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A blowdown stack is an elevated vent or vertical stack that is used to vent the pressure of components of a chemical, refinery or other plant if there is a process problem or emergency. A blowdown stack can be used to complement a flare stack or as an alternative. The purpose is to prevent 'loss of containment' of volatile liquids and gases. Blowdown from several systems may be combined in a blowdown header prior to the stack. A knock-out pot may be provided at the base of the stack to remove any liquids. Blowdown stacks may either be ignited (like a flare) or un-ignited (a ‘cold’ vent). The height of the blowdown stack must be tall enough to ensure the safe dispersal of vapour.