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 7,854 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
Checked against published worked examples from: MATCOR, Inc.
Details, formula, and sources
ScopeBoth 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.
Formula or tablecurrent = 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.
Source and editionAPI RP 651, NACE SP0193 (now AMPP), and the ribbon manufacturer's rating govern.
Free accessAPI RP 651 is a purchased standard; ribbon ratings are published by manufacturers.
What governsEstimate. AHJ and licensed professional govern.
API field namestank_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