What is a common safety device used in boilers to prevent overpressure?

A safety valve is a critical safety device found in boilers designed to prevent overpressure. Its primary function is to automatically open when the pressure within the boiler exceeds a predetermined level, thereby allowing steam or liquid to escape. This action reduces the pressure inside the boiler, preventing potential damage to the system or catastrophic failures that could result from excessive pressure.

In contrast, a pressure gauge simply measures the current pressure within the boiler but does not actively relieve pressure. A flow meter measures the flow rate of the fluid but does not address pressure concerns. A temperature sensor monitors the temperature within the system, which is important for overall operations but does not directly control pressure. Therefore, the unique function of the safety valve in automatically mitigating excess pressure makes it the correct answer in this context.