In petroleum marketing operations, the term ballast has two meanings.

The first has been derived from the term used to describe additional weight introduced into the hull of a ship to provide greater stability. When an underground storage tank is being installed, there are occasions when groundwater or rainwater might enter the excavation and cause the empty tank to float.

During the installation process, it sometimes becomes necessary to add weight to the tank to keep it from floating. This can be accomplished by dispensing into the tank either water or the product the tank is ultimately destined to contain. The liquid thus introduced into the tank is referred to as ballast.

It is important that the level of liquid used as ballast not be higher than the backfill on the outside of the tank at the time the ballast is added. Otherwise, the weight of the liquid inside the tank– unsupported by backfill on the outside–could cause distortions or ruptures in the tank shell. 

The term ballast is also used to describe an assembly used in fluorescent light fixtures. The device provides the voltage which starts a fluorescent lamp. In addition to starting the lamp, the ballast regulates the flow of current in the lamp circuit.