An apparatus for controlling the flow of a fluid includes a LP fluid inlet (18) and a LP fluid outlet (9) connected to a flare/vent system. A LP flow line (20) connects the LP fluid inlet (18) to the LP inlet 6a of a jet pump (6). A supply of HP fluid is connected to jet pump inlet (6b), and a MP line (8) is connected to jet pump outlet (6c). A control unit (28) is configured to control operation of first and second LP flow control valves (4, 3) according to signals received from a flow meter (12), so that LP gas is diverted from the flare system to the LP inlet (6a) of the jet pump for pressurisation.