Secondary containment is the second barrier around a tank: a dike, a basin, or an outer wall that catches whatever the tank lets go. The practical questions are whether a site needs it and how big it has to be. Both answers depend on what is in the tank.
Which rule applies depends on what is stored
Two federal rules spell out containment for aboveground tanks in plain numbers. Neither covers every chemical tank.
- Hazardous waste tanks. Under 40 CFR 264.193, an outside containment system has to hold 100 percent of the capacity of the largest tank inside it. If rain can get in, it also has to hold the rain from a 25-year, 24-hour storm.
- Oil. Under the federal oil spill rule (40 CFR Part 112), a bulk oil tank needs containment for the entire capacity of the largest single container, plus enough freeboard for precipitation. The rule only applies to a site storing more than 1,320 gallons of oil above ground, counting containers of 55 gallons or more.
A tank of acid, caustic or bleach you bought to use in your process is a product, not a waste, and it is not oil. For those tanks, containment usually comes from the fire code your city or county adopted (in most of Texas that is an edition of the International Fire Code), state rules, your permit, or your insurer. The fire marshal or the agency with jurisdiction decides which apply to a given site, and the answer can change with the quantity and the hazard class.
How much a dike has to hold
Take one of our standard vertical tanks: 5,000 gallons, 102 inches across. Say it sits alone in a square concrete dike 16 feet by 16 feet inside the walls.
| Step | Number |
|---|---|
| Tank volume | 5,000 gal = 668 cubic feet |
| Dike floor area | 16 ft x 16 ft = 256 sq ft |
| Area the tank itself covers | about 57 sq ft |
| Floor left to hold liquid | about 199 sq ft |
| Wall height if you ignore the tank | about 31 inches |
| Wall height with the tank counted | about 40 inches |
That 9 inch gap is easy to miss. The tank stands inside the dike and takes up room below the wall line, so a dike sized on floor area alone comes up short.
Then the rain. Every inch of rain that falls on a 256 square foot dike adds about 160 gallons, which takes about 1.3 more inches of wall in this one. The 25-year, 24-hour storm depth for a given county comes from NOAA Atlas 14. If that number were 10 inches, this dike would need about 13 more inches of wall, so roughly 53 inches in total.
The zero case: put a roof over the dike so no rain reaches it, and the rain term drops out. The wall goes back to about 40 inches.
If nobody plans for it
Containment that is added after the tank is set usually means pouring a dike around equipment that is already plumbed, or moving the tank. And an open dike fills with rain. Under the oil rule, that rainwater cannot just be let out: the drain valve stays sealed shut, someone inspects the water first, opens and reseals the valve under supervision, and keeps a record each time.
Dike or double-wall tank
A double-wall tank puts the containment inside the product: an inner tank inside an outer shell sized to hold everything the inner one carries. We build them in polyethylene and fiberglass.
| Open dike or basin | Double-wall tank | |
|---|---|---|
| Footprint | Tank plus the dike | The tank alone |
| Rain | Collects in the dike, has to be managed | Kept out by the outer shell |
| Several tanks | One dike can serve them all, sized for the largest | Each tank carries its own |
| Seeing a leak | The dike floor is in plain view | Watched through the space between the walls |
| Fittings | Standard tank fittings | Every pipe passes through both walls, so fitting locations are fixed at order |
For hazardous waste, 40 CFR 264.193 accepts a double-walled tank when the inner tank is completely enclosed by the outer shell and there is continuous leak detection that can find a release within 24 hours.
What to have ready for a quote
- What will be in the tank, at what concentration, and whether it is a product, a waste, or oil.
- Who regulates the site: the permit, the fire marshal, or an insurer requirement, if you know it.
- Indoor or outdoor, and whether a roof is planned.
- How many tanks will share the space, and how often someone can inspect it.
This article explains what the rules say and how the sizing works. It is not legal or compliance advice; the agency or fire marshal with jurisdiction decides what your site needs.
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