03.07 What equipment is needed to outfit aboveground tanks?

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.

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.