A tank that holds over 500 gallons must have what safety feature?

For a tank that holds over 500 gallons, having a bottom drain that opens in the event of fire is crucial for safety. This feature is designed to help control the hazards associated with the tank's contents during emergency situations, such as a fire. If a fire occurs, the ability to vent the contents of the tank can prevent the tank from becoming a pressure vessel that could rupture or explode, reducing the risk of further damage and enhancing overall fire safety.

This safety measure is primarily aimed at managing flammable or hazardous materials. When such materials are exposed to heat, they can ignite or create toxic fumes. The bottom drain facilitates the removal of these materials to a safe location, allowing for better control during a fire incident.

While the other options, like a pressure gauge or lockable cap, provide important safety measures in different contexts, they do not specifically address the immediate risks associated with holding large volumes of potentially dangerous liquids during a fire. A backup pump, although useful in certain situations for transferring liquids, does not directly relate to the quick response needed in a fire scenario. Therefore, the bottom drain serves a vital role in enhancing the safety and effective response to emergencies related to large storage tanks.