What describes the function of pressure vacuum relief valves?

The function of pressure vacuum relief valves is crucial for the safe operation of tanks, particularly in tanker systems. These valves are designed to protect tanks from both excess pressure and vacuum conditions, which can occur during the loading and unloading of cargo, as well as from temperature changes that affect the cargo inside.

When a tank is filled, the liquid expands and could create pressure that needs to be released. Conversely, when the liquid is withdrawn, the volume decrease can create a vacuum that can damage the tank. The pressure vacuum relief valves address both scenarios: they open to release excess pressure to prevent tank rupture and they also permit air to enter the tank when a vacuum develops, ensuring structural integrity.

Furthermore, the valves are typically located either directly at each tank or at a central vent pipe, allowing for effective management of pressure and vacuum conditions in the system. They are designed to be normally closed to maintain the tank's pressure integrity, only opening when necessary to relieve pressure or allow air in.

Thus, recognizing that pressure vacuum relief valves serve multiple essential functions, including their strategic placement and operational state, is why the option stating "all of these" accurately describes their role in tanker safety and operation.