Knock out drum

A knock-out drum is a pressure vessel that separates entrained liquid from a gas or vapor stream by sudden velocity drop, letting droplets settle out before the gas goes downstream. At an LNG or chemical terminal it sits in the vapor-return and flare or vent system: it catches condensed liquid carried in boil-off gas or in the vapor displaced during a tanker transfer, protecting compressors, the flare tip, and the vent stack from liquid slugs. Sizing follows the Souders-Brown gas-load equation for the allowable vapor velocity above the liquid surface, with a demister pad to coalesce fine droplets. It is process plant on the terminal, not cargo-handling equipment, sized to the peak vapor rate of the loading arms.