Aboveground Storage Tank Trends

The words "aboveground storage tanks" (ASTs) bring different pictures to mind depending on the individual. While the term AST generally refers to a wide variety of storage units containing various fuels and chemicals over a capacity range of 100 gallons to 20 million gallons, the federal government has defined an AST as any tank with contents 90 percent or more above the surface of the ground.

Most people are familiar with ASTs but don't think about them. They are used for heating oil at homes, by farmers, by fleets (government, utility and private), marinas, golf courses, universities, fire stations, fuel delivery companies, airports, oil fields, refineries, storage "tank farms," in manufacturing and sometimes at retail gas stations. They provide fuel for backup power at hospitals, emergency buildings, food storage warehouses, hotels, condos, shopping malls and computer companies. And this is only a partial list. If you watch, you will probably see several this week.

This article will not cover any pressurized tanks; it instead will cover only tanks designed to be operated at atmospheric or close to atmospheric pressure. Additionally, only tanks designed for storage of flammable and combustible liquids such as petroleum fuels are included in this discussion.

In the United States, ASTs are built and tested according to nationally recognized standards. Let us divide ASTs into smaller categories according to the testing standard.

When buying an AST, check with your insurance company's agents for their input because -- like them -- you are trying to reduce your liability.

Field Erected

The largest ASTs are usually field erected vertical cylindrical tanks and are built according to American Petroleum Institute (API) Standards 620 or 650. These tanks are typically used by oil companies (for storing fuel during production, refining and wholesale distribution) and large industrial users. These tanks are sometimes referred to as "on ground storage tanks" because the entire bottom of the unit rests on the ground or foundation. Sizes are usually measured in barrels (one barrel = 42 gallons) due to the large capacities. Dikes or berms built around the tank or set of tanks provide secondary containment (in case of leak or rupture). The advantage of these systems is the ability for high capacity at the most reasonable cost.


A vault system, usually installed below grade, has recently become popular for larger fleet and retail gas stations. These systems consist of a shop-fabricated steel tank inside a larger concrete vault, which is assembled on site. The system is designed so the primary tank can be visually inspected by entry into the vault and is therefore considered an aboveground tank even though the tank is not visible to the public. The concrete vault provides the required secondary containment. These systems are built according to Underwriter's Laboratories (UL) Standard 2245, Below Grade Vaults for Flammable Liquid Storage Tanks and usually range in sizes of 500 to 15,000 gallons. The advantage of these systems is that tanks do not use up valuable real estate, while certain tax and environmental liability advantages of aboveground tanks are retained.

Shop Fabricated

The third category contains the greatest number of tanks and has the widest variety of users. Tank sizes typically range from 100 to 12,000 gallons, although some manufacturers may produce up to 50,000 gallons under special circumstances. These ASTs are usually shop-fabricated, so the reasonable size is limited by shipping restrictions. They are normally mounted on a stand or saddle or have built-in supports allowing visual inspection under the entire unit. Secondary containment on newer units is provided by a second wall that is an integral part of the system. Installations may be vertical or horizontal and tanks may be either cylindrical or rectangular.

The advantages of these units are ease of installation and flexibility, because the units can be moved if requirements change. They are also considered to be an asset instead of a liability to the property. The majority of the owners of these tanks are only involved in fuel storage, because it is necessary in the performance of their job or as part of the physical plant operation. They are not part of a tank owner organization or petroleum distributor group and have, until recently, largely been ignored by big oil and federal regulators. Today, tanks in these sizes can be subdivided according to the following manufacturing standards:

Prepare and implement a Spill Prevention Control and Countermeasures (SPCC) plan if required, as the penalty for failure to comply may cost more than the tank system.
  • UL Standard 142
  • , Steel Aboveground Tanks for Flammable and Combustible Liquids;
  • Southwest Research Institute's (SwRI) 97-04, Testing Requirements for Fire Resistant Aboveground Flammable Liquid/Fuel Storage Tanks in accordance with Section 2-4.5 of NFPA 30A (1996);
  • UL Standard 2080
  • , Fire Resistant Tanks for Flammable Liquids;
  • SwRI 93-01
  • Testing Requirements for Protected Aboveground Flammable Liquid/Fuel Storage Tanks; and
  • UL Standard 2085
  • , Protected Aboveground Tanks for Flammable and Combustible Liquids.

