Valve with vortex chamber and a mechanical member to shut off flow

A valve assembly comprises a vortex chamber with an axial outlet port, a main inlet port for a fluid to be controlled, and a substantially tangential inlet port; the fluid enters through an inlet chamber in which is a mechanical valve movable so as to obstruct fluid flow into the vortex chamber. A duct links the inlet chamber to the tangential inlet port of the vortex chamber. The position of the mechanical valve affects the flow of fluid through the duct, so the vortex chamber amplifies the effect of the mechanical valve. Further movement of the valve closes off flow altogether. The assembly may include a weir in the outlet, to separate gas and liquid phases.