Which item is installed on propane cylinders to allow excess pressure to escape?

The correct response is the relief valve, which is an essential safety device installed on propane cylinders. Its primary function is to manage excess pressure that builds up within the cylinder. When the internal pressure exceeds a specific threshold, the relief valve opens to allow gas to escape, thereby preventing the risk of rupture or explosion of the cylinder due to over-pressurization. This ensures the safety and integrity of the propane storage.

Relief valves are designed to automatically reset once the pressure normalizes, making them a reliable safety feature in maintaining the proper functioning of propane systems. This mechanism is critical in both residential and commercial settings where propane is used as a fuel source.

The other options, while they may also be related to pressure control or safety in some contexts, do not fulfill the same role within a propane cylinder. A blowout disk is typically used in other contexts for bursting under specific conditions, a purge valve allows for the release of gas for maintenance but does not serve a pressure regulation function, and a frangible plug is often used in different types of systems to rupture under pressure but is not commonly used in propane cylinders for pressure release.