Abstract: Automatically operable protection is afforded to eliminate pollution to the atmosphere caused by failure of fabric filter bags in a filter network. This is achieved by increases after bag failure in aerodynamic flow volume of fluid in the flow path to the individual filter bags past a closure valve structure. Critical and rapid response is achieved by an aerodynamic substantially flat plate member pivotably mounted off center to initiate movement toward a closure position. A counterweight on the smaller area side moving toward closure assures closure of the flow path. The valve may be latched close, and may telemeter closure to a central location visible to an operator. For reverse jet type systems the valve is mounted in a fluid output flow path from the filter bags in a location where the reverse jet flow does not pass through the valve. Consistent operation of a plurality of networked valves is assured by locating them in flow path positions having little turbulence.