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Selecting fuel storage tanks.

UNTIL the use of underground storage tanks (USTs) for fuel storage was mandated by the 1970 Uniform Fire code, above-ground storage tanks (ASTs) were widely used. The tanks were relatively crude by today's standards so the technical superiority and fire protection afforded by use of underground tanks soon made USTs the system of choice for almost all uses.

As a result, tens of thousands of tanks have been underground for more than 20 years, and at some point, many of them began leaking. Often, the first sign of these leaks appeared when groundwater became contaminated. The EPA responded to this major environmental problem by strictly regulating the use of below-ground tanks to store flammable liquids.

These added regulations have had a severe effect on both service stations and private fueling. The removal of underground tanks and the removal and disposal of any contaminated soil is an extremely expensive proposition. Furthermore, new Uniform Fire Code regulations have added to the costs, imposing requirements for double-walled tanks, corrosion protection, electronic leak monitoring, and annual tank testing. These requirements, plus the financial responsibility requirements the EPA imposed on owners and users of below-ground tanks, led directly to a reconsideration of the use of above-ground tanks for some applications.

When determining whether to use AST or UST storage systems, a thorough evaluation of the user's needs is necessary. Generally, ASTs make sense for vehicular fueling when the volume stored is less than 6,000 gallons. Larger sizes may be appropriate for emergency power generation or other situations where a large supply is critical, but use is infrequent. For installations requiring emergency or standby fueling or other low volume applications, ASTs are an excellent means of avoiding some of the environmental problems associated with the typical UST installation.

Re-Emergence of Above-Ground Tanks

In recognition of this fact, the Western Fire Chiefs amended the Uniform Fire Code to stipulate requirements for above-ground tanks used to store fuel for dispensing into motor vehicles. The present code is stringent, requiring secondary containment systems and six in. of reinforced concrete to give the structure both a two-hour firewall rating and protection equivalent to that of a tank four ft underground.

Even with these stringent requirements, above-ground tanks today are inherently more practical than underground tanks for many applications. Not only is the installation (and removal) far less expensive, but the financial responsibility requirements represent a much lower risk. Any leaks that might occur can be readily detected long before they contaminate any groundwater. As a result, a number of manufacturers brought above-ground tanks to market. Models of up to 12,000 gallon capacity are now readily available, all advertised as meeting the current Uniform Fire Code requirements for "Special Enclosures for Dispensing Fuel into Motor Vehicles."

While this Uniform Fire Code regulation stipulated a number of macro requirements which all models must meet, there are significant differences in how the various manufacturers designed their products. Moreover, an understanding of these differences--and their implications--is extremely important both to engineers specifying above-ground tanks and to owner/operators who acquire them.

Secondary Containment

Perhaps the most important difference is in the way various manufacturers have engineered their products to meet the basic Code requirements of 1) secondary containment, and 2) the six in. of reinforced concrete. In practice, these macro requirements have been met by products currently on the market in two fundamentally different ways.

One design approach uses a structure consisting of a steel primary tank, surrounded by a high-density polyethylene liner, which serves as the secondary containment system, all encased in six in. of reinforced concrete. The other design approach is to fabricate both the primary tank and a secondary containment tank from steel. The inner tank is enclosed in a polyethylene bladder, and the annular, six-in. space between the inner and outer steel tanks is filled with reinforced concrete. In this case, there is a system of secondary containment using two levels--the polyethylene bladder and the exterior steel tank.

The problem with above-ground tanks designed with concrete as the outer layer is the fact that all exposed concrete is subject to expansion and contraction due to temperature fluctuation and to damage from external physical elements. This leads to cracking and spalling that could compromise the concrete as a fire protective barrier. It is, therefore, difficult to predict the life of exposed concrete and most manufacturers do not cover exposed concrete in their warranties.

The other design shields the concrete in an exterior steel tank, thereby eliminating the vulnerability of the structure to weathering and external physical damage. Unlike concrete, moreover, an exterior steel tank can be pressure-tested to check for integrity. Thus, the integrity of both the primary and the secondary containments can be verified.

