Abstract: A valve, with a ball having a shaft (5), comprises, at least on the "upstream" side, a seat (14) that can move in the direction in which the fluid flows (F). Seat (14) is connected to body (1) of the valve by an annular membrane (21), constituting a wall of an annular chamber (22) relatively isolated from the "upstream" conditions for example, by a calibrated hole (23). Equilibration means (26) are provided, with a channel (28) and a conduit (29) connecting annular chamber (22) to central volume (27) of the valve, a communication being formed when the valve is opened, thus moving seat (14) away from ball (5). Said communication can be caused to form by control rod (10) of ball (5) when the valve is adjusted for opening.

Abstract: A ball valve has a movable upstream valve seat with a metal lip engageable with the valve ball under pressure generated by a bellows section or metal membrane connected between the movable valve seat and the housing. The force of fluid pressure against this moveable valve seat is relieved by operation of a bypass valve connecting opposite sides of this seat. The ball can also have a deformable metal lip.