UST systems are commonly inerted with which substances?

UST systems, or Underground Storage Tank systems, are commonly inerted with dry ice and compressed nitrogen. Inerting these systems is crucial for safety and environmental protection; it helps to minimize the risk of fires and explosions by displacing the oxygen that could potentially support combustion.

Dry ice, which is solid carbon dioxide, sublimates to gas at room temperature and does not support combustion. Compressed nitrogen, being an inert gas, similarly does not react chemically and is used to displace any flammable vapors present in the tank. Both of these substances help in creating an inert atmosphere within the tank, ensuring that any gases present do not reach levels that might ignite.

The other substances listed in the options are not suitable for inerting UST systems. For instance, using helium and oxygen would not be safe, as oxygen is highly reactive and can contribute to combustion when mixed with flammable vapors. Water and fuel would not serve this purpose effectively, as introducing water could create an environment conducive to microbial growth and corrosion of the tank, while fuel can obviously introduce flammability risks. Thus, the combination of dry ice and compressed nitrogen effectively meets the requirements for safe operation of UST systems.