Storage Tanks
Aboveground

Aboveground tanks for fuel in the US are of steel construction. While fiberglass and polyethylene tanks are often used for non-fuel liquids, they are typically not approved because of the relatively low temperature failure mode versus steel.

Aboveground steel tanks may be classified as stationary or portable. Facilities installations are nearly always stationary tanks.

United Laboratories (UL) has standards and certifications for fuel tanks and all US building codes recognize these standards. The basic standard is UL 142 Steel Aboveground Tanks for Flammable and Combustible Liquids. The standard includes single wall, double wall, and diked tanks. The standard covers horizontal tanks and also vertical tanks. The standard covers cylindrical and rectangular tanks. So a proper description of a tank would include descriptors such as: UL 142 Steel Aboveground Tank, Double Wall Horizontal Rectangular Construction.

UL 2085 is a standard for UL 142 Aboveground Tanks that also have Fire Rated properties. The fire rating is typically achieved by installing a lightweight porous cement based material between the inner and outer steel shells of the tank. These tanks are commonly required by State and Local fire marshalls to address their concerns about fire safety.

All fire, building, and environmental codes will require secondary containment of aboveground storage tanks. Double wall tanks are commonly used to provide this containment. Double wall tanks have the benefit of excluding rainwater from the containment when they are installed outside.

Diked tanks are single wall tanks with integral steel containment provided by an open top dike. Most double wall tanks will have a secondary containment volume of 5-20% of the primary tanks. In some places the codes may require a secondary containment of 125% or 150% of the primary tank and diked tanks can fulfill this requirement. Diked tanks should be shielded or covered outside to prevent rainwater accumulation.

Single wall tanks are commonly used, either inside or outside of a building, where concrete structures provide the required secondary containment. Typically the concrete is coated with an epoxy or other material to make it impervious to fuel.

Fire rated steel tanks provide the benefits of fire- protection and also enhanced physical protection for aboveground fuel tanks. The fire rating is meant to protect the tank from a building fire or from a liquid fire from spilled fuel. The fire rating is typically a 2 hour standard, meaning that if the tank is engulfed in a fire that the contents will be protected for at least 2 hours, based the common standard of response for fire fighting equipment and personnel.

The fire rated tanks also meet a standard for physical protection, typically tested with a rifle bullet. Both rural and urban installations have the possibility for damage by accident or vandalism.

Fire Rated tanks are more expensive that double wall tanks in the range of 50-100% more than a double wall tank. They are also because of their weight more expensive to transport and install.

General Location:

The primary site consideration for aboveground tanks is finding a place to put them. Fire and building codes specify required tank separation distances from buildings and property lines. These codes are often based on NFPA 30 and 30A Fire Codes Flammable and Combustible Liquids Code.

Aesthetic / Architectural and Security

Other important site considerations are Aesthetic / Architectural and Security. Aesthetic concerns are often addressed by installing architectural walls around the tank area sometimes in conjunction with landscaping. Security may be inherent if the entire property is protected by fencing and security personnel / security systems, but typically the tank area itself is also secured by lockable doors or fencing.

Access to Tank Area:

Consideration should also be given to access to the site for fire fighting personnel, and for tank re-filling. If truck access adjacent to the tanks is not practical, then fill piping may be installed from the tanks to a remote fill station.

Drainage from Tank and Tank Loading Area:

An important consideration for aboveground tanks is where rainwater or spilled fuel will flow. This is especially important for the fill area where a fuel delivery truck will park and operate. A fuel spill or rainwater with trace amounts of spilled fuel will flow toward the site drainage features. Typically there should be separation of stormwater inlets from tank and tank loading areas, so that if there is a small spill during transfer, that it can be stopped and cleaned prior to flowing into the storm drain. This issue can be addressed in detailed fill procedures, and precautions such as curbing or temporary inlet covers.

Canopies for Tank Areas

Large outside tank areas are typically constructed with concrete containment dikes or curbing. Even with double wall tanks, there may be tank connections, equipment, or piping that may leak – and there is the potential for tank overfills. Rainwater or snow accumulation in these diked and curbed areas can be a maintenance issue. The tank areas may be covered with a roof or canopy to alleviate this problem. The canopies are typically open on the sides so that they are not classified as buildings, however there will be some building code requirements for the canopy structures that needs to be considered.

Pipe Routing to Building

Tanks will be separated from buildings and will require consideration of pipe routing from the tank area to the building. Pipe supports will be required for fuel piping and sometimes for electrical conduit as well. Portions of he pipe routing may be underground requiring the transition detailing for aboveground to underground piping. Piping installed in concrete trenches is sometimes used instead of underground piping, however water intrusion into these trenches can be problematic.

The primary construction issue is providing a suitable foundation for the tank to avoid settlement issues. A reinforced concrete slab on grade with compacted sub-base will often be sufficient in competent soils. Some installation may require footings beneath the tank supports, or in extreme circumstances piers or piling. In any case the tank foundation should be designed by a competent person.

