 As pressure rises it pushes the tubes against the frame of the valve closing the opening.These valves are typically unidirectional in airflow and are employed in large wall structures where large amounts of airflow is required.One form of blast valve, popularized by the book Nuclear War Survival Skills and tested by ORNL is worn flat rubber tire treads nailed or bolted to frames strong enough to resist the maximum overpressure, with tested closing times cited as being identical to commercial grade blast valves, however the use of this form of blast valve design must also take the risk of the flammable rubber catching fire into consideration.