Whenever a process requires that entrained droplets be removed from a vapor stream, a knockout drum should be integrated into the system. Condensation can occur when hot process gases cool considerably in the flare gas header and riser, and some gases also go to dew point at ambient temperatures and pressures and can therefore generate liquids. These droplets must be removed in order to avoid the phenomenon known as flaming rain, which occurs when liquid droplets are blown out the top of a flare stack along with ignited process gases. These vessels can be either horizontal or vertical.
The liquid seal drum is a specially designed vessel containing a predetermined level of water in the base of the drum. The main purpose of the liquid seal drum is to stop flame propagation in the unlikely event of flashback. The liquid seal also acts as a large check valve so that gas can not travel upstream for any reason. A third function of the liquid seal is to disentrain liquid droplets. Liquid seals can be designed as independent vessels or as an integral part of flare risers.