What is “flash steam”?

Flash steam refers to the phenomenon that occurs when hot water is introduced to a lower-pressure environment, leading to the rapid conversion of some of that water into steam. This occurs because the boiling point of water decreases as pressure decreases. Thus, when hot water suddenly finds itself in a lower pressure area, it can "flash" into steam even without any increase in heat. This principle is critical in various applications within mechanical and process engineering, particularly in steam generation and heating systems, where efficient utilization of heat and water is essential.

In contrast, the other choices relate to different steam phenomena or situations: steam from a broken pipe represents a leak rather than a specific steam type, cold water generating steam is inaccurate without sufficient heat, and excess steam during operation does not define the scenario of water transitioning into steam under lowered pressure conditions.