Compensated valve

Claim 1

A VALVE UNIT ADAPTED TO CONTROL THE FLOW RATE OF GAS UNDER PRESSURE COMPRISING, IN COMBINATION, A VALVE BODY WHICH FORMS AN INLET DUCT DUCT OF TAPERING CROSS-SECTION, AN OUTLET DUCT LATERALLY DISPOSED WITH RESPECT TO SAID INLET DUCT, MEANS IN SAID OUTLET DUCT FORMING A SONIC THROAT, AND AXIALLY MOVABLE VALVE MEANS COMPRISING A VALVE HEAD AND A COMPENSATING PISTON AXIALLY SPACED APART AND SUPPORTED ON A CONTROL ROD, SAID VALVE MEANS BEING AXIALLY MOVABLE TOWARDS AND AWAY FROM SAID INLET DUCT, SAID VALVE HEAD COMPRISING RESPECTIVE UPSTREAM AND DOWNSTREAM SIDES, SAID COMPENSATING PISTON COMPRISING AN EFFECTIVE PRESSURE AREA FACING THE DOWNSTREAM SIDE OF SAID HEAD, THE DOWNSTREAM SIDE OF SAID HEAD COMPRISING AN EFFECTIVE PRESSURE AREA SMALLER THAN THE FACING SAID AREA ON SAID PISTON, SAID VALVE HEAD FORMING WITH SAID TAPERING CROSS-SECTION DUCT A VARIABLE CONVERGENT NOZZLE EXTENDED DOWNSTREAM BY A SUDDEN WIDENING, SAID NOZZLE BEING SONIC NOZZLE WHEN SAID VALVE HEAD IS AT THE BEGINNING OF ITS AXIAL MOVEMENT AWAY FROM SAID TAPERING CROSS-SECTION DUCT, THE UPSTREAM AND DOWNSTREAM EFFECTIVE PRESSURE AREAS ON SAID VALVE HEAD BEING SO PROPORTIONED RELATIVE TO THE EFFECTIVE PRESSURE AREA ON SAID PISTON THAT FOR ALL AXIAL POSITIONS OF SAID VALVE MEANS RELATIVE TO SAID INLET DUCT, THE TOTAL FORCE ACTING AGAINST SAID VALVE MEANS IN AN AXIAL DIRECTION AWAY FROM SAID INLET DUCT REMAINS CONSTANT.