Primary Tank Construction

Obtaining a UL 142 Standard listing on the primary steel wall does not require more than 1/8-in. wall thickness. Accordingly, some manufacturers use a 0.125-in. or a 0.135-in. wall thickness while others use a 0.1875-in. wall thickness. Is this additional steel thickness necessary? The life spans of 1/8-in. or 3/16-in. steel walls exposed to petroleum products are not precisely known. However, they are subject to pinpoint corrosion from bacteria buildup or water buildup in diesel fuel. Eventually, this corrosion will compromise the integrity of either wall, but a 3/16-in. wall should provide 50 percent greater security than a 1/8-in. wall. Since there is not a substantial cost differential among products, it is obviously advantageous to install tanks with 3/16-in. primary steel tanks.

Leak Detection. Since the life span of the primary tank is not precisely known, positive leak detection is a highly desirable feature. The use of a sealed polyethylene bladder or membrane layer around the primary vessel will prevent leaking fuel from being absorbed into the concrete and be undetected at the leak point. Instead it will give the leaking fuel a migration path to the low point where a vapor or liquid monitoring sensor is located. The better manufacturers have this type of system.

Warranty. Manufacturers warrant their products for either 20 or 30 years. Either period seems consistent with the probable life of this type of product, although a 30-year warranty is obviously preferable to a 20-year warranty.

The main issue in warranties regarding products of this kind are how inclusive they are. One warranty stipulates that "this |20-year~ warranty is limited to the tank only and does not include..." Another warranty is far more inclusive, stipulating a "30-year warranty which includes the concrete vaulting materials as well as the primary and secondary tanks." Because of such a wide difference in warranty provisions from product to product, it is advisable to ask for and analyze each manufacturer's warranty before making a purchase or specification decision.

Fire Rating. There is no UL fire rating on any above-ground fuel tank system yet, primarily because these products are so new to the market. However, some manufacturers have commissioned experiments and tests on their own. A typical test or simulated test exposes the above-ground system to the 2000 |degrees~ fire exposure condition of UL 1709 and measures (or predicts), after two hours, the internal temperature of 1) the tank when empty, 2) the tank when 90 percent full of gasoline, and 3) the tank when full of water.

The manufacturer should make available to you the test reports of the independent laboratory that performed the test. It is advisable to read this report carefully. Not only will it provide you with the test results, but it may also give you additional information such as "an outer steel tank which should provide protection to the concrete protective layer and should reduce the effects of spalling."

Configuration. Tanks are available from most manufacturers in standard sizes of 250, 500, 1,000 and 2,000 gallons. Custom sizes up to 12,000 gallons can also be acquired. Also, 500, 1,000, and 2,000 gallon tanks are usually available in divided configurations with half the tank containing one fuel and half the tank containing another (e.g., gasoline and diesel). At least one manufacturer offers a split configuration with non-equal tank compartment sizes (e.g., 750/250 gallons).

Typically, the tanks are four to five ft high to facilitate operating the pump and reading the gauges, which are all on the top of the storage tank. The tank should rest on skids with earthquake/hurricane restraint tie-downs, bolted to a properly designed foundation.

Installation. There are several options in installations involving above-ground vaulted storage tanks. Some vaulted tanks can be field-poured with concrete at the job site. Some have the concrete already precast when they are shipped, and arrive ready to be placed on a concrete pad or foundation. There are cost differences and many variables, such as a remote location, that can make a difference in a successful installation. For a field-poured tank, it is recommended that the installer follow instructions from the manufacturer. Such features as earthquake tiedowns, overfill protection, and venting are generally offered either as standard features or as options.

Although there are some differences in installation methods required by most manufacturers, all conform to the Uniform Fire Code, NFPA guidelines, and local codes that specify minimum clearances from property lines and buildings. Currently, organizations such as the Petroleum Equipment Institute are publishing their recommendations for AST installation.

Conclusions

In the brief period since the 1991 Uniform Fire Code's Appendix Chapter II F covering Above-Ground Storage Tanks for Fuel Dispensing was published, an active industry began to engineer and develop technology to meet the new codes. Selection from among available models must be made with considerable care, but the best of today's above-ground tanks meet the implicit as well as the explicit requirements of that document. As a result, the use of such tanks will result in life-cycle costs that are substantially lower than those of underground tanks, with far less danger to the environment and liability to the owner.
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Author:Doherty, Rick
Publication:Public Works
Date:Jul 1, 1993
Words:1658
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