Vapor–liquid separator

Lore & Background

In vapor-liquid separators, gravity causes the denser fluid (liquid) to settle to the bottom of the vessel where it is withdrawn, while the less dense fluid (vapor) is withdrawn from the top. In low gravity environments such as a space station, centrifugal force is utilized in a spinning centrifugal separator to drive liquid towards the outer edge for removal, with gaseous components migrating toward the center.

An inlet diffuser reduces velocity and spreads the incoming mixture across the full cross-section of the vessel. A mesh pad in the upper part traps entrained liquid droplets, allowing them to coalesce until large enough to fall through the up-flowing vapor to the bottom. Vane packs and cyclonic separators are also used to remove liquid from the outlet vapor.

The separator is only effective as long as there is a vapor space inside the chamber. It can fail if the mixed inlet is overwhelmed or the liquid drain cannot handle the volume collected. The separator may be combined with a sight glass or float sensor to regulate supply and drain flow.

Reader's Guide

Vapor–liquid separators are widely used across many industries, including oil refineries, offshore platforms, natural-gas processing plants, petrochemical and chemical plants, refrigeration systems, air conditioning, compressor systems, gas pipelines, steam condensate flash drums, geothermal power plants, combined cycle power plants, flare stacks, soil vapor extraction, paper mills, and liquid ring pumps. Their significance lies in protecting downstream equipment, such as compressors, from liquid damage. For instance, in refrigeration systems, some compressor types like the scroll compressor use a continuously shrinking compression volume; if liquid completely fills this volume, the pump may stall, overload, or suffer warped or damaged chambers. The separator prevents such damage by removing liquid before it reaches the compressor. The design of these separators varies by application: vertical separators are used when the gas-liquid ratio is high or gas volumes are high, while horizontal separators are used where large volumes of liquid are involved. A slug catcher is a type of vapor–liquid separator that can receive a large inflow of liquid at random times, typically found at the end of gas pipelines where condensate may appear as slugs of liquid.

Did You Know?

• A vapor–liquid separator may also be referred to as a flash drum, breakpot, knock-out drum, knock-out pot, compressor suction drum, suction scrubber, compressor inlet drum, or vent scrubber.
• In low gravity environments such as a space station, centrifugal force is used instead of gravity to separate vapor and liquid.
• A vortex breaker on the liquid outlet prevents the formation of vortices and of vapor being drawn into the liquid outlet.
• A slug catcher is a type of vapor–liquid separator that can receive a large inflow of liquid at random times, often found at the end of gas pipelines.