Flashing in a fluid system occurs under which condition?

• A. A portion of a liquid converts to a vapor as it passes through a control valve because the pressure has fallen below the vapor pressure of the liquid.
• B. Flashing never occurs in practical systems.
• C. A liquid expands but does not vaporize when pressure drops.
• D. Flashing refers to a sudden leak of gas into the atmosphere.

Answer: (B)

Flashing happens when the liquid is depressurized to below its vapor pressure at the current temperature, causing part of the liquid to vaporize. Vapor pressure is the equilibrium pressure at which liquid and its vapor coexist; if the surrounding pressure drops below that value, the liquid boils or “flashes” into vapor, producing a two-phase mixture of liquid and gas. This is a common occurrence in throttling devices, control valves, and rapid depressurization in piping systems, where the pressure drop is enough to push some of the liquid into the vapor phase.

So, the description that matches flashing is the one where a portion of the liquid converts to vapor as the pressure falls below the liquid’s vapor pressure. The notion that flashing never occurs in practical systems isn’t accurate, since flashing is a standard phenomenon in many real-world processes. The idea that the liquid merely expands without vaporizing doesn’t describe flashing, and flashing isn’t simply a leak of gas into the atmosphere.