Underground Fuel Storage Tanks VS Aboveground Fuel Storage Tanks
How to Decide which Type of Storage Tank is the Best Option for Upgrading Your Facilities Bulk Fuel Storage
Site specifics and local and state regulations play an important role in deciding which storage system to select when comparing bulk storage options for fleet fueling, emergency diesel generators, and fuel oil systems. Speaking in general terms, the initial cost of an underground storage tank (UST) installation is approximately 15-25% more than an aboveground storage tank (AST). These numbers only paint a partial picture. Facility and property managers need to look at the long-term maintenance as well to determine the real cost of fuel storage tank ownership.
What is the Cost of Ownership for an Underground Storage Tank?
USTs require a monthly inspection (typically by personnel in house) and yearly testing of the different components of system. The approximate cost of testing is $1,500/per year. A Compliance Certification is required by the Massachusetts Department of Environmental Protection (MassDEP) and New Hampshire Department of Environmental Services (NHDES) every 3 years. The purpose of the Compliance Certification is to assist with regulatory compliance and to encourage good UST system management practices. You can submit this form on your own or get assistance through a tank management program for a cost of $800.
Underground Storage Tank Repair Costs
The choices made during the design and planning will directly impact the long term maintenance of a UST so it’s important to think about future maintenance and replacement. The number and diameter of manways make a big difference when it comes time to test and repair a buried tank. Unions within piping sumps allow easy disconnection of pipes. A pipe chase that has extra slack also allows for easy disconnection and replacement if required.
Steel components that come in contact with water will corrode so ensuring easy access can reduce or eliminate the need to cut concrete and excavate a pipe or fitting for repair or replacement. The NH Department of Environmental Services recently issued a warning about the use of steel flex connectors in direct buried applications. Flexible connectors allow for pipe repairs in UST sumps where there is no slack or unions in the piping but the manufacturer did not intend for the pipe to come in contact with soil. According to the Underwriters Laboratory listing UL-2039 flex connectors may be used underground when accessible “within chase piping” or “enclosed tank, transition or dispenser sumps”.
New construction materials provide superior long-term reliability which lowers the risk of corrosion. Installation materials such as nylon, high-density polyethylene and fiberglass-reinforced composite will not rust or corrode. Water tight double-wall spill buckets contain fuel while allowing surface water to drain into the backfill and provide increased durability. A buried tank is also protected from elements like wind, snow and sun.
What is the Cost of Ownership for an Aboveground Storage Tank?
ASTs require a monthly walk through inspection (by in house personnel) and a yearly inspection of the tank monitoring system by a certified inspector. The approximate cost of this is $450/per year. If you have an AST, the facility is required to have and maintain an SPCC plan as well. SPCC plans vary based on the complexity of the facility. Developing a plan from scratch can cost between $8,000 and $12,000 and updating an existing plan may cost between $500 and $1,500. The SPCC plan will specify the inspection cycle of your AST by an STI SP001 certified inspector. The frequency schedule will be based on the age and condition of your tank. For example; a large field erected tank without secondary containment installed near saltwater may require yearly inspections but a shop-built tank installed within secondary containment may have a 10-year inspection cycle. The state of Massachusetts requires annual inspections of all regulated ASTs as part of the Use Permit program. The cost of an inspection ranges from $250-$800 depending upon tank size.
Aboveground Storage Tank Repair Costs
Let’s take a look at common recurring repair costs for ASTs. The biggest hit when looking at AST maintenance is painting or coating. The exterior paint should be re-touched or repainted every 5-10 years. To repaint an AST so that the coating meets the paint manufacturer’s requirements and provides the best protection to the steel, the tank should be sandblasted, primed and painted. The cost of recoating a tank that is brought to near white metal via sandblasting costs approximately $4.4 per square foot.
How to Select an Aboveground Storage Tank
Before you purchase a storage tank it is important that certain requirements are met for the location and type of fuel stored.
- Tanks installed outdoors must have a UL listing for exterior installation.
- Secondary containment must have 110% capacity of the primary.
- Double-walled tanks provide secondary containment and eliminate rainwater and snow collection.
- Tanks that store gasoline must have a 2-hour fire rating.
- Tank that store diesel do not require a 2-hour fire rating because the flash point is higher than 100F.
How to Select an Underground Storage Tank
Underground storage tank are manufactured to provide different attributes. Fiberglass Reinforced Plastic (FRP) is a non-corrosive material that is the most widely used in underground tank installations.
