An apparatus for rapidly terminating of the flow of any fluid or particulate material through a conduit responsive to predefined dangerous conditions. A particular application will terminate fuel flow through a large fuel supply conduit in a racing vehicle responsive to a fire or accident or other dangerous condition. The apparatus includes a gas supply source that is opened responsive to actuation of the apparatus such that gas passes into a cylinder which moves to force a liquid to hydraulically expand an inflatable member which is operative to expand within the flow conduit to instantaneously terminate material flow therethrough.