| 摘要 |
A ball valve having a housing; a movable ball for opening and closing a fluid passage within the housing; an annular seat retainer fitted in the housing around the fluid passage in opposed relation to, the ball; an annular valve seat mounted on the seat retainer in opposed relation to the ball; a stepped portion formed in the outer peripheral portion of the seat retainer where the seat retainer is in sliding contact with the inner wall surface of the housing, the stepped portion extending from a surface portion having a diameter larger than a seal point diameter of the valve seat to a surface portion having a diameter smaller than said seal point diameter, said larger- and smallerdiameter surface portions forming a sealed chamber in conjunction with the inner wall of the housing which is in sliding contact therewith, the sealed chamber serving as an O-ring slot: and an O-ring fitted in said O-ring slot, characterized in that: a change-over valve having a main cylinder and a sub cylinder is used; a main piston which is axially movable within the main cylinder and a sub piston which is axially movable within the sub cylinder are coupled together; and the interior of the main cylinder is partitioned by the main piston into a primary chamber which communicates with the fluid passage and a secondary chamber which communicates with the body cavity, while the interior of the sub cylinder is partitioned by the sub piston into a communication chamber which communicates with the primary chamber or the atmosphere and a control chamber which communicates with the body cavity, the sub piston being biased toward the control chamber by means of a compression spring in the communication chamber, whereby when the fluid pressure is higher than the internal pressure of the body cavity and the body cavity internal pressure is within an allowable range, the sub piston is held in said biased position to cut off the communication between the fluid passage and the body cavity, while when the body cavity internal pressure reaches a predetermined level, the sub piston is moved against the compression spring to bring the body cavity into communication with the fluid passage through said primary chamber. |