Steel USTs coated with fiberglass reinforced plastic provide high structural integrity but are more susceptible to corrosion long term from accumulation of water internally. USTs constructed entirely of FRP have a lower structural strength and require proper backfill material to provide up to 90% of the tank’s support. The benefit of %100 FRP construction is corrosion-resistance, both inside and out. Following the manufacturer’s instructions is critical to eliminating structural damage and meeting the product warranty. Additional installation guidance is provided by the Petroleum Equipment Institute (PEI) and American Petroleum Institute (API).
Importance of Using a Certified Tank Installer
A certified tank installer is trained by the manufacturer on the proper installation techniques and recommended components for a new fuel storage tank. Building owners who hire a tank installer that doesn’t have these certifications run the risk of equipment being installed that was not intended for the application. Fuel tanks, piping and monitoring systems are intended to be periodically inspected and maintained for continued service by a qualified fuel system technician in accordance with manufacture instructions and/or industry practices. Some states require additional certifications from the fuel tank contractor prior to working on the installation of a new or replacement AST system.
Proper planning is another important component to new AST installations and an experienced tank installer will know the regulations required for your installation. The NH Department of Environmental Services requires complete plans and specifications to be submitted for review and approval, 45 days prior to a new AST installation. Plans must be approved before commencing construction and the site must pass a successful inspection by the NHDES to confirm the fuel system was installed according to the approved plans at project completion.
Do USTs require more periodic fuel storage tank cleaning than ASTs?
This question addresses the perception that USTs require frequent tank cleanings because they are installed below ground. After reviewing this question with field technicians at CommTank and examining customer records, we believe that USTs are more likely to accumulate water than an AST. This water accumulation is more likely to occur through an improperly maintained fill, vent or sensor pipe, than through the transfer of vapor through the air.
In Massachusetts and New Hampshire, the temperature of fuel stored in USTs is typically measured at 55 degrees during July and August and the air aboveground can reach the mid to upper 90’s. USTs expand as the soil above them is heated during the day which allows hot air to enter a cool tank. When the warm air mixes with the fuel, condensation forms on the tank walls. But this only occurs if you let your fuel level drop. More ullage space allows for a larger transfer of air. If an UST fuel level is kept between 80-90% full then there is virtually no ullage space for air to move in and out of. Storage tanks for backup generators are typically kept at capacity so they will provide maximum run time in case of an emergency. We rarely find water in a properly maintained generator tank.
A new delivery of fuel is the most common source of water in storage tanks. Bulk storage tanks have a large amount of turnover which means they have larger ullage space in their tanks on a regular basis. The air exchange in the tanks along with temperature differences in the fuel being transferred is a recipe for condensation and water buildup. Facility managers and tank owners should check their tanks after every delivery to ensure that no water was introduced into their storage tank.
CommTank recommends cleaning your tanks at least once of year if you turn your fuel inventory over fairly quickly. If you don't turn over inventory quickly, we recommend cleaning the tank once every three years. CommTank also recommends using an additive for diesel fuel to ensure the lubricity factor stays high and to eliminate fuel microbial growth. The requirement that facilities use Ultra Low Sulfur Diesel (ULSD) has also created a concern for long term fuel storage. USLD fuel is more likely to suffer from oxidation. The gum that forms when low sulfur fuel mixes with air can clog fuel filters and injectors. Tank owners can avoid this issue by eliminating water and keeping sludge buildup to a minimum.
Snow removal for storage tanks
Is there a requirement to keep ASTs clear of snow or is it just a best management practice? The answer may vary based on local codes and fire department rules. Typically emergency vents (primary and interstitial space) and tank gauges must be kept clear in addition to the fill port and vent. Fire departments now require remote fill ports for large ASTs instead of stairs that access a fill port on top of the tank. This change reduces the chance of a delivery driver slipping while carrying a fuel hose up metal stairs in ice and snowy conditions. Spill kits are required to be accessible at all times so they must be located near the AST and kept clear of snow.
USTs are usually located in parking areas and snowplows keep lots clear for vehicle traffic and access to delivery bays. For USTs located on lawns, the top of the tank pad must be kept clear for maintenance and filling. Many facilities install USTs as close to the building as possible to minimize piping runs. Shrubs and trees are used to hide the fact that a UST is located there, but this only makes it more difficult to keep the fill pipe and manways clear of ice and snow. We recommend maintaining a 3 foot clearance around manways and using only bark mulch to prevent weeds and erosion.
Is there a leak risk when storing fuel in Underground Storage Tanks?
One common misconception is that USTs have a greater possibility of a leak as compared to AST’s. This misconception stems from the single wall steel tanks and pipes that were found to have been leaking during the 80’s and 90’s. The EPA made significant changes to UST regulations in the past 30-years that now require operator training, secondary containment, walkthrough inspections, spill and overfill prevention equipment, containment sump and tank monitoring equipment testing.
