Abstract: A valve is provided for controlling fluid flow between the inlet and outlet of a closed valve housing. A stationary insert body in the housing includes a through channel with cross channels extending transversely therefrom. The latter open on the inner side of an expansible U-shaped ring which surrounds the periphery of the insert body and is spaced from the wall of the valve housing. A piston is movable to close the outlet of the through channel so as to increase the pressure in the cross channels and cause expansion of the ring to close off the passage around the insert body between the inlet and outlet.