. 4.1.1, Criteria for protection. The anode array shall provide and maintain low-circuit resistance consistent with the design life of the anode system and be designed to prevent electrical shorting to the tank.
4.1.2.4 Riser circuit. For an elevated tank riser 30 in. (750 mm) in diameter or larger, additional system capacity shall be provided. A separately controlled circuit shall be provided for energizing the riser anode assembly.
4.2.2 Reference electrode(s). The permanent reference electrode used to mea- sure the tank-to-water potential for controlling the current output of the rectifier shall be designed to maintain a stable potential in continuous immersion in the water for a minimum of 10 years with only minimal maintenance as recommended by the manufacturer. The reference electrode(s) shall have a potential drift of less than 10 mv. The reference electrode lead wire shall be insulated.AWWA D104-97 pdf download.