Abstract: Fluid flow through a conduit is selectively blocked upon the occurrence of an alarm condition by an inflatable member normally folded when not in use to occupy as small a volume as is practicable. The small size volume together with securement of the inflatable member in a preferred location minimizes the effect of the inflatable member to normal fluid flow. A thin, tear-away sleeve maintains the inflatable member in the folded state and quickly tears apart during inflation enabling the inflatable member to fill and plug the interior of the conduit. The shape of the inflatable member, together with the fluid source utilized for inflation assures rapid, liquid-tight sealing. Alarm signal monitoring and inflation initiation is guaranteed under substantially all conditions through the employment of an electronic controller with a stand-by battery.