CP System Design for a Storage Tank Bottom
A new above-ground storage tank (30 m diameter) will store brackish water. The tank bottom is on a sand pad with a soil resistivity of 10,000 ohm-cm. The tank bottom is coated with a dielectric coating, but the coating may have holidays. The CP system must protect the soil-side of the tank bottom. The tank has a secondary containment liner that is electrically insulating. The design life is 25 years. The available CP options are: (1) impressed current with mixed metal oxide (MMO) anodes in a grid, (2) galvanic anodes (zinc or magnesium), or (3) a hybrid system. The tank will be hydrotested before commissioning.
1. Which CP system is most suitable for this tank bottom, and why?
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For a tank bottom with a dielectric coating, the current requirement is low due to the coating. However, holidays require protection. Galvanic anodes (zinc or magnesium) are limited by their driving voltage and may not be sufficient in high-resistivity soil (10,000 ohm-cm). Magnesium anodes have a higher driving voltage but may not last 25 years. Impressed current with MMO anodes in a grid is more suitable because it can provide consistent current distribution and is not limited by soil resistivity. The secondary containment liner is insulating, so the anode grid must be placed between the tank bottom and the liner, typically in a sand layer. A grid of MMO anodes with a coke breeze backfill can ensure uniform protection. The system can be designed with a rectifier and junction boxes. The hydrotest water may affect readings, so the system should be commissioned after hydrotest. Therefore, impressed current with MMO anodes is recommended.
2. What design parameters and installation considerations are critical for the selected CP system?
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Critical parameters: (1) Current requirement: based on coating quality and holiday size. Assume 1-2 mA/m² for coated tank bottom, but with holidays, calculate based on bare area. For a 30 m diameter tank, area is ~707 m². If 1% bare, bare area is 7 m². Current density for bare steel in soil is 10-20 mA/m². So current requirement is 0.07-0.14 A. (2) Anode resistance: calculate using Dwight's equation for horizontal anodes. (3) Rectifier voltage: based on anode resistance and current. (4) Anode spacing: typically 1-2 m grid. (5) Backfill: coke breeze around anodes to lower resistance. (6) Reference electrodes: install permanent Cu/CuSO4 electrodes for potential monitoring. (7) Test stations: for current and potential measurements. (8) Installation: anodes must be placed on the liner, covered with sand, and the tank bottom placed on top. Care must be taken not to damage the liner. (9) Hydrotest: ensure CP is off during hydrotest to avoid interference. (10) Safety: proper grounding and lightning protection.