1. IN A PNEUMATIC CONVEYING SYSTEM A VESSEL FOR THE MATERIAL TO BE CONVEYED; SAID VESSEL HAVING AN OUTLET OPENING; A GAS PIPE DISPOSED IN SAID VESSEL AND EXTENDING TOWARD SAID OUTLET OPENING; MEANS FOR SUPPLYING GAS TO SAID GAS PIPE; A NOZZLE CONNECTED TO SAID GAS PIPE AND DISPOSED IN SUBSTANTIALLY AXIAL ALIGNMENT WITH THE OUTLET OPENING IN SAID VESSEL HOPPER; A VENTURI-SHAPED VALVE BODY COMMUNICATING WITH SAID OUTLET OPENING AND WITH THE CONDUIT OF SAID PNEUMATIC CONVEYOR; A FLEXIBLE LINING DISPOSED IN SAID VENTURI-SHAPED VALVE BODY AND ADAPTED FOR MOVEMENT INWARDLY WITH RESPECT THERETO; AND MEANS FOR SUPPLYING GAS UNDER PRESSURE TO THE INTERIOR OF SAID VALVE BODY TO MOVE SAID FLEXIBLE LINING THEREFROM AND CONSTRICT THE THROAT OF THE VENTURI-SHAPED OPENING PROVIDED THEREBY.