Method and system for controlling gas flow

A well system and method including applying suction to a wellhead housing outlet (8) whose top is open to the atmosphere, to divert the flow of subterranean gas from flowing through a gas conduit through the wellhead housing (4). An operation can then safely be performed on a component (e.g. removing a hanger) of the wellhead apparatus. Well gas can be diverted to a flare system (200). Suction can be applied by a venturi system including eductors (104,106). The method includes opening the gas conduit outlet once a pressure sensed at the conduit outlet is negative. Suction may also be applied to an upper outlet (14). Also claimed is: decreasing the suction applied to the upper outlet to pull a hanger assembly into position.