By Jonathan Saucedo, Co-Founder of Tank Supply Direct
A spill berm needs to hold the full contents of the largest container you put inside it. If the berm sits outside, it also needs room for rain. If other containers stand inside the walls, they take up space a spill can't use, so you add that space to the total the berm has to hold. To check a berm against that total, multiply its length, width and wall height in feet, then multiply by 7.48 to get gallons. Outdoors in a wet climate, the rain can need more room than the spill itself.
Spill containment is one of the two largest categories we sell at Tank Supply Direct. It's the same math I'd walk through with you on the phone, with real numbers for drums, totes, a fuel tank and a frac tank.
Does a spill berm need to hold 100 or 110 percent?
The federal rule asks for 100 percent of the largest container plus enough room for rain, not 110 percent. That rule is EPA's SPCC rule (Spill Prevention, Control, and Countermeasure), which covers sites that store oil. For bulk storage tanks, 40 CFR 112.8(c)(2) calls for containment of "the entire capacity of the largest single container and sufficient freeboard to contain precipitation." Drums, totes and other portable containers fall under paragraph (c)(11) of the same section, which uses the same standard: "the capacity of the largest single compartment or container with sufficient freeboard to contain precipitation." Freeboard is the extra space in the berm that's there to catch rain.
The 110 percent figure comes from design practice and from state law. EPA's SPCC guidance for regional inspectors calls 110 percent a "rule of thumb" that "may be an acceptable design criterion in many situations." The same guidance notes that "aboveground storage tank regulations in many states require secondary containment to be sized to contain at least 110 percent of the volume of the largest tank." It also warns that 110 percent "may not provide enough volume to contain precipitation from storm events." So if your state uses 110 percent, treat it as the minimum. If the berm is outside, run the rain math too, because a storm can add far more than the extra 10 percent.
Your SPCC plan and your state and local rules set the number your berm has to meet. The amount of rain room is decided by the professional engineer who certifies your SPCC plan, or by the owner or operator at what EPA calls a qualified facility. The plan records how that amount was decided, so check your plan for a freeboard figure before you size the berm.
How to calculate spill berm capacity in gallons
To size a berm, add up the gallons it has to hold, then check that the berm's capacity in gallons is at least that total. Here are the steps:
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Start with the largest single container. Use its full capacity, not how full you usually keep it. A 55-gallon drum counts as 55 gallons. If your state or plan uses 110 percent, multiply this number by 1.1.
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Add room for rain if the berm sits outside. You work it out from your location's 25-year, 24-hour rainfall.
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Add displacement for the other containers inside the walls. Containers that aren't leaking still stand in the berm and take up room the spill would otherwise fill. Count the part of each one that sits below the top of the wall.
- Convert the berm's size to gallons and compare. Multiply length x width x wall height, all in feet, then multiply by 7.48, the number of gallons in a cubic foot (USGS). The result has to be equal to or larger than your total from steps 1 to 3.
Any measurement in inches has to be converted to feet first, by dividing by 12. Two examples show how it works:
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An 8 x 10 foot berm with a 12-inch wall: 8 x 10 x 1 x 7.48 = 598 gallons. That matches the 598 gallons listed for the 8 x 10 x 1 L-bracket berm from Enpac, one of the berm makers we carry. Enpac's other listed capacities follow the same formula.
- A 12 x 40 foot foam-wall berm with a 4-inch wall: 4 inches is a third of a foot (4 / 12), so the berm holds 12 x 40 x 4/12 x 7.48, or about 1,197 gallons. The common mistake is entering the wall height as 4, as if it were 4 feet, which gives 14,362 gallons and overstates the berm twelve times over.
You can also run the formula backward to find the floor size you need. Divide the gallons you need by 7.48 to get cubic feet. Then divide that by the wall height in feet. The result is the floor area in square feet.
How much room do you need for rain?
The usual measure of how much rain room you need is the 25-year, 24-hour storm, which is the heaviest 24-hour rainfall you'd expect about once every 25 years on average. In Houston, that storm is 11.6 inches.
Every square foot of berm floor catches the same inches of rain, so rain fills a berm to the same depth whatever the size of its floor. In Houston, the 25-year storm would fill 11.6 inches of a 12-inch wall and leave 0.4 inches for a spill. On a 10 x 20 x 1 berm listed at 1,496 gallons, that's 1,446 gallons of rain and 50 gallons left over.
EPA considers room for that storm to be enough: in its words, "the proper standard of 'sufficient freeboard' to contain precipitation is that amount necessary to contain a 25-year, 24-hour storm event" (EPA). EPA did not write that storm into the rule itself, so it is the usual starting point rather than a fixed requirement.
Sizing for rain is a matter of trying a berm size and checking it. You work out the rain that size would catch, add it to your total, and compare the total with that berm's capacity. If the berm falls short, try a bigger one. Here's how to turn your local rainfall into gallons:
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Look up your rainfall depth. Go to NOAA's Precipitation Frequency Data Server, pick your state, click your location on the map, and read the value in the 24-hour row under the 25-year column. It's given in inches.
