27.05 What are the causes of tank overfills and the appropriate protections?

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, (c) 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%.