It is the gas sealing device uses the density difference between air and gas to prevent air from flowing back into the flare header. It is installed on the flare stack just below the flare tip. The design includes a drain to grade level with a suitable seal for removal of trapped condensables.
The mol seal should have a hand hole near the drain line to permit cleaning when the seal or the drain line is plugged. The bottom of the mol seal and the entire drain line should be electrically traced and insulated or steam traced and insulated in areas where temperatures fall below 35°F.
If the knockout drum is immediately below the flare stack, the drain line can be run directly to it. Valves on these drain lines should be normally open. Consideration for toxic hazards should be considered in design of these drains in sour/acid gas flare designs.