An improved rotary plug valve of the type having a valve body with a fluid inlet conduit and a fluid outlet conduit therein has a rotary plug disposed for rotation in the valve body between an open position and a closed position.PLUG VALVES In the closed position the plug has a seat seal disposed in a groove in the cylindrical surface thereof which seat seal extends adjacent and surrounds the fluid inlet conduit to seal the inlet conduit against fluid flow. The groove in the plug has a radially inner wall,Safety valves a radially outer wall and a base wall. The base wall extends at an angle so that the radially outer wall extends more deeply into the plug than the radially inner wall. Preferably the groove and the seat seal have an annular, cylindrical shape. Also preferably the seat seal has a shape so that a pressure space is formed adjacent the base wall of the groove and a gap is formed adjacent the radially inner wall of the groove. Fluid pressure in these urge the seat seal into sealing positions.Check Valves Also preferably the seat seal has a greater curvature in the direction of its leading and trailing edges, when not under pressure, than the surface of the plug so that the seat seal is less prone to cutting as it moves over the inlet opening.

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