Tank Bottom Cathodic Protection Anode Layout
Compares an under-tank ribbon grid with a perimeter ring on current requirement and ribbon loading.
Example
You enter
- Tank diameter (ft) 100
- Design current density (mA/sq ft) 1
- Grid line spacing (ft) 10
- Ribbon rated output (mA/ft) 30
You get
- Bottom area (ft²) 7853.98
- Current requirement 7.85 A over 7854 sq ft
- Grid ribbon 793 ft in 10 lines at 10 ft
- Grid loading 9.9 mA/ft -- within the 30 mA/ft rating
- Ring ribbon (ft) 314.159
- Perimeter ring loading 25
- Ring saving (%) 60.3833
- The check that passes and the one that does not 50
Details, formula, and sources
Both pass the loading check, and the ring leaves the tank centre 50 ft from any anode -- where bottoms perforate. Over a containment liner the ring does nothing at all.
current = bottom area x design current density; ribbon loading = current / ribbon length, for a grid of chords at the entered spacing and for a perimeter ring.
API RP 651, NACE SP0193 (now AMPP), and the ribbon manufacturer's rating govern.
API RP 651 is a purchased standard; ribbon ratings are published by manufacturers.
Estimate. AHJ and licensed professional govern.
Field names used by the API: tank_diameter_ft, current_density_ma_per_sqft, grid_spacing_ft, ribbon_rating_ma_per_ft, bottom_area_sqft, current_requirement_a, grid_ribbon_ft, grid_loading_ma_per_ft, ring_ribbon_ft, ring_loading_ma_per_ft, ring_saving_pct, ring_to_centre_ft
- Distribution ribbon loading is checked; current spread through the pad is not modelledspec-v1771 scope
- Liner a closed containment blocks any anode outside itAPI RP 651