Relief system disposal “processes” usually include some type of separation process to ensure the safe operation of the ultimate disposal methods. To avoid the occurrence of “burning rain”, flare systems always contain a vapor-liquid separator (the “flare knockout drum”) upstream of the flare itself. In other cases, relief disposal systems may contain a counter-current absorption contactor to remove one or more hazardous or valuable components from the relief stream. For example, refinery alkylation units route relief and vent streams acid (HF or sulfuric) through a caustic scrubber to neutralize the acid; the scrubbed vent stream is then sent through the remainder of the collection system to a flare.
Flare Gas Recovery. In some cases, environmental or economic considerations may justify installation of equipment to recover noxious or valuable components from a relief stream that might otherwise simply be flared. These are typically sized to recover such components only from relatively routine venting activity, and should therefore be designed to direct the relief stream directly to the flare in emergency situations. See API Recommended Practice 521 for further discussion of these systems.