Knockout drums normally are operated at atmospheric pressure. An emergency sump (disposal) pile is recommended to dispose of the liquid, and a seal in the pile is recommended to contain the backpressure in the drum. For offshore operations, it is recommended to provide normal separation-retention times (1 to 3 minutes on the basis of API gravity) and an emergency dump design to handle the maximum liquid flow with no valves. In onshore operations, it is recommended to take 20% of the maximum potential liquid stream and provide a 10-minute liquid holdup. This is not practical in upstream operations. RP 521 suggests 20 to 30 minutes of liquid holdup. 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. 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. Most knockout drums are horizontal with a slenderness ratio (length-to-diameter ratio) between 2 and 4. RP 14J suggests sizing for liquid droplets between 400 and 500 μm. Most flares require that the particle size be reduced to a minimum of less than 300 μm. The knockout drum removes any liquid droplets that carry over with the gas relief sent to the flare. All flare, vent, and relief systems must include a liquid knockout drum. RP 521, paragraph 5.4.2, provides detailed guidance for the design of knockout drums (also called relief drums or flare or vent scrubbers).

RP 521, Appendix C, provides sample calculations for sizing a flare stack. 4 and 5, cover disposal and depressuring system design. The actual configuration of the flare or vent system depends on the hazards assessment for the specific installation. 5.3.5 Flame distortion caused by lateral windĪ flare or vent system from an atmospheric source may include:.5.3.1 Flare tip diameter and exit gas velocity.

3.5 velocity stack