Abstract: A gas/liquid separator for separating an inlet stream of a mixture of a gas and a liquid into a substantially liquid stream and a substantially gaseous stream, the separator having a controllable, variable liquid level, includes an elongate cylindrical body portion having first and second closed ends which define a separation chamber, tangentially oriented inlet and outlet nozzles positioned near the ends of the vessel, and an overflow-discharge nozzle extending from inside of the separation chamber and longitudinally exiting from the separator through the inlet end, proximate to the inlet nozzle. The separator further includes a vent subassembly positioned proximate to the overflow-discharge nozzle. The subassembly includes a nozzle in flow communication with the separation chamber, and a flow-stop device operable between an open condition and a closed condition. The flow-stop device is positioned on the nozzle.