Abstract: A seal for flow passages has directional characteristics allowing gases or fluids to flow freely in one direction while restricting the flow in the opposite direction. The seal includes a flow diverter cone (or other inclined surface) which is spacedly mounted on a flow inverter cone (or other inclined surface) in a passage to effectively reverse the direction of undesired counterflow of gases without restricting normal gas flow. The seal has no moving parts but serves as an effective aerodynamic valve. Addition of this seal to the stack of a flare system, or to other chimneys, restricts the flow of air into the stack while allowing the waste gases to be exhausted with negligible restriction.