NOVELTY - The valve has a mobile seat (150) formed by a circular ring centered on an axis (X0X0) of a body and movable in sliding along the axis relative to the body so as to be applied against a fixed seat (230) of a spherical cap (221) of a large-sized spherical shutter (200). A stop (116) for stopping end of travel of opening of the shutter is placed on a path of an edge (223A) of a cylindrical passage (210) at a side of a spherical crown (220) cooperating with the mobile seat. The edge of the passage is supported against the stop in the closing position.    USE - Valve i.e. guard valve (claimed), for a hydroelectric control turbine in a hydroelectric power station.    ADVANTAGE - The valve is provided with the large-sized spherical shutter for protection of hydroelectric control turbine of hydroelectric power stations, which are robust, resist water having variable quality and reliability. The valve allows the natural closing of the spherical shutter even in the absence of energy and guarantees perfect sealing while presenting the reduced encumbrance. The valve generates closing couple, which compensates resistant couple generated by the friction of the bearings, with the help of double offsets of the spherical shutter. The valve is placed in the closing position in the absence of energy to control an actuating jack of the shutter irrespective of the flow speed in the passage of the sphere. The fixed seat of the shutter is formed by the spherical stainless steel crown initially at the end of the closing, and the mobile seat is auto-controlled and pushed against the fixed seat of the sphere by springs, thus avoiding the need of an additional couple for generating frictions of the seats applied in an uninterrupted manner to the spherical rotation. The shutter does not require a high surface quality state such that rotation of the shutter is simple.    DETAILED DESCRIPTION - The mobile seat is formed by a principal ring with T-shaped section receiving a wear ring that is made of nickel aluminum-bronze. The principal ring and the wear ring form a joint that is made of polyurethane.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view illustrating an operating principle of a spherical shutter with double offsets.    Rotational axis (CR)    Offset (delta a)    Median plane (MM)    Body axis (X0X0)    Passage axis (X1X1)    Stop (116)    Mobile seat (150)    Large-sized spherical shutter (200)    Cylindrical passage (210)    Spherical crown (220)    Spherical cap (221)    Passage edge (223A)    Fixed seat (230)