- Work out the gallons of rain for that berm size. Divide the rainfall inches by 12, multiply by that berm's length and width in feet, then multiply by 7.48.
The rainfall depth changes the answer a lot from one city to the next. In Houston, the storm works out to 7.23 gallons for every square foot of berm floor (11.6 / 12 x 7.48). Denver's 25-year, 24-hour storm is 3.52 inches, or 2.19 gallons per square foot. On the same 10 x 20 berm, that's 439 gallons of rain in Denver against 1,446 in Houston.
In a high-rainfall area, making a 1-foot berm wider barely helps. Rain uses up the same inches of wall however wide the berm is, so a wider 1-foot berm in Houston still has only 0.4 inches left for a spill. A taller wall is what leaves room. Once you've picked a wall height, the backward formula finds the floor area you need in two steps:
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Find the wall height left for the spill. Subtract the rain depth in feet from the wall height.
- Find the floor area. Divide the gallons you need for the spill (the largest container plus any displacement) by 7.48, then divide by the wall height left from step 1.
If the berm sits under a roof or cover that keeps rain out, that can reduce or remove the rain allowance. Your SPCC plan decides by how much. A berm indoors needs no rain allowance at all.
What is displacement, and why does it matter?
Displacement is the reason four drums in a berm need more room than one drum's 55 gallons. If one drum in a group of four leaks, the other three are still standing in the berm. The part of each drum below the top of the wall takes up room the spill would otherwise fill. The part above the wall doesn't matter, because the spill can't rise past the wall anyway. EPA's worked containment example adds this displaced volume for each container that isn't leaking to the total the berm has to hold. The leaking container isn't counted as displacement, because its full capacity is already in the total.
In a berm with a 1-foot wall, the part of a 55-gallon drum below the wall is a cylinder 1 foot tall and up to 24 inches across. That's 3.14 cubic feet, or about 23.5 gallons for each drum still standing. A drum stands up to 35 inches tall, so with a 2-foot wall the bottom 2 feet of each standing drum sit below the wall. That doubles the displacement to about 47 gallons per drum. Anything else you store inside the walls, such as pallets, pumps or spare parts, takes up space the same way.
Spill berm sizing examples
These examples run the full calculation for common setups, indoors and outdoors in a wet city (Houston) and a dry one (Denver). Each total depends on the berm you try. The floor area sets how much rain the berm catches, and the wall height sets how much of each standing drum counts as displacement. So in each cell, "Need" is the total for that setup and "Berm" is the size it was checked against. "Listed" means the capacity on a real listing on our site. "By formula" means length x width x wall height x 7.48.
|
Setup |
Indoors |
Outdoors, Denver |
Outdoors, Houston |
|---|---|---|---|
|
Four 55-gallon drums |
Need 126 gal. Berm: 4 x 8 x 1 ft holds 239 gal (listed) |
Need 196 gal. Berm: 4 x 8 x 1 ft holds 239 gal (listed) |
Need 427 gal. Berm: 4 x 8 x 2 ft holds 479 gal (by formula) |
|
One 275-gallon tote |
Need 275 gal. Berm: 4 x 5 x 2 ft IBC berm holds 299 gal (listed) |
Need 328 gal. Berm: 4 x 6 x 2 ft holds 359 gal (by formula) |
Need 622 gal. Berm: 6 x 8 x 2 ft holds 718 gal (by formula) |
|
One 330-gallon tote |
Need 330 gal. Berm: 4 x 6 x 2 ft holds 359 gal (by formula) |
Need 396 gal. Berm: 5 x 6 x 2 ft holds 449 gal (by formula) |
Need 677 gal. Berm: 6 x 8 x 2 ft holds 718 gal (by formula) |
|
One 1,000-gallon fuel tank |
Need 1,000 gal. Berm: 12 x 12 x 1 ft holds 1,077 gal (listed) |
Need 1,439 gal. Berm: 10 x 20 x 1 ft holds 1,496 gal (listed) |
Need 2,446 gal. Berm: 10 x 20 x 2 ft holds 2,992 gal (by formula) |
Four 55-gallon drums
With a 1-foot wall, the berm has to hold one drum's 55 gallons plus 23.5 gallons for each of the three drums still standing. Outdoors, rain on the 32-square-foot floor of a 4 x 8 berm adds 70 gallons in Denver and 231 gallons in Houston. In Houston, that brings the total to 357 gallons, which is more than a 4 x 8 x 1 berm holds. Raising the wall to 2 feet fixes it. Each standing drum then displaces about 47 gallons, so the total becomes 55 + 141 + 231 = 427 gallons. A 4 x 8 berm with a 2-foot wall holds 479 gallons by the formula.
A 275- or 330-gallon tote
A single tote is the only container in the berm, so there is no displacement to add. A 330-gallon tote holds 31 gallons more than the 299-gallon 4 x 5 x 2 IBC berm, so even indoors it needs a bigger floor. Outdoors, the rain each floor catches pushes the total up, which is why the table moves to wider floors with 2-foot walls. Measure your tote's base before you settle on a floor size, so the tote fits inside the walls.