An important construction safety issue is access to the top of tanks to install equipment and instrumentation. A horizontal cylindrical tank does not provide a proper working surface for personnel Proper access and safety procedures, such as personnel lifts and fall protection restraints need to be implemented.

Some local ordinances may severely limit aboveground tank capacity or even prohibit them. Special rules for fuel dispensing into motor vehicles will also limit capacities for these applications.

Tank Construction

Typically regulations will require that tanks be built to UL 142 or UL 2085 standards.

Tank Location

Regulations will specify distances of separation for aboveground tanks from buildings and property lines.

Tank Equipment

Regulations will typically require proper vent devices for aboveground tanks, overfill prevention valves, Anti-siphon and emergency valves.

Fire Protection / Suppression

Fire Protection / Suppression measures may be required by local regulations. Typical requirements may include a piping system to allow for the remote dispensing of foam by fire fighters into the tank containment area.

Leak, Spill, and Overfill Protection

Regulations leak monitoring procedures and devices. These may be visual with recordkeeping or continuous electronic monitors. Spill protection is provided by curbs and diking. Overfill protection is usually required as redundant methods such as a high level alarm device and a fill pipe overfill prevention valve.

Spill Prevention Control and Countermeasure Plans (SPCC)

SPCC Rules are US Federal and State regulations that apply to certain aboveground tanks. They were originally enacted to prevent problems at large marine oil terminals, which had the experience in several instances of discharging large volumes of oil into adjacent rivers. The regulations apply to tanks over 500 gallons where a discharge could impact a navigable waterway. Since the concept of navigable waterway is broadly interpreted, the regulations will apply to most tanks. The standards require a written plan for the facility with the plan reviewed by a professional engineer, and periodically updated.

Emergency Vents

Emergency vents are required by UL 142 and NFPA Standards. The emergency vent is a normally closed device that opens at maximum 2.5 PSI to relieve any pressure accumulating in the tank. The normal vent will maintain the tank at atmospheric pressure under normal condition. The emergency vent is sized so that in a fire condition the vapors generated from the fire heat will not cause pressure to build up in the tanks – they prevent a dangerous explosion. On double wall tanks a second emergency vent is required for the containment space to provide the same protection in the event that there is leaked fuel in that space.

Standard Vents

Standard vent caps allow the tank to remain at atmospheric pressure as the fluid level changes. Air enters as the level goes down, and exits as the level goes up. Vent devices are designed to provide the required air flow without obstruction, and prevent water or animals / insects from entering the vent pipe.

Overfills

Overfills are the most common cause of release from aboveground tanks. The key idea in prevention is redundancy. What several methods should be used to prevent overfills: (a) a method – such as an electronic tank gauge- of determining the fuel volume in the tank, and calculation of the available free space to the 90% fluid level, (b) a secondary method for levels / volumes such as a direct reading level gauge, © a third method for level / volume such as a manual gauge stick, (d) procedures for filling tanks only when facility personnel are present for observation and monitoring and lockable fill equipment to control access, (e) a high level audible and visual alarm to warn the fill operation personnel when the tank reaches 85% capacity, (f) an overfill prevention valve to close and stop flow when the tank level reaches 90%.

Accidental Transfer Between Tanks

An opportunity for overfills exists when multiple fuel supply tanks are included in a system. System designs should include valve control to allow fuel returns from boiler or day tank overflows to return to the active supply tank. Similarly fuel filtration / polishing systems that serve more than one tank should have the same controls. It is recommended that systems be designed with redundancy of control sensors and methods to assure that accidental transfer between tanks cannot occur.

Spills During Tank Fills

Spills during tank filling are likely to result from a failure of some aspect of the overfill prevention system. These failures might include: (a) an overfill valve that does not close, (b) a high level sensor that does activate, © a level measurement that gives an erroneous reading, or (d) a high level alarm that is ignored. These are reasons why redundancy in prevention methods is important.

Water Accumulation

Water naturally accumulates in the bottom of tanks because the specific gravity of water exceeds that of fuel. Water enters the tank as moisture in the air which condenses as it cools in the tank environment. Air is drawn into the tank as the fluid level decrease from consumption, or from temperature changes. In areas of high humidity, the removal of accumulated water can become a regular maintenance item for aboveground tanks

Vent Flame Arrestors

Flame arrestors are not typically used on normal vents for diesel tanks but are sometimes required by local fire marshalls. Where required the flame arrestors would have particular application where vents discharge on the roofs of buildings. A flame arrestor is a device with a series of metal elements that disperse a flame.

Fill Pipe / Spill Containers

In some applications filling of tanks is by connection of the fuel delivery truck hose to a top-of-tank fitting. The fitting is located within a spill containment device, typically of 5-7 gallon capacity.