The Trend

Although incidents involving large on-ground tanks are more widely reported than smaller tank incidents due to the catastrophic nature of the event, most fire jurisdictions had prohibited or greatly restricted the use of shop fabricated ASTs for many years due to fires involving ASTs. These prohibitions and restrictions remained in place until the late 1980s when a UL 142 tank was equipped with integral secondary containment and encapsulated in six inches of concrete for protection. Marketed under the trade name of ConVault, it gradually gained acceptance from the fire prevention officials in California and then across the United States. During the early 1990s, with U.S. Environmental Protection Agency's (EPA) December 1998 deadline for owners and operator to upgrade underground storage tanks looming just a few years away, other tank manufacturers (over one hundred) began developing systems with varying methods and levels of environmental and fire protection. In response to manufacturers and regulators req uests, UL Standard 2085 was promulgated in December 1994. This document set down manufacturing and testing standards for the new "Protected" tank design. An alternate set of criteria for "Fire Resistant" rating was published as UL Standard 2080 in 1997, which allowed an 800 degree Fahrenheit average temperature rise (vs. 260 degrees for UL 2085) and did not require any impact testing. Also in 1997, Southwest Research Institute published SwRI 97-04, an AST standard that had no maximum temperature limit criteria for the fire test and required no impact testing.

In 1997, Southwest Research Institute published SwRI 97-04, an AST standard that had no maximum temperature limit criteria for the fire test and required no impact testing.

What does all this mean to the average business that needs fuel on-site? An example might be a business that is inside city limits. Until at least 1986, an aboveground gas tank probably would not have been allowed, so the owners of the business had an underground storage tank (UST). When they removed their leaking tank (and cleaned up the contamination), it was determined that it was prudent to place the next tank aboveground. If this took place prior to December 1991, they probably met with considerable opposition from regulators with their only AST choice being a protected type tank (although only listed as UL 142) from one of a few closely monitored manufacturers. By mid-1995 they could quite easily get approval for a UL 2085 listed tank, and by the end of 1998 they could choose from several listings and over one hundred manufacturers with a range of quality. Today, these listings are available, although not all are allowed in all circumstances. The surge of manufacturers that occurred in the mid to lat e '90s has dwindled due to excess competition in the market, the number of which is still changing and hard to determine due to buyouts, companies changing their focus to other products or just going out of business.

Recently there has been a push for more liberal interpretations of codes to allow tanks with lower testing criteria to be allowed in more situations due to cost factors. Thus, we have seen the pendulum swing from a very critical and conservative attitude by fire regulators to a quite liberal approval in less than 15 years. This is partly due to the demand for an answer to the leaking underground fuel storage tank problem and partly due to the excellent track record that the new technology of ASTs has compiled since 1986. The pendulum swing is a broad generalization, but a look at the overall picture reveals this strong trend.

What to Do

So what should you do if your business needs to store fuel on-site? My recommendation would be to buy the highest rated tank you can afford. When buying an AST, check with your insurance company's agents for their input because -- like them -- you are trying to reduce your liability. Consider life cycle cost, not just initial cost, and take a close look at what the warranty covers. Make sure you include a determination of the strength of the manufacturer, because a 30-year warranty is only valuable if the company remains in business for 30 years. Contact your local authority having jurisdiction (often the local fire marshal) for their input, even though they usually are not allowed to make formal recommendations. Use your own common sense. Just because you are allowed to install a UL 142 or SwRI 93-01 listed tank doesn't reduce the cost of a cleanup.

Make sure that you have all the proper approvals, and prepare and implement a Spill Prevention Control and Countermeasures (SPCC) plan if required, as the penalty for failure to comply may cost more than the tank system. Consider that the "approval pendulum" will probably swing more conservative again some day. And lastly, accidents happen -- but if you can prevent a spill and/or fire catastrophe, it is not only good for the pocketbook, it is good for the entire community.

I have only touched on standards for historical purposes. More information on standards and fire codes as well as some discussion of federal and state government regulations will be presented in a later article.


Most reputable tank manufacturers now have Web sites where you can become more familiar with their products. Their URL's can be found by looking up the manufacturer by name on the Petroleum Equipment Institute's Web site at www.pei.org/search/ (use Internet Explorer). Links to regulatory organization, listing organization and petroleum related organization Web sites can be found at www.convault.com/links3.html. For Underwriter's Laboratories go to www.ul.com and for SPCC information go to www.epa.gov/oilspill/spcc/index.htm.

This article originally appeared in the 07/01/2001 issue of Environmental Protection.