SPCC Secondary Containment Volume and Freeboard

The requirement is the capacity of the LARGEST SINGLE container, plus freeboard for precipitation -- not the sum of the tanks, because the design event is one container failing rather than all of them. Freeboard is commonly sized for a 25-year, 24-hour storm or stated as a percentage, and which applies is a matter for the rule and the certifying professional engineer rather than for arithmetic. The DISPLACEMENT subtraction is what gets missed, and it is why this exists. A dike sized to hold the largest tank looks adequate until you notice that the other tanks, their ring foundations, and a pump skid all stand inside it and occupy volume that oil cannot. Net capacity is the gross volume minus everything inside it up to the containment height, and on a congested tank farm that displacement can be a substantial fraction of the total -- large enough to turn a comfortable margin into a deficiency, which is exactly what the gross-versus-net comparison here reports. Two conditions matter as much as the volume and neither is arithmetic. A dike with an open drain valve has NO capacity at all: valves are normally closed and secured, and their operation is a plan requirement with recordkeeping attached. And the containment must be sufficiently impervious to hold the oil long enough to recover it -- a bermed area over permeable soil holds a number on paper and nothing in practice. This computes volumes from ENTERED dimensions and treats the dike as a rectangular prism, which a real dike with sloped walls or an irregular footprint is not. It does not determine SPCC applicability or the required freeboard, address the alternative sized-containment provisions for loading racks, mobile containers or oil-filled equipment, evaluate imperviousness, drainage control or the general containment requirements, or substitute for the PE certification the rule requires. 40 CFR Part 112, the facility's SPCC plan, and the certifying professional engineer govern.

required volume = the largest single container's capacity x (1 + freeboard); gross dike volume = length x width x wall height; displacement = the volume of tanks and equipment standing inside up to that height; net capacity = gross minus displacement, at 1728/231 cubic inches per gallon.

Secondary containment sizing as SPCC practice states it -- capacity for the LARGEST SINGLE container plus freeboard, with NET capacity being gross minus the displacement of everything inside. It treats the dike as a rectangular prism. It does not determine SPCC applicability or the required freeboard, address the alternative sized-containment provisions for loading racks, mobile containers or oil-filled equipment, evaluate imperviousness or drainage control, or substitute for the PE certification the rule requires.

One volume minus another; no rule text is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: largest_tank_gal, freeboard_pct, dike_length_ft, dike_width_ft, dike_height_ft, other_tank_count, other_tank_diameter_ft, other_equipment_ft3, required_gal, required_ft3, gross_ft3, displacement_ft3, net_gal, displacement_pct

• A rectangular prism sloped walls and irregular footprints are not modelledthe site drawings
• Freeboard is entered the applicable storm event or percentage is a rule question40 CFR Part 112
• Not a compliance determination the PE certification the rule requires is not replacedthe certifying professional engineer