Knockout drums (also known as relief drums or flare or vent scrubbers) removes any liquid droplets that carry over with the gas relief sent to the flare. Most flares require that the particle size be reduced to a minimum of less than 300 μm. Normative suggests sizing for liquid droplets between 400 and 500 μm. Most knockout drums are horizontal with a length-to-diameter ratio of 2 to 4. A horizontal knockout drum must have a diameter large enough to keep the vapor velocity low enough to allow entrained liquids to settle or drop out.
Knockout drums operated at atmospheric pressure should be sized to handle the greatest liquid volume expected at the maximum rates of liquid buildup and pump out. Onshore, it is recommended to take 20% of the maximum potential liquid stream and provide a 10-minute liquid holdup. An emergency disposal pile is recommended to dispose of the liquid, and a seal in the pile is recommended to contain the backpressure in the drum.