Abstract: A throttle valve has a control member which is angularly positioned by two stepper motors which drive the control member through differential gearing. The drives to the control member are arranged so that a single step displacement of the respective motors in the same direction drives the control member by different amounts in opposite directions, providing a high resolution drive. The control member may be driven at a high speed by reversing one of the motors.

Abstract: A valve arrangement for a pressurizable vessel includes a valve member mountable on a vessel for movement relative thereto. The valve member is formed with a passage which extends over part of the length of the valve member. A valve-actuating assembly is removably mountable on the vessel. The assembly includes a fitting surrounding the valve member with clearance at the exterior of the vessel, a housing movably mounted within the fitting, and force-transmitting elements partially received within the housing and operative for displacing the valve member between an open position in which the passage communicates the vessel interior with the clearance so as to permit fluid flow between the interior and exterior of the vessel, and a closed position in which such fluid flow is prevented.