A positively controlled ball valve for flowing fluids, in particular water, for instance for use in snow making machines, and comprising a valve housing (1) having a flow passageway (3) for the water (7), a valve seat (9) arranged in said flow passageway (3), and a valve ball (10) which is movable towards and from said valve seat (9), and a force actuation means (4) having a piston rod (20) for positively acting on said valve ball (10), which is movable in a race (11) in the valve housing (1), which race extends from the valve seat (9) obliquely outwards-rearwards, upstream the flow of water (7), and in which the force actuation means (4) is formed with a piston rod (20) arranged for being able to force the valve ball (10) in the direction away from the valve seat (4) and into the race (11) for the ball. The piston rod (20) is arranged to act on the valve ball (10), in the first stage of the opening of the valve, so that said ball at least partly rolls out of the valve seat (9). The valve seat (9) is formed with a valve sealing (14) of an elastomeric material having a metal ring (16) engaging the inner surface and the rear end of the valve sealing (14).
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