What does a pressure relief valve do?

A pressure relief valve is a crucial safety device designed to prevent excessive pressure buildup in a tank or system. When the internal pressure exceeds predetermined safe levels, the valve opens to release some of the vapor or gas, thereby reducing pressure and preventing potential explosions or equipment failure. This ability to release vapor pressure ensures the safety and integrity of the system by maintaining operating conditions within designated limits.

The other options describe functions that do not align with the purpose of a pressure relief valve. For instance, holding the tank closed under all conditions or automatically refilling the tank involves mechanisms distinct from pressure relief management. Maintaining a constant internal pressure is typically the role of other devices, such as pressure regulators, rather than a pressure relief valve, which specifically reacts to overpressure situations.