According to one aspect of the present invention, in an upstream fluid passageway of a valve housing there is fitted a valve seat retainer movable forwardly and backwardly within a given distance along the direction of movement of the pressure fluid.Ball Valves A sealed chamber is formed in a portion of a spacing defined by the rear wall of the valve seat retainer and the inner wall of the valve housing. The valve housing is provided with a channel which communicates between the sealed chamber and the upstream fluid passageway. The channel is opened or closed by a switch valve which in turn allows or prohibits movement of fluid within the upstream fluid passageway toward the sealed chamber, varying the pressure force of the pressurized fluid exerted on the valve seat retainer in the direction of movement of the pressure fluid. When the pressurized fluid is allowed to enter into the sealed chamber, the valve seat retainer and the valve seat are displaced in the direction of movement of the fluid to come into contact with the periphery of the ball.Ball Valves On the other hand, if the pressurized fluid is precluded, the valve seat retainer and the valve seat are shifted in the direction opposite to the fluid movement and thus disengaged from the periphery of the ball. While the pressurized fluid within the upstream fluid passageway acts on the valve seat retainer both in the forward and backward direction of fluid movement, it is obvious that the direction of movement of the valve seat retainer is dependent upon not only the area but also the direction where pressure of the pressurized fluid is effective upon the retainer.

The sealed chamber of the ball valve embodying the present invention forming a portion of the pressure acting area can be affected or released from the pressure of the pressurized fluid by the action of the switch valve. In other words, when the pressure fluid flows into the sealed chamber, the area where the fluid pressure acts in the direction of movement of the fluid or toward the ball is extended so that the pressure urging the valve seat retainer toward the ball is increased as a whole and unbalanced with respect to the pressure in the opposite direction so as to shift the valve seat retainer toward the ball. BALL VALVESOn the other hand, when the pressurized fluid is precluded from the sealed chamber, the pressure acting area where the fluid pressure is exerted toward the ball is limited correspondingly so that the pressure urging the valve seat retainer toward the ball is reduced as a whole. Consequently, the opposite pressure is relatively greater such that the valve seat retainer moves away from the ball.

With the above summarized structure of the ball valve, the switch valve is operated to space the valve seat retainer away from the periphery of the ball during the opening and closing actions of the ball. Therefore, the opening and closing actions can be accomplished with an extremely small torque while not being in contact with the valve seat and the valve seat retainer. Thus, the valve seat is hardly worn in operation. Moreover, since the valve seat is uniformly in contact with the periphery of the ball under the same condition, the valve seat is hardly locally worn or damaged and leakage of the pressurized fluid is thus prevented.

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