According to a ‘Cause of Leak Study’ performed by the Florida Department of Environmental Protection, 90% of leaks reported between 1998 and 2005 were devices other than the UST. Of the data collected, 2% of double-wall tanks were the source of a leak and 9% were related to double-wall piping. Most striking is that 39% were attributed to overfills/spills and spill buckets. If you compare current spill protection components and leak detection technology to tanks monitored during this 2005 study, you will see it’s virtually impossible that a properly installed double wall fiberglass tank with double wall piping, containment sumps and leak detection systems will develop a leak that impacts the environment.
The Pros and Cons of ASTs versus USTs
In the case of an AST, other than the space of the tank, pad and protection bollards, there are also set back requirements found on NFPA 30A. Depending on the size and type of tank, ASTs need to be installed away from property lines, buildings and dispensers. UST’s don’t have set back requirements and trucks can drive over them. Placing a tank underground means you can convert the space above to parking, driveways or storage for emergency generators.
Setback Requirements for Aboveground Storage Tanks Larger than 110-gallons
- IFC (2015) requires that dispensing devices be located 10 feet or more from lot lines and buildings having combustible exterior wall surfaces or buildings having non-combustible exterior wall surfaces that are not part of a 1-hour fire resistance-rated assembly or buildings having combustible overhangs.
- NFPA 30 (2015) requires that unprotected above ground storage tanks of 275 gallons or less be located ten feet from property lines and five feet from important buildings
- NFPA 30A (2015) requires that dispensing devices be located 10 feet or more from property lines and buildings having combustible exterior wall surfaces or buildings having non-combustible exterior wall surfaces that are not part of a 1-hour fire resistance-rated assembly or buildings having combustible overhangs.
Another consideration is the aesthetics which may be important depending on the area of the proposed installation. An aboveground storage tank, the concrete pad it is installed on and connecting pipe may be unsightly for adjacent property owners. The visual impact to your property can be reduced by surrounding ASTs with plantings and screened fencing or by painting the tanks to blend in with the building. A solid high fence will hide the AST, piping and fill port but metal, wood or Trex fences add significant time and cost to the AST installation. Plantings take time to mature and must be maintained so they don’t interfere with the filling and maintenance of the tanks. Dark paint colors will increase internal temperatures within a tank and could cause additional vapor loss through the vent pipe or increase microbial growth.
Aboveground storage tanks have a higher risk of vandalism, theft and damage due to vehicle impacts or severe weather events. Hurricanes, Tornadoes, lightning and flooding are examples of weather that could damage an AST while falling snow and ice from buildings can damage piping. ASTs are installed with bollards to protect them from vehicle impact and tall fencing will keep out trespassers. Proper anchoring and lightning protection will protect ASTs from storm damage.
Aboveground tanks must be labeled to indicate volume and type of product stored. Proper labeling is important for accurate filling and emergency response actions. Product signage should be 5” x 14“, with black lettering on a white background. Placards in indicating flammability “Flammable: Keep Fire and Flame Away” should be 10”x 14”, with red lettering on a white background. “Dyed Fuel” and “Undyed Fuel” should be 10”x 10”, black on white.
Underground tanks are installed below the frost line. The soil around a UST stays at a consistent range (50-55 degrees) during summer and winter. This means that diesel fuel won’t gel, and accumulated water will not freeze and clog inlet filters, in subzero temperatures. Installing below ground sometimes comes with extra costs. In New England bedrock can be seen jutting out from parks and lawns alike. Encountering bedrock during a UST installation can add significant costs to a project due to the blasting and jack hammering required when removing it. High groundwater tables are another common occurrence in Massachusetts. Many cities and towns were built on wetland areas that were backfilled. It’s not uncommon to encounter groundwater from 3’ to 10’ in many areas. Dewatering an excavation during installation increases the cost, time and difficulty of access to the UST. Underground tank manufacturers design their products with these conditions in mind. There a specific requirements for keeping underground tanks where they belong including concrete deadmen and straps to tie down the tank. It’s important to test the soil type and groundwater levels before choosing a UST for your facility.
The Bottom Line
Aboveground storage tanks are less expensive when comparing cost of ownership. ASTs are easier to install, maintain and repair and complete access makes them easier to relocate if you ever need to move your tank. Underground storage tank installation is a viable option where the value of creating additional space for parking, delivery/loading bays and or green space outweighs the cost differential. Ultimately, the choice will come down to site requirements and financial considerations.