03.08 What are the primary problems or failures of aboveground tanks?

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.