For situations where fuel oil tanks can not be elevated, flood protection can be provided by replacing the tank with one that can withstand floods and flood forces or by putting the tank in a dry flood-proofed location.
Tank replacement. Replacing the tank with one that can withstand flood forces offers flood protection by ensuring the fuel tank can withstand the hydrostatic pressures and buoyancy (uplifting) forces for the flood event design. This could involve ensuring that the application of the tank is within the boundary conditions set out in tests performed on it, requirements it complies with, or instructions given by the manufacturer.
- Hydrostatic pressure can cause imploding of tanks. For example, a tank that extends 10 feet below the level of the flood must withstand pressures as high as 640 pounds per square foot. Also tanks listed for underground applications can not be designed to withstand hydrostatic pressures if they are mounted at greater depths than the manufacturer specifies.
- The buoyant force on a tank may cause it to break from its base or may push it out of the ground in the case of a buried tank. For example, when submerged, an empty 25,000-gallon fuel oil tank can experience more than 200,000 pounds of buoyant force. Using this example, if concrete ballast was used to stabilize the tank and the ballast was inundated with floodwaters, it would take almost 2,400 cubic feet of concrete (about 14 feet square by 14 feet high) to overcome the buoyant force. The weight of liquid material in a tank balances the buoyant force and decreases lift. Nevertheless, since the amount of fuel in a tank can vary, it is not advisable to depend on the weight of the material within the tank to resist the buoyant forces. Tanks should be properly secured where possible to avoid flotation when the tank is completely empty. If the amount of anchorage needed is not necessary, anchorage should be adequate to prevent the tank from floating when it contains the least amount of fuel oil it will usually hold. Anchorage points will be spread around the tank to prevent the tank from experiencing disproportionate uplift forces.
Dry floodproof. Providing protection by dry floodproofing means placing the tank in a watertight space. Tanks that can not withstand hydrostatic pressures or which can not be anchored to withstand floating can be put in reinforced rooms built to withstand hydrostatic pressures and anchored to prevent flotation. The spaces, also called vaults, are usually made of reinforced concrete because its density helps offset the buoyancy, and concrete can withstand hydrostatic pressures with proper reinforcement. Also useable are steel vaults, which are usually lighter than concrete, but also require additional mass or anchorage to withstand buoyant forces.
Because rooms that contain tanks need access and ventilation to prevent the explosive concentration of gases from accumulating, they should be fitted with specially built, watertight submarine doors and ventilation equipment that vents above the level of flood size. Also, even if dry floodproofing means rendering a building or area “substantially impermeable” inside a house, meaning that no more than 4 inches of water depth may accumulate over a 24-hour span, some water can accumulate so that an internal drainage collection system is needed. It’s recommended to use sump pumps supplied by emergency power sources.
Fuel pumps should also be protected from floodwaters and their controls. Two general types of pumps exist submersible pumps, and external pumps. Submersible pumps that are installed inside the fuel tank are typically used in underground tanks, and sometimes in building tanks above ground. External fuel pumps are usually not floodwater tolerant and can only be used if they are installed in dry floodproof areas. For both types of pumps the pump controls and power should be elevated, dry floodproofed, or designed for submerged operation.
Fill Lines and Tank Vents
All filling lines, pipes, and connections should include suitable components ( i.e. valves) to prevent floodwaters from contaminating fuel tanks and to prevent fuel from escaping during a flood. Tank vents should also be either extended above flood levels or provided with inspection valves that prevent floodwaters from entering the vents when they are submerged. Since a check valve failure can lead to contaminated fuel, it is preferable to extend the vent lines above the DFE. Those guidelines comply with the specifications of NFPA 30.