How to Select a Steel Fuel Tank for an Underground Installation

Selecting a fuel tank for an underground installation will probably have you looking at either an all-fiberglass tank, or a carbon steel tank. If you like the durability and strength of steel, there are several steel-tank designs for you to consider. This article describes three underground storage tanks (UST) that provide the necessary secondary containment and leak monitoring, as well as the corrosion-protection so critical to these installations.

As some background… all of these tanks are built per specifications published by the Steel Tank Institute (STI). The tanks are shop-fabricated, under STI license, by independent manufacturers across the United States.

Here are your options for the best steel tank designs currently offered by the STI specifications.

Steel Primary Tank with Fiberglass-Jacket Secondary Containment

Permatank™ is an F-922 (Fiberglass Secondary Containment), UL-58 and UL-1746 labeled tank, designed for underground storage of flammable and combustible liquids. These tanks are manufactured of carbon steel, with an exterior FRP jacket that is applied over a mesh-like material. The FRP jacket provides corrosion protection and secondary containment with an interstitial monitoring space. These tanks are available in capacities up to 50,000 gallons, with single or multiple compartments. Here are the Installation Instructions for this type of tank.

Steel Primary Tank with Steel Secondary Containment

ACT-100™ tanks are F-894 (External Corrosion Protection of Composite Steel USTs), UL-58 and UL-1746 labeled and designed for underground storage of flammable and combustible liquids. ACT-100 tanks are manufactured of double-wall carbon steel, with a 100-mil FRP laminate coating. The double-wall steel construction offers secondary containment with an interstitial space for leak monitoring, and the exterior is clad in FRP to provide protection against corrosion. Tanks are available in capacities up to 50,000 gallons, with single or multiple compartments. Here are the Installation Instructions for this type of tank.

ACT-100U tanks are F-961 (External Corrosion Protection of Composite Steel USTs), UL-58 and UL-1746 labeled and designed for underground storage of flammable and combustible liquids. Similar to the ACT-100 design, the ACT-100U tanks are manufactured of double-wall carbon steel, but their exterior features a 70-mil urethane coating. These tanks are available in capacities up to 50,000 gallons, with single or multiple compartments. Here are the Installation Instructions for this type of tank.

A Solution for Every Budget

Given the above choices, which is best suited for your project? All of these tanks are compliant with NFPA codes, UL, and environmental regulations. So, in the end, it may be your client’s budget that tilts the scale toward one or the other. Here is a simple Cost vs. Benefit chart:

	Permatank	ACT-100	ACT-100U
Construction Type	Steel Primary / FRP Secondary	Double-wall Steel / FRP-clad Exterior	Double-wall Steel / Urethane-coated Exterior
Initial Cost	Best	Better	Good
Durability	Good	Better	Best
Ease of Installation	Good	Better	Best

In my opinion, the double-wall tanks provide a more rugged construction, with lower potential for damage during transportation and installation. Given a reasonable budget, I would choose the ACT-100 as the best value, with its solid double-wall steel construction and excellent corrosion protection provided by the FRP exterior cladding. For projects with tighter budgets, the Permatank design offers a great alternative, as long as the installation is performed with careful attention to the manufacturer’s recommended procedures. With a generous budget, and for the longest life expectancy, I would choose the ACT-100U, with its superb 70-mil urethane coating.

Here are a few points specific to steel fuel tanks installed underground:

• All of the above tank designs eliminate the need for cathodic protection. Whether by use of FRP jacket, FRP cladding or urethane coating, these tanks offer a much simpler and lower-cost installation than earlier designs relying on cathodic systems for corrosion protection.
• Compared to above-ground tanks, underground tanks are more susceptible to water contamination from moisture condensation. This is due to the cooler temperatures surrounding these buried tanks. With this in mind, and considering the higher water-absorption rate of today’s biofuel blends, it is important that certain procedures be put in place if long-term fuel storage is expected. While steel tanks are fully-compatible with biofuel blends, it is the water contamination that can cause problems to the tank and related equipment. You can follow this link for my recommendations on the subject of fuel quality.
• Steel tanks do not rely on the fill substrate for support, and they can be designed to be buried at any depth. Even so, care must still be taken during installation. Although not so much a structural concern (as in the case of fiberglass tanks), a poor choice of backfill material can damage the exterior of the tank.
• The Steel Tank Institute offers a 30-Year Warranty for Permatank, ACT-100 and ACT-100U tanks, but this warranty must be assigned to each tank individually and, unless requested, it may not be automatically included by the independent tank fabricator. If you are a specifying engineer, be sure to include this requirement in your specifications.

As a final point, be aware that fuel tanks will “float” out of the ground in installations that have water-saturated soil, due to excessive rain or due to a naturally-high water table. Concrete deadmen and nylon straps should be specified to keep underground fuel storage tanks securely in place. This applies to any type of underground fuel tank.