Pivoted gate valve

Claim 1

A VALVE COMPRISING, IN COMBINATION, A HOUSING INCLUDING TWO PLATES HAVING OPPOSED FLAT PARALLEL SURFACES DEFINING SIDES OF A CAVITY WITHIN THE HOUSING; OPPOSED FLOW OPENINGS IN EACH PLATE COMBINING WITH THE CAVITY TO PROVIDE A FLOWWAY THROUGH THE HOUSING; OPPOSED ANNULAR GROOVES IN EACH PLATE AROUND AND SPACED FROM THE FLOW OPENINGS; A ROTATABLE SHAFT EXTENDING ACROSS SAID CAVITY AND THROUGH OPENINGS IN THE TWO FLAT PLATES AND EXPOSED TO PRESSURE IN SAID CAVITY; MEANS FOR SEALING BETWEEN THE SHAFT AND EACH PLATE, THE SEALING MEANS ENCOMPASSING EQUAL CROSS-SECTIONAL AREAS TO BALANCE OUT THE EFFECT OF FLUID PRESSURE IN THE CAVITY ON THE SHAFT, THE SHAFT HAVING A NONCIRCULAR PORTION BETWEEN THE PLATES; A FLAT PLAT VALVE CLOSURE MEMBER MOUNTED ON THE SHAFT AND HAVING A NONCIRCULAR OPENING ENGAGING THE NONCIRCULAR PORTION OF THE SHAFT TO PREVENT RELATIVE ROTATION BETWEEN THE CLOSURE MEMBER AND THE SHAFT AND TO ALLOW RELATIVE LONGITUDINAL MOVEMENT BETWEEN THE CLOSURE MEMBER AND THE SHAFT, THE CLOSURE MEMBER BEING ROTATABLE BY THE SHAFT FROM A FIRST POSITION ACROSS THE FLOW OPENINGS TO A SECOND POSITON OUT OF ALIGNMENT WITH THE FLOW OPENINGS, AN ANNULAR O-RING TYPE SEAL IN EACH OF SAID GROOVES AND BEING OF SUFFICIENT CROSS-SECTIONAL AREA RELATIVE TO THE DEPTH OF THE GROOVE TO ENGAGE AN INNER WALL OF THE GROOVE AND TO EXTEND FROM THE GROOVE TO ENGAGE A SIDE OF THE CLOSURE MEMBER WHEN THE LATTER IS IN SAID FIRST POSITION, THE CLOSURE MEMBER BEING OF SUFFICIENT THICKNESS TO ENGAGE BOTH OF THE SAID ANNULAR SEALS ON OPPOSITE SIDES OF THE CLOSURE MEMBER WHEN IN SAID FIRST POSITION, SAID CLOSURE MEMBER BEING FREE TO BE MOVED LONGITUDINALLY INTO SEALING ENGAGEMENT WITH THE DOWNSTREAM ANNULAR SEAL BY LINE PRESSURE.