What causes a safety valve to open?

A safety valve is designed to protect a system from excessive pressure by automatically venting the excess pressure when it exceeds a predetermined level, known as the set limit.

When the internal pressure within a vessel or system reaches this set limit, the safety valve opens to allow some of the steam or gas to escape. This action helps prevent potential accidents or damage that could arise from overpressure, such as explosions or ruptures. The valve is calibrated to open precisely at the set pressure, ensuring that safety mechanisms function properly and maintain the integrity of the system.

In contrast, manually opening a valve or addressing high temperature conditions does not trigger the safety valve operation, and cutting off the fuel supply does not directly relate to pressure management within a boiler or pressure vessel. Therefore, the mechanism that specifically activates the safety valve is its response to excess pressure, making this the correct understanding of its function.