Overfill Prevention Valves

Overfill prevention valves are installed in the fill pipe to automatically close the fill pipe at a 90% tank level. The valve is actuated by a mechanical float located within the tank. Mechanical overfill valves for aboveground tanks differ from thos used for underground tanks. Fuel delivery for underground tanks is typically by gravity at a low pressure of 5-15 PSI, and the overfill valves are rated for pressures up to about 25 PSI. Aboveground tanks are often filled with pups on the fuel delivery trucks which can generate pressures of 100 PSI, and the AST fill valves are rated for this higher pressure. An alternate overfill prevention method is to provide a solenoid valve, or actuated ball / butterfly valve in the fill pipe, with a high level sensor controlling the closure of the valve.

Ground Mounted Fill Stations

Larger aboveground tanks are usually filled from the ground through hose connections and valves located within a steel containment box. The piping includes a hose tight-fill adapter (camlock), manual valve, check valve, and overfill prevention valve. Fill Station containment capacity ranges from 5-20 gallons. A hand pump is often included to transfer spilled fuel from the containment into the fill pipe and tank. A high level audible visual alarm is mounted in or adjacent to the fill station, and this may or may not include a tank level indicator gauge.

Direct Reading Gauges

Direct reading level gauges are provided for aboveground tanks as a check for tank level / volumes. When an electronic gauge is also used, the manual gauge provides a secondary check. The gauge face must be large enough to be read from ground level.

Submersible Pumps

Submersible pumps are used with aboveground tanks in the same way that they are used for underground tanks. They install into the tank and mount to it through a 4” top opening. Since fuel can siphon through the submersible pump, an anti-siphon valve must be provided.

Foot Valves

Foot valves are vertical check valves installed at the bottom of suction pipes to maintain prime in suction piping for non-submersible pumps. Extractor fittings are sometimes used at the top of tank fitting to allow for easier removal and maintenance of the valves.

Anti-Siphon Valves

Anti-siphon valves are used in suction pipes or at submersible pump discharge pipes to prevent siphon flow from the tank to a lower elevation discharge. Standard anti-siphon valves are spring loaded angle check valves, with the spring sized to resist the static siphon head – typically 5-20 feet of static head. When flow is required, force from the suction pump or submersible pump overcomes the spring force to allow flow. An alternative is to use a normally closed solenoid valve that opens when the pump is activated. The solenoid valve can be less problematic on suction pump applications where the suction pipe characteristics have already placed a significant load on the pump.

Emergency Valves

Emergency valves are normally open valves that automatically close based on fusible links. The valves are held open by a link that fails at about 165 degrees F, to shutdown fuel flow in a fire condition. The valves are required for all tank openings below liquid level and these are often a style that internally mounts in the tank fitting. External valves are installed in fuel supply piping systems at the top of tanks, and where piping penetrates through walls into building rooms. The valves may be standard or FM approved valves. A special version of the valve allows activation / closure through fire alarm inputs.

Level Transmitters, Level Switches, Leak Sensors

Level transmitters for aboveground tanks may be magnestrictuve type probes as used on underground storage tanks. Since aboveground tanks have a larger temperature change during the day, the in-tank tightness testing capability of the level system, as used on UST systems, is not valid for aboveground systems. Ultrasonic and capacitance based level transmitters are also used.

Level switches are used for high and low level alarm signals. The most common type is a float switch, although capacitance and optical types are also used.

Leak sensors used for tank interstitial monitoring are the same as used for underground tanks and are typically float switches, capacitance type, or optical.

Heaters

Heaters are sometimes used for aboveground tanks to maintain diesel fuel temperatures in cold conditions. Heater types are typically an immersed electric heating element installed through the top or side wall of the tank. Steam coils are also available. An alternative method is a circulation heater where fuel is re-circulated through a heater vessel with an internal heating element. Heaters should be properly designed for fuel oil so the temperature at the element itself is limited to a same value. Heaters are typically controlled with a series of thermostats or electronic controller and temperature elements to allow for shutdown on high temperature. Heaters are also controlled to shutdown on low fluid level, that would expose heating elements to air causing overheating, and to shutdown based on ground fault protection of the electrical supply.

Access Steps, Ladders, Platforms

Access Steps, Ladders, and Platforms may be required to allow inspection, maintenance, manual gauging, and sometimes filling of aboveground tanks. These structures should be designed in accordance with OSHA regulations that specify their minimum design and construction requirements.

Visual Inspection

The primary inspection requirement for aboveground tanks is regular visual inspection for equipment damage, wear, or leaks. For SPCC plan compliance a written check list and inspection record would be part of the plan.

Maintenance items would include removal of accumulated water from tank bottoms (typically annually), checking of sensors and instruments, and maintenance of coatings.

Tightness Testing

State regulations may require tightness testing of aboveground tanks at 1-5 year intervals.

Recordkeeping

SPCC regulations require a written record of the plan, records of tests and inspections, records of training facility personnel, response to spill incidents, and periodic management review.

RELATED:

• Storage Tanks – General
• Storage Tanks – Underground
• Storage Tanks in Building