In order to size the knockout drum or scrubber appropriately, you should look at the potential liquid carryover for removing liquids from the vapor stream prior to vapor being sent to the flare or vent.
Liquid retention capacity should be provided to allow for ESD system reaction time or manual intervention in the event of control or ESD malfunction. Sufficient vapor space should be provided such that, with maximum liquid level in the knockout vessel and under the worst case combination of liquid and gas being relieved, the maximum liquid droplet size leaving the vapor outlet can be completely burned by the flare or ignited vent. Liquid retention of 20 minutes inland and 10 minutes offshore, and 450-micron maximum liquid droplet size are recommended as the initial design selection. (See API RP 521).
Knockout drums should be sized per ASME and API RP 521. The scrubber and downstream vent/flare stack piping should be designed for at least 150% of calculated operating pressure at maximum operating flow rate but not less than 150 psi (1035 kPa) unless the maximum design pressure of all sources tying into the system is less than or equal to 150 psig (1035 kPa gauge). Pressure rise in the event of flashback should be accounted for in selecting design pressure of the knockout vessel. A minimum design pressure of 100 psig (690 kPa gauge) may be used for low pressure flares. The vessel will typically be protected from flashback if designed to 100 psig and operating at atmospheric pressure.
Knockout drums should be gravity drained either completely or through level control valves. Knockout drums can be gravity drained to closed drain tanks if adequate pressure differential exists under all conditions to ensure adequate flow. Pumps controlled by level control devices can be utilized to pump out knockout drums.
When pumps are used to remove liquid from the flare KO drum, they should be sized to empty the KO drum from the high-high level within 2 hours. Two 100% pumps should be installed. The discharge piping should be sized for simultaneous operation of both pumps.
Knockout drum can also be used as the closed drain tank. Sufficient normal liquid volume in addition to the emergency liquid retention volume should be provided.
Drums should be equipped with reflex gage glasses, which cover the full range of operating levels, as well as all other normal pressure vessel connections and accessories, including manual drain, man way, pressure gauge, and indicating thermometer.