In process instrumentation, flashing occurs when which condition is met?

• A. A process where liquids are heated until all components vaporize.
• B. 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.
• C. The rapid flashing of a gauge indicator due to pressure spike.
• D. The boiling of a liquid at its boiling point under constant pressure.

Answer: (B)

Flashing happens when a liquid experiences a sudden drop in pressure that goes below its vapor pressure at the current temperature. Liquids have a vapor pressure curve: if the downstream pressure is high enough, the liquid stays as a liquid; but when the pressure falls below the vapor pressure, part of the liquid cannot remain liquid and changes into vapor. This creates a two-phase mixture (both liquid and vapor) in the flow.

This is why the best description is the one that says a portion of liquid turns to vapor as it passes through a valve due to the pressure dropping below the liquid’s vapor pressure. It's not about all of the liquid vaporizing or about the gauge itself flickering, and it’s not simply boiling at the boiling point under constant pressure.