In process instrumentation, flashing refers to which of the following?

• A. A process where liquids are heated until all components vaporize.
• B. The boiling of a liquid at its boiling point under constant pressure.
• C. The rapid flashing of a gauge indicator due to pressure spike.
• D. A portion of liquid that turns to vapor when passing through a valve due to the pressure dropping below the vapor pressure of the liquid.

Answer: (D)

Flashing happens when a liquid is throttled through a restriction and experiences a sudden drop in pressure. If the downstream pressure falls below the liquid’s vapor pressure at that temperature, part of the liquid rapidly vaporizes, creating a two‑phase mixture of liquid and vapor. This is different from simply heating a liquid to its boiling point under constant pressure, which would boil the liquid rather than flash as a result of a pressure change. It’s also not about a gauge reading or about all the liquid turning to vapor.

So the description that matches flashing is a portion of the liquid turning to vapor as it passes through a valve due to the pressure dropping below the liquid’s vapor pressure. This illustrates why a pressure drop in a throttling process can cause rapid vapor formation, affecting flow, energy balance, and instrument readings downstream.