What device prevents a boiler explosion (BLEVE) if the boiler runs out of water?

The low water cut off (LWCO) is designed to protect a boiler from potentially dangerous situations, particularly the risk of a Boiling Liquid Expanding Vapor Explosion (BLEVE) that can occur if the boiler runs out of water. When the water level drops below a certain threshold, the LWCO automatically shuts off the fuel supply to the boiler. This action prevents the boiler from operating without water, which is crucial for keeping the system cool and preventing overheating that could lead to an explosion.

The other devices listed, while important for boiler safety, serve different functions. A pressure relief valve is designed to release excess pressure from the system to prevent overpressure situations, and a safety valve also releases pressure but is primarily focused on ensuring that the operating pressure does not exceed safe levels. A water level gauge, on the other hand, provides a visual indication of the water level in the boiler but does not actively prevent a malfunction or explosion on its own. Therefore, the role of the LWCO in actively maintaining safe operational conditions makes it the correct answer in this scenario.