Which type of safety valve can expel steam without allowing the pressure to rise excessively?

The correct choice of safety valve that can expel steam without allowing the pressure to rise excessively is the safety valve. Safety valves are specifically designed to protect pressure vessels and systems from overpressure situations. They operate by automatically opening at a predetermined set pressure to allow excess fluid or steam to escape, thereby preventing potential damage due to overpressure.

When the internal pressure of a system exceeds the set limit, the safety valve opens to release the steam or fluid, which helps maintain pressure at safe levels. This mechanism is vital in preventing catastrophic failures in systems, especially those involving steam, as excessive pressure can lead to dangerous situations, including explosions.

On the other hand, a pressure relief valve also manages excess pressure, but it functions differently by allowing a controlled release of pressure rather than simply letting excess material escape. A vacuum relief valve serves to prevent vacuum conditions, and a flow control valve regulates the flow rate rather than controlling pressure directly. Thus, the safety valve is the best choice for ensuring that pressure does not rise excessively while expelling excess steam.