Water Seals (Liquid Seals) and / or Knock-Out Drums may be designed as vertical vessels, in which case they can be incorporated at the base of the flare system, or as horizontal vessels located at a reasonable distance from the flare. It is, however, advisable to locate the water seal and knock-out drum vessels as close to the stack as possible to avoid condensation in the flare header between the vessel and the flare.

Water Seals (Liquid Seals) provide a positive means of flash-back prevention in addition to enabling the upstream flare system header to operate at a slight positive pressure at all times. This is especially of use when an elevated flare is used in combination with another flare or with a flare gas recovery system.

The Water Seal (Liquid Seal) vessel is fitted with a special saw-tooth dip leg and anti-pulsation baffle to minimise pulsing. Within the Water Seal, the water level is preferably maintained by a constant overflow weir in combination with a suitable 'S' bend drainpipe. Filling rates will be sufficient to re-establish the seal within 5 minutes if the liquid seal is broken.

he Water Seal vessel may be equipped with an internal steam coil / sparger for winterisation purposes as required.

Knock-Out Drums are designed to effectively remove hydrocarbon liquids from the main flare relief gas to prevent the possibility of liquid carryover and “flaming rain” from the flare tip.

The sizing of these knock-out drum vessels is generally based on the criteria defined in API RP 521 considering residence time and knock-out velocity of the liquid particles in the vapour flow. Since flare tips can handle small liquid droplets, the allowable velocity in the drum may be based on that necessary to knock out droplets from 300 microns to 600 microns in diameter; typically 450 microns.