A 1,000-gallon fuel tank
Outdoors in Denver, rain on the 144-square-foot floor of a 12 x 12 x 1 berm adds 316 gallons, for a total of 1,316 gallons, which is more than its 1,077. A 10 x 20 x 1 berm catches 439 gallons of rain, for a total of 1,439 gallons. It holds 1,496 gallons, so it covers that total with 57 gallons to spare. In Houston, rain fills all but 0.4 inches of any 1-foot wall, so the berm needs a taller wall. With a 2-foot wall, a 10 x 20 floor holds the tank plus the Houston storm.
A 500-barrel frac tank
A 500-barrel frac tank holds 21,000 gallons, since a barrel is 42 gallons. That's about 2,807 cubic feet of liquid (21,000 / 7.48). The frac tank berm on our site measures 10 x 50 x 1 feet and holds 3,740 gallons, about 18 percent of a full 500-barrel tank. Its listing describes uses such as containment under vehicles, tankers and heavy equipment, and temporary containment for drums, totes and tanks.
Containing a full 500-barrel tank takes a much bigger berm than that. The table runs the backward formula for a 2-foot wall: subtract the rain depth from the wall height, then divide 2,807 cubic feet by the height that's left.
|
Location |
Rain depth |
Wall height left for the spill |
Floor area needed |
|---|---|---|---|
|
Denver |
3.52 in (0.29 ft) |
1.71 ft |
about 1,650 sq ft |
|
Houston |
11.6 in (0.97 ft) |
1.03 ft |
about 2,720 sq ft |
If a second tank shares the berm, add its displacement too: its footprint times the wall height. Frac tank dimensions vary, so take the footprint from your own tank.
Why a berm's listed capacity is a best case
A berm's listed capacity is the most it can hold under ideal conditions. Enpac's L-bracket berm listings say so directly: "The berm capacity is based on dimensions, low flow rate, a level surface and no displaced volume." That means the number assumes liquid fills the berm all the way to the top of the wall, on flat ground, with nothing else inside. On a sloped pad, liquid reaches the top of the low side and runs over before the high side fills. So when the total you calculated lands close to a berm's listed capacity, step up a size or a wall height rather than counting on every last gallon.
When does the SPCC rule apply?
Storage size is one of the tests for whether the SPCC rule applies to a site. The exemption has an aboveground test. Under 40 CFR 112.1(d)(2)(ii), a site's aboveground oil containers, added together, must hold "1,320 U.S. gallons or less of oil." Only containers of 55 gallons or more count toward that total, and some kinds of containers are left out of it. The exemption also requires completely buried storage of 42,000 gallons or less. Twenty-four 55-gallon drums add up to exactly 1,320 gallons, so a 25th drum puts a site over the aboveground limit.
Rules for hazardous waste and flammable liquids
If you store hazardous waste or flammable liquids, a different federal rule may set the number your berm has to meet. Here is how they compare with SPCC:
|
Rule |
What it covers |
Containment it calls for |
|---|---|---|
|
EPA SPCC, 40 CFR 112.8 |
Oil storage at SPCC-regulated sites |
The largest single container, plus room for rain |
|
EPA RCRA, 40 CFR 264.175(b)(3) |
Hazardous waste stored in containers at facilities covered by EPA's Part 264 hazardous waste standards |
10 percent of the total volume of the containers or the volume of the largest container, whichever is greater. Containers that hold no free liquids don't need to be counted |
|
OSHA, 29 CFR 1910.106 |
Diked areas around aboveground tanks (not drums or totes) under OSHA's flammable liquids standard |
Generally the largest tank, assuming it is full, with an exception the rule spells out. The volume of other tanks below the wall height (their displacement) is subtracted from the capacity |
The hazardous waste rule can call for more than one container's worth when you store many small containers. Twenty 55-gallon drums hold 1,100 gallons. Ten percent of that is 110 gallons, so the berm has to hold at least 110 gallons, twice what one drum holds.
What to have ready when you choose a berm
A few measurements make the choice quick, whether you're shopping on your own or calling us. Have these ready:
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The capacity of every container that will go inside, and which one is largest.
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The footprint and height of each container, so you can work out displacement and make sure everything physically fits.
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Whether the berm will sit indoors or outdoors, and your location's 25-year, 24-hour rainfall from NOAA if it's outside.
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The number your plan or state uses, whether that's 100 or 110 percent, and any freeboard amount your engineer has set.
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Whether vehicles or forklifts need to drive in. If they do, look at drive-thru berms. For containers that stay put, L-bracket berms are the common choice.
- How level the ground is where the berm will sit.
Once you have your number, you can compare sizes across all of our spill berms or browse the rest of our spill containment line. If your setup doesn't fit neatly into the examples here, say several tanks in one berm or a high-rainfall site, I'd be glad to run the numbers with you before you buy. Call us at 833-465-8265 or reach out through our contact page.
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