Is Your Knockout Drum Adequately Sized?

A knockout drum is an integral part of a facility’s pressure relief and flare system. The knockout drum is designed to remove any liquid being sent to the flare due to pressure relief devices opening, or intentional venting.

If liquid droplets are unintentionally emitted through the flare, typically referred to as "flare rainout", these can have safety and environmental consequences affecting the site or neighboring community. Hazards such as “flaming rain” can occur, where the liquid droplets ignite as they pass through the flare flame and rain down to the ground while still on fire. Flaming rain can create onsite and offsite personnel hazards as well as becoming ignition sources for secondary fires. Furthermore, if liquid droplets are emitted through the flare tip, then the combustion efficiency may be affected, resulting in the potential for unburnt toxic vapors being emitted to atmosphere.

API Standard 521, Section 5.9, provides detailed guidance for the design of knockout drums. Most flares require that the particle size be reduced to a minimum of less than 300-600 μm, though API RP 14J suggests sizing for liquid droplets between 400 and 500 μm. Knockout drums can be either vertical or horizontal, depending on process requirements and available space. Either way, the equipment diameter must be large enough to keep the vapor velocity sufficiently low to allow entrained liquids to settle or drop out.

Knockout drums should be sized to handle the greatest liquid volume expected at the maximum rates of liquid buildup and pump out. API Standard 521 suggests 20-30 min of liquid holdup. The maximum allowable working pressure (MAWP) of the knockout drum is usually around 50 psig (3.4 barg) as this is expected sufficient to withstand an internal deflagration.

There are many factors to consider when designing a flare knockout drum; and the consequences of getting it wrong can be significant. This paper discusses the best practices to consider when sizing a knockout drum. Additionally, mitigation options for existing inadequately sized knockout drums